chore: bump version

cmd/root_test.go

@@ -0,0 +1,383 @@
+package cmd
+
+import (
+	"bytes"
+	"io"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"sub-cli/internal/config"
+)
+
+// setupTestEnv creates a testing environment with redirected stdout
+// and returns the output buffer and cleanup function
+func setupTestEnv() (*bytes.Buffer, func()) {
+	// Save original stdout
+	oldStdout := os.Stdout
+	
+	// Create pipe to capture stdout
+	r, w, _ := os.Pipe()
+	os.Stdout = w
+	
+	// Create buffer to store output
+	outBuf := &bytes.Buffer{}
+	
+	// Create cleanup function
+	cleanup := func() {
+		// Restore original stdout
+		os.Stdout = oldStdout
+		
+		// Close writer
+		w.Close()
+		
+		// Read from pipe
+		io.Copy(outBuf, r)
+		r.Close()
+	}
+	
+	return outBuf, cleanup
+}
+
+// TestExecute_Version tests the version command
+func TestExecute_Version(t *testing.T) {
+	// Save original args
+	oldArgs := os.Args
+	defer func() { os.Args = oldArgs }()
+	
+	// Set up test environment
+	outBuf, cleanup := setupTestEnv()
+	
+	// Set args for version command
+	os.Args = []string{"sub-cli", "version"}
+	
+	// Execute command
+	Execute()
+	
+	// Get output
+	cleanup()
+	output := outBuf.String()
+	
+	// Verify output
+	expectedOutput := "sub-cli version " + config.Version
+	if !strings.Contains(output, expectedOutput) {
+		t.Errorf("Expected version output to contain '%s', got '%s'", expectedOutput, output)
+	}
+}
+
+// TestExecute_Help tests the help command
+func TestExecute_Help(t *testing.T) {
+	// Save original args
+	oldArgs := os.Args
+	defer func() { os.Args = oldArgs }()
+	
+	// Set up test environment
+	outBuf, cleanup := setupTestEnv()
+	
+	// Set args for help command
+	os.Args = []string{"sub-cli", "help"}
+	
+	// Execute command
+	Execute()
+	
+	// Get output
+	cleanup()
+	output := outBuf.String()
+	
+	// Verify output contains usage information
+	if !strings.Contains(output, "Usage:") {
+		t.Errorf("Expected help output to contain usage information")
+	}
+	
+	if !strings.Contains(output, "Commands:") {
+		t.Errorf("Expected help output to contain commands information")
+	}
+}
+
+// TestExecute_NoArgs tests execution with no arguments
+func TestExecute_NoArgs(t *testing.T) {
+	// Save original args
+	oldArgs := os.Args
+	defer func() { os.Args = oldArgs }()
+	
+	// Set up test environment
+	outBuf, cleanup := setupTestEnv()
+	
+	// Set args with no command
+	os.Args = []string{"sub-cli"}
+	
+	// Execute command
+	Execute()
+	
+	// Get output
+	cleanup()
+	output := outBuf.String()
+	
+	// Verify output contains usage information
+	if !strings.Contains(output, "Usage:") {
+		t.Errorf("Expected output to contain usage information when no args provided")
+	}
+}
+
+// TestExecute_UnknownCommand tests execution with unknown command
+func TestExecute_UnknownCommand(t *testing.T) {
+	// Save original args
+	oldArgs := os.Args
+	defer func() { os.Args = oldArgs }()
+	
+	// Set up test environment
+	outBuf, cleanup := setupTestEnv()
+	
+	// Set args with unknown command
+	os.Args = []string{"sub-cli", "unknown-command"}
+	
+	// Execute command
+	Execute()
+	
+	// Get output
+	cleanup()
+	output := outBuf.String()
+	
+	// Verify output
+	if !strings.Contains(output, "Unknown command") {
+		t.Errorf("Expected output to contain 'Unknown command' message")
+	}
+	
+	if !strings.Contains(output, "Usage:") {
+		t.Errorf("Expected output to contain usage information when unknown command provided")
+	}
+}
+
+// TestHandleSync tests the sync command
+func TestHandleSync(t *testing.T) {
+	// Create temporary test directory
+	tempDir := t.TempDir()
+	
+	// Create source file
+	sourceContent := `[00:01.00]This is line one.
+[00:05.00]This is line two.`
+	sourceFile := filepath.Join(tempDir, "source.lrc")
+	if err := os.WriteFile(sourceFile, []byte(sourceContent), 0644); err != nil {
+		t.Fatalf("Failed to create source file: %v", err)
+	}
+	
+	// Create target file
+	targetContent := `[00:10.00]This is target line one.
+[00:20.00]This is target line two.`
+	targetFile := filepath.Join(tempDir, "target.lrc")
+	if err := os.WriteFile(targetFile, []byte(targetContent), 0644); err != nil {
+		t.Fatalf("Failed to create target file: %v", err)
+	}
+	
+	// Set up test environment
+	outBuf, cleanup := setupTestEnv()
+	
+	// Execute sync command
+	handleSync([]string{sourceFile, targetFile})
+	
+	// Get output
+	cleanup()
+	output := outBuf.String()
+	
+	// Verify no error message
+	if strings.Contains(output, "Error:") {
+		t.Errorf("Expected no error, but got: %s", output)
+	}
+	
+	// Verify target file has been modified
+	modifiedContent, err := os.ReadFile(targetFile)
+	if err != nil {
+		t.Fatalf("Failed to read modified target file: %v", err)
+	}
+	
+	// Check that target file now has source timings
+	if !strings.Contains(string(modifiedContent), "[00:01.000]") {
+		t.Errorf("Expected modified target to contain source timing [00:01.000], got: %s", string(modifiedContent))
+	}
+	
+	// Check that target content is preserved
+	if !strings.Contains(string(modifiedContent), "This is target line one.") {
+		t.Errorf("Expected modified target to preserve content 'This is target line one.', got: %s", string(modifiedContent))
+	}
+}
+
+// TestHandleSync_NoArgs tests sync command with insufficient arguments
+func TestHandleSync_NoArgs(t *testing.T) {
+	// Set up test environment
+	outBuf, cleanup := setupTestEnv()
+	
+	// Execute sync command with no args
+	handleSync([]string{})
+	
+	// Get output
+	cleanup()
+	output := outBuf.String()
+	
+	// Verify output contains usage information
+	if !strings.Contains(output, "Usage: sub-cli sync") {
+		t.Errorf("Expected sync usage information when no args provided")
+	}
+}
+
+// TestHandleSync_OneArg tests sync command with only one argument
+func TestHandleSync_OneArg(t *testing.T) {
+	// Set up test environment
+	outBuf, cleanup := setupTestEnv()
+	
+	// Execute sync command with one arg
+	handleSync([]string{"source.lrc"})
+	
+	// Get output
+	cleanup()
+	output := outBuf.String()
+	
+	// Verify output contains usage information
+	if !strings.Contains(output, "Usage: sub-cli sync") {
+		t.Errorf("Expected sync usage information when only one arg provided")
+	}
+}
+
+// TestHandleConvert tests the convert command
+func TestHandleConvert(t *testing.T) {
+	// Create temporary test directory
+	tempDir := t.TempDir()
+	
+	// Create source file
+	sourceContent := `1
+00:00:01,000 --> 00:00:04,000
+This is a test subtitle.`
+	sourceFile := filepath.Join(tempDir, "source.srt")
+	if err := os.WriteFile(sourceFile, []byte(sourceContent), 0644); err != nil {
+		t.Fatalf("Failed to create source file: %v", err)
+	}
+	
+	// Define target file
+	targetFile := filepath.Join(tempDir, "target.vtt")
+	
+	// Set up test environment
+	outBuf, cleanup := setupTestEnv()
+	
+	// Execute convert command
+	handleConvert([]string{sourceFile, targetFile})
+	
+	// Get output
+	cleanup()
+	output := outBuf.String()
+	
+	// Verify no error message
+	if strings.Contains(output, "Error:") {
+		t.Errorf("Expected no error, but got: %s", output)
+	}
+	
+	// Verify target file has been created
+	if _, err := os.Stat(targetFile); os.IsNotExist(err) {
+		t.Errorf("Target file was not created")
+	}
+	
+	// Verify target file content
+	targetContent, err := os.ReadFile(targetFile)
+	if err != nil {
+		t.Fatalf("Failed to read target file: %v", err)
+	}
+	
+	// Check that target file has VTT format
+	if !strings.Contains(string(targetContent), "WEBVTT") {
+		t.Errorf("Expected target file to have WEBVTT header, got: %s", string(targetContent))
+	}
+	
+	// Check that content is preserved
+	if !strings.Contains(string(targetContent), "This is a test subtitle.") {
+		t.Errorf("Expected target file to preserve content, got: %s", string(targetContent))
+	}
+}
+
+// TestHandleConvert_NoArgs tests convert command with insufficient arguments
+func TestHandleConvert_NoArgs(t *testing.T) {
+	// Set up test environment
+	outBuf, cleanup := setupTestEnv()
+	
+	// Execute convert command with no args
+	handleConvert([]string{})
+	
+	// Get output
+	cleanup()
+	output := outBuf.String()
+	
+	// Verify output contains usage information
+	if !strings.Contains(output, "Usage: sub-cli convert") {
+		t.Errorf("Expected convert usage information when no args provided")
+	}
+}
+
+// TestHandleFormat tests the fmt command
+func TestHandleFormat(t *testing.T) {
+	// Create temporary test directory
+	tempDir := t.TempDir()
+	
+	// Create test file with non-sequential numbers
+	content := `2
+00:00:05,000 --> 00:00:08,000
+This is the second line.
+
+1
+00:00:01,000 --> 00:00:04,000
+This is the first line.`
+	testFile := filepath.Join(tempDir, "test.srt")
+	if err := os.WriteFile(testFile, []byte(content), 0644); err != nil {
+		t.Fatalf("Failed to create test file: %v", err)
+	}
+	
+	// Set up test environment
+	outBuf, cleanup := setupTestEnv()
+	
+	// Execute fmt command
+	handleFormat([]string{testFile})
+	
+	// Get output
+	cleanup()
+	output := outBuf.String()
+	
+	// Verify no error message
+	if strings.Contains(output, "Error:") {
+		t.Errorf("Expected no error, but got: %s", output)
+	}
+	
+	// Verify file has been modified
+	modifiedContent, err := os.ReadFile(testFile)
+	if err != nil {
+		t.Fatalf("Failed to read modified file: %v", err)
+	}
+	
+	// Check that entries are correctly numbered - don't assume ordering by timestamp
+	contentStr := string(modifiedContent)
+	
+	// Just check that identifiers 1 and 2 exist and content is preserved
+	if !strings.Contains(contentStr, "1\n00:00:") && !strings.Contains(contentStr, "2\n00:00:") {
+		t.Errorf("Output should contain sequential identifiers (1 and 2)")
+	}
+	
+	// Check content preservation
+	if !strings.Contains(contentStr, "This is the first line.") || 
+	   !strings.Contains(contentStr, "This is the second line.") {
+		t.Errorf("Output should preserve all content")
+	}
+}
+
+// TestHandleFormat_NoArgs tests fmt command with no arguments
+func TestHandleFormat_NoArgs(t *testing.T) {
+	// Set up test environment
+	outBuf, cleanup := setupTestEnv()
+	
+	// Execute fmt command with no args
+	handleFormat([]string{})
+	
+	// Get output
+	cleanup()
+	output := outBuf.String()
+	
+	// Verify output contains usage information
+	if !strings.Contains(output, "Usage: sub-cli fmt") {
+		t.Errorf("Expected fmt usage information when no args provided")
+	}
+}

docs/commands.md

@@ -114,15 +114,18 @@ Currently, synchronization only works between files of the same format:
 #### For LRC Files:
 
 - **When entry counts match**: The source timeline is directly applied to the target content.
-- **When entry counts differ**: The source timeline is scaled to match the target content using linear interpolation.
+- **When entry counts differ**: The source timeline is scaled to match the target content using linear interpolation:
+  - For each target entry position, a corresponding position in the source timeline is calculated
+  - Times are linearly interpolated between the nearest source entries
+  - This ensures smooth and proportional timing across entries of different counts
 - **Preserved from target**: All content text and metadata (artist, title, etc.).
 - **Modified in target**: Only timestamps are updated.
 
 #### For SRT Files:
 
 - **When entry counts match**: Both start and end times from the source are directly applied to the target entries.
-- **When entry counts differ**: A scaled approach is used:
-  - Start times are taken from proportionally matched source entries
+- **When entry counts differ**: A scaled approach using linear interpolation is used:
+  - Start times are calculated using linear interpolation between the nearest source entries
   - End times are calculated based on source entry durations
   - The timing relationship between entries is preserved
 - **Preserved from target**: All subtitle text content.
@@ -131,17 +134,35 @@ Currently, synchronization only works between files of the same format:
 #### For VTT Files:
 
 - **When entry counts match**: Both start and end times from the source are directly applied to the target entries.
-- **When entry counts differ**: A scaled approach is used, similar to SRT synchronization:
-  - Start times are taken from proportionally matched source entries
+- **When entry counts differ**: A scaled approach using linear interpolation is used, similar to SRT synchronization:
+  - Start times are calculated using linear interpolation between the nearest source entries
   - End times are calculated based on source entry durations
   - The timing relationship between entries is preserved
 - **Preserved from target**: All subtitle text content, formatting, cue settings, and styling.
 - **Modified in target**: Timestamps are updated and cue identifiers are standardized (sequential from 1).
 
+### Timeline Interpolation Details
+
+The sync command uses linear interpolation to handle different entry counts between source and target files:
+
+- **What is linear interpolation?** It's a mathematical technique for estimating values between two known points. For timeline synchronization, it creates a smooth transition between source timestamps when applied to a different number of target entries.
+
+- **How it works:**
+  1. The algorithm maps each target entry position to a corresponding position in the source timeline
+  2. For each target position, it calculates a timestamp by interpolating between the nearest source timestamps
+  3. The calculation ensures proportionally distributed timestamps that maintain the rhythm of the original
+
+- **Example:** If source file has entries at 1s, 5s, and 9s (3 entries), and target has 5 entries, the interpolated timestamps would be approximately 1s, 3s, 5s, 7s, and 9s, maintaining even spacing.
+
+- **Benefits of linear interpolation:**
+  - More accurate timing when entry counts differ significantly
+  - Preserves the pacing and rhythm of the source timeline
+  - Handles both expanding (target has more entries) and contracting (target has fewer entries) scenarios
+
 ### Edge Cases
 
 - If the source file has no timing information, the target remains unchanged.
-- If source duration calculations result in negative values, a default 3-second duration is applied.
+- If source duration calculations result in negative values, a default duration of zero is applied (improved from previous 3-second default).
 - The command displays a warning when entry counts differ but proceeds with the scaled synchronization.
 - Format-specific features from the target file (such as styling, alignment, metadata) are preserved. The sync operation only replaces timestamps, not any other formatting or content features.
 

docs/zh-Hans/commands.md

@@ -114,15 +114,18 @@ sub-cli sync <源文件> <目标文件>
 #### 对于LRC文件：
 
 - **当条目数匹配时**: 源时间线直接应用于目标内容。
-- **当条目数不同时**: 源时间线使用线性插值进行缩放以匹配目标内容。
+- **当条目数不同时**: 源时间线使用线性插值进行缩放以匹配目标内容：
+  - 对于每个目标条目位置，计算源时间线中的对应位置
+  - 在最近的源条目之间进行线性插值计算时间
+  - 这确保了在不同数量的条目间实现平滑和比例化的时间分布
 - **从目标保留**: 所有内容文本和元数据（艺术家、标题等）。
 - **在目标中修改**: 只更新时间戳。
 
 #### 对于SRT文件：
 
 - **当条目数匹配时**: 源的开始和结束时间直接应用于目标条目。
-- **当条目数不同时**: 使用缩放方法：
-  - 开始时间取自按比例匹配的源条目
+- **当条目数不同时**: 使用基于线性插值的缩放方法：
+  - 开始时间使用源条目之间的线性插值计算
   - 结束时间根据源条目时长计算
   - 保持条目之间的时间关系
 - **从目标保留**: 所有字幕文本内容。
@@ -131,17 +134,35 @@ sub-cli sync <源文件> <目标文件>
 #### 对于VTT文件：
 
 - **当条目数匹配时**: 源的开始和结束时间直接应用于目标条目。
-- **当条目数不同时**: 使用缩放方法：
-  - 开始时间取自按比例匹配的源条目
+- **当条目数不同时**: 使用基于线性插值的缩放方法，类似于SRT同步：
+  - 开始时间使用源条目之间的线性插值计算
   - 结束时间根据源条目时长计算
   - 保持条目之间的时间关系
 - **从目标保留**: 所有字幕文本内容和样式信息。
 - **在目标中修改**: 更新时间戳并标准化提示标识符。
 
+### 时间线插值详情
+
+同步命令使用线性插值来处理源文件和目标文件之间不同的条目数量：
+
+- **什么是线性插值？** 这是一种估计两个已知点之间值的数学技术。对于时间线同步，它在应用于不同数量的目标条目时，可以在源时间戳之间创建平滑过渡。
+
+- **工作原理：**
+  1. 算法将每个目标条目位置映射到源时间线中的对应位置
+  2. 对于每个目标位置，通过插值计算最近的源时间戳之间的时间戳
+  3. 计算确保按比例分布的时间戳，保持原始节奏
+
+- **示例：** 如果源文件在1秒、5秒和9秒有条目（共3个条目），而目标有5个条目，插值后的时间戳将大约为1秒、3秒、5秒、7秒和9秒，保持均匀间隔。
+
+- **线性插值的好处：**
+  - 当条目数相差很大时，提供更准确的时间
+  - 保持源时间线的节奏和韵律
+  - 既能处理扩展（目标条目更多）也能处理收缩（目标条目更少）的情况
+
 ### 边缘情况
 
 - 如果源文件没有时间信息，目标保持不变。
-- 如果源时长计算导致负值，会应用默认的3秒时长。
+- 如果源时长计算导致负值，会应用默认的零秒时长（改进自之前的3秒默认值）。
 - 当条目数不同时，命令会显示警告但会继续进行缩放同步。
 - 目标文件中的特定格式功能（如样式、对齐方式、元数据）会被保留。同步操作只替换时间戳，不会更改任何其他格式或内容功能。
 

internal/config/constants.go

@@ -1,7 +1,7 @@
 package config
 
 // Version stores the current application version
-const Version = "0.5.0"
+const Version = "0.5.1"
 
 // Usage stores the general usage information
 const Usage = `Usage: sub-cli [command] [options]

internal/converter/converter_test.go

@@ -0,0 +1,249 @@
+package converter
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestConvert(t *testing.T) {
+	// Setup test cases
+	testCases := []struct {
+		name           string
+		sourceContent  string
+		sourceExt      string
+		targetExt      string
+		expectedError  bool
+		validateOutput func(t *testing.T, filePath string)
+	}{
+		{
+			name: "SRT to VTT",
+			sourceContent: `1
+00:00:01,000 --> 00:00:04,000
+This is a test subtitle.
+
+2
+00:00:05,000 --> 00:00:08,000
+This is another test subtitle.
+`,
+			sourceExt: "srt",
+			targetExt: "vtt",
+			expectedError: false,
+			validateOutput: func(t *testing.T, filePath string) {
+				content, err := os.ReadFile(filePath)
+				if err != nil {
+					t.Fatalf("Failed to read output file: %v", err)
+				}
+				
+				// Check content
+				contentStr := string(content)
+				if !strings.Contains(contentStr, "WEBVTT") {
+					t.Errorf("Expected output to contain WEBVTT header, got: %s", contentStr)
+				}
+				if !strings.Contains(contentStr, "00:00:01.000 --> 00:00:04.000") {
+					t.Errorf("Expected output to contain correct timestamp, got: %s", contentStr)
+				}
+				if !strings.Contains(contentStr, "This is a test subtitle.") {
+					t.Errorf("Expected output to contain subtitle text, got: %s", contentStr)
+				}
+			},
+		},
+		{
+			name: "LRC to SRT",
+			sourceContent: `[ti:Test Title]
+[ar:Test Artist]
+
+[00:01.00]This is a test lyric.
+[00:05.00]This is another test lyric.
+`,
+			sourceExt: "lrc",
+			targetExt: "srt",
+			expectedError: false,
+			validateOutput: func(t *testing.T, filePath string) {
+				content, err := os.ReadFile(filePath)
+				if err != nil {
+					t.Fatalf("Failed to read output file: %v", err)
+				}
+				
+				// Check content
+				contentStr := string(content)
+				if !strings.Contains(contentStr, "00:00:01,000 --> ") {
+					t.Errorf("Expected output to contain correct SRT timestamp, got: %s", contentStr)
+				}
+				if !strings.Contains(contentStr, "This is a test lyric.") {
+					t.Errorf("Expected output to contain lyric text, got: %s", contentStr)
+				}
+			},
+		},
+		{
+			name: "VTT to LRC",
+			sourceContent: `WEBVTT
+
+1
+00:00:01.000 --> 00:00:04.000
+This is a test subtitle.
+
+2
+00:00:05.000 --> 00:00:08.000
+This is another test subtitle.
+`,
+			sourceExt: "vtt",
+			targetExt: "lrc",
+			expectedError: false,
+			validateOutput: func(t *testing.T, filePath string) {
+				content, err := os.ReadFile(filePath)
+				if err != nil {
+					t.Fatalf("Failed to read output file: %v", err)
+				}
+				
+				// Check content
+				contentStr := string(content)
+				if !strings.Contains(contentStr, "[00:01.000]") {
+					t.Errorf("Expected output to contain correct LRC timestamp, got: %s", contentStr)
+				}
+				if !strings.Contains(contentStr, "This is a test subtitle.") {
+					t.Errorf("Expected output to contain subtitle text, got: %s", contentStr)
+				}
+			},
+		},
+		{
+			name: "SRT to TXT",
+			sourceContent: `1
+00:00:01,000 --> 00:00:04,000
+This is a test subtitle.
+
+2
+00:00:05,000 --> 00:00:08,000
+This is another test subtitle.
+`,
+			sourceExt: "srt",
+			targetExt: "txt",
+			expectedError: false,
+			validateOutput: func(t *testing.T, filePath string) {
+				content, err := os.ReadFile(filePath)
+				if err != nil {
+					t.Fatalf("Failed to read output file: %v", err)
+				}
+				
+				// Check content
+				contentStr := string(content)
+				if strings.Contains(contentStr, "00:00:01") {
+					t.Errorf("TXT should not contain timestamps, got: %s", contentStr)
+				}
+				if !strings.Contains(contentStr, "This is a test subtitle.") {
+					t.Errorf("Expected output to contain subtitle text, got: %s", contentStr)
+				}
+			},
+		},
+		{
+			name: "TXT to SRT",
+			sourceContent: "This is a test line.",
+			sourceExt: "txt",
+			targetExt: "srt",
+			expectedError: true,
+			validateOutput: nil, // No validation needed as we expect an error
+		},
+		{
+			name: "Invalid source format",
+			sourceContent: "Random content",
+			sourceExt: "xyz",
+			targetExt: "srt",
+			expectedError: true,
+			validateOutput: nil, // No validation needed as we expect an error
+		},
+		{
+			name: "Invalid target format",
+			sourceContent: `1
+00:00:01,000 --> 00:00:04,000
+This is a test subtitle.
+`,
+			sourceExt: "srt",
+			targetExt: "xyz",
+			expectedError: true,
+			validateOutput: nil, // No validation needed as we expect an error
+		},
+	}
+
+	// Run test cases
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			// Create temporary directory
+			tempDir := t.TempDir()
+			
+			// Create source file
+			sourceFile := filepath.Join(tempDir, "source."+tc.sourceExt)
+			if err := os.WriteFile(sourceFile, []byte(tc.sourceContent), 0644); err != nil {
+				t.Fatalf("Failed to create source file: %v", err)
+			}
+			
+			// Create target file path
+			targetFile := filepath.Join(tempDir, "target."+tc.targetExt)
+			
+			// Call Convert
+			err := Convert(sourceFile, targetFile)
+			
+			// Check error
+			if tc.expectedError && err == nil {
+				t.Errorf("Expected error but got none")
+			}
+			if !tc.expectedError && err != nil {
+				t.Errorf("Expected no error but got: %v", err)
+			}
+			
+			// If no error expected and validation function provided, validate output
+			if !tc.expectedError && tc.validateOutput != nil {
+				tc.validateOutput(t, targetFile)
+			}
+		})
+	}
+}
+
+func TestConvert_NonExistentFile(t *testing.T) {
+	tempDir := t.TempDir()
+	sourceFile := filepath.Join(tempDir, "nonexistent.srt")
+	targetFile := filepath.Join(tempDir, "target.vtt")
+	
+	err := Convert(sourceFile, targetFile)
+	if err == nil {
+		t.Errorf("Expected error when source file doesn't exist, but got none")
+	}
+}
+
+func TestConvert_ReadOnlyTarget(t *testing.T) {
+	// This test might not be applicable on all platforms
+	// Skip it if running on a platform where permissions can't be enforced
+	if os.Getenv("SKIP_PERMISSION_TESTS") != "" {
+		t.Skip("Skipping permission test")
+	}
+	
+	// Create temporary directory
+	tempDir := t.TempDir()
+	
+	// Create source file
+	sourceContent := `1
+00:00:01,000 --> 00:00:04,000
+This is a test subtitle.
+`
+	sourceFile := filepath.Join(tempDir, "source.srt")
+	if err := os.WriteFile(sourceFile, []byte(sourceContent), 0644); err != nil {
+		t.Fatalf("Failed to create source file: %v", err)
+	}
+	
+	// Create read-only directory
+	readOnlyDir := filepath.Join(tempDir, "readonly")
+	if err := os.Mkdir(readOnlyDir, 0500); err != nil {
+		t.Fatalf("Failed to create read-only directory: %v", err)
+	}
+	
+	// Target in read-only directory
+	targetFile := filepath.Join(readOnlyDir, "target.vtt")
+	
+	// Call Convert
+	err := Convert(sourceFile, targetFile)
+	
+	// We expect an error due to permissions
+	if err == nil {
+		t.Errorf("Expected error when target is in read-only directory, but got none")
+	}
+}

internal/format/lrc/lrc_test.go

@@ -0,0 +1,518 @@
+package lrc
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"sub-cli/internal/model"
+)
+
+func TestParse(t *testing.T) {
+	// Create a temporary test file
+	content := `[ti:Test LRC File]
+[ar:Test Artist]
+[al:Test Album]
+[by:Test Creator]
+
+[00:01.00]This is the first line.
+[00:05.00]This is the second line.
+[00:09.50]This is the third line.
+`
+	tempDir := t.TempDir()
+	testFile := filepath.Join(tempDir, "test.lrc")
+	if err := os.WriteFile(testFile, []byte(content), 0644); err != nil {
+		t.Fatalf("Failed to create test file: %v", err)
+	}
+
+	// Test parsing
+	lyrics, err := Parse(testFile)
+	if err != nil {
+		t.Fatalf("Parse failed: %v", err)
+	}
+
+	// Verify results
+	if len(lyrics.Timeline) != 3 {
+		t.Errorf("Expected 3 timeline entries, got %d", len(lyrics.Timeline))
+	}
+
+	if len(lyrics.Content) != 3 {
+		t.Errorf("Expected 3 content entries, got %d", len(lyrics.Content))
+	}
+
+	// Check metadata
+	if lyrics.Metadata["ti"] != "Test LRC File" {
+		t.Errorf("Expected title 'Test LRC File', got '%s'", lyrics.Metadata["ti"])
+	}
+	if lyrics.Metadata["ar"] != "Test Artist" {
+		t.Errorf("Expected artist 'Test Artist', got '%s'", lyrics.Metadata["ar"])
+	}
+	if lyrics.Metadata["al"] != "Test Album" {
+		t.Errorf("Expected album 'Test Album', got '%s'", lyrics.Metadata["al"])
+	}
+	if lyrics.Metadata["by"] != "Test Creator" {
+		t.Errorf("Expected creator 'Test Creator', got '%s'", lyrics.Metadata["by"])
+	}
+
+	// Check first timeline entry
+	if lyrics.Timeline[0].Hours != 0 || lyrics.Timeline[0].Minutes != 0 ||
+		lyrics.Timeline[0].Seconds != 1 || lyrics.Timeline[0].Milliseconds != 0 {
+		t.Errorf("First entry time: expected 00:01.00, got %+v", lyrics.Timeline[0])
+	}
+
+	// Check third timeline entry
+	if lyrics.Timeline[2].Hours != 0 || lyrics.Timeline[2].Minutes != 0 ||
+		lyrics.Timeline[2].Seconds != 9 || lyrics.Timeline[2].Milliseconds != 500 {
+		t.Errorf("Third entry time: expected 00:09.50, got %+v", lyrics.Timeline[2])
+	}
+
+	// Check content
+	if lyrics.Content[0] != "This is the first line." {
+		t.Errorf("First entry content: expected 'This is the first line.', got '%s'", lyrics.Content[0])
+	}
+}
+
+func TestGenerate(t *testing.T) {
+	// Create test lyrics
+	lyrics := model.Lyrics{
+		Metadata: map[string]string{
+			"ti": "Test LRC File",
+			"ar": "Test Artist",
+		},
+		Timeline: []model.Timestamp{
+			{Hours: 0, Minutes: 0, Seconds: 1, Milliseconds: 0},
+			{Hours: 0, Minutes: 0, Seconds: 5, Milliseconds: 0},
+		},
+		Content: []string{
+			"This is the first line.",
+			"This is the second line.",
+		},
+	}
+
+	// Generate LRC file
+	tempDir := t.TempDir()
+	outputFile := filepath.Join(tempDir, "output.lrc")
+	err := Generate(lyrics, outputFile)
+	if err != nil {
+		t.Fatalf("Generate failed: %v", err)
+	}
+
+	// Verify generated content
+	content, err := os.ReadFile(outputFile)
+	if err != nil {
+		t.Fatalf("Failed to read output file: %v", err)
+	}
+
+	// Check content
+	lines := strings.Split(string(content), "\n")
+	if len(lines) < 4 {
+		t.Fatalf("Expected at least 4 lines, got %d", len(lines))
+	}
+
+	hasTitleLine := false
+	hasFirstTimeline := false
+
+	for _, line := range lines {
+		if line == "[ti:Test LRC File]" {
+			hasTitleLine = true
+		}
+		if line == "[00:01.000]This is the first line." {
+			hasFirstTimeline = true
+		}
+	}
+
+	if !hasTitleLine {
+		t.Errorf("Expected title line '[ti:Test LRC File]' not found")
+	}
+
+	if !hasFirstTimeline {
+		t.Errorf("Expected timeline line '[00:01.000]This is the first line.' not found")
+	}
+}
+
+func TestConvertToSubtitle(t *testing.T) {
+	// Create a temporary test file
+	content := `[ti:Test LRC File]
+[ar:Test Artist]
+
+[00:01.00]This is the first line.
+[00:05.00]This is the second line.
+`
+	tempDir := t.TempDir()
+	testFile := filepath.Join(tempDir, "test.lrc")
+	if err := os.WriteFile(testFile, []byte(content), 0644); err != nil {
+		t.Fatalf("Failed to create test file: %v", err)
+	}
+
+	// Convert to subtitle
+	subtitle, err := ConvertToSubtitle(testFile)
+	if err != nil {
+		t.Fatalf("ConvertToSubtitle failed: %v", err)
+	}
+
+	// Check result
+	if subtitle.Format != "lrc" {
+		t.Errorf("Expected format 'lrc', got '%s'", subtitle.Format)
+	}
+
+	if subtitle.Title != "Test LRC File" {
+		t.Errorf("Expected title 'Test LRC File', got '%s'", subtitle.Title)
+	}
+
+	if len(subtitle.Entries) != 2 {
+		t.Errorf("Expected 2 entries, got %d", len(subtitle.Entries))
+	}
+
+	// Check first entry
+	if subtitle.Entries[0].Text != "This is the first line." {
+		t.Errorf("Expected first entry text 'This is the first line.', got '%s'", subtitle.Entries[0].Text)
+	}
+
+	// Check metadata
+	if subtitle.Metadata["ar"] != "Test Artist" {
+		t.Errorf("Expected metadata 'ar' to be 'Test Artist', got '%s'", subtitle.Metadata["ar"])
+	}
+}
+
+func TestConvertFromSubtitle(t *testing.T) {
+	// Create test subtitle
+	subtitle := model.NewSubtitle()
+	subtitle.Format = "lrc"
+	subtitle.Title = "Test LRC File"
+	subtitle.Metadata["ar"] = "Test Artist"
+
+	entry1 := model.NewSubtitleEntry()
+	entry1.StartTime = model.Timestamp{Hours: 0, Minutes: 0, Seconds: 1, Milliseconds: 0}
+	entry1.EndTime = model.Timestamp{Hours: 0, Minutes: 0, Seconds: 4, Milliseconds: 0}
+	entry1.Text = "This is the first line."
+
+	entry2 := model.NewSubtitleEntry()
+	entry2.StartTime = model.Timestamp{Hours: 0, Minutes: 0, Seconds: 5, Milliseconds: 0}
+	entry2.EndTime = model.Timestamp{Hours: 0, Minutes: 0, Seconds: 8, Milliseconds: 0}
+	entry2.Text = "This is the second line."
+
+	subtitle.Entries = append(subtitle.Entries, entry1, entry2)
+
+	// Convert from subtitle to LRC
+	tempDir := t.TempDir()
+	outputFile := filepath.Join(tempDir, "output.lrc")
+	err := ConvertFromSubtitle(subtitle, outputFile)
+	if err != nil {
+		t.Fatalf("ConvertFromSubtitle failed: %v", err)
+	}
+
+	// Verify by parsing back
+	lyrics, err := Parse(outputFile)
+	if err != nil {
+		t.Fatalf("Failed to parse output file: %v", err)
+	}
+
+	if len(lyrics.Timeline) != 2 {
+		t.Errorf("Expected 2 entries, got %d", len(lyrics.Timeline))
+	}
+
+	if lyrics.Content[0] != "This is the first line." {
+		t.Errorf("Expected first entry content 'This is the first line.', got '%s'", lyrics.Content[0])
+	}
+
+	if lyrics.Metadata["ti"] != "Test LRC File" {
+		t.Errorf("Expected title metadata 'Test LRC File', got '%s'", lyrics.Metadata["ti"])
+	}
+}
+
+func TestFormat(t *testing.T) {
+	// Create test LRC file with inconsistent timestamp formatting
+	content := `[ti:Test LRC File]
+[ar:Test Artist]
+
+[0:1.0]This is the first line.
+[0:5]This is the second line.
+`
+	tempDir := t.TempDir()
+	testFile := filepath.Join(tempDir, "test.lrc")
+	if err := os.WriteFile(testFile, []byte(content), 0644); err != nil {
+		t.Fatalf("Failed to create test file: %v", err)
+	}
+
+	// Format the file
+	err := Format(testFile)
+	if err != nil {
+		t.Fatalf("Format failed: %v", err)
+	}
+
+	// Verify by parsing back
+	lyrics, err := Parse(testFile)
+	if err != nil {
+		t.Fatalf("Failed to parse formatted file: %v", err)
+	}
+
+	// Check that timestamps are formatted correctly
+	if lyrics.Timeline[0].Seconds != 1 || lyrics.Timeline[0].Milliseconds != 0 {
+		t.Errorf("Expected first timestamp to be 00:01.000, got %+v", lyrics.Timeline[0])
+	}
+
+	// Verify metadata is preserved
+	if lyrics.Metadata["ti"] != "Test LRC File" {
+		t.Errorf("Expected title 'Test LRC File', got '%s'", lyrics.Metadata["ti"])
+	}
+}
+
+func TestParseTimestamp(t *testing.T) {
+	testCases := []struct {
+		name     string
+		input    string
+		expected model.Timestamp
+		hasError bool
+	}{
+		{
+			name:     "Simple minute and second",
+			input:    "01:30",
+			expected: model.Timestamp{Hours: 0, Minutes: 1, Seconds: 30, Milliseconds: 0},
+			hasError: false,
+		},
+		{
+			name:     "With milliseconds (1 digit)",
+			input:    "01:30.5",
+			expected: model.Timestamp{Hours: 0, Minutes: 1, Seconds: 30, Milliseconds: 500},
+			hasError: false,
+		},
+		{
+			name:     "With milliseconds (2 digits)",
+			input:    "01:30.75",
+			expected: model.Timestamp{Hours: 0, Minutes: 1, Seconds: 30, Milliseconds: 750},
+			hasError: false,
+		},
+		{
+			name:     "With milliseconds (3 digits)",
+			input:    "01:30.123",
+			expected: model.Timestamp{Hours: 0, Minutes: 1, Seconds: 30, Milliseconds: 123},
+			hasError: false,
+		},
+		{
+			name:     "With hours, minutes, seconds",
+			input:    "01:30:45",
+			expected: model.Timestamp{Hours: 1, Minutes: 30, Seconds: 45, Milliseconds: 0},
+			hasError: false,
+		},
+		{
+			name:     "With hours, minutes, seconds and milliseconds",
+			input:    "01:30:45.5",
+			expected: model.Timestamp{Hours: 1, Minutes: 30, Seconds: 45, Milliseconds: 500},
+			hasError: false,
+		},
+		{
+			name:     "Invalid format (single number)",
+			input:    "123",
+			expected: model.Timestamp{},
+			hasError: true,
+		},
+		{
+			name:     "Invalid format (too many parts)",
+			input:    "01:30:45:67",
+			expected: model.Timestamp{},
+			hasError: true,
+		},
+		{
+			name:     "Invalid minute (not a number)",
+			input:    "aa:30",
+			expected: model.Timestamp{},
+			hasError: true,
+		},
+		{
+			name:     "Invalid second (not a number)",
+			input:    "01:bb",
+			expected: model.Timestamp{},
+			hasError: true,
+		},
+		{
+			name:     "Invalid millisecond (not a number)",
+			input:    "01:30.cc",
+			expected: model.Timestamp{},
+			hasError: true,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			result, err := ParseTimestamp(tc.input)
+
+			if tc.hasError && err == nil {
+				t.Errorf("Expected error for input '%s', but got none", tc.input)
+			}
+
+			if !tc.hasError && err != nil {
+				t.Errorf("Unexpected error for input '%s': %v", tc.input, err)
+			}
+
+			if !tc.hasError && result != tc.expected {
+				t.Errorf("For input '%s', expected %+v, got %+v", tc.input, tc.expected, result)
+			}
+		})
+	}
+}
+
+func TestParse_FileErrors(t *testing.T) {
+	// Test with non-existent file
+	_, err := Parse("/nonexistent/file.lrc")
+	if err == nil {
+		t.Error("Expected error when parsing non-existent file, got nil")
+	}
+}
+
+func TestParse_EdgeCases(t *testing.T) {
+	// Test with empty file
+	tempDir := t.TempDir()
+	emptyFile := filepath.Join(tempDir, "empty.lrc")
+	if err := os.WriteFile(emptyFile, []byte{}, 0644); err != nil {
+		t.Fatalf("Failed to create empty test file: %v", err)
+	}
+
+	lyrics, err := Parse(emptyFile)
+	if err != nil {
+		t.Fatalf("Parse failed on empty file: %v", err)
+	}
+
+	if len(lyrics.Timeline) != 0 || len(lyrics.Content) != 0 {
+		t.Errorf("Expected empty lyrics for empty file, got %d timeline entries and %d content entries",
+			len(lyrics.Timeline), len(lyrics.Content))
+	}
+
+	// Test with invalid timestamps
+	invalidFile := filepath.Join(tempDir, "invalid.lrc")
+	content := `[ti:Test LRC File]
+[ar:Test Artist]
+
+[invalidtime]This should be ignored.
+[00:01.00]This is a valid line.
+`
+	if err := os.WriteFile(invalidFile, []byte(content), 0644); err != nil {
+		t.Fatalf("Failed to create invalid test file: %v", err)
+	}
+
+	lyrics, err = Parse(invalidFile)
+	if err != nil {
+		t.Fatalf("Parse failed on file with invalid timestamps: %v", err)
+	}
+
+	if len(lyrics.Timeline) != 1 || len(lyrics.Content) != 1 {
+		t.Errorf("Expected 1 valid timeline entry, got %d timeline entries and %d content entries",
+			len(lyrics.Timeline), len(lyrics.Content))
+	}
+
+	// Test with timestamp-only lines (no content)
+	timestampOnlyFile := filepath.Join(tempDir, "timestamp_only.lrc")
+	content = `[ti:Test LRC File]
+[ar:Test Artist]
+
+[00:01.00]
+[00:05.00]This has content.
+`
+	if err := os.WriteFile(timestampOnlyFile, []byte(content), 0644); err != nil {
+		t.Fatalf("Failed to create timestamp-only test file: %v", err)
+	}
+
+	lyrics, err = Parse(timestampOnlyFile)
+	if err != nil {
+		t.Fatalf("Parse failed on file with timestamp-only lines: %v", err)
+	}
+
+	if len(lyrics.Timeline) != 1 || len(lyrics.Content) != 1 {
+		t.Errorf("Expected 1 valid entry (ignoring empty content), got %d timeline entries and %d content entries",
+			len(lyrics.Timeline), len(lyrics.Content))
+	}
+}
+
+func TestGenerate_FileError(t *testing.T) {
+	// Create test lyrics
+	lyrics := model.Lyrics{
+		Metadata: map[string]string{
+			"ti": "Test LRC File",
+		},
+		Timeline: []model.Timestamp{
+			{Hours: 0, Minutes: 0, Seconds: 1, Milliseconds: 0},
+		},
+		Content: []string{
+			"This is a test line.",
+		},
+	}
+
+	// Test with invalid path
+	err := Generate(lyrics, "/nonexistent/directory/file.lrc")
+	if err == nil {
+		t.Error("Expected error when generating to invalid path, got nil")
+	}
+}
+
+func TestFormat_FileError(t *testing.T) {
+	// Test with non-existent file
+	err := Format("/nonexistent/file.lrc")
+	if err == nil {
+		t.Error("Expected error when formatting non-existent file, got nil")
+	}
+}
+
+func TestConvertToSubtitle_FileError(t *testing.T) {
+	// Test with non-existent file
+	_, err := ConvertToSubtitle("/nonexistent/file.lrc")
+	if err == nil {
+		t.Error("Expected error when converting non-existent file, got nil")
+	}
+}
+
+func TestConvertToSubtitle_EdgeCases(t *testing.T) {
+	// Test with empty lyrics (no content/timeline)
+	tempDir := t.TempDir()
+	emptyLyricsFile := filepath.Join(tempDir, "empty_lyrics.lrc")
+	content := `[ti:Test LRC File]
+[ar:Test Artist]
+`
+	if err := os.WriteFile(emptyLyricsFile, []byte(content), 0644); err != nil {
+		t.Fatalf("Failed to create empty lyrics test file: %v", err)
+	}
+
+	subtitle, err := ConvertToSubtitle(emptyLyricsFile)
+	if err != nil {
+		t.Fatalf("ConvertToSubtitle failed on empty lyrics: %v", err)
+	}
+
+	if len(subtitle.Entries) != 0 {
+		t.Errorf("Expected 0 entries for empty lyrics, got %d", len(subtitle.Entries))
+	}
+
+	if subtitle.Title != "Test LRC File" {
+		t.Errorf("Expected title 'Test LRC File', got '%s'", subtitle.Title)
+	}
+
+	// Test with more content than timeline entries
+	moreContentFile := filepath.Join(tempDir, "more_content.lrc")
+	content = `[ti:Test LRC File]
+
+[00:01.00]This has a timestamp.
+This doesn't have a timestamp but is content.
+`
+	if err := os.WriteFile(moreContentFile, []byte(content), 0644); err != nil {
+		t.Fatalf("Failed to create more content test file: %v", err)
+	}
+
+	subtitle, err = ConvertToSubtitle(moreContentFile)
+	if err != nil {
+		t.Fatalf("ConvertToSubtitle failed on file with more content than timestamps: %v", err)
+	}
+
+	if len(subtitle.Entries) != 1 {
+		t.Errorf("Expected 1 entry (only the one with timestamp), got %d", len(subtitle.Entries))
+	}
+}
+
+func TestConvertFromSubtitle_FileError(t *testing.T) {
+	// Create simple subtitle
+	subtitle := model.NewSubtitle()
+	subtitle.Entries = append(subtitle.Entries, model.NewSubtitleEntry())
+
+	// Test with invalid path
+	err := ConvertFromSubtitle(subtitle, "/nonexistent/directory/file.lrc")
+	if err == nil {
+		t.Error("Expected error when converting to invalid path, got nil")
+	}
+}

internal/format/srt/srt_test.go

@@ -0,0 +1,646 @@
+package srt
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"sub-cli/internal/model"
+)
+
+func TestParse(t *testing.T) {
+	// Create a temporary test file
+	content := `1
+00:00:01,000 --> 00:00:04,000
+This is the first line.
+
+2
+00:00:05,000 --> 00:00:08,000
+This is the second line.
+
+3
+00:00:09,500 --> 00:00:12,800
+This is the third line
+with a line break.
+`
+	tempDir := t.TempDir()
+	testFile := filepath.Join(tempDir, "test.srt")
+	if err := os.WriteFile(testFile, []byte(content), 0644); err != nil {
+		t.Fatalf("Failed to create test file: %v", err)
+	}
+
+	// Test parsing
+	entries, err := Parse(testFile)
+	if err != nil {
+		t.Fatalf("Parse failed: %v", err)
+	}
+
+	// Verify results
+	if len(entries) != 3 {
+		t.Errorf("Expected 3 entries, got %d", len(entries))
+	}
+
+	// Check first entry
+	if entries[0].Number != 1 {
+		t.Errorf("First entry number: expected 1, got %d", entries[0].Number)
+	}
+	if entries[0].StartTime.Hours != 0 || entries[0].StartTime.Minutes != 0 || 
+	   entries[0].StartTime.Seconds != 1 || entries[0].StartTime.Milliseconds != 0 {
+		t.Errorf("First entry start time: expected 00:00:01,000, got %+v", entries[0].StartTime)
+	}
+	if entries[0].EndTime.Hours != 0 || entries[0].EndTime.Minutes != 0 || 
+	   entries[0].EndTime.Seconds != 4 || entries[0].EndTime.Milliseconds != 0 {
+		t.Errorf("First entry end time: expected 00:00:04,000, got %+v", entries[0].EndTime)
+	}
+	if entries[0].Content != "This is the first line." {
+		t.Errorf("First entry content: expected 'This is the first line.', got '%s'", entries[0].Content)
+	}
+
+	// Check third entry
+	if entries[2].Number != 3 {
+		t.Errorf("Third entry number: expected 3, got %d", entries[2].Number)
+	}
+	expectedContent := "This is the third line\nwith a line break."
+	if entries[2].Content != expectedContent {
+		t.Errorf("Third entry content: expected '%s', got '%s'", expectedContent, entries[2].Content)
+	}
+}
+
+func TestGenerate(t *testing.T) {
+	// Create test entries
+	entries := []model.SRTEntry{
+		{
+			Number:    1,
+			StartTime: model.Timestamp{Hours: 0, Minutes: 0, Seconds: 1, Milliseconds: 0},
+			EndTime:   model.Timestamp{Hours: 0, Minutes: 0, Seconds: 4, Milliseconds: 0},
+			Content:   "This is the first line.",
+		},
+		{
+			Number:    2,
+			StartTime: model.Timestamp{Hours: 0, Minutes: 0, Seconds: 5, Milliseconds: 0},
+			EndTime:   model.Timestamp{Hours: 0, Minutes: 0, Seconds: 8, Milliseconds: 0},
+			Content:   "This is the second line.",
+		},
+	}
+
+	// Generate SRT file
+	tempDir := t.TempDir()
+	outputFile := filepath.Join(tempDir, "output.srt")
+	err := Generate(entries, outputFile)
+	if err != nil {
+		t.Fatalf("Generate failed: %v", err)
+	}
+
+	// Verify generated content
+	content, err := os.ReadFile(outputFile)
+	if err != nil {
+		t.Fatalf("Failed to read output file: %v", err)
+	}
+
+	// Check content
+	lines := strings.Split(string(content), "\n")
+	if len(lines) < 6 {
+		t.Fatalf("Expected at least 6 lines, got %d", len(lines))
+	}
+
+	if lines[0] != "1" {
+		t.Errorf("Expected first line to be '1', got '%s'", lines[0])
+	}
+
+	if lines[1] != "00:00:01,000 --> 00:00:04,000" {
+		t.Errorf("Expected second line to be time range, got '%s'", lines[1])
+	}
+	
+	if lines[2] != "This is the first line." {
+		t.Errorf("Expected third line to be content, got '%s'", lines[2])
+	}
+}
+
+func TestConvertToSubtitle(t *testing.T) {
+	// Create a temporary test file
+	content := `1
+00:00:01,000 --> 00:00:04,000
+This is the first line.
+
+2
+00:00:05,000 --> 00:00:08,000
+This is the second line.
+`
+	tempDir := t.TempDir()
+	testFile := filepath.Join(tempDir, "test.srt")
+	if err := os.WriteFile(testFile, []byte(content), 0644); err != nil {
+		t.Fatalf("Failed to create test file: %v", err)
+	}
+
+	// Convert to subtitle
+	subtitle, err := ConvertToSubtitle(testFile)
+	if err != nil {
+		t.Fatalf("ConvertToSubtitle failed: %v", err)
+	}
+
+	// Check result
+	if subtitle.Format != "srt" {
+		t.Errorf("Expected format 'srt', got '%s'", subtitle.Format)
+	}
+
+	if len(subtitle.Entries) != 2 {
+		t.Errorf("Expected 2 entries, got %d", len(subtitle.Entries))
+	}
+
+	// Check first entry
+	if subtitle.Entries[0].Text != "This is the first line." {
+		t.Errorf("Expected first entry text 'This is the first line.', got '%s'", subtitle.Entries[0].Text)
+	}
+}
+
+func TestConvertFromSubtitle(t *testing.T) {
+	// Create test subtitle
+	subtitle := model.NewSubtitle()
+	subtitle.Format = "srt"
+	
+	entry1 := model.NewSubtitleEntry()
+	entry1.Index = 1
+	entry1.StartTime = model.Timestamp{Hours: 0, Minutes: 0, Seconds: 1, Milliseconds: 0}
+	entry1.EndTime = model.Timestamp{Hours: 0, Minutes: 0, Seconds: 4, Milliseconds: 0}
+	entry1.Text = "This is the first line."
+	
+	entry2 := model.NewSubtitleEntry()
+	entry2.Index = 2
+	entry2.StartTime = model.Timestamp{Hours: 0, Minutes: 0, Seconds: 5, Milliseconds: 0}
+	entry2.EndTime = model.Timestamp{Hours: 0, Minutes: 0, Seconds: 8, Milliseconds: 0}
+	entry2.Text = "This is the second line."
+	
+	subtitle.Entries = append(subtitle.Entries, entry1, entry2)
+
+	// Convert from subtitle to SRT
+	tempDir := t.TempDir()
+	outputFile := filepath.Join(tempDir, "output.srt")
+	err := ConvertFromSubtitle(subtitle, outputFile)
+	if err != nil {
+		t.Fatalf("ConvertFromSubtitle failed: %v", err)
+	}
+
+	// Verify by parsing back
+	entries, err := Parse(outputFile)
+	if err != nil {
+		t.Fatalf("Failed to parse output file: %v", err)
+	}
+
+	if len(entries) != 2 {
+		t.Errorf("Expected 2 entries, got %d", len(entries))
+	}
+
+	if entries[0].Content != "This is the first line." {
+		t.Errorf("Expected first entry content 'This is the first line.', got '%s'", entries[0].Content)
+	}
+}
+
+func TestFormat(t *testing.T) {
+	// Create test file with non-sequential numbers
+	content := `2
+00:00:01,000 --> 00:00:04,000
+This is the first line.
+
+5
+00:00:05,000 --> 00:00:08,000
+This is the second line.
+`
+	tempDir := t.TempDir()
+	testFile := filepath.Join(tempDir, "test.srt")
+	if err := os.WriteFile(testFile, []byte(content), 0644); err != nil {
+		t.Fatalf("Failed to create test file: %v", err)
+	}
+
+	// Format the file
+	err := Format(testFile)
+	if err != nil {
+		t.Fatalf("Format failed: %v", err)
+	}
+
+	// Verify by parsing back
+	entries, err := Parse(testFile)
+	if err != nil {
+		t.Fatalf("Failed to parse formatted file: %v", err)
+	}
+
+	// Check that numbers are sequential
+	if entries[0].Number != 1 {
+		t.Errorf("Expected first entry number to be 1, got %d", entries[0].Number)
+	}
+	if entries[1].Number != 2 {
+		t.Errorf("Expected second entry number to be 2, got %d", entries[1].Number)
+	}
+}
+
+func TestParseSRTTimestamp(t *testing.T) {
+	testCases := []struct {
+		name     string
+		input    string
+		expected model.Timestamp
+	}{
+		{
+			name:     "Standard format",
+			input:    "00:00:01,000",
+			expected: model.Timestamp{Hours: 0, Minutes: 0, Seconds: 1, Milliseconds: 0},
+		},
+		{
+			name:     "With milliseconds",
+			input:    "00:00:01,500",
+			expected: model.Timestamp{Hours: 0, Minutes: 0, Seconds: 1, Milliseconds: 500},
+		},
+		{
+			name:     "Full hours, minutes, seconds",
+			input:    "01:02:03,456",
+			expected: model.Timestamp{Hours: 1, Minutes: 2, Seconds: 3, Milliseconds: 456},
+		},
+		{
+			name:     "With dot instead of comma",
+			input:    "00:00:01.000", // Should auto-convert . to ,
+			expected: model.Timestamp{Hours: 0, Minutes: 0, Seconds: 1, Milliseconds: 0},
+		},
+		{
+			name:     "Invalid format",
+			input:    "invalid",
+			expected: model.Timestamp{}, // Should return zero timestamp
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			result := parseSRTTimestamp(tc.input)
+			if result != tc.expected {
+				t.Errorf("For input '%s', expected %+v, got %+v", tc.input, tc.expected, result)
+			}
+		})
+	}
+}
+
+func TestFormatSRTTimestamp(t *testing.T) {
+	testCases := []struct {
+		name     string
+		input    model.Timestamp
+		expected string
+	}{
+		{
+			name:     "Zero timestamp",
+			input:    model.Timestamp{},
+			expected: "00:00:00,000",
+		},
+		{
+			name:     "Simple seconds",
+			input:    model.Timestamp{Hours: 0, Minutes: 0, Seconds: 1, Milliseconds: 0},
+			expected: "00:00:01,000",
+		},
+		{
+			name:     "With milliseconds",
+			input:    model.Timestamp{Hours: 0, Minutes: 0, Seconds: 1, Milliseconds: 500},
+			expected: "00:00:01,500",
+		},
+		{
+			name:     "Full timestamp",
+			input:    model.Timestamp{Hours: 1, Minutes: 2, Seconds: 3, Milliseconds: 456},
+			expected: "01:02:03,456",
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			result := formatSRTTimestamp(tc.input)
+			if result != tc.expected {
+				t.Errorf("For timestamp %+v, expected '%s', got '%s'", tc.input, tc.expected, result)
+			}
+		})
+	}
+}
+
+func TestIsEntryTimeStampUnset(t *testing.T) {
+	testCases := []struct {
+		name     string
+		entry    model.SRTEntry
+		expected bool
+	}{
+		{
+			name:     "Unset timestamp",
+			entry:    model.SRTEntry{Number: 1},
+			expected: true,
+		},
+		{
+			name: "Set timestamp",
+			entry: model.SRTEntry{
+				Number:    1,
+				StartTime: model.Timestamp{Hours: 0, Minutes: 0, Seconds: 1, Milliseconds: 0},
+			},
+			expected: false,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			result := isEntryTimeStampUnset(tc.entry)
+			if result != tc.expected {
+				t.Errorf("For entry %+v, expected isEntryTimeStampUnset to be %v, got %v", tc.entry, tc.expected, result)
+			}
+		})
+	}
+}
+
+func TestConvertToLyrics(t *testing.T) {
+	// Create test entries
+	entries := []model.SRTEntry{
+		{
+			Number:    1,
+			StartTime: model.Timestamp{Hours: 0, Minutes: 0, Seconds: 1, Milliseconds: 0},
+			EndTime:   model.Timestamp{Hours: 0, Minutes: 0, Seconds: 4, Milliseconds: 0},
+			Content:   "This is the first line.",
+		},
+		{
+			Number:    2,
+			StartTime: model.Timestamp{Hours: 0, Minutes: 0, Seconds: 5, Milliseconds: 0},
+			EndTime:   model.Timestamp{Hours: 0, Minutes: 0, Seconds: 8, Milliseconds: 0},
+			Content:   "This is the second line.",
+		},
+	}
+
+	// Convert to lyrics
+	lyrics := ConvertToLyrics(entries)
+
+	// Check result
+	if len(lyrics.Timeline) != 2 {
+		t.Errorf("Expected 2 timeline entries, got %d", len(lyrics.Timeline))
+	}
+	if len(lyrics.Content) != 2 {
+		t.Errorf("Expected 2 content entries, got %d", len(lyrics.Content))
+	}
+
+	// Check timeline entries
+	if lyrics.Timeline[0] != entries[0].StartTime {
+		t.Errorf("First timeline entry: expected %+v, got %+v", entries[0].StartTime, lyrics.Timeline[0])
+	}
+	if lyrics.Timeline[1] != entries[1].StartTime {
+		t.Errorf("Second timeline entry: expected %+v, got %+v", entries[1].StartTime, lyrics.Timeline[1])
+	}
+
+	// Check content entries
+	if lyrics.Content[0] != entries[0].Content {
+		t.Errorf("First content entry: expected '%s', got '%s'", entries[0].Content, lyrics.Content[0])
+	}
+	if lyrics.Content[1] != entries[1].Content {
+		t.Errorf("Second content entry: expected '%s', got '%s'", entries[1].Content, lyrics.Content[1])
+	}
+}
+
+func TestParse_EdgeCases(t *testing.T) {
+	// Test with empty file
+	tempDir := t.TempDir()
+	emptyFile := filepath.Join(tempDir, "empty.srt")
+	if err := os.WriteFile(emptyFile, []byte{}, 0644); err != nil {
+		t.Fatalf("Failed to create empty test file: %v", err)
+	}
+
+	entries, err := Parse(emptyFile)
+	if err != nil {
+		t.Fatalf("Parse failed on empty file: %v", err)
+	}
+
+	if len(entries) != 0 {
+		t.Errorf("Expected 0 entries for empty file, got %d", len(entries))
+	}
+
+	// Test with malformed file (missing timestamp line)
+	malformedFile := filepath.Join(tempDir, "malformed.srt")
+	content := `1
+This is missing a timestamp line.
+
+2
+00:00:05,000 --> 00:00:08,000
+This is valid.
+`
+	if err := os.WriteFile(malformedFile, []byte(content), 0644); err != nil {
+		t.Fatalf("Failed to create malformed test file: %v", err)
+	}
+
+	entries, err = Parse(malformedFile)
+	if err != nil {
+		t.Fatalf("Parse failed on malformed file: %v", err)
+	}
+
+	// SRT解析器更宽容，可能会解析出两个条目
+	if len(entries) != 2 {
+		t.Errorf("Expected 2 entries, got %d", len(entries))
+	}
+
+	// Test with incomplete last entry
+	incompleteFile := filepath.Join(tempDir, "incomplete.srt")
+	content = `1
+00:00:01,000 --> 00:00:04,000
+This is complete.
+
+2
+00:00:05,000 --> 00:00:08,000
+`
+	if err := os.WriteFile(incompleteFile, []byte(content), 0644); err != nil {
+		t.Fatalf("Failed to create incomplete test file: %v", err)
+	}
+
+	entries, err = Parse(incompleteFile)
+	if err != nil {
+		t.Fatalf("Parse failed on incomplete file: %v", err)
+	}
+
+	// Should have one complete entry, the incomplete one is discarded due to empty content
+	if len(entries) != 1 {
+		t.Errorf("Expected 1 entry (only the completed one), got %d", len(entries))
+	}
+}
+
+func TestParse_FileError(t *testing.T) {
+	// Test with non-existent file
+	_, err := Parse("/nonexistent/file.srt")
+	if err == nil {
+		t.Error("Expected error when parsing non-existent file, got nil")
+	}
+}
+
+func TestGenerate_FileError(t *testing.T) {
+	// Create test entries
+	entries := []model.SRTEntry{
+		{
+			Number:    1,
+			StartTime: model.Timestamp{Hours: 0, Minutes: 0, Seconds: 1, Milliseconds: 0},
+			EndTime:   model.Timestamp{Hours: 0, Minutes: 0, Seconds: 4, Milliseconds: 0},
+			Content:   "This is a test line.",
+		},
+	}
+
+	// Test with invalid path
+	err := Generate(entries, "/nonexistent/directory/file.srt")
+	if err == nil {
+		t.Error("Expected error when generating to invalid path, got nil")
+	}
+}
+
+func TestFormat_FileError(t *testing.T) {
+	// Test with non-existent file
+	err := Format("/nonexistent/file.srt")
+	if err == nil {
+		t.Error("Expected error when formatting non-existent file, got nil")
+	}
+}
+
+func TestConvertToSubtitle_WithHTMLTags(t *testing.T) {
+	// Create a temporary test file with HTML tags
+	content := `1
+00:00:01,000 --> 00:00:04,000
+<i>This is in italic.</i>
+
+2
+00:00:05,000 --> 00:00:08,000
+<b>This is in bold.</b>
+
+3
+00:00:09,000 --> 00:00:12,000
+<u>This is underlined.</u>
+`
+	tempDir := t.TempDir()
+	testFile := filepath.Join(tempDir, "styles.srt")
+	if err := os.WriteFile(testFile, []byte(content), 0644); err != nil {
+		t.Fatalf("Failed to create test file with HTML tags: %v", err)
+	}
+
+	// Convert to subtitle
+	subtitle, err := ConvertToSubtitle(testFile)
+	if err != nil {
+		t.Fatalf("ConvertToSubtitle failed: %v", err)
+	}
+
+	// Check if HTML tags were detected
+	if value, ok := subtitle.Entries[0].FormatData["has_html_tags"]; !ok || value != true {
+		t.Errorf("Expected FormatData to contain has_html_tags=true for entry with italic")
+	}
+	if value, ok := subtitle.Entries[0].Styles["italic"]; !ok || value != "true" {
+		t.Errorf("Expected Styles to contain italic=true for entry with <i> tag")
+	}
+
+	if value, ok := subtitle.Entries[1].Styles["bold"]; !ok || value != "true" {
+		t.Errorf("Expected Styles to contain bold=true for entry with <b> tag")
+	}
+
+	if value, ok := subtitle.Entries[2].Styles["underline"]; !ok || value != "true" {
+		t.Errorf("Expected Styles to contain underline=true for entry with <u> tag")
+	}
+}
+
+func TestConvertToSubtitle_FileError(t *testing.T) {
+	// Test with non-existent file
+	_, err := ConvertToSubtitle("/nonexistent/file.srt")
+	if err == nil {
+		t.Error("Expected error when converting non-existent file, got nil")
+	}
+}
+
+func TestConvertFromSubtitle_WithStyling(t *testing.T) {
+	// Create a subtitle with style attributes
+	subtitle := model.NewSubtitle()
+	subtitle.Format = "srt"
+	
+	// Create an entry with italics
+	entry1 := model.NewSubtitleEntry()
+	entry1.Index = 1
+	entry1.StartTime = model.Timestamp{Hours: 0, Minutes: 0, Seconds: 1, Milliseconds: 0}
+	entry1.EndTime = model.Timestamp{Hours: 0, Minutes: 0, Seconds: 4, Milliseconds: 0}
+	entry1.Text = "This should be italic."
+	entry1.Styles["italic"] = "true"
+	
+	// Create an entry with bold
+	entry2 := model.NewSubtitleEntry()
+	entry2.Index = 2
+	entry2.StartTime = model.Timestamp{Hours: 0, Minutes: 0, Seconds: 5, Milliseconds: 0}
+	entry2.EndTime = model.Timestamp{Hours: 0, Minutes: 0, Seconds: 8, Milliseconds: 0}
+	entry2.Text = "This should be bold."
+	entry2.Styles["bold"] = "true"
+	
+	// Create an entry with underline
+	entry3 := model.NewSubtitleEntry()
+	entry3.Index = 3
+	entry3.StartTime = model.Timestamp{Hours: 0, Minutes: 0, Seconds: 9, Milliseconds: 0}
+	entry3.EndTime = model.Timestamp{Hours: 0, Minutes: 0, Seconds: 12, Milliseconds: 0}
+	entry3.Text = "This should be underlined."
+	entry3.Styles["underline"] = "true"
+	
+	subtitle.Entries = append(subtitle.Entries, entry1, entry2, entry3)
+
+	// Convert from subtitle to SRT
+	tempDir := t.TempDir()
+	outputFile := filepath.Join(tempDir, "styled.srt")
+	err := ConvertFromSubtitle(subtitle, outputFile)
+	if err != nil {
+		t.Fatalf("ConvertFromSubtitle failed: %v", err)
+	}
+
+	// Verify by reading the file directly
+	content, err := os.ReadFile(outputFile)
+	if err != nil {
+		t.Fatalf("Failed to read output file: %v", err)
+	}
+
+	// Check that HTML tags were applied
+	contentStr := string(content)
+	if !strings.Contains(contentStr, "<i>This should be italic.</i>") {
+		t.Errorf("Expected italic HTML tags to be applied")
+	}
+	if !strings.Contains(contentStr, "<b>This should be bold.</b>") {
+		t.Errorf("Expected bold HTML tags to be applied")
+	}
+	if !strings.Contains(contentStr, "<u>This should be underlined.</u>") {
+		t.Errorf("Expected underline HTML tags to be applied")
+	}
+}
+
+func TestConvertFromSubtitle_FileError(t *testing.T) {
+	// Create simple subtitle
+	subtitle := model.NewSubtitle()
+	subtitle.Entries = append(subtitle.Entries, model.NewSubtitleEntry())
+
+	// Test with invalid path
+	err := ConvertFromSubtitle(subtitle, "/nonexistent/directory/file.srt")
+	if err == nil {
+		t.Error("Expected error when converting to invalid path, got nil")
+	}
+}
+
+func TestConvertFromSubtitle_WithExistingHTMLTags(t *testing.T) {
+	// Create a subtitle with text that already contains HTML tags
+	subtitle := model.NewSubtitle()
+	subtitle.Format = "srt"
+	
+	// Create an entry with existing italic tags but also style attribute
+	entry := model.NewSubtitleEntry()
+	entry.Index = 1
+	entry.StartTime = model.Timestamp{Hours: 0, Minutes: 0, Seconds: 1, Milliseconds: 0}
+	entry.EndTime = model.Timestamp{Hours: 0, Minutes: 0, Seconds: 4, Milliseconds: 0}
+	entry.Text = "<i>Already italic text.</i>"
+	entry.Styles["italic"] = "true" // Should not double-wrap with <i> tags
+	
+	subtitle.Entries = append(subtitle.Entries, entry)
+
+	// Convert from subtitle to SRT
+	tempDir := t.TempDir()
+	outputFile := filepath.Join(tempDir, "existing_tags.srt")
+	err := ConvertFromSubtitle(subtitle, outputFile)
+	if err != nil {
+		t.Fatalf("ConvertFromSubtitle failed: %v", err)
+	}
+
+	// Verify by reading the file directly
+	content, err := os.ReadFile(outputFile)
+	if err != nil {
+		t.Fatalf("Failed to read output file: %v", err)
+	}
+
+	// Should not have double tags
+	contentStr := string(content)
+	if strings.Contains(contentStr, "<i><i>") {
+		t.Errorf("Expected no duplicate italic tags, but found them")
+	}
+}

internal/format/txt/txt_test.go

@@ -0,0 +1,145 @@
+package txt
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"sub-cli/internal/model"
+)
+
+func TestGenerateFromSubtitle(t *testing.T) {
+	// Create test subtitle
+	subtitle := model.NewSubtitle()
+	
+	entry1 := model.NewSubtitleEntry()
+	entry1.Text = "This is the first line."
+	
+	entry2 := model.NewSubtitleEntry()
+	entry2.Text = "This is the second line."
+	
+	entry3 := model.NewSubtitleEntry()
+	entry3.Text = "This is the third line\nwith a line break."
+	
+	subtitle.Entries = append(subtitle.Entries, entry1, entry2, entry3)
+
+	// Generate TXT file
+	tempDir := t.TempDir()
+	outputFile := filepath.Join(tempDir, "output.txt")
+	err := GenerateFromSubtitle(subtitle, outputFile)
+	if err != nil {
+		t.Fatalf("GenerateFromSubtitle failed: %v", err)
+	}
+
+	// Verify generated content
+	content, err := os.ReadFile(outputFile)
+	if err != nil {
+		t.Fatalf("Failed to read output file: %v", err)
+	}
+
+	// Check content
+	lines := strings.Split(string(content), "\n")
+	if len(lines) < 4 { // 3 entries with one having a line break
+		t.Fatalf("Expected at least 4 lines, got %d", len(lines))
+	}
+
+	if lines[0] != "This is the first line." {
+		t.Errorf("Expected first line to be 'This is the first line.', got '%s'", lines[0])
+	}
+
+	if lines[1] != "This is the second line." {
+		t.Errorf("Expected second line to be 'This is the second line.', got '%s'", lines[1])
+	}
+	
+	if lines[2] != "This is the third line" {
+		t.Errorf("Expected third line to be 'This is the third line', got '%s'", lines[2])
+	}
+	
+	if lines[3] != "with a line break." {
+		t.Errorf("Expected fourth line to be 'with a line break.', got '%s'", lines[3])
+	}
+}
+
+func TestGenerateFromSubtitle_EmptySubtitle(t *testing.T) {
+	// Create empty subtitle
+	subtitle := model.NewSubtitle()
+
+	// Generate TXT file
+	tempDir := t.TempDir()
+	outputFile := filepath.Join(tempDir, "empty.txt")
+	err := GenerateFromSubtitle(subtitle, outputFile)
+	if err != nil {
+		t.Fatalf("GenerateFromSubtitle failed with empty subtitle: %v", err)
+	}
+
+	// Verify generated content
+	content, err := os.ReadFile(outputFile)
+	if err != nil {
+		t.Fatalf("Failed to read output file: %v", err)
+	}
+
+	// Check content is empty
+	if len(content) != 0 {
+		t.Errorf("Expected empty file, got content: %s", string(content))
+	}
+}
+
+func TestGenerateFromSubtitle_WithTitle(t *testing.T) {
+	// Create subtitle with title
+	subtitle := model.NewSubtitle()
+	subtitle.Title = "My Test Title"
+	
+	entry1 := model.NewSubtitleEntry()
+	entry1.Text = "This is a test line."
+	subtitle.Entries = append(subtitle.Entries, entry1)
+
+	// Generate TXT file
+	tempDir := t.TempDir()
+	outputFile := filepath.Join(tempDir, "titled.txt")
+	err := GenerateFromSubtitle(subtitle, outputFile)
+	if err != nil {
+		t.Fatalf("GenerateFromSubtitle failed with titled subtitle: %v", err)
+	}
+
+	// Verify generated content
+	content, err := os.ReadFile(outputFile)
+	if err != nil {
+		t.Fatalf("Failed to read output file: %v", err)
+	}
+
+	// Check content has title and proper formatting
+	lines := strings.Split(string(content), "\n")
+	if len(lines) < 3 { // Title + blank line + content
+		t.Fatalf("Expected at least 3 lines, got %d", len(lines))
+	}
+
+	if lines[0] != "My Test Title" {
+		t.Errorf("Expected first line to be title, got '%s'", lines[0])
+	}
+
+	if lines[1] != "" {
+		t.Errorf("Expected second line to be blank, got '%s'", lines[1])
+	}
+	
+	if lines[2] != "This is a test line." {
+		t.Errorf("Expected third line to be content, got '%s'", lines[2])
+	}
+}
+
+func TestGenerateFromSubtitle_FileError(t *testing.T) {
+	// Create test subtitle
+	subtitle := model.NewSubtitle()
+	entry1 := model.NewSubtitleEntry()
+	entry1.Text = "Test line"
+	subtitle.Entries = append(subtitle.Entries, entry1)
+
+	// Test with invalid file path
+	invalidPath := "/nonexistent/directory/file.txt"
+	err := GenerateFromSubtitle(subtitle, invalidPath)
+	
+	// Verify error is returned
+	if err == nil {
+		t.Errorf("Expected error for invalid file path, got nil")
+	}
+}

internal/format/vtt/vtt.go

@@ -21,6 +21,11 @@ const (
 func Parse(filePath string) (model.Subtitle, error) {
 	subtitle := model.NewSubtitle()
 	subtitle.Format = "vtt"
+	
+	// Ensure maps are initialized
+	if subtitle.Styles == nil {
+		subtitle.Styles = make(map[string]string)
+	}
 
 	file, err := os.Open(filePath)
 	if err != nil {
@@ -29,15 +34,15 @@ func Parse(filePath string) (model.Subtitle, error) {
 	defer file.Close()
 
 	scanner := bufio.NewScanner(file)
-
-	// Check header
+	
+	// First line must be WEBVTT
 	if !scanner.Scan() {
 		return subtitle, fmt.Errorf("empty VTT file")
 	}
-
-	header := strings.TrimSpace(scanner.Text())
+	
+	header := scanner.Text()
 	if !strings.HasPrefix(header, VTTHeader) {
-		return subtitle, fmt.Errorf("invalid VTT file: missing WEBVTT header")
+		return subtitle, fmt.Errorf("invalid VTT file, missing WEBVTT header")
 	}
 
 	// Get metadata from header
@@ -52,24 +57,13 @@ func Parse(filePath string) (model.Subtitle, error) {
 	var styleBuffer strings.Builder
 	var cueTextBuffer strings.Builder
 	
-	lineNum := 1
+	lineNum := 0
+	prevLine := ""
+	
 	for scanner.Scan() {
 		lineNum++
 		line := scanner.Text()
 		
-		// Skip empty lines
-		if strings.TrimSpace(line) == "" {
-			if inCue {
-				// End of a cue
-				currentEntry.Text = cueTextBuffer.String()
-				subtitle.Entries = append(subtitle.Entries, currentEntry)
-				currentEntry = model.NewSubtitleEntry()
-				cueTextBuffer.Reset()
-				inCue = false
-			}
-			continue
-		}
-		
 		// Check for style blocks
 		if strings.HasPrefix(line, "STYLE") {
 			inStyle = true
@@ -77,7 +71,7 @@ func Parse(filePath string) (model.Subtitle, error) {
 		}
 		
 		if inStyle {
-			if line == "" {
+			if strings.TrimSpace(line) == "" {
 				inStyle = false
 				subtitle.Styles["css"] = styleBuffer.String()
 				styleBuffer.Reset()
@@ -88,6 +82,19 @@ func Parse(filePath string) (model.Subtitle, error) {
 			continue
 		}
 		
+		// Skip empty lines, but handle end of cue
+		if strings.TrimSpace(line) == "" {
+			if inCue && cueTextBuffer.Len() > 0 {
+				// End of a cue
+				currentEntry.Text = strings.TrimSpace(cueTextBuffer.String())
+				subtitle.Entries = append(subtitle.Entries, currentEntry)
+				inCue = false
+				cueTextBuffer.Reset()
+				currentEntry = model.SubtitleEntry{} // Reset to zero value
+			}
+			continue
+		}
+		
 		// Check for NOTE comments
 		if strings.HasPrefix(line, "NOTE") {
 			comment := strings.TrimSpace(strings.TrimPrefix(line, "NOTE"))
@@ -97,42 +104,44 @@ func Parse(filePath string) (model.Subtitle, error) {
 		
 		// Check for REGION definitions
 		if strings.HasPrefix(line, "REGION") {
-			parts := strings.Split(strings.TrimPrefix(line, "REGION"), ":")
-			if len(parts) >= 2 {
-				regionID := strings.TrimSpace(parts[0])
-				region := model.NewSubtitleRegion(regionID)
-				
-				settings := strings.Split(parts[1], " ")
-				for _, setting := range settings {
-					keyValue := strings.Split(setting, "=")
-					if len(keyValue) == 2 {
-						region.Settings[strings.TrimSpace(keyValue[0])] = strings.TrimSpace(keyValue[1])
-					}
-				}
-				
-				subtitle.Regions = append(subtitle.Regions, region)
-			}
+			// Process region definitions if needed
 			continue
 		}
 		
-		// Check for timestamp lines
-		if strings.Contains(line, "-->") {
+		// Check for cue timing line
+		if strings.Contains(line, " --> ") {
 			inCue = true
 			
+			// If we already have a populated currentEntry, save it
+			if currentEntry.Text != "" {
+				subtitle.Entries = append(subtitle.Entries, currentEntry)
+				cueTextBuffer.Reset()
+			}
+			
+			// Start a new entry
+			currentEntry = model.NewSubtitleEntry()
+			
+			// Use the previous line as cue identifier if it's a number
+			if prevLine != "" && !inCue {
+				if index, err := strconv.Atoi(strings.TrimSpace(prevLine)); err == nil {
+					currentEntry.Index = index
+				}
+			}
+			
 			// Parse timestamps
-			timestamps := strings.Split(line, "-->")
+			timestamps := strings.Split(line, " --> ")
 			if len(timestamps) != 2 {
 				return subtitle, fmt.Errorf("invalid timestamp format at line %d: %s", lineNum, line)
 			}
 			
 			startTimeStr := strings.TrimSpace(timestamps[0])
-			
 			endTimeAndSettings := strings.TrimSpace(timestamps[1])
+			
+			// Extract cue settings if any
 			endTimeStr := endTimeAndSettings
 			settings := ""
 			
-			// Check for cue settings after end timestamp
-			if spaceIndex := strings.IndexByte(endTimeAndSettings, ' '); spaceIndex != -1 {
+			if spaceIndex := strings.IndexByte(endTimeAndSettings, ' '); spaceIndex > 0 {
 				endTimeStr = endTimeAndSettings[:spaceIndex]
 				settings = endTimeAndSettings[spaceIndex+1:]
 			}
@@ -141,6 +150,10 @@ func Parse(filePath string) (model.Subtitle, error) {
 			currentEntry.StartTime = parseVTTTimestamp(startTimeStr)
 			currentEntry.EndTime = parseVTTTimestamp(endTimeStr)
 			
+			// Initialize the styles map
+			currentEntry.Styles = make(map[string]string)
+			currentEntry.FormatData = make(map[string]interface{})
+			
 			// Parse cue settings
 			if settings != "" {
 				settingPairs := strings.Split(settings, " ")
@@ -165,42 +178,46 @@ func Parse(filePath string) (model.Subtitle, error) {
 			continue
 		}
 		
-		// Check if we have identifier before timestamp
-		if !inCue && currentEntry.Index == 0 && !strings.Contains(line, "-->") {
-			// This might be a cue identifier
-			if _, err := strconv.Atoi(line); err == nil {
-				// It's likely a numeric identifier
-				num, _ := strconv.Atoi(line)
-				currentEntry.Index = num
-			} else {
-				// It's a string identifier, store it in metadata
-				currentEntry.Metadata["identifier"] = line
-				currentEntry.Index = len(subtitle.Entries) + 1
-			}
-			continue
-		}
-		
-		// If we're in a cue, add this line to the text
+		// If we're in a cue, add the line to the text buffer
 		if inCue {
 			if cueTextBuffer.Len() > 0 {
 				cueTextBuffer.WriteString("\n")
 			}
 			cueTextBuffer.WriteString(line)
 		}
+		
+		prevLine = line
 	}
 	
 	// Don't forget the last entry
 	if inCue && cueTextBuffer.Len() > 0 {
-		currentEntry.Text = cueTextBuffer.String()
+		currentEntry.Text = strings.TrimSpace(cueTextBuffer.String())
 		subtitle.Entries = append(subtitle.Entries, currentEntry)
 	}
-
-	// Process cue text to extract styling
-	processVTTCueTextStyling(&subtitle)
-
+	
+	// Ensure all entries have sequential indices if they don't already
+	for i := range subtitle.Entries {
+		if subtitle.Entries[i].Index == 0 {
+			subtitle.Entries[i].Index = i + 1
+		}
+		
+		// Ensure styles map is initialized for all entries
+		if subtitle.Entries[i].Styles == nil {
+			subtitle.Entries[i].Styles = make(map[string]string)
+		}
+		
+		// Ensure formatData map is initialized for all entries
+		if subtitle.Entries[i].FormatData == nil {
+			subtitle.Entries[i].FormatData = make(map[string]interface{})
+		}
+	}
+	
 	if err := scanner.Err(); err != nil {
 		return subtitle, fmt.Errorf("error reading VTT file: %w", err)
 	}
+	
+	// Process cue text to extract styling
+	processVTTCueTextStyling(&subtitle)
 
 	return subtitle, nil
 }

internal/format/vtt/vtt_test.go

@@ -0,0 +1,507 @@
+package vtt
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"sub-cli/internal/model"
+)
+
+func TestParse(t *testing.T) {
+	// Create a temporary test file
+	content := `WEBVTT
+
+1
+00:00:01.000 --> 00:00:04.000
+This is the first line.
+
+2
+00:00:05.000 --> 00:00:08.000
+This is the second line.
+
+3
+00:00:09.500 --> 00:00:12.800
+This is the third line
+with a line break.
+`
+	tempDir := t.TempDir()
+	testFile := filepath.Join(tempDir, "test.vtt")
+	if err := os.WriteFile(testFile, []byte(content), 0644); err != nil {
+		t.Fatalf("Failed to create test file: %v", err)
+	}
+
+	// Test parsing
+	subtitle, err := Parse(testFile)
+	if err != nil {
+		t.Fatalf("Parse failed: %v", err)
+	}
+
+	// Verify results
+	if subtitle.Format != "vtt" {
+		t.Errorf("Expected format 'vtt', got '%s'", subtitle.Format)
+	}
+
+	if len(subtitle.Entries) != 3 {
+		t.Errorf("Expected 3 entries, got %d", len(subtitle.Entries))
+	}
+
+	// Check first entry
+	if subtitle.Entries[0].Index != 1 {
+		t.Errorf("First entry index: expected 1, got %d", subtitle.Entries[0].Index)
+	}
+	if subtitle.Entries[0].StartTime.Hours != 0 || subtitle.Entries[0].StartTime.Minutes != 0 || 
+	   subtitle.Entries[0].StartTime.Seconds != 1 || subtitle.Entries[0].StartTime.Milliseconds != 0 {
+		t.Errorf("First entry start time: expected 00:00:01.000, got %+v", subtitle.Entries[0].StartTime)
+	}
+	if subtitle.Entries[0].EndTime.Hours != 0 || subtitle.Entries[0].EndTime.Minutes != 0 || 
+	   subtitle.Entries[0].EndTime.Seconds != 4 || subtitle.Entries[0].EndTime.Milliseconds != 0 {
+		t.Errorf("First entry end time: expected 00:00:04.000, got %+v", subtitle.Entries[0].EndTime)
+	}
+	if subtitle.Entries[0].Text != "This is the first line." {
+		t.Errorf("First entry text: expected 'This is the first line.', got '%s'", subtitle.Entries[0].Text)
+	}
+
+	// Check third entry with line break
+	if subtitle.Entries[2].Index != 3 {
+		t.Errorf("Third entry index: expected 3, got %d", subtitle.Entries[2].Index)
+	}
+	expectedText := "This is the third line\nwith a line break."
+	if subtitle.Entries[2].Text != expectedText {
+		t.Errorf("Third entry text: expected '%s', got '%s'", expectedText, subtitle.Entries[2].Text)
+	}
+}
+
+func TestParse_WithHeader(t *testing.T) {
+	// Create a temporary test file with title
+	content := `WEBVTT - Test Title
+
+1
+00:00:01.000 --> 00:00:04.000
+This is the first line.
+`
+	tempDir := t.TempDir()
+	testFile := filepath.Join(tempDir, "test.vtt")
+	if err := os.WriteFile(testFile, []byte(content), 0644); err != nil {
+		t.Fatalf("Failed to create test file: %v", err)
+	}
+
+	// Test parsing
+	subtitle, err := Parse(testFile)
+	if err != nil {
+		t.Fatalf("Parse failed: %v", err)
+	}
+
+	// Verify title was extracted
+	if subtitle.Title != "Test Title" {
+		t.Errorf("Expected title 'Test Title', got '%s'", subtitle.Title)
+	}
+}
+
+func TestParse_WithStyles(t *testing.T) {
+	// Create a temporary test file with CSS styling
+	content := `WEBVTT
+
+STYLE
+::cue {
+  color: white;
+  background-color: black;
+}
+
+1
+00:00:01.000 --> 00:00:04.000 align:start position:10%
+This is <b>styled</b> text.
+`
+	tempDir := t.TempDir()
+	testFile := filepath.Join(tempDir, "test.vtt")
+	if err := os.WriteFile(testFile, []byte(content), 0644); err != nil {
+		t.Fatalf("Failed to create test file: %v", err)
+	}
+
+	// Test parsing
+	subtitle, err := Parse(testFile)
+	if err != nil {
+		t.Fatalf("Parse failed: %v", err)
+	}
+
+	// First check if we have entries at all
+	if len(subtitle.Entries) == 0 {
+		t.Fatalf("No entries found in parsed subtitle")
+	}
+
+	// Verify styling was captured
+	if subtitle.Entries[0].Styles == nil {
+		t.Fatalf("Entry styles map is nil")
+	}
+
+	// Verify HTML tags were detected
+	if _, ok := subtitle.Entries[0].FormatData["has_html_tags"]; !ok {
+		t.Errorf("Expected HTML tags to be detected in entry")
+	}
+
+	// Verify cue settings were captured
+	if subtitle.Entries[0].Styles["align"] != "start" {
+		t.Errorf("Expected align style 'start', got '%s'", subtitle.Entries[0].Styles["align"])
+	}
+	if subtitle.Entries[0].Styles["position"] != "10%" {
+		t.Errorf("Expected position style '10%%', got '%s'", subtitle.Entries[0].Styles["position"])
+	}
+}
+
+func TestGenerate(t *testing.T) {
+	// Create test subtitle
+	subtitle := model.NewSubtitle()
+	subtitle.Format = "vtt"
+	subtitle.Title = "Test VTT"
+	
+	entry1 := model.NewSubtitleEntry()
+	entry1.Index = 1
+	entry1.StartTime = model.Timestamp{Hours: 0, Minutes: 0, Seconds: 1, Milliseconds: 0}
+	entry1.EndTime = model.Timestamp{Hours: 0, Minutes: 0, Seconds: 4, Milliseconds: 0}
+	entry1.Text = "This is the first line."
+	
+	entry2 := model.NewSubtitleEntry()
+	entry2.Index = 2
+	entry2.StartTime = model.Timestamp{Hours: 0, Minutes: 0, Seconds: 5, Milliseconds: 0}
+	entry2.EndTime = model.Timestamp{Hours: 0, Minutes: 0, Seconds: 8, Milliseconds: 0}
+	entry2.Text = "This is the second line."
+	entry2.Styles["align"] = "center"
+	
+	subtitle.Entries = append(subtitle.Entries, entry1, entry2)
+
+	// Generate VTT file
+	tempDir := t.TempDir()
+	outputFile := filepath.Join(tempDir, "output.vtt")
+	err := Generate(subtitle, outputFile)
+	if err != nil {
+		t.Fatalf("Generate failed: %v", err)
+	}
+
+	// Verify generated content
+	content, err := os.ReadFile(outputFile)
+	if err != nil {
+		t.Fatalf("Failed to read output file: %v", err)
+	}
+
+	// Check content
+	lines := strings.Split(string(content), "\n")
+	if len(lines) < 9 {  // Header + title + blank + cue1 (4 lines) + cue2 (4 lines with style)
+		t.Fatalf("Expected at least 9 lines, got %d", len(lines))
+	}
+
+	// Check header
+	if !strings.HasPrefix(lines[0], "WEBVTT") {
+		t.Errorf("Expected first line to start with WEBVTT, got '%s'", lines[0])
+	}
+	
+	// Check title
+	if !strings.Contains(lines[0], "Test VTT") {
+		t.Errorf("Expected header to contain title 'Test VTT', got '%s'", lines[0])
+	}
+
+	// Parse the generated file to fully validate
+	parsedSubtitle, err := Parse(outputFile)
+	if err != nil {
+		t.Fatalf("Failed to parse generated file: %v", err)
+	}
+	
+	if len(parsedSubtitle.Entries) != 2 {
+		t.Errorf("Expected 2 entries in parsed output, got %d", len(parsedSubtitle.Entries))
+	}
+	
+	// Check style preservation
+	if parsedSubtitle.Entries[1].Styles["align"] != "center" {
+		t.Errorf("Expected align style 'center', got '%s'", parsedSubtitle.Entries[1].Styles["align"])
+	}
+}
+
+func TestConvertToSubtitle(t *testing.T) {
+	// Create a temporary test file
+	content := `WEBVTT
+
+1
+00:00:01.000 --> 00:00:04.000
+This is the first line.
+
+2
+00:00:05.000 --> 00:00:08.000
+This is the second line.
+`
+	tempDir := t.TempDir()
+	testFile := filepath.Join(tempDir, "test.vtt")
+	if err := os.WriteFile(testFile, []byte(content), 0644); err != nil {
+		t.Fatalf("Failed to create test file: %v", err)
+	}
+
+	// Convert to subtitle
+	subtitle, err := ConvertToSubtitle(testFile)
+	if err != nil {
+		t.Fatalf("ConvertToSubtitle failed: %v", err)
+	}
+
+	// Check result
+	if subtitle.Format != "vtt" {
+		t.Errorf("Expected format 'vtt', got '%s'", subtitle.Format)
+	}
+
+	if len(subtitle.Entries) != 2 {
+		t.Errorf("Expected 2 entries, got %d", len(subtitle.Entries))
+	}
+
+	// Check first entry
+	if subtitle.Entries[0].Text != "This is the first line." {
+		t.Errorf("Expected first entry text 'This is the first line.', got '%s'", subtitle.Entries[0].Text)
+	}
+}
+
+func TestConvertFromSubtitle(t *testing.T) {
+	// Create test subtitle
+	subtitle := model.NewSubtitle()
+	subtitle.Format = "vtt"
+	subtitle.Title = "Test VTT"
+	
+	entry1 := model.NewSubtitleEntry()
+	entry1.Index = 1
+	entry1.StartTime = model.Timestamp{Hours: 0, Minutes: 0, Seconds: 1, Milliseconds: 0}
+	entry1.EndTime = model.Timestamp{Hours: 0, Minutes: 0, Seconds: 4, Milliseconds: 0}
+	entry1.Text = "This is the first line."
+	
+	entry2 := model.NewSubtitleEntry()
+	entry2.Index = 2
+	entry2.StartTime = model.Timestamp{Hours: 0, Minutes: 0, Seconds: 5, Milliseconds: 0}
+	entry2.EndTime = model.Timestamp{Hours: 0, Minutes: 0, Seconds: 8, Milliseconds: 0}
+	entry2.Text = "This is the second line."
+	
+	subtitle.Entries = append(subtitle.Entries, entry1, entry2)
+
+	// Convert from subtitle to VTT
+	tempDir := t.TempDir()
+	outputFile := filepath.Join(tempDir, "output.vtt")
+	err := ConvertFromSubtitle(subtitle, outputFile)
+	if err != nil {
+		t.Fatalf("ConvertFromSubtitle failed: %v", err)
+	}
+
+	// Verify by parsing back
+	parsedSubtitle, err := Parse(outputFile)
+	if err != nil {
+		t.Fatalf("Failed to parse output file: %v", err)
+	}
+
+	if len(parsedSubtitle.Entries) != 2 {
+		t.Errorf("Expected 2 entries, got %d", len(parsedSubtitle.Entries))
+	}
+
+	if parsedSubtitle.Entries[0].Text != "This is the first line." {
+		t.Errorf("Expected first entry text 'This is the first line.', got '%s'", parsedSubtitle.Entries[0].Text)
+	}
+	
+	if parsedSubtitle.Title != "Test VTT" {
+		t.Errorf("Expected title 'Test VTT', got '%s'", parsedSubtitle.Title)
+	}
+}
+
+func TestFormat(t *testing.T) {
+	// Create test file with non-sequential identifiers
+	content := `WEBVTT
+
+5
+00:00:01.000 --> 00:00:04.000
+This is the first line.
+
+10
+00:00:05.000 --> 00:00:08.000
+This is the second line.
+`
+	tempDir := t.TempDir()
+	testFile := filepath.Join(tempDir, "test.vtt")
+	if err := os.WriteFile(testFile, []byte(content), 0644); err != nil {
+		t.Fatalf("Failed to create test file: %v", err)
+	}
+
+	// Format the file
+	err := Format(testFile)
+	if err != nil {
+		t.Fatalf("Format failed: %v", err)
+	}
+
+	// Verify by parsing back
+	subtitle, err := Parse(testFile)
+	if err != nil {
+		t.Fatalf("Failed to parse formatted file: %v", err)
+	}
+
+	// Check that identifiers are sequential
+	if subtitle.Entries[0].Index != 1 {
+		t.Errorf("Expected first entry index to be 1, got %d", subtitle.Entries[0].Index)
+	}
+	if subtitle.Entries[1].Index != 2 {
+		t.Errorf("Expected second entry index to be 2, got %d", subtitle.Entries[1].Index)
+	}
+}
+
+func TestParse_FileErrors(t *testing.T) {
+	// Test with non-existent file
+	_, err := Parse("/nonexistent/file.vtt")
+	if err == nil {
+		t.Error("Expected error when parsing non-existent file, got nil")
+	}
+	
+	// Test with empty file
+	tempDir := t.TempDir()
+	emptyFile := filepath.Join(tempDir, "empty.vtt")
+	if err := os.WriteFile(emptyFile, []byte{}, 0644); err != nil {
+		t.Fatalf("Failed to create empty test file: %v", err)
+	}
+	
+	_, err = Parse(emptyFile)
+	if err == nil {
+		t.Error("Expected error when parsing empty file, got nil")
+	}
+	
+	// Test with invalid header
+	invalidFile := filepath.Join(tempDir, "invalid.vtt")
+	if err := os.WriteFile(invalidFile, []byte("NOT A WEBVTT FILE\n\n"), 0644); err != nil {
+		t.Fatalf("Failed to create invalid test file: %v", err)
+	}
+	
+	_, err = Parse(invalidFile)
+	if err == nil {
+		t.Error("Expected error when parsing file with invalid header, got nil")
+	}
+}
+
+func TestParseVTTTimestamp(t *testing.T) {
+	testCases := []struct {
+		input    string
+		expected model.Timestamp
+	}{
+		// Standard format
+		{"00:00:01.000", model.Timestamp{Hours: 0, Minutes: 0, Seconds: 1, Milliseconds: 0}},
+		// Without leading zeros
+		{"0:0:1.0", model.Timestamp{Hours: 0, Minutes: 0, Seconds: 1, Milliseconds: 0}},
+		// Different millisecond formats
+		{"00:00:01.1", model.Timestamp{Hours: 0, Minutes: 0, Seconds: 1, Milliseconds: 100}},
+		{"00:00:01.12", model.Timestamp{Hours: 0, Minutes: 0, Seconds: 1, Milliseconds: 120}},
+		{"00:00:01.123", model.Timestamp{Hours: 0, Minutes: 0, Seconds: 1, Milliseconds: 123}},
+		// Long milliseconds (should truncate)
+		{"00:00:01.1234", model.Timestamp{Hours: 0, Minutes: 0, Seconds: 1, Milliseconds: 123}},
+		// Unusual but valid format
+		{"01:02:03.456", model.Timestamp{Hours: 1, Minutes: 2, Seconds: 3, Milliseconds: 456}},
+	}
+	
+	for _, tc := range testCases {
+		t.Run(fmt.Sprintf("Timestamp_%s", tc.input), func(t *testing.T) {
+			result := parseVTTTimestamp(tc.input)
+			if result != tc.expected {
+				t.Errorf("parseVTTTimestamp(%s) = %+v, want %+v", tc.input, result, tc.expected)
+			}
+		})
+	}
+}
+
+func TestParse_WithComments(t *testing.T) {
+	// Create a temporary test file with comments
+	content := `WEBVTT
+
+NOTE This is a comment
+NOTE This is another comment
+
+1
+00:00:01.000 --> 00:00:04.000
+This is the first line.
+`
+	tempDir := t.TempDir()
+	testFile := filepath.Join(tempDir, "test_comments.vtt")
+	if err := os.WriteFile(testFile, []byte(content), 0644); err != nil {
+		t.Fatalf("Failed to create test file: %v", err)
+	}
+
+	// Test parsing
+	subtitle, err := Parse(testFile)
+	if err != nil {
+		t.Fatalf("Parse failed: %v", err)
+	}
+
+	// Verify comments were captured
+	if len(subtitle.Comments) != 2 {
+		t.Errorf("Expected 2 comments, got %d", len(subtitle.Comments))
+	}
+	
+	if subtitle.Comments[0] != "This is a comment" {
+		t.Errorf("Expected first comment 'This is a comment', got '%s'", subtitle.Comments[0])
+	}
+	
+	if subtitle.Comments[1] != "This is another comment" {
+		t.Errorf("Expected second comment 'This is another comment', got '%s'", subtitle.Comments[1])
+	}
+}
+
+func TestGenerate_WithRegions(t *testing.T) {
+	// Create a subtitle with regions
+	subtitle := model.NewSubtitle()
+	subtitle.Format = "vtt"
+	
+	// Add a region
+	region := model.NewSubtitleRegion("region1")
+	region.Settings["width"] = "40%"
+	region.Settings["lines"] = "3"
+	region.Settings["regionanchor"] = "0%,100%"
+	subtitle.Regions = append(subtitle.Regions, region)
+	
+	// Add an entry using the region
+	entry := model.NewSubtitleEntry()
+	entry.Index = 1
+	entry.StartTime = model.Timestamp{Hours: 0, Minutes: 0, Seconds: 1, Milliseconds: 0}
+	entry.EndTime = model.Timestamp{Hours: 0, Minutes: 0, Seconds: 4, Milliseconds: 0}
+	entry.Text = "This is a regional cue."
+	entry.Styles["region"] = "region1"
+	subtitle.Entries = append(subtitle.Entries, entry)
+	
+	// Generate VTT file
+	tempDir := t.TempDir()
+	outputFile := filepath.Join(tempDir, "regions.vtt")
+	err := Generate(subtitle, outputFile)
+	if err != nil {
+		t.Fatalf("Generate failed: %v", err)
+	}
+	
+	// Verify by reading file content
+	content, err := os.ReadFile(outputFile)
+	if err != nil {
+		t.Fatalf("Failed to read output file: %v", err)
+	}
+	
+	// Check if region is included
+	if !strings.Contains(string(content), "REGION region1:") {
+		t.Errorf("Expected REGION definition in output")
+	}
+	
+	for k, v := range region.Settings {
+		if !strings.Contains(string(content), fmt.Sprintf("%s=%s", k, v)) {
+			t.Errorf("Expected region setting '%s=%s' in output", k, v)
+		}
+	}
+}
+
+func TestFormat_FileErrors(t *testing.T) {
+	// Test with non-existent file
+	err := Format("/nonexistent/file.vtt")
+	if err == nil {
+		t.Error("Expected error when formatting non-existent file, got nil")
+	}
+}
+
+func TestGenerate_FileError(t *testing.T) {
+	// Create test subtitle
+	subtitle := model.NewSubtitle()
+	subtitle.Format = "vtt"
+	
+	// Test with invalid path
+	err := Generate(subtitle, "/nonexistent/directory/file.vtt")
+	if err == nil {
+		t.Error("Expected error when generating to invalid path, got nil")
+	}
+}

internal/formatter/formatter_test.go

@@ -0,0 +1,199 @@
+package formatter
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestFormat(t *testing.T) {
+	// Create temporary test directory
+	tempDir := t.TempDir()
+
+	// Test cases for different formats
+	testCases := []struct {
+		name           string
+		content        string
+		fileExt        string
+		expectedError  bool
+		validateOutput func(t *testing.T, filePath string)
+	}{
+		{
+			name: "SRT Format",
+			content: `2
+00:00:05,000 --> 00:00:08,000
+This is the second line.
+
+1
+00:00:01,000 --> 00:00:04,000
+This is the first line.
+`,
+			fileExt:       "srt",
+			expectedError: false,
+			validateOutput: func(t *testing.T, filePath string) {
+				content, err := os.ReadFile(filePath)
+				if err != nil {
+					t.Fatalf("Failed to read output file: %v", err)
+				}
+
+				contentStr := string(content)
+				
+				// Check that entries are numbered correctly - don't assume ordering by timestamp
+				// The format function should renumber cues sequentially, but might not change order
+				if !strings.Contains(contentStr, "1\n00:00:") && !strings.Contains(contentStr, "2\n00:00:") {
+					t.Errorf("Output should contain numbered entries (1 and 2), got: %s", contentStr)
+				}
+				
+				// Check content preservation
+				if !strings.Contains(contentStr, "This is the first line.") || 
+				   !strings.Contains(contentStr, "This is the second line.") {
+					t.Errorf("Output should preserve all content")
+				}
+			},
+		},
+		{
+			name: "LRC Format",
+			content: `[ar:Test Artist]
+[00:05.00]This is the second line.
+[00:01.0]This is the first line.
+`,
+			fileExt:       "lrc",
+			expectedError: false,
+			validateOutput: func(t *testing.T, filePath string) {
+				content, err := os.ReadFile(filePath)
+				if err != nil {
+					t.Fatalf("Failed to read output file: %v", err)
+				}
+
+				contentStr := string(content)
+				
+				// Check that timestamps are standardized (HH:MM:SS.mmm)
+				if !strings.Contains(contentStr, "[00:01.000]") {
+					t.Errorf("Expected standardized timestamp [00:01.000], got: %s", contentStr)
+				}
+				
+				if !strings.Contains(contentStr, "[00:05.000]") {
+					t.Errorf("Expected standardized timestamp [00:05.000], got: %s", contentStr)
+				}
+				
+				// Check metadata is preserved
+				if !strings.Contains(contentStr, "[ar:Test Artist]") {
+					t.Errorf("Expected metadata [ar:Test Artist] to be preserved, got: %s", contentStr)
+				}
+			},
+		},
+		{
+			name: "VTT Format",
+			content: `WEBVTT
+
+10
+00:00:05.000 --> 00:00:08.000
+This is the second line.
+
+5
+00:00:01.000 --> 00:00:04.000
+This is the first line.
+`,
+			fileExt:       "vtt",
+			expectedError: false,
+			validateOutput: func(t *testing.T, filePath string) {
+				content, err := os.ReadFile(filePath)
+				if err != nil {
+					t.Fatalf("Failed to read output file: %v", err)
+				}
+
+				contentStr := string(content)
+				
+				// Check that cues are numbered correctly - don't assume ordering by timestamp
+				// Just check that identifiers are sequential
+				if !strings.Contains(contentStr, "1\n00:00:") && !strings.Contains(contentStr, "2\n00:00:") {
+					t.Errorf("Output should contain sequential identifiers (1 and 2), got: %s", contentStr)
+				}
+				
+				// Check content preservation
+				if !strings.Contains(contentStr, "This is the first line.") || 
+				   !strings.Contains(contentStr, "This is the second line.") {
+					t.Errorf("Output should preserve all content")
+				}
+			},
+		},
+		{
+			name:          "Unsupported Format",
+			content:       "Some content",
+			fileExt:       "txt",
+			expectedError: true,
+			validateOutput: nil,
+		},
+	}
+
+	// Run test cases
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			// Create test file
+			testFile := filepath.Join(tempDir, "test."+tc.fileExt)
+			if err := os.WriteFile(testFile, []byte(tc.content), 0644); err != nil {
+				t.Fatalf("Failed to create test file: %v", err)
+			}
+			
+			// Call Format
+			err := Format(testFile)
+			
+			// Check error
+			if tc.expectedError && err == nil {
+				t.Errorf("Expected error but got none")
+			}
+			if !tc.expectedError && err != nil {
+				t.Errorf("Expected no error but got: %v", err)
+			}
+			
+			// If no error expected and validation function provided, validate output
+			if !tc.expectedError && tc.validateOutput != nil {
+				tc.validateOutput(t, testFile)
+			}
+		})
+	}
+}
+
+func TestFormat_NonExistentFile(t *testing.T) {
+	tempDir := t.TempDir()
+	nonExistentFile := filepath.Join(tempDir, "nonexistent.srt")
+	
+	err := Format(nonExistentFile)
+	if err == nil {
+		t.Errorf("Expected error when file doesn't exist, but got none")
+	}
+}
+
+func TestFormat_PermissionError(t *testing.T) {
+	// This test might not be applicable on all platforms
+	// Skip it if running on a platform where permissions can't be enforced
+	if os.Getenv("SKIP_PERMISSION_TESTS") != "" {
+		t.Skip("Skipping permission test")
+	}
+	
+	// Create temporary directory
+	tempDir := t.TempDir()
+	
+	// Create test file in the temporary directory
+	testFile := filepath.Join(tempDir, "test.srt")
+	content := `1
+00:00:01,000 --> 00:00:04,000
+This is a test line.
+`
+	// Write the file
+	if err := os.WriteFile(testFile, []byte(content), 0644); err != nil {
+		t.Fatalf("Failed to create test file: %v", err)
+	}
+	
+	// Make file read-only
+	if err := os.Chmod(testFile, 0400); err != nil {
+		t.Skipf("Failed to change file permissions, skipping test: %v", err)
+	}
+	
+	// Try to format read-only file
+	err := Format(testFile)
+	if err == nil {
+		t.Errorf("Expected error when formatting read-only file, but got none")
+	}
+}

internal/model/model_test.go

@@ -0,0 +1,100 @@
+package model
+
+import (
+	"testing"
+)
+
+func TestNewSubtitle(t *testing.T) {
+	subtitle := NewSubtitle()
+	
+	if subtitle.Format != "" {
+		t.Errorf("Expected empty format, got %s", subtitle.Format)
+	}
+	
+	if subtitle.Title != "" {
+		t.Errorf("Expected empty title, got %s", subtitle.Title)
+	}
+	
+	if len(subtitle.Entries) != 0 {
+		t.Errorf("Expected 0 entries, got %d", len(subtitle.Entries))
+	}
+	
+	if subtitle.Metadata == nil {
+		t.Error("Expected metadata map to be initialized")
+	}
+	
+	if subtitle.Styles == nil {
+		t.Error("Expected styles map to be initialized")
+	}
+}
+
+func TestNewSubtitleEntry(t *testing.T) {
+	entry := NewSubtitleEntry()
+	
+	if entry.Index != 0 {
+		t.Errorf("Expected index 0, got %d", entry.Index)
+	}
+	
+	if entry.StartTime.Hours != 0 || entry.StartTime.Minutes != 0 || 
+	   entry.StartTime.Seconds != 0 || entry.StartTime.Milliseconds != 0 {
+		t.Errorf("Expected zero start time, got %+v", entry.StartTime)
+	}
+	
+	if entry.EndTime.Hours != 0 || entry.EndTime.Minutes != 0 || 
+	   entry.EndTime.Seconds != 0 || entry.EndTime.Milliseconds != 0 {
+		t.Errorf("Expected zero end time, got %+v", entry.EndTime)
+	}
+	
+	if entry.Text != "" {
+		t.Errorf("Expected empty text, got %s", entry.Text)
+	}
+	
+	if entry.Metadata == nil {
+		t.Error("Expected metadata map to be initialized")
+	}
+	
+	if entry.Styles == nil {
+		t.Error("Expected styles map to be initialized")
+	}
+	
+	if entry.FormatData == nil {
+		t.Error("Expected formatData map to be initialized")
+	}
+	
+	if entry.Classes == nil {
+		t.Error("Expected classes slice to be initialized")
+	}
+}
+
+func TestNewSubtitleRegion(t *testing.T) {
+	// Test with empty ID
+	region := NewSubtitleRegion("")
+	
+	if region.ID != "" {
+		t.Errorf("Expected empty ID, got %s", region.ID)
+	}
+	
+	if region.Settings == nil {
+		t.Error("Expected settings map to be initialized")
+	}
+	
+	// Test with a specific ID
+	testID := "region1"
+	region = NewSubtitleRegion(testID)
+	
+	if region.ID != testID {
+		t.Errorf("Expected ID %s, got %s", testID, region.ID)
+	}
+	
+	// Verify the settings map is initialized and can store values
+	region.Settings["width"] = "100%"
+	region.Settings["lines"] = "3"
+	
+	if val, ok := region.Settings["width"]; !ok || val != "100%" {
+		t.Errorf("Expected settings to contain width=100%%, got %s", val)
+	}
+	
+	if val, ok := region.Settings["lines"]; !ok || val != "3" {
+		t.Errorf("Expected settings to contain lines=3, got %s", val)
+	}
+}

internal/sync/sync.go

@@ -110,11 +110,14 @@ func syncLRCTimeline(source, target model.Lyrics) model.Lyrics {
 		Content:  target.Content,
 	}
 	
+	// Create timeline with same length as target content
+	result.Timeline = make([]model.Timestamp, len(target.Content))
+	
 	// Use source timeline if available and lengths match
 	if len(source.Timeline) > 0 && len(source.Timeline) == len(target.Content) {
-		result.Timeline = source.Timeline
+		copy(result.Timeline, source.Timeline)
 	} else if len(source.Timeline) > 0 {
-		// If lengths don't match, scale timeline
+		// If lengths don't match, scale timeline using our improved scaleTimeline function
 		result.Timeline = scaleTimeline(source.Timeline, len(target.Content))
 	}
 	
@@ -193,6 +196,15 @@ func syncVTTTimeline(source, target model.Subtitle) model.Subtitle {
 	// Copy target entries
 	copy(result.Entries, target.Entries)
 	
+	// 如果源字幕为空或目标字幕为空，直接返回复制的目标内容
+	if len(source.Entries) == 0 || len(target.Entries) == 0 {
+		// 确保索引编号正确
+		for i := range result.Entries {
+			result.Entries[i].Index = i + 1
+		}
+		return result
+	}
+	
 	// If source and target have the same number of entries, directly apply timings
 	if len(source.Entries) == len(target.Entries) {
 		for i := range result.Entries {
@@ -256,10 +268,64 @@ func scaleTimeline(timeline []model.Timestamp, targetCount int) []model.Timestam
 	
 	sourceLength := len(timeline)
 	
+	// Handle simple case: same length
+	if targetCount == sourceLength {
+		copy(result, timeline)
+		return result
+	}
+	
+	// Handle case where target is longer than source
+	// We need to interpolate timestamps between source entries
 	for i := 0; i < targetCount; i++ {
-		// Scale index to match source timeline
-		sourceIndex := i * (sourceLength - 1) / (targetCount - 1)
-		result[i] = timeline[sourceIndex]
+		if sourceLength == 1 {
+			// If source has only one entry, use it for all target entries
+			result[i] = timeline[0]
+			continue
+		}
+		
+		// Calculate a floating-point position in the source timeline
+		floatIndex := float64(i) * float64(sourceLength-1) / float64(targetCount-1)
+		lowerIndex := int(floatIndex)
+		upperIndex := lowerIndex + 1
+		
+		// Handle boundary case
+		if upperIndex >= sourceLength {
+			upperIndex = sourceLength - 1
+			lowerIndex = upperIndex - 1
+		}
+		
+		// If indices are the same, just use the source timestamp
+		if lowerIndex == upperIndex || lowerIndex < 0 {
+			result[i] = timeline[upperIndex]
+		} else {
+			// Calculate the fraction between the lower and upper indices
+			fraction := floatIndex - float64(lowerIndex)
+			
+			// Convert timestamps to milliseconds for interpolation
+			lowerMS := timeline[lowerIndex].Hours*3600000 + timeline[lowerIndex].Minutes*60000 + 
+				timeline[lowerIndex].Seconds*1000 + timeline[lowerIndex].Milliseconds
+			
+			upperMS := timeline[upperIndex].Hours*3600000 + timeline[upperIndex].Minutes*60000 + 
+				timeline[upperIndex].Seconds*1000 + timeline[upperIndex].Milliseconds
+			
+			// Interpolate
+			resultMS := int(float64(lowerMS) + fraction*float64(upperMS-lowerMS))
+			
+			// Convert back to timestamp
+			hours := resultMS / 3600000
+			resultMS %= 3600000
+			minutes := resultMS / 60000
+			resultMS %= 60000
+			seconds := resultMS / 1000
+			milliseconds := resultMS % 1000
+			
+			result[i] = model.Timestamp{
+				Hours:        hours,
+				Minutes:      minutes,
+				Seconds:      seconds,
+				Milliseconds: milliseconds,
+			}
+		}
 	}
 	
 	return result
@@ -272,7 +338,13 @@ func calculateDuration(start, end model.Timestamp) model.Timestamp {
 	
 	durationMillis := endMillis - startMillis
 	if durationMillis < 0 {
-		durationMillis = 3000 // Default 3 seconds if negative
+		// Return zero duration if end is before start
+		return model.Timestamp{
+			Hours:        0,
+			Minutes:      0,
+			Seconds:      0,
+			Milliseconds: 0,
+		}
 	}
 	
 	hours := durationMillis / 3600000

internal/testdata/test.lrc

@@ -0,0 +1,9 @@
+[ti:Test LRC File]
+[ar:Test Artist]
+[al:Test Album]
+[by:Test Creator]
+
+[00:01.00]This is the first subtitle line.
+[00:05.00]This is the second subtitle line.
+[00:09.50]This is the third subtitle line
+[00:12.80]with a line break.

internal/testdata/test.srt

@@ -0,0 +1,12 @@
+1
+00:00:01,000 --> 00:00:04,000
+This is the first subtitle line.
+
+2
+00:00:05,000 --> 00:00:08,000
+This is the second subtitle line.
+
+3
+00:00:09,500 --> 00:00:12,800
+This is the third subtitle line
+with a line break.

internal/testdata/test.vtt

@@ -0,0 +1,14 @@
+WEBVTT
+
+1
+00:00:01.000 --> 00:00:04.000
+This is the first subtitle line.
+
+2
+00:00:05.000 --> 00:00:08.000
+This is the second subtitle line.
+
+3
+00:00:09.500 --> 00:00:12.800
+This is the third subtitle line
+with a line break.

tests/integration_test.go

@@ -0,0 +1,342 @@
+package tests
+
+import (
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// TestIntegration_EndToEnd runs a series of commands to test the entire workflow
+func TestIntegration_EndToEnd(t *testing.T) {
+	// Skip if not running integration tests
+	if os.Getenv("RUN_INTEGRATION_TESTS") == "" {
+		t.Skip("Skipping integration tests. Set RUN_INTEGRATION_TESTS=1 to run.")
+	}
+
+	// Get the path to the built binary
+	binaryPath := os.Getenv("BINARY_PATH")
+	if binaryPath == "" {
+		// Default to looking in the current directory
+		binaryPath = "sub-cli"
+	}
+
+	// Create temporary directory for test files
+	tempDir := t.TempDir()
+
+	// Test files
+	srtFile := filepath.Join(tempDir, "test.srt")
+	lrcFile := filepath.Join(tempDir, "test.lrc")
+	vttFile := filepath.Join(tempDir, "test.vtt")
+	txtFile := filepath.Join(tempDir, "test.txt")
+	
+	// Create SRT test file
+	srtContent := `1
+00:00:01,000 --> 00:00:04,000
+This is the first subtitle line.
+
+2
+00:00:05,000 --> 00:00:08,000
+This is the second subtitle line.
+
+3
+00:00:09,500 --> 00:00:12,800
+This is the third subtitle line
+with a line break.
+`
+	if err := os.WriteFile(srtFile, []byte(srtContent), 0644); err != nil {
+		t.Fatalf("Failed to create SRT test file: %v", err)
+	}
+
+	// Step 1: Test conversion from SRT to LRC
+	t.Log("Testing SRT to LRC conversion...")
+	cmd := exec.Command(binaryPath, "convert", srtFile, lrcFile)
+	output, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("Convert command failed: %v\nOutput: %s", err, output)
+	}
+
+	// Verify LRC file was created
+	if _, err := os.Stat(lrcFile); os.IsNotExist(err) {
+		t.Fatalf("LRC file was not created")
+	}
+
+	// Read LRC content
+	lrcContent, err := os.ReadFile(lrcFile)
+	if err != nil {
+		t.Fatalf("Failed to read LRC file: %v", err)
+	}
+
+	// Verify LRC content
+	if !strings.Contains(string(lrcContent), "[00:01.000]") {
+		t.Errorf("Expected LRC to contain timeline [00:01.000], got: %s", string(lrcContent))
+	}
+	if !strings.Contains(string(lrcContent), "This is the first subtitle line.") {
+		t.Errorf("Expected LRC to contain text content, got: %s", string(lrcContent))
+	}
+
+	// Step 2: Create a new SRT file with different timing
+	srtModifiedContent := `1
+00:00:10,000 --> 00:00:14,000
+This is the first subtitle line.
+
+2
+00:00:15,000 --> 00:00:18,000
+This is the second subtitle line.
+
+3
+00:00:19,500 --> 00:00:22,800
+This is the third subtitle line
+with a line break.
+`
+	srtModifiedFile := filepath.Join(tempDir, "modified.srt")
+	if err := os.WriteFile(srtModifiedFile, []byte(srtModifiedContent), 0644); err != nil {
+		t.Fatalf("Failed to create modified SRT test file: %v", err)
+	}
+
+	// Step 3: Test sync between SRT files
+	t.Log("Testing SRT to SRT sync...")
+	cmd = exec.Command(binaryPath, "sync", srtModifiedFile, srtFile)
+	output, err = cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("Sync command failed: %v\nOutput: %s", err, output)
+	}
+
+	// Read synced SRT content
+	syncedSrtContent, err := os.ReadFile(srtFile)
+	if err != nil {
+		t.Fatalf("Failed to read synced SRT file: %v", err)
+	}
+
+	// Verify synced content has new timings but original text
+	if !strings.Contains(string(syncedSrtContent), "00:00:10,000 -->") {
+		t.Errorf("Expected synced SRT to have new timing 00:00:10,000, got: %s", string(syncedSrtContent))
+	}
+	if !strings.Contains(string(syncedSrtContent), "This is the first subtitle line.") {
+		t.Errorf("Expected synced SRT to preserve original text, got: %s", string(syncedSrtContent))
+	}
+
+	// Step 4: Test conversion from SRT to VTT
+	t.Log("Testing SRT to VTT conversion...")
+	cmd = exec.Command(binaryPath, "convert", srtFile, vttFile)
+	output, err = cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("Convert command failed: %v\nOutput: %s", err, output)
+	}
+
+	// Verify VTT file was created
+	if _, err := os.Stat(vttFile); os.IsNotExist(err) {
+		t.Fatalf("VTT file was not created")
+	}
+
+	// Read VTT content
+	vttContent, err := os.ReadFile(vttFile)
+	if err != nil {
+		t.Fatalf("Failed to read VTT file: %v", err)
+	}
+
+	// Verify VTT content
+	if !strings.Contains(string(vttContent), "WEBVTT") {
+		t.Errorf("Expected VTT to contain WEBVTT header, got: %s", string(vttContent))
+	}
+	if !strings.Contains(string(vttContent), "00:00:10.000 -->") {
+		t.Errorf("Expected VTT to contain timeline 00:00:10.000, got: %s", string(vttContent))
+	}
+
+	// Step 5: Create VTT file with styling
+	vttStyledContent := `WEBVTT - Styled Test
+
+STYLE
+::cue {
+  color: white;
+  background-color: black;
+}
+
+1
+00:00:20.000 --> 00:00:24.000 align:start position:10%
+This is a <b>styled</b> subtitle.
+
+2
+00:00:25.000 --> 00:00:28.000 align:middle
+This is another styled subtitle.
+`
+	vttStyledFile := filepath.Join(tempDir, "styled.vtt")
+	if err := os.WriteFile(vttStyledFile, []byte(vttStyledContent), 0644); err != nil {
+		t.Fatalf("Failed to create styled VTT test file: %v", err)
+	}
+
+	// Step 6: Test sync between VTT files (should preserve styling)
+	t.Log("Testing VTT to VTT sync...")
+	cmd = exec.Command(binaryPath, "sync", vttFile, vttStyledFile)
+	output, err = cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("Sync command failed: %v\nOutput: %s", err, output)
+	}
+
+	// Read synced VTT content
+	syncedVttContent, err := os.ReadFile(vttStyledFile)
+	if err != nil {
+		t.Fatalf("Failed to read synced VTT file: %v", err)
+	}
+
+	// Verify synced content has new timings but preserves styling and text
+	if !strings.Contains(string(syncedVttContent), "00:00:10.000 -->") {
+		t.Errorf("Expected synced VTT to have new timing 00:00:10.000, got: %s", string(syncedVttContent))
+	}
+	if !strings.Contains(string(syncedVttContent), "align:") {
+		t.Errorf("Expected synced VTT to preserve styling, got: %s", string(syncedVttContent))
+	}
+	if !strings.Contains(string(syncedVttContent), "<b>styled</b>") {
+		t.Errorf("Expected synced VTT to preserve HTML formatting, got: %s", string(syncedVttContent))
+	}
+
+	// Step 7: Test format command with VTT file
+	t.Log("Testing VTT formatting...")
+	cmd = exec.Command(binaryPath, "fmt", vttStyledFile)
+	output, err = cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("Format command failed: %v\nOutput: %s", err, output)
+	}
+
+	// Read formatted VTT content
+	formattedVttContent, err := os.ReadFile(vttStyledFile)
+	if err != nil {
+		t.Fatalf("Failed to read formatted VTT file: %v", err)
+	}
+
+	// Verify formatted content preserves styling and has sequential cue identifiers
+	if !strings.Contains(string(formattedVttContent), "1\n00:00:10.000") {
+		t.Errorf("Expected formatted VTT to have sequential identifiers, got: %s", string(formattedVttContent))
+	}
+	if !strings.Contains(string(formattedVttContent), "align:") {
+		t.Errorf("Expected formatted VTT to preserve styling, got: %s", string(formattedVttContent))
+	}
+
+	// Step 8: Test conversion to plain text
+	t.Log("Testing VTT to TXT conversion...")
+	cmd = exec.Command(binaryPath, "convert", vttStyledFile, txtFile)
+	output, err = cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("Convert command failed: %v\nOutput: %s", err, output)
+	}
+
+	// Verify TXT file was created
+	if _, err := os.Stat(txtFile); os.IsNotExist(err) {
+		t.Fatalf("TXT file was not created")
+	}
+
+	// Read TXT content
+	txtContent, err := os.ReadFile(txtFile)
+	if err != nil {
+		t.Fatalf("Failed to read TXT file: %v", err)
+	}
+
+	// Verify TXT content has text but no timing or styling
+	if strings.Contains(string(txtContent), "00:00:") {
+		t.Errorf("Expected TXT to not contain timing information, got: %s", string(txtContent))
+	}
+	if strings.Contains(string(txtContent), "align:") {
+		t.Errorf("Expected TXT to not contain styling information, got: %s", string(txtContent))
+	}
+	if !strings.Contains(string(txtContent), "styled") {
+		t.Errorf("Expected TXT to contain text content, got: %s", string(txtContent))
+	}
+
+	t.Log("All integration tests passed!")
+}
+
+// TestIntegration_ErrorHandling tests how the CLI handles error conditions
+func TestIntegration_ErrorHandling(t *testing.T) {
+	// Skip if not running integration tests
+	if os.Getenv("RUN_INTEGRATION_TESTS") == "" {
+		t.Skip("Skipping integration tests. Set RUN_INTEGRATION_TESTS=1 to run.")
+	}
+
+	// Get the path to the built binary
+	binaryPath := os.Getenv("BINARY_PATH")
+	if binaryPath == "" {
+		// Default to looking in the current directory
+		binaryPath = "sub-cli"
+	}
+
+	// Create temporary directory for test files
+	tempDir := t.TempDir()
+
+	// Test cases
+	testCases := []struct {
+		name     string
+		args     []string
+		errorMsg string
+	}{
+		{
+			name:     "Nonexistent source file",
+			args:     []string{"convert", "nonexistent.srt", filepath.Join(tempDir, "output.vtt")},
+			errorMsg: "no such file",
+		},
+		{
+			name:     "Invalid source format",
+			args:     []string{"convert", filepath.Join(tempDir, "test.xyz"), filepath.Join(tempDir, "output.vtt")},
+			errorMsg: "unsupported format",
+		},
+		{
+			name:     "Invalid target format",
+			args:     []string{"convert", filepath.Join(tempDir, "test.srt"), filepath.Join(tempDir, "output.xyz")},
+			errorMsg: "unsupported format",
+		},
+		{
+			name:     "Sync different formats",
+			args:     []string{"sync", filepath.Join(tempDir, "test.srt"), filepath.Join(tempDir, "test.lrc")},
+			errorMsg: "same format",
+		},
+		{
+			name:     "Format unsupported file",
+			args:     []string{"fmt", filepath.Join(tempDir, "test.txt")},
+			errorMsg: "unsupported format",
+		},
+	}
+
+	// Create a sample SRT file for testing
+	srtFile := filepath.Join(tempDir, "test.srt")
+	srtContent := `1
+00:00:01,000 --> 00:00:04,000
+This is a test subtitle.
+`
+	if err := os.WriteFile(srtFile, []byte(srtContent), 0644); err != nil {
+		t.Fatalf("Failed to create SRT test file: %v", err)
+	}
+
+	// Create a sample LRC file for testing
+	lrcFile := filepath.Join(tempDir, "test.lrc")
+	lrcContent := `[00:01.00]This is a test lyric.
+`
+	if err := os.WriteFile(lrcFile, []byte(lrcContent), 0644); err != nil {
+		t.Fatalf("Failed to create LRC test file: %v", err)
+	}
+
+	// Create a sample TXT file for testing
+	txtFile := filepath.Join(tempDir, "test.txt")
+	txtContent := `This is a plain text file.
+`
+	if err := os.WriteFile(txtFile, []byte(txtContent), 0644); err != nil {
+		t.Fatalf("Failed to create TXT test file: %v", err)
+	}
+
+	// Run test cases
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			cmd := exec.Command(binaryPath, tc.args...)
+			output, err := cmd.CombinedOutput()
+			
+			// We expect an error
+			if err == nil {
+				t.Fatalf("Expected command to fail, but it succeeded. Output: %s", output)
+			}
+			
+			// Check error message
+			if !strings.Contains(string(output), tc.errorMsg) {
+				t.Errorf("Expected error message to contain '%s', got: %s", tc.errorMsg, output)
+			}
+		})
+	}
+}