feat(tui): add support for custom line number starting position

- Add `StartLine` field to `CodeHighlightInfo` struct
- Support `--start-at-line` flag in markdown code blocks
- Update line number calculation to use custom starting position
- Fix highlight range logic to work with relative line numbers
- Add tests for custom line number offset functionality

Author: tharropoulos

internal/tui/highlight.go

@@ -24,6 +24,7 @@ type CodeHighlightInfo struct {
 	Language        string
 	Ranges          []LineRange
 	ShowLineNumbers bool
+	StartLine       int
 }
 
 func parseHighlightSyntax(syntax string) []LineRange {
@@ -92,7 +93,8 @@ func renderWithStyle(content string, info CodeHighlightInfo, lexer chroma.Lexer,
 
 	lineNumberWidth := 0
 	if info.ShowLineNumbers {
-		lineNumberWidth = len(strconv.Itoa(len(lines))) + 2
+		maxLineNum := info.StartLine + len(lines) - 1
+		lineNumberWidth = len(strconv.Itoa(maxLineNum)) + 2
 	}
 
 	lineNumberStyle := lipgloss.NewStyle().
@@ -101,19 +103,19 @@ func renderWithStyle(content string, info CodeHighlightInfo, lexer chroma.Lexer,
 
 	var result strings.Builder
 	for lineNum, line := range lines {
-		lineNumDisplay := lineNum + 1
+		displayLineNum := info.StartLine + lineNum
+		relativeLineNum := lineNum + 1
 
 		// Add line number if requested
 		if info.ShowLineNumbers {
-			lineNumStr := strconv.Itoa(lineNumDisplay)
+			lineNumStr := strconv.Itoa(displayLineNum)
 			paddedLineNum := fmt.Sprintf("%*s", lineNumberWidth-1, lineNumStr)
 			result.WriteString(lineNumberStyle.Render(paddedLineNum))
 		}
 
-		shouldHighlight := len(info.Ranges) == 0 || shouldHighlightLine(lineNumDisplay, info.Ranges)
+		shouldHighlight := len(info.Ranges) == 0 || shouldHighlightLine(relativeLineNum, info.Ranges)
 
 		if !shouldHighlight {
-			// No syntax highlighting for this line
 			result.WriteString(line)
 			if lineNum < len(lines)-1 {
 				result.WriteString("\n")
@@ -159,7 +161,7 @@ func renderWithStyle(content string, info CodeHighlightInfo, lexer chroma.Lexer,
 }
 
 func processMarkdownWithHighlighting(markdown string, themeName string) (string, error) {
-	re := regexp.MustCompile("(?s)```([a-zA-Z0-9_+-]*)({[^}]*})?(\\s+--numbered)?\\s*\n(.*?)\n```")
+	re := regexp.MustCompile("(?s)```([a-zA-Z0-9_+-]*)({[^}]*})?(\\s+--numbered)?(\\s+--start-at-line\\s+(\\d+))?\\s*\n(.*?)\n```")
 
 	matches := re.FindAllStringSubmatch(markdown, -1)
 	if len(matches) == 0 {
@@ -172,7 +174,7 @@ func processMarkdownWithHighlighting(markdown string, themeName string) (string,
 	indices := re.FindAllStringIndex(markdown, -1)
 
 	for i, match := range matches {
-		if len(match) < 5 {
+		if len(match) < 7 {
 			continue
 		}
 
@@ -200,12 +202,21 @@ func processMarkdownWithHighlighting(markdown string, themeName string) (string,
 			highlightSyntax = match[2] // e.g., "{1-2}"
 		}
 		numberedFlag := strings.TrimSpace(match[3]) // " --numbered"
-		content := match[4]                         // The actual code content
+		startLineStr := match[5]                    // The line number from --start-at-line
+		content := match[6]                         // The actual code content
+
+		startLine := 1
+		if startLineStr != "" {
+			if parsed, err := strconv.Atoi(startLineStr); err == nil && parsed > 0 {
+				startLine = parsed
+			}
+		}
 
 		info := CodeHighlightInfo{
 			Language:        language,
 			Ranges:          parseHighlightSyntax(highlightSyntax),
 			ShowLineNumbers: strings.Contains(numberedFlag, "--numbered"),
+			StartLine:       startLine,
 		}
 
 		if language != "" || info.ShowLineNumbers {

internal/tui/highlight_test.go

@@ -1,6 +1,7 @@
 package tui
 
 import (
+	"strconv"
 	"strings"
 	"testing"
 
@@ -186,6 +187,17 @@ func TestRenderCustomCodeBlock(t *testing.T) {
 			},
 			themeName: "dark",
 		},
+		{
+			name:    "code with start line offset",
+			content: "console.log('hello');\nconsole.log('world');\nvar x = 1;",
+			info: CodeHighlightInfo{
+				Language:        "javascript",
+				Ranges:          []LineRange{{Start: 2, End: 2}}, // Should highlight the second line
+				ShowLineNumbers: true,
+				StartLine:       10, // Line numbers should start at 10
+			},
+			themeName: "dark",
+		},
 	}
 
 	for _, tt := range tests {
@@ -204,9 +216,15 @@ func TestRenderCustomCodeBlock(t *testing.T) {
 
 			// Check for line numbers if requested
 			if tt.info.ShowLineNumbers {
-				if !strings.Contains(result, "1") {
+				if !strings.Contains(result, "1") && tt.info.StartLine == 1 {
 					t.Error("renderCustomCodeBlock with ShowLineNumbers should contain line number 1")
 				}
+				if tt.info.StartLine > 1 {
+					expectedFirstLine := strconv.Itoa(tt.info.StartLine)
+					if !strings.Contains(result, expectedFirstLine) {
+						t.Errorf("renderCustomCodeBlock with StartLine %d should contain line number %s", tt.info.StartLine, expectedFirstLine)
+					}
+				}
 			}
 
 			// For known languages, check that it contains some recognizable elements
@@ -358,6 +376,24 @@ func TestProcessMarkdownWithHighlighting(t *testing.T) {
 			theme:    "dark",
 			wantErr:  false,
 		},
+		{
+			name:     "markdown with --start-at-line flag",
+			markdown: "```javascript --numbered --start-at-line 10\nconsole.log('hello');\nconsole.log('world');\n```",
+			theme:    "dark",
+			wantErr:  false,
+		},
+		{
+			name:     "markdown with highlighting and --start-at-line",
+			markdown: "```python{1-2} --numbered --start-at-line 5\ndef hello():\n    print('hello')\n    return True\n```",
+			theme:    "dark",
+			wantErr:  false,
+		},
+		{
+			name:     "markdown with --start-at-line only (no --numbered)",
+			markdown: "```typescript --start-at-line 100\nconst x = 1;\nconst y = 2;\n```",
+			theme:    "dark",
+			wantErr:  false,
+		},
 	}
 
 	for _, tt := range tests {
@@ -396,6 +432,7 @@ func TestCodeHighlightInfo(t *testing.T) {
 		Language:        "javascript",
 		Ranges:          []LineRange{{Start: 1, End: 3}, {Start: 5, End: 5}},
 		ShowLineNumbers: true,
+		StartLine:       10,
 	}
 
 	if info.Language != "javascript" {
@@ -414,6 +451,10 @@ func TestCodeHighlightInfo(t *testing.T) {
 	if !info.ShowLineNumbers {
 		t.Error("Expected ShowLineNumbers to be true")
 	}
+
+	if info.StartLine != 10 {
+		t.Errorf("Expected StartLine to be 10, got %d", info.StartLine)
+	}
 }
 
 func TestLineRange(t *testing.T) {