fix(edit): correct diff @@ hunk headers at file boundaries

hunkBounds approximated one-sided start lines by reusing the opposite side's number, and re-assigned it every iteration inside the count==0 guard. For pure insertion/deletion hunks (e.g. writing into an empty file, or deleting a whole file) this produced wrong headers like '@@ -2,0 +1,2 @@' instead of '@@ -0,0 +1,2 @@'. flattenDiffLines now records both running line numbers on every line, so hunkBounds takes the start from the first line directly and applies GNU's zero-count convention (line-before-change, clamped to 0). Adds TestGenerateDiffHunkHeaderBounds.

internal/edit/edit.go

@@ -642,16 +642,19 @@ func flattenDiffLines(diffs []diffmatchpatch.Diff) (lines []diffLine, maxOldN, m
 			// Strip the terminator for display; also drop a trailing CR so CRLF files
 			// do not leak raw carriage returns into the rendered diff.
 			text := strings.TrimSuffix(strings.TrimSuffix(raw, "\n"), "\r")
+			// Record both running line numbers on every line (not just the active
+			// side): the opposite-side counter is the position the change sits at,
+			// which hunkBounds needs to compute correct @@ header start lines.
 			switch d.Type {
 			case diffmatchpatch.DiffEqual:
 				lines = append(lines, diffLine{op: d.Type, text: text, oldN: oldLine, newN: newLine})
 				oldLine++
 				newLine++
 			case diffmatchpatch.DiffDelete:
-				lines = append(lines, diffLine{op: d.Type, text: text, oldN: oldLine})
+				lines = append(lines, diffLine{op: d.Type, text: text, oldN: oldLine, newN: newLine})
 				oldLine++
 			case diffmatchpatch.DiffInsert:
-				lines = append(lines, diffLine{op: d.Type, text: text, newN: newLine})
+				lines = append(lines, diffLine{op: d.Type, text: text, oldN: oldLine, newN: newLine})
 				newLine++
 			}
 		}
@@ -680,38 +683,31 @@ func diffHunkRanges(lines []diffLine) []indexRange {
 }
 
 // hunkBounds computes the unified-diff hunk header line numbers and counts for
-// lines[start:end+1]. newStart/oldStart for one-sided lines are approximate.
+// lines[start:end+1]. Each line carries both running line numbers (see
+// flattenDiffLines), so the hunk start is simply the first line's numbers. When a
+// side has zero lines in the hunk (pure insertion or deletion), the header start
+// for that side follows GNU's convention: the line number *before* the change,
+// clamped to 0 at the start of the file (e.g. "@@ -0,0 +1,N @@").
 func hunkBounds(lines []diffLine, start, end int) (oldStart, oldCount, newStart, newCount int) {
+	oldStart = lines[start].oldN
+	newStart = lines[start].newN
 	for i := start; i <= end; i++ {
-		l := lines[i]
-		switch l.op {
+		switch lines[i].op {
 		case diffmatchpatch.DiffEqual:
-			if oldCount == 0 {
-				oldStart = l.oldN
-			}
-			if newCount == 0 {
-				newStart = l.newN
-			}
 			oldCount++
 			newCount++
 		case diffmatchpatch.DiffDelete:
-			if oldCount == 0 {
-				oldStart = l.oldN
-			}
-			if newCount == 0 {
-				newStart = l.oldN // approximate
-			}
 			oldCount++
 		case diffmatchpatch.DiffInsert:
-			if newCount == 0 {
-				newStart = l.newN
-			}
-			if oldCount == 0 {
-				oldStart = l.newN // approximate
-			}
 			newCount++
 		}
 	}
+	if oldCount == 0 {
+		oldStart = max(0, oldStart-1)
+	}
+	if newCount == 0 {
+		newStart = max(0, newStart-1)
+	}
 	return
 }
 

internal/edit/edit_test.go

@@ -427,6 +427,29 @@ func TestGenerateDiffLineLevelAccuracy(t *testing.T) {
 	}
 }
 
+// Hunk @@ headers must be correct at file boundaries (previously approximate).
+func TestGenerateDiffHunkHeaderBounds(t *testing.T) {
+	e := &Engine{dmp: diffmatchpatch.New()}
+	tests := []struct {
+		name     string
+		original string
+		modified string
+		wantHdr  string
+	}{
+		{name: "insert into empty file", original: "", modified: "X\nY\n", wantHdr: "@@ -0,0 +1,2 @@"},
+		{name: "delete entire file", original: "a\nb\n", modified: "", wantHdr: "@@ -1,2 +0,0 @@"},
+		{name: "mid-file change keeps 1-based start", original: "l1\nl2\nl3\nl4\nl5\n", modified: "l1\nl2\nCHANGED\nl4\nl5\n", wantHdr: "@@ -1,5 +1,5 @@"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			diff := e.generateDiff(tt.original, tt.modified, "f.txt")
+			if !strings.Contains(diff, tt.wantHdr) {
+				t.Errorf("expected hunk header %q, got:\n%s", tt.wantHdr, diff)
+			}
+		})
+	}
+}
+
 func TestGenerateDiffNoPhantomChanges(t *testing.T) {
 	// Regression test: replacing a line range should not produce phantom
 	// +/- lines for unchanged code after the edit region.