Fix malformed @@ header in formatHunkHeader fallback

.changeset/clean-hunk-header.md

@@ -0,0 +1,5 @@
+---
+"hunkdiff": patch
+---
+
+Fix malformed `@@` hunk headers so each side's line range and count are emitted correctly.

src/core/hunkHeader.test.ts

@@ -0,0 +1,77 @@
+import { describe, expect, test } from "bun:test";
+import type { Hunk } from "@pierre/diffs";
+import { formatHunkHeader } from "./hunkHeader";
+
+/**
+ * Minimal synthesized Hunk that forces the fallback branch of
+ * `formatHunkHeader` (no parsed `hunkSpecs`). Only the numeric header fields
+ * the fallback reads are populated; the rest are stubbed to satisfy the type.
+ */
+function fallbackHunk(overrides: Partial<Hunk> = {}): Hunk {
+  return {
+    collapsedBefore: 0,
+    additionStart: 0,
+    additionCount: 0,
+    additionLines: 0,
+    additionLineIndex: 0,
+    deletionStart: 0,
+    deletionCount: 0,
+    deletionLines: 0,
+    deletionLineIndex: 0,
+    hunkContent: [],
+    splitLineStart: 0,
+    splitLineCount: 0,
+    unifiedLineStart: 0,
+    unifiedLineCount: 0,
+    ...overrides,
+  } as unknown as Hunk;
+}
+
+describe("formatHunkHeader", () => {
+  test("fallback uses the per-side line count (context + changes) for the @@ ranges", () => {
+    // A context-bearing hunk where git renders  @@ -10,4 +10,4 @@:
+    //   old side = lines 10, 11 (context) + 12 (removed) + 13 (context) = 4
+    //   new side = lines 10, 11 (context) + 13 (context) + 14 (added)   = 4
+    // `*Count` is the header count (context + changes); `*Lines` is only the
+    // changed `+`/`-` lines (1 each here), so they diverge — exactly the shape
+    // that used to emit a malformed `@@ -10,1 +10,1 @@`.
+    const hunk = fallbackHunk({
+      deletionStart: 10,
+      deletionCount: 4,
+      deletionLines: 1,
+      additionStart: 10,
+      additionCount: 4,
+      additionLines: 1,
+    });
+
+    const header = formatHunkHeader(hunk);
+
+    expect(header).toBe("@@ -10,4 +10,4 @@");
+    expect(header).toMatch(/^@@ -\d+,\d+ \+\d+,\d+ @@$/);
+  });
+
+  test("fallback renders a new-file (pure-addition) hunk with a 0,0 old range", () => {
+    const hunk = fallbackHunk({
+      deletionStart: 0,
+      deletionCount: 0,
+      deletionLines: 0,
+      additionStart: 1,
+      additionCount: 3,
+      additionLines: 3,
+    });
+
+    expect(formatHunkHeader(hunk)).toBe("@@ -0,0 +1,3 @@");
+  });
+
+  test("fallback appends hunkContext after the @@ markers when present", () => {
+    const hunk = fallbackHunk({
+      deletionStart: 1,
+      deletionCount: 1,
+      additionStart: 1,
+      additionCount: 1,
+      hunkContext: "function name()",
+    });
+
+    expect(formatHunkHeader(hunk)).toBe("@@ -1,1 +1,1 @@ function name()");
+  });
+});

src/core/hunkHeader.ts

@@ -4,6 +4,9 @@ import type { Hunk } from "@pierre/diffs";
 export function formatHunkHeader(hunk: Hunk) {
   const specs =
     hunk.hunkSpecs ??
-    `@@ -${hunk.deletionStart},${hunk.deletionLines} +${hunk.additionStart},${hunk.additionLines} @@`;
+    // The header count is the per-side line total (context + changes), i.e.
+    // `*Count` parsed from `-X,count` / `+X,count` — not `*Lines`, which is
+    // only the changed `+`/`-` lines and would undercount a context-bearing hunk.
+    `@@ -${hunk.deletionStart},${hunk.deletionCount} +${hunk.additionStart},${hunk.additionCount} @@`;
   return hunk.hunkContext ? `${specs} ${hunk.hunkContext}` : specs;
 }