Another try at adding Git support, this time with Claude Opus 4.6

README.md

@@ -184,9 +184,11 @@ jsdiff's diff functions all take an old text and a new text and perform three st
 
     Once all patches have been applied or an error occurs, the `options.complete(err)` callback is made.
 
-* `parsePatch(diffStr)` - Parses a patch into structured data
+* `parsePatch(diffStr)` - Parses a unified diff format patch into a structured patch object.
 
-    Return a JSON object representation of the a patch, suitable for use with the `applyPatch` method. This parses to the same structure returned by `structuredPatch`.
+    Return a JSON object representation of the a patch, suitable for use with the `applyPatch` method. This parses to the same structure returned by `structuredPatch`, except that `oldFileName` and `newFileName` may be `undefined` if the patch doesn't contain enough information to determine them (e.g. a hunk-only patch with no file headers).
+
+    `parsePatch` has some understanding of [Git's particular dialect of unified diff format](https://git-scm.com/docs/git-diff#generate_patch_text_with_p). In particular, it can extract filenames from the patch headers or extended headers of Git patches that contain no hunks and no file headers, including ones representing a file being renamed without changes. (However, it ignores many extended headers that describe things irrelevant to jsdiff's patch representation, like mode changes.)
 
 * `reversePatch(patch)` - Returns a new structured patch which when applied will undo the original `patch`.
 
@@ -360,6 +362,70 @@ applyPatches(patch, {
 });
 ```
 
+##### Applying a multi-file Git patch that may include renames
+
+[Git patches](https://git-scm.com/docs/git-diff#generate_patch_text_with_p) can include file renames and copies (with or without content changes), which need to be handled in the callbacks you provide to `applyPatches`. `parsePatch` sets `isRename` or `isCopy` on the structured patch object so you can distinguish these cases. Patches can also potentially include file *swaps* (renaming `a → b` and `b → a`), in which case it is incorrect to simply apply each change atomically in sequence. The pattern with the `pendingWrites` Map below handles all of these nuances:
+
+```
+const {applyPatches} = require('diff');
+const patch = fs.readFileSync("git-diff.patch").toString();
+const DELETE = Symbol('delete');
+const pendingWrites = new Map(); // filePath → {content, mode} or DELETE sentinel
+applyPatches(patch, {
+    loadFile: (patch, callback) => {
+        if (patch.isCreate) {
+            // Newly created file — no old content to load
+            callback(undefined, '');
+            return;
+        }
+        try {
+            // Git diffs use a/ and b/ prefixes; strip them to get the real path
+            const filePath = patch.oldFileName.replace(/^a\//, '');
+            callback(undefined, fs.readFileSync(filePath).toString());
+        } catch (e) {
+            callback(`No such file: ${patch.oldFileName}`);
+        }
+    },
+    patched: (patch, patchedContent, callback) => {
+        if (patchedContent === false) {
+            callback(`Failed to apply patch to ${patch.oldFileName}`);
+            return;
+        }
+        const oldPath = patch.oldFileName.replace(/^a\//, '');
+        const newPath = patch.newFileName.replace(/^b\//, '');
+        if (patch.isDelete) {
+            if (!pendingWrites.has(oldPath)) {
+                pendingWrites.set(oldPath, DELETE);
+            }
+        } else {
+            pendingWrites.set(newPath, {content: patchedContent, mode: patch.newMode});
+            // For renames, delete the old file (but not for copies,
+            // where the old file should be kept)
+            if (patch.isRename && !pendingWrites.has(oldPath)) {
+                pendingWrites.set(oldPath, DELETE);
+            }
+        }
+        callback();
+    },
+    complete: (err) => {
+        if (err) {
+            console.log("Failed with error:", err);
+            return;
+        }
+        for (const [filePath, entry] of pendingWrites) {
+            if (entry === DELETE) {
+                fs.unlinkSync(filePath);
+            } else {
+                fs.writeFileSync(filePath, entry.content);
+                if (entry.mode) {
+                    fs.chmodSync(filePath, parseInt(entry.mode, 8) & 0o777);
+                }
+            }
+        }
+    }
+});
+```
+
 ## Compatibility
 
 jsdiff should support all ES5 environments. If you find one that it doesn't support, please [open an issue](https://github.com/kpdecker/jsdiff/issues).

release-notes.md

@@ -1,5 +1,41 @@
 # Release Notes
 
+## 9.0.0 (prerelease)
+
+TODO:
+- Tidy up AI slop below the ---
+- Note support for parsing quoted filenames in +++ and --- headers (even outside Git patches as `diff -u` outputs these)
+- Note fixes to #640 and #648
+
+---
+
+
+- **`parsePatch` now robustly handles Git-style diffs.** Previously, `parsePatch` had inconsistent regex usage that caused several bugs when parsing `diff --git` output:
+  * Multi-file Git diffs containing hunk-less entries (e.g. mode-only changes, binary files, rename-only entries without content changes) could cause file entries to be merged together or lost entirely.
+  * Git extended headers (`rename from`/`rename to`, `copy from`/`copy to`, `old mode`/`new mode`, `index`, etc.) were not parsed and could cause parse errors.
+  * `diff --git` was not consistently recognized as a diff header, leading to missing `index` entries and other subtle issues.
+
+  The parser now:
+  * Correctly recognizes `diff --git` headers and parses filenames from them, including C-style quoted filenames (used by Git when paths contain tabs, newlines, backslashes, or double quotes).
+  * Consumes Git extended headers (`rename from`/`rename to`, `copy from`/`copy to`, mode changes, similarity index, etc.) without choking.
+  * Handles hunk-less entries (rename-only, mode-only, binary) as distinct file entries rather than merging them into adjacent entries.
+  * Sets metadata flags on the resulting `StructuredPatch`: `isGit` (always, for Git diffs), `isRename`, `isCopy`, `isCreate`, `isDelete` (when the corresponding extended headers are present), and `oldMode`/`newMode` (parsed from `old mode`, `new mode`, `deleted file mode`, or `new file mode` headers). This lets consumers distinguish renames (where the old file should be deleted) from copies (where it should be kept), detect file creations and deletions, and preserve file mode information.
+  * Uses consistent, centralized helper functions for header detection instead of duplicated regexes.
+
+- **`reversePatch` now correctly reverses copy patches.** Reversing a copy produces a deletion (the reversed patch has `newFileName` set to `'/dev/null'` and `isCopy`/`isRename` unset), since undoing a copy means deleting the file that was created. Reversing a rename still produces a rename in the opposite direction, as before.
+
+- **`formatPatch` now supports Git-style patches.** When a `StructuredPatch` has `isGit: true`, `formatPatch` emits a `diff --git` header (instead of `Index:` / underline) and the appropriate Git extended headers (`rename from`/`rename to`, `copy from`/`copy to`, `deleted file mode`, `new file mode`, `old mode`/`new mode`) based on the patch's metadata flags. File headers (`---`/`+++`) are omitted on hunk-less Git patches (e.g. pure renames, mode-only changes), matching Git's own output. This means `parsePatch` output can be round-tripped through `formatPatch`.
+
+- **`formatPatch` now gracefully handles patches with undefined filenames** instead of emitting nonsensical headers like `--- undefined`. If `oldFileName` or `newFileName` is `undefined`, the `---`/`+++` file headers and the `Index:` line are silently omitted. This is consistent with how such patches can arise from parsing Git diffs that lack `---`/`+++` lines.
+
+- **README: added documentation and example for applying Git patches that include renames, copies, deletions, and file creations** using `applyPatches`.
+
+### Breaking changes
+
+- **The `oldFileName` and `newFileName` fields of `StructuredPatch` are now typed as `string | undefined` instead of `string`.** This reflects the reality that `parsePatch` can produce patches without filenames (e.g. when parsing a Git diff with an unparseable `diff --git` header and no `---`/`+++` fallback). TypeScript users who access these fields without null checks will see type errors and should update their code to handle the `undefined` case.
+
+- **`StructuredPatch` has new optional fields for Git metadata:** `isGit`, `isRename`, `isCopy`, `isCreate`, `isDelete`, `oldMode`, and `newMode`. These are set by `parsePatch` when parsing Git diffs. Code that does exact deep-equality checks (e.g. `assert.deepEqual`) against `StructuredPatch` objects from `parsePatch` may need updating to account for the new fields.
+
 ## 8.0.4 (prerelease)
 
 - [#667](https://github.com/kpdecker/jsdiff/pull/667) - **fix another bug in `diffWords` when used with an `Intl.Segmenter`**. If the text to be diffed included a combining mark after a whitespace character (i.e. roughly speaking, an accented space), `diffWords` would previously crash. Now this case is handled correctly.

src/patch/create.ts

@@ -292,13 +292,42 @@ export function formatPatch(patch: StructuredPatch | StructuredPatch[], headerOp
   }
 
   const ret = [];
-  if (headerOptions.includeIndex && patch.oldFileName == patch.newFileName) {
-    ret.push('Index: ' + patch.oldFileName);
-  }
-  if (headerOptions.includeUnderline) {
-    ret.push('===================================================================');
+
+  if (patch.isGit) {
+    // Emit Git-style diff --git header and extended headers
+    ret.push('diff --git ' + (patch.oldFileName ?? '') + ' ' + (patch.newFileName ?? ''));
+    if (patch.isDelete) {
+      ret.push('deleted file mode ' + (patch.oldMode ?? '100644'));
+    }
+    if (patch.isCreate) {
+      ret.push('new file mode ' + (patch.newMode ?? '100644'));
+    }
+    if (patch.oldMode && patch.newMode && !patch.isDelete && !patch.isCreate) {
+      ret.push('old mode ' + patch.oldMode);
+      ret.push('new mode ' + patch.newMode);
+    }
+    if (patch.isRename) {
+      ret.push('rename from ' + (patch.oldFileName ?? '').replace(/^a\//, ''));
+      ret.push('rename to ' + (patch.newFileName ?? '').replace(/^b\//, ''));
+    }
+    if (patch.isCopy) {
+      ret.push('copy from ' + (patch.oldFileName ?? '').replace(/^a\//, ''));
+      ret.push('copy to ' + (patch.newFileName ?? '').replace(/^b\//, ''));
+    }
+  } else {
+    if (headerOptions.includeIndex && patch.oldFileName == patch.newFileName && patch.oldFileName !== undefined) {
+      ret.push('Index: ' + patch.oldFileName);
+    }
+    if (headerOptions.includeUnderline) {
+      ret.push('===================================================================');
+    }
   }
-  if (headerOptions.includeFileHeaders) {
+
+  // Emit --- / +++ file headers. For Git patches with no hunks (e.g.
+  // pure renames, mode-only changes), Git omits these, so we do too.
+  const hasHunks = patch.hunks.length > 0;
+  if (headerOptions.includeFileHeaders && patch.oldFileName !== undefined && patch.newFileName !== undefined
+      && (!patch.isGit || hasHunks)) {
     ret.push('--- ' + patch.oldFileName + (typeof patch.oldHeader === 'undefined' ? '' : '\t' + patch.oldHeader));
     ret.push('+++ ' + patch.newFileName + (typeof patch.newHeader === 'undefined' ? '' : '\t' + patch.newHeader));
   }