feat(index): add ability to create diff between two indexes

lib/src/bindings/diff.dart

@@ -5,6 +5,42 @@ import 'package:libgit2dart/src/bindings/libgit2_bindings.dart';
 import 'package:libgit2dart/src/error.dart';
 import 'package:libgit2dart/src/util.dart';
 
+/// Create a diff with the difference between two index objects.
+///
+/// Throws a [LibGit2Error] if error occured.
+Pointer<git_diff> indexToIndex({
+  required Pointer<git_repository> repoPointer,
+  required Pointer<git_index> oldIndexPointer,
+  required Pointer<git_index> newIndexPointer,
+  required int flags,
+  required int contextLines,
+  required int interhunkLines,
+}) {
+  final out = calloc<Pointer<git_diff>>();
+  final opts = _diffOptionsInit(
+    flags: flags,
+    contextLines: contextLines,
+    interhunkLines: interhunkLines,
+  );
+
+  final error = libgit2.git_diff_index_to_index(
+    out,
+    repoPointer,
+    oldIndexPointer,
+    newIndexPointer,
+    opts,
+  );
+
+  calloc.free(opts);
+
+  if (error < 0) {
+    calloc.free(out);
+    throw LibGit2Error(libgit2.git_error_last());
+  } else {
+    return out.value;
+  }
+}
+
 /// Create a diff between the repository index and the workdir directory.
 Pointer<git_diff> indexToWorkdir({
   required Pointer<git_repository> repoPointer,
@@ -80,14 +116,15 @@ Pointer<git_diff> treeToWorkdir({
     opts,
   );
 
-  if (error < 0) {
-    throw LibGit2Error(libgit2.git_error_last());
-  }
-
   calloc.free(opts);
 
+  if (error < 0) {
+    calloc.free(out);
+    throw LibGit2Error(libgit2.git_error_last());
+  } else {
     return out.value;
   }
+}
 
 /// Create a diff between a tree and the working directory using index data to
 /// account for staged deletes, tracked files, etc.
@@ -118,14 +155,15 @@ Pointer<git_diff> treeToWorkdirWithIndex({
     opts,
   );
 
-  if (error < 0) {
-    throw LibGit2Error(libgit2.git_error_last());
-  }
-
   calloc.free(opts);
 
+  if (error < 0) {
+    calloc.free(out);
+    throw LibGit2Error(libgit2.git_error_last());
+  } else {
     return out.value;
   }
+}
 
 /// Create a diff with the difference between two tree objects.
 ///
@@ -153,14 +191,15 @@ Pointer<git_diff> treeToTree({
     opts,
   );
 
-  if (error < 0) {
-    throw LibGit2Error(libgit2.git_error_last());
-  }
-
   calloc.free(opts);
 
+  if (error < 0) {
+    calloc.free(out);
+    throw LibGit2Error(libgit2.git_error_last());
+  } else {
     return out.value;
   }
+}
 
 /// Query how many diff records are there in a diff.
 int length(Pointer<git_diff> diff) => libgit2.git_diff_num_deltas(diff);

lib/src/index.dart

@@ -300,6 +300,37 @@ class Index with IterableMixin<IndexEntry> {
   void removeAll(List<String> path) =>
       bindings.removeAll(indexPointer: _indexPointer, pathspec: path);
 
+  /// Creates a diff with the difference between two index objects.
+  ///
+  /// [index] is the [Index] object to diff to.
+  ///
+  /// [flags] is a combination of [GitDiff] flags. Defaults to [GitDiff.normal].
+  ///
+  /// [contextLines] is the number of unchanged lines that define the boundary
+  /// of a hunk (and to display before and after). Defaults to 3.
+  ///
+  /// [interhunkLines] is the maximum number of unchanged lines between hunk
+  /// boundaries before the hunks will be merged into one. Defaults to 0.
+  ///
+  /// Throws a [LibGit2Error] if error occured.
+  Diff diffToIndex({
+    required Index index,
+    Set<GitDiff> flags = const {GitDiff.normal},
+    int contextLines = 3,
+    int interhunkLines = 0,
+  }) {
+    return Diff(
+      diff_bindings.indexToIndex(
+        repoPointer: bindings.owner(_indexPointer),
+        oldIndexPointer: _indexPointer,
+        newIndexPointer: index.pointer,
+        flags: flags.fold(0, (acc, e) => acc | e.value),
+        contextLines: contextLines,
+        interhunkLines: interhunkLines,
+      ),
+    );
+  }
+
   /// Creates a diff between the repository index and the workdir directory.
   ///
   /// [flags] is a combination of [GitDiff] flags. Defaults to [GitDiff.normal].

test/diff_test.dart

@@ -71,6 +71,21 @@ void main() {
     'subdir/modified_file',
   ];
 
+  const indexToIndex = [
+    'current_file',
+    'file_deleted',
+    'modified_file',
+    'staged_changes',
+    'staged_changes_file_deleted',
+    'staged_changes_file_modified',
+    'staged_new',
+    'staged_new_file_deleted',
+    'staged_new_file_modified',
+    'subdir/current_file',
+    'subdir/deleted_file',
+    'subdir/modified_file',
+  ];
+
   const patchText = """
 diff --git a/subdir/modified_file b/subdir/modified_file
 index e69de29..c217c63 100644
@@ -237,6 +252,45 @@ index e69de29..c217c63 100644
       head.free();
     });
 
+    test(
+        'throws when trying to diff between tree and workdir with index and '
+        'error occurs', () {
+      expect(
+        () => Diff.treeToWorkdirWithIndex(
+          repo: Repository(nullptr),
+          tree: null,
+        ),
+        throwsA(isA<LibGit2Error>()),
+      );
+    });
+
+    test('returns diff between index and index', () {
+      final index = repo.index;
+      final emptyIndex = Index.newInMemory();
+
+      final diff = index.diffToIndex(index: emptyIndex);
+
+      expect(diff.length, 12);
+      for (var i = 0; i < diff.deltas.length; i++) {
+        expect(diff.deltas[i].newFile.path, indexToIndex[i]);
+      }
+
+      index.free();
+      emptyIndex.free();
+    });
+
+    test('throws when trying to diff between index and index and error occurs',
+        () {
+      final index = repo.index;
+
+      expect(
+        () => index.diffToIndex(index: Index(nullptr)),
+        throwsA(isA<LibGit2Error>()),
+      );
+
+      index.free();
+    });
+
     test('merges diffs', () {
       final head = repo.head;
       final commit = repo.lookupCommit(head.target);