thodg/libgit2

diff --git a/src/index.c b/src/index.c
index 0d2a03e..7c0c310 100644
--- a/src/index.c
+++ b/src/index.c
@@ -2451,6 +2451,104 @@ int git_index_read_tree(git_index *index, const git_tree *tree)
 	return error;
 }
 
+int git_index_read_index(
+	git_index *index,
+	const git_index *new_index)
+{
+	git_vector new_entries = GIT_VECTOR_INIT,
+		remove_entries = GIT_VECTOR_INIT;
+	git_iterator *index_iterator = NULL;
+	git_iterator *new_iterator = NULL;
+	const git_index_entry *old_entry, *new_entry;
+	git_index_entry *entry;
+	size_t i;
+	int error;
+
+	if ((error = git_vector_init(&new_entries, new_index->entries.length, index->entries._cmp)) < 0 ||
+		(error = git_vector_init(&remove_entries, index->entries.length, NULL)) < 0)
+		goto done;
+
+	if ((error = git_iterator_for_index(&index_iterator,
+			index, GIT_ITERATOR_DONT_IGNORE_CASE, NULL, NULL)) < 0 ||
+		(error = git_iterator_for_index(&new_iterator,
+			(git_index *)new_index, GIT_ITERATOR_DONT_IGNORE_CASE, NULL, NULL)) < 0)
+		goto done;
+
+	if (((error = git_iterator_current(&old_entry, index_iterator)) < 0 && 
+			error != GIT_ITEROVER) ||
+		((error = git_iterator_current(&new_entry, new_iterator)) < 0 && 
+			error != GIT_ITEROVER))
+		goto done;
+
+	while (true) {
+		int diff;
+
+		if (old_entry && new_entry)
+			diff = git_index_entry_cmp(old_entry, new_entry);
+		else if (!old_entry && new_entry)
+			diff = 1;
+		else if (old_entry && !new_entry)
+			diff = -1;
+		else
+			break;
+
+		if (diff < 0) {
+			git_vector_insert(&remove_entries, (git_index_entry *)old_entry);
+		} else if (diff > 0) {
+			if ((error = index_entry_dup(&entry, git_index_owner(index), new_entry)) < 0)
+				goto done;
+
+			git_vector_insert(&new_entries, entry);
+		} else {
+			/* Path and stage are equal, if the OID is equal, keep it to
+			 * keep the stat cache data.
+			 */
+			if (git_oid_equal(&old_entry->id, &new_entry->id)) {
+				git_vector_insert(&new_entries, (git_index_entry *)old_entry);
+			} else {
+				if ((error = index_entry_dup(&entry, git_index_owner(index), new_entry)) < 0)
+					goto done;
+
+				git_vector_insert(&new_entries, entry);
+				git_vector_insert(&remove_entries, (git_index_entry *)old_entry);
+			}
+		}
+
+		if (diff <= 0) {
+			if ((error = git_iterator_advance(&old_entry, index_iterator)) < 0 &&
+				error != GIT_ITEROVER)
+				goto done;
+		}
+
+		if (diff >= 0) {
+			if ((error = git_iterator_advance(&new_entry, new_iterator)) < 0 &&
+				error != GIT_ITEROVER)
+				goto done;
+		}
+	}
+
+	git_index_name_clear(index);
+	git_index_reuc_clear(index);
+
+	git_vector_swap(&new_entries, &index->entries);
+
+	git_vector_foreach(&remove_entries, i, entry) {
+		if (index->tree)
+			git_tree_cache_invalidate_path(index->tree, entry->path);
+
+		index_entry_free(entry);
+	}
+
+	error = 0;
+
+done:
+	git_vector_free(&new_entries);
+	git_vector_free(&remove_entries);
+	git_iterator_free(index_iterator);
+	git_iterator_free(new_iterator);
+	return error;
+}
+
 git_repository *git_index_owner(const git_index *index)
 {
 	return INDEX_OWNER(index);
diff --git a/src/index.h b/src/index.h
index 6d2904f..0f6f4e8 100644
--- a/src/index.h
+++ b/src/index.h
@@ -93,6 +93,8 @@ extern int git_index_snapshot_find(
 	size_t *at_pos, git_vector *snap, git_vector_cmp entry_srch,
 	const char *path, size_t path_len, int stage);
 
+/* Replace an index with a new index */
+int git_index_read_index(git_index *index, const git_index *new_index);
 
 typedef struct {
 	git_index *index;
diff --git a/tests/index/read_index.c b/tests/index/read_index.c
new file mode 100644
index 0000000..82a771d
--- /dev/null
+++ b/tests/index/read_index.c
@@ -0,0 +1,73 @@
+#include "clar_libgit2.h"
+#include "posix.h"
+#include "index.h"
+
+static git_repository *_repo;
+static git_index *_index;
+
+void test_index_read_index__initialize(void)
+{
+	git_object *head;
+	git_reference *head_ref;
+
+	_repo = cl_git_sandbox_init("testrepo");
+	cl_git_pass(git_revparse_ext(&head, &head_ref, _repo, "HEAD"));
+	cl_git_pass(git_reset(_repo, head, GIT_RESET_HARD, NULL));
+	cl_git_pass(git_repository_index(&_index, _repo));
+
+	git_reference_free(head_ref);
+	git_object_free(head);
+}
+
+void test_index_read_index__cleanup(void)
+{
+	git_index_free(_index);
+	cl_git_sandbox_cleanup();
+}
+
+void test_index_read_index__maintains_stat_cache(void)
+{
+	git_index *new_index;
+	git_oid index_id;
+	git_index_entry new_entry;
+	const git_index_entry *e;
+	git_tree *tree;
+	size_t i;
+
+	cl_assert_equal_i(4, git_index_entrycount(_index));
+
+	/* write-tree */
+	cl_git_pass(git_index_write_tree(&index_id, _index));
+
+	/* read-tree, then read index */
+	git_tree_lookup(&tree, _repo, &index_id);
+	cl_git_pass(git_index_new(&new_index));
+	cl_git_pass(git_index_read_tree(new_index, tree));
+	git_tree_free(tree);
+
+	/* add a new entry that will not have stat data */
+	memset(&new_entry, 0, sizeof(git_index_entry));
+	new_entry.path = "Hello";
+	git_oid_fromstr(&new_entry.id, "0123456789012345678901234567890123456789");
+	new_entry.file_size = 1234;
+	new_entry.mode = 0100644;
+	cl_git_pass(git_index_add(new_index, &new_entry));
+	cl_assert_equal_i(5, git_index_entrycount(new_index));
+
+	cl_git_pass(git_index_read_index(_index, new_index));
+	git_index_free(new_index);
+
+	cl_assert_equal_i(5, git_index_entrycount(_index));
+
+	for (i = 0; i < git_index_entrycount(_index); i++) {
+		e = git_index_get_byindex(_index, i);
+
+		if (strcmp(e->path, "Hello") == 0) {
+			cl_assert_equal_i(0, e->ctime.seconds);
+			cl_assert_equal_i(0, e->mtime.seconds);
+		} else {
+			cl_assert(0 != e->ctime.seconds);
+			cl_assert(0 != e->mtime.seconds);
+		}
+	}
+}