thodg/libgit2

diff --git a/include/git2.h b/include/git2.h
index e182ce9..9239b48 100644
--- a/include/git2.h
+++ b/include/git2.h
@@ -9,6 +9,7 @@
 #define INCLUDE_git_git_h__
 
 #include "git2/annotated_commit.h"
+#include "git2/apply.h"
 #include "git2/attr.h"
 #include "git2/blob.h"
 #include "git2/blame.h"
diff --git a/include/git2/apply.h b/include/git2/apply.h
new file mode 100644
index 0000000..a4ec4fc
--- /dev/null
+++ b/include/git2/apply.h
@@ -0,0 +1,41 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#ifndef INCLUDE_git_apply_h__
+#define INCLUDE_git_apply_h__
+
+#include "common.h"
+#include "types.h"
+#include "oid.h"
+#include "diff.h"
+
+/**
+ * @file git2/apply.h
+ * @brief Git patch application routines
+ * @defgroup git_apply Git patch application routines
+ * @ingroup Git
+ * @{
+ */
+GIT_BEGIN_DECL
+
+/**
+ * Apply a `git_diff` to a `git_tree`, and return the resulting image
+ * as an index.
+ *
+ * @param out the postimage of the application
+ * @param repo the repository to apply
+ * @param preimage the tree to apply the diff to
+ * @param diff the diff to apply
+ */
+GIT_EXTERN(int) git_apply_to_tree(
+	git_index **out,
+	git_repository *repo,
+	git_tree *preimage,
+	git_diff *diff);
+
+/** @} */
+GIT_END_DECL
+#endif
diff --git a/src/apply.c b/src/apply.c
index 8c7bb6b..be8c7b6 100644
--- a/src/apply.c
+++ b/src/apply.c
@@ -11,6 +11,8 @@
 
 #include "git2/patch.h"
 #include "git2/filter.h"
+#include "git2/blob.h"
+#include "git2/index.h"
 #include "array.h"
 #include "patch.h"
 #include "fileops.h"
@@ -376,3 +378,101 @@ done:
 
 	return error;
 }
+
+static int apply_one(
+	git_repository *repo,
+	git_tree *preimage,
+	git_index *postimage,
+	git_diff *diff,
+	size_t i)
+{
+	git_patch *patch = NULL;
+	git_tree_entry *tree_entry = NULL;
+	git_blob *blob = NULL;
+	git_buf contents = GIT_BUF_INIT;
+	const git_diff_delta *delta;
+	char *filename = NULL;
+	unsigned int mode;
+	git_oid blob_id;
+	git_index_entry index_entry;
+	int error;
+
+	if ((error = git_patch_from_diff(&patch, diff, i)) < 0)
+		goto done;
+
+	delta = git_patch_get_delta(patch);
+
+	if ((error = git_tree_entry_bypath(&tree_entry, preimage,
+			delta->old_file.path)) < 0 ||
+		(error = git_blob_lookup(&blob, repo,
+			git_tree_entry_id(tree_entry))) < 0 ||
+		(error = git_apply__patch(&contents, &filename, &mode,
+			git_blob_rawcontent(blob), git_blob_rawsize(blob), patch)) < 0 ||
+		(error = git_blob_create_frombuffer(&blob_id, repo,
+			contents.ptr, contents.size)) < 0)
+		goto done;
+
+	memset(&index_entry, 0, sizeof(git_index_entry));
+	index_entry.path = filename;
+	index_entry.mode = mode;
+	git_oid_cpy(&index_entry.id, &blob_id);
+
+	if ((error = git_index_add(postimage, &index_entry)) < 0)
+		goto done;
+
+done:
+	git_buf_free(&contents);
+	git__free(filename);
+	git_blob_free(blob);
+	git_tree_entry_free(tree_entry);
+	git_patch_free(patch);
+
+	return error;
+}
+
+int git_apply_to_tree(
+	git_index **out,
+	git_repository *repo,
+	git_tree *preimage,
+	git_diff *diff)
+{
+	git_index *postimage = NULL;
+	const git_diff_delta *delta;
+	size_t i;
+	int error = 0;
+
+	assert(out && repo && preimage && diff);
+
+	*out = NULL;
+
+	if ((error = git_index_new(&postimage)) < 0 ||
+		(error = git_index_read_tree(postimage, preimage)) < 0)
+		goto done;
+
+	/*
+	 * Remove the old paths from the index before applying diffs -
+	 * we need to do a full pass to remove them before adding deltas,
+	 * in order to handle rename situations.
+	 */
+	for (i = 0; i < git_diff_num_deltas(diff); i++) {
+		delta = git_diff_get_delta(diff, i);
+
+		if ((error = git_index_remove(postimage,
+				delta->old_file.path, 0)) < 0)
+			goto done;
+	}
+
+	for (i = 0; i < git_diff_num_deltas(diff); i++) {
+		if ((error = apply_one(repo, preimage, postimage, diff, i)) < 0)
+			goto done;
+	}
+
+	*out = postimage;
+
+done:
+	if (error < 0)
+		git_index_free(postimage);
+
+	return error;
+}
+
diff --git a/tests/apply/tree.c b/tests/apply/tree.c
new file mode 100644
index 0000000..38c64c0
--- /dev/null
+++ b/tests/apply/tree.c
@@ -0,0 +1,58 @@
+#include "clar_libgit2.h"
+#include "../merge/merge_helpers.h"
+
+static git_repository *repo;
+
+#define TEST_REPO_PATH "merge-recursive"
+
+
+void test_apply_tree__initialize(void)
+{
+	repo = cl_git_sandbox_init(TEST_REPO_PATH);
+}
+
+void test_apply_tree__cleanup(void)
+{
+	cl_git_sandbox_cleanup();
+}
+
+void test_apply_tree__one(void)
+{
+	git_oid a_oid, b_oid;
+	git_commit *a_commit, *b_commit;
+	git_tree *a_tree, *b_tree;
+	git_diff *diff;
+	git_index *index = NULL;
+	git_diff_options opts = GIT_DIFF_OPTIONS_INIT;
+
+	struct merge_index_entry expected[] = {
+		{ 0100644, "ffb36e513f5fdf8a6ba850a20142676a2ac4807d", 0, "asparagus.txt" },
+		{ 0100644, "68f6182f4c85d39e1309d97c7e456156dc9c0096", 0, "beef.txt" },
+		{ 0100644, "4b7c5650008b2e747fe1809eeb5a1dde0e80850a", 0, "bouilli.txt" },
+		{ 0100644, "c4e6cca3ec6ae0148ed231f97257df8c311e015f", 0, "gravy.txt" },
+		{ 0100644, "68af1fc7407fd9addf1701a87eb1c95c7494c598", 0, "oyster.txt" },
+		{ 0100644, "a7b066537e6be7109abfe4ff97b675d4e077da20", 0, "veal.txt" },
+	};
+
+	git_oid_fromstr(&a_oid, "539bd011c4822c560c1d17cab095006b7a10f707");
+	git_oid_fromstr(&b_oid, "7c7bf85e978f1d18c0566f702d2cb7766b9c8d4f");
+
+	cl_git_pass(git_commit_lookup(&a_commit, repo, &a_oid));
+	cl_git_pass(git_commit_lookup(&b_commit, repo, &b_oid));
+
+	cl_git_pass(git_commit_tree(&a_tree, a_commit));
+	cl_git_pass(git_commit_tree(&b_tree, b_commit));
+
+	cl_git_pass(git_diff_tree_to_tree(&diff, repo, a_tree, b_tree, &opts));
+
+	cl_git_pass(git_apply_to_tree(&index, repo, a_tree, diff));
+	merge_test_index(index, expected, 6);
+
+	git_index_free(index);
+	git_diff_free(diff);
+	git_tree_free(a_tree);
+	git_tree_free(b_tree);
+	git_commit_free(a_commit);
+	git_commit_free(b_commit);
+}
+