thodg/libgit2

diff --git a/include/git2/reset.h b/include/git2/reset.h
index 4c75ae7..c7c9519 100644
--- a/include/git2/reset.h
+++ b/include/git2/reset.h
@@ -53,6 +53,29 @@ typedef enum {
 GIT_EXTERN(int) git_reset(
 	git_repository *repo, git_object *target, git_reset_t reset_type);
 
+/**
+ * Updates some entries in the index from the target commit tree.
+ *
+ * The scope of the updated entries is determined by the paths
+ * being passed in the `pathspec` parameters.
+ *
+ * Passing a NULL `target` will result in removing
+ * entries in the index matching the provided pathspecs.
+ *
+ * @param repo Repository where to perform the reset operation.
+ *
+ * @param target The committish which content will be used to reset the content
+ * of the index.
+ *
+ * @param pathspecs List of pathspecs to operate on.
+ *
+ * @return 0 on success or an error code < 0
+ */
+GIT_EXTERN(int) git_reset_default(
+    git_repository *repo,
+    git_object *target,
+    git_strarray* pathspecs);
+
 /** @} */
 GIT_END_DECL
 #endif
diff --git a/src/reset.c b/src/reset.c
index b637e37..700aac8 100644
--- a/src/reset.c
+++ b/src/reset.c
@@ -9,6 +9,7 @@
 #include "commit.h"
 #include "tag.h"
 #include "merge.h"
+#include "diff.h"
 #include "git2/reset.h"
 #include "git2/checkout.h"
 #include "git2/merge.h"
@@ -54,6 +55,79 @@ cleanup:
 	return error;
 }
 
+int git_reset_default(
+	git_repository *repo,
+	git_object *target,
+	git_strarray* pathspecs)
+{
+	git_object *commit = NULL;
+	git_tree *tree = NULL;
+	git_diff_list *diff = NULL;
+	git_diff_options opts = GIT_DIFF_OPTIONS_INIT;
+	size_t i;
+	git_diff_delta *delta;
+	git_index_entry entry;
+	int error;
+	git_index *index = NULL;
+
+	assert(pathspecs != NULL && pathspecs->count > 0); 
+
+	memset(&entry, 0, sizeof(git_index_entry));
+
+	if ((error = git_repository_index(&index, repo)) < 0)
+		goto cleanup;
+
+	if (target) {
+		if (git_object_owner(target) != repo) {
+			giterr_set(GITERR_OBJECT,
+				"%s_default - The given target does not belong to this repository.", ERROR_MSG);
+			return -1;
+		}
+
+		if ((error = git_object_peel(&commit, target, GIT_OBJ_COMMIT)) < 0 ||
+			(error = git_commit_tree(&tree, (git_commit *)commit)) < 0)
+			goto cleanup;
+	}
+
+	opts.pathspec = *pathspecs;
+	opts.flags = GIT_DIFF_REVERSE;
+
+	if ((error = git_diff_tree_to_index(
+		&diff, repo, tree, index, &opts)) < 0)
+			goto cleanup;
+
+	git_vector_foreach(&diff->deltas, i, delta) {
+		if ((error = git_index_conflict_remove(index, delta->old_file.path)) < 0)
+			goto cleanup;
+
+		assert(delta->status == GIT_DELTA_ADDED ||
+			delta->status == GIT_DELTA_MODIFIED ||
+			delta->status == GIT_DELTA_DELETED);
+
+		if (delta->status == GIT_DELTA_DELETED) {
+			if ((error = git_index_remove(index, delta->old_file.path, 0)) < 0)
+				goto cleanup;
+		} else {
+			entry.mode = delta->new_file.mode;
+			git_oid_cpy(&entry.oid, &delta->new_file.oid);
+			entry.path = (char *)delta->new_file.path;
+
+			if ((error = git_index_add(index, &entry)) < 0)
+				goto cleanup;
+		}
+	}
+
+	error = git_index_write(index);
+
+cleanup:
+	git_object_free(commit);
+	git_tree_free(tree);
+	git_index_free(index);
+	git_diff_list_free(diff);
+
+	return error;
+}
+
 int git_reset(
 	git_repository *repo,
 	git_object *target,
diff --git a/tests-clar/reset/default.c b/tests-clar/reset/default.c
new file mode 100644
index 0000000..506d971
--- /dev/null
+++ b/tests-clar/reset/default.c
@@ -0,0 +1,180 @@
+#include "clar_libgit2.h"
+#include "posix.h"
+#include "reset_helpers.h"
+#include "path.h"
+
+static git_repository *_repo;
+static git_object *_target;
+static git_strarray _pathspecs;
+static git_index *_index;
+
+static void initialize(const char *repo_name)
+{
+	_repo = cl_git_sandbox_init(repo_name);
+	cl_git_pass(git_repository_index(&_index, _repo));
+
+	_target = NULL;
+
+	_pathspecs.strings = NULL;
+	_pathspecs.count = 0;
+}
+
+void test_reset_default__initialize(void)
+{
+	initialize("status");
+}
+
+void test_reset_default__cleanup(void)
+{
+	git_object_free(_target);
+	_target = NULL;
+
+	git_index_free(_index);
+	_index = NULL;
+
+	cl_git_sandbox_cleanup();
+}
+
+static void assert_content_in_index(
+	git_strarray *pathspecs,
+	bool should_exist,
+	git_strarray *expected_shas)
+{
+	size_t i, pos;
+	int error;
+
+	for (i = 0; i < pathspecs->count; i++) {
+		error = git_index_find(&pos, _index, pathspecs->strings[i]);
+
+		if (should_exist) {
+			const git_index_entry *entry;
+
+			cl_assert(error != GIT_ENOTFOUND);
+
+			entry = git_index_get_byindex(_index, pos);
+			cl_assert(entry != NULL);
+
+			if (!expected_shas)
+				continue;
+
+			cl_git_pass(git_oid_streq(&entry->oid, expected_shas->strings[i]));
+		} else
+			cl_assert_equal_i(should_exist, error != GIT_ENOTFOUND);
+	}
+}
+
+void test_reset_default__resetting_filepaths_against_a_null_target_removes_them_from_the_index(void)
+{
+	char *paths[] = { "staged_changes", "staged_new_file" };
+
+	_pathspecs.strings = paths;
+	_pathspecs.count = 2;
+
+	assert_content_in_index(&_pathspecs, true, NULL);
+
+	cl_git_pass(git_reset_default(_repo, NULL, &_pathspecs));
+
+	assert_content_in_index(&_pathspecs, false, NULL);
+}
+
+/*
+ * $ git ls-files --cached -s --abbrev=7 -- "staged*"
+ * 100644 55d316c 0        staged_changes
+ * 100644 a6be623 0        staged_changes_file_deleted
+ * ...
+ *
+ * $ git reset 0017bd4 -- staged_changes staged_changes_file_deleted
+ * Unstaged changes after reset:
+ * ...
+ *
+ * $ git ls-files --cached -s --abbrev=7 -- "staged*"
+ * 100644 32504b7 0        staged_changes
+ * 100644 061d42a 0        staged_changes_file_deleted
+ * ...
+ */
+void test_reset_default__resetting_filepaths_replaces_their_corresponding_index_entries(void)
+{
+	git_strarray before, after;
+
+	char *paths[] = { "staged_changes", "staged_changes_file_deleted" };
+	char *before_shas[] = { "55d316c9ba708999f1918e9677d01dfcae69c6b9",
+		"a6be623522ce87a1d862128ac42672604f7b468b" };
+	char *after_shas[] = { "32504b727382542f9f089e24fddac5e78533e96c",
+		"061d42a44cacde5726057b67558821d95db96f19" };
+
+	_pathspecs.strings = paths;
+	_pathspecs.count = 2;
+	before.strings = before_shas;
+	before.count = 2;
+	after.strings = after_shas;
+	after.count = 2;
+
+	cl_git_pass(git_revparse_single(&_target, _repo, "0017bd4"));
+	assert_content_in_index(&_pathspecs, true, &before);
+
+	cl_git_pass(git_reset_default(_repo, _target, &_pathspecs));
+
+	assert_content_in_index(&_pathspecs, true, &after);
+}
+
+/*
+ * $ git ls-files --cached -s --abbrev=7 -- conflicts-one.txt
+ * 100644 1f85ca5 1        conflicts-one.txt
+ * 100644 6aea5f2 2        conflicts-one.txt
+ * 100644 516bd85 3        conflicts-one.txt
+ *
+ * $  git reset 9a05ccb -- conflicts-one.txt
+ * Unstaged changes after reset:
+ * ...
+ *
+ * $ git ls-files --cached -s --abbrev=7 -- conflicts-one.txt
+ * 100644 1f85ca5 0        conflicts-one.txt
+ *
+ */
+void test_reset_default__resetting_filepaths_clears_previous_conflicts(void)
+{
+	git_index_entry *conflict_entry[3];
+	git_strarray after;
+
+	char *paths[] = { "conflicts-one.txt" };
+	char *after_shas[] = { "1f85ca51b8e0aac893a621b61a9c2661d6aa6d81" };
+
+	test_reset_default__cleanup();
+	initialize("mergedrepo");
+
+	_pathspecs.strings = paths;
+	_pathspecs.count = 1;
+	after.strings = after_shas;
+	after.count = 1;
+
+	cl_git_pass(git_index_conflict_get(&conflict_entry[0], &conflict_entry[1],
+		&conflict_entry[2], _index, "conflicts-one.txt"));
+
+	cl_git_pass(git_revparse_single(&_target, _repo, "9a05ccb"));
+	cl_git_pass(git_reset_default(_repo, _target, &_pathspecs));
+
+	assert_content_in_index(&_pathspecs, true, &after);
+
+	cl_assert_equal_i(GIT_ENOTFOUND, git_index_conflict_get(&conflict_entry[0],
+		&conflict_entry[1], &conflict_entry[2], _index, "conflicts-one.txt"));
+}
+
+/*
+$  git reset HEAD -- "I_am_not_there.txt" "me_neither.txt"
+Unstaged changes after reset:
+...
+*/
+void test_reset_default__resetting_unknown_filepaths_does_not_fail(void)
+{
+	char *paths[] = { "I_am_not_there.txt", "me_neither.txt" };
+
+	_pathspecs.strings = paths;
+	_pathspecs.count = 2;
+
+	assert_content_in_index(&_pathspecs, false, NULL);
+
+	cl_git_pass(git_revparse_single(&_target, _repo, "HEAD"));
+	cl_git_pass(git_reset_default(_repo, _target, &_pathspecs));
+
+	assert_content_in_index(&_pathspecs, false, NULL);
+}