thodg/libgit2

diff --git a/include/git2/merge.h b/include/git2/merge.h
index 38e3408..1d30a5a 100644
--- a/include/git2/merge.h
+++ b/include/git2/merge.h
@@ -235,6 +235,46 @@ GIT_EXTERN(int) git_merge_tree_init_opts(
 	int version);
 
 /**
+ * The results of `git_merge_status` indicate the state of a merge scenario.
+ */
+typedef enum {
+	/**
+	 * A "normal" merge; both HEAD and the given merge input have diverged
+	 * from their common ancestor.  The divergent commits must be merged.
+	 */
+	GIT_MERGE_STATUS_NORMAL = 0,
+
+	/**
+	 * The repository is already up-to-date and no merge needs to be
+	 * performed.  The given merge input already exists as a parent of HEAD.
+	 */
+	GIT_MERGE_STATUS_UP_TO_DATE = (1 << 0),
+
+	/**
+	 * The given merge input is a fast-forward from HEAD and no merge
+	 * needs to be performed.  Instead, the client can check out the
+	 * given merge input.
+	 */
+	GIT_MERGE_STATUS_FASTFORWARD = (1 << 1),
+} git_merge_status_t;
+
+/**
+ * Determine the status of the merge between the given branch(es) and the
+ * HEAD of the repository.
+ *
+ * @param status_out status enumeration that the result is written into
+ * @param repo the repository to merge
+ * @param their_heads the heads to merge into
+ * @param their_heads_len the number of heads to merge
+ * @return 0 on success or error code
+ */
+GIT_EXTERN(int) git_merge_status(
+	git_merge_status_t *status_out,
+	git_repository *repo,
+	const git_merge_head **their_heads,
+	size_t their_heads_len);
+
+/**
  * Option flags for `git_merge`.
  */
 typedef enum {
diff --git a/src/merge.c b/src/merge.c
index 8a33edb..cdc1921 100644
--- a/src/merge.c
+++ b/src/merge.c
@@ -2497,6 +2497,79 @@ static int merge_state_cleanup(git_repository *repo)
 	return git_repository__cleanup_files(repo, state_files, ARRAY_SIZE(state_files));
 }
 
+static int merge_heads(
+	git_merge_head **ancestor_head_out,
+	git_merge_head **our_head_out,
+	git_repository *repo,
+	const git_merge_head **their_heads,
+	size_t their_heads_len)
+{
+	git_merge_head *ancestor_head = NULL, *our_head = NULL;
+	git_reference *our_ref = NULL;
+	int error = 0;
+
+	*ancestor_head_out = NULL;
+	*our_head_out = NULL;
+
+	if ((error = git_repository__ensure_not_bare(repo, "merge")) < 0)
+		goto done;
+
+	if ((error = git_reference_lookup(&our_ref, repo, GIT_HEAD_FILE)) < 0 ||
+		(error = git_merge_head_from_ref(&our_head, repo, our_ref)) < 0)
+		goto done;
+
+	if ((error = merge_ancestor_head(&ancestor_head, repo, our_head, their_heads, their_heads_len)) < 0) {
+		if (error != GIT_ENOTFOUND)
+			goto done;
+
+		giterr_clear();
+		error = 0;
+	}
+
+	*ancestor_head_out = ancestor_head;
+	*our_head_out = our_head;
+
+done:
+	if (error < 0) {
+		git_merge_head_free(ancestor_head);
+		git_merge_head_free(our_head);
+	}
+
+	git_reference_free(our_ref);
+
+	return error;
+}
+
+int git_merge_status(
+	git_merge_status_t *out,
+	git_repository *repo,
+	const git_merge_head **their_heads,
+	size_t their_heads_len)
+{
+	git_merge_head *ancestor_head = NULL, *our_head = NULL;
+	int error;
+
+	assert(out && repo && their_heads);
+
+	*out = GIT_MERGE_STATUS_NORMAL;
+
+	if ((error = merge_heads(&ancestor_head, &our_head, repo, their_heads, their_heads_len)) < 0)
+		goto done;
+
+	if (their_heads_len == 1 && ancestor_head != NULL) {
+		/* We're up-to-date if we're trying to merge our own common ancestor. */
+		if (git_oid_equal(&ancestor_head->oid, &their_heads[0]->oid))
+			*out = GIT_MERGE_STATUS_UP_TO_DATE;
+
+		/* We're fastforwardable if we're our own common ancestor. */
+		else if (git_oid_equal(&ancestor_head->oid, &our_head->oid))
+			*out = GIT_MERGE_STATUS_FASTFORWARD;
+	}
+
+done:
+	return error;
+}
+
 int git_merge(
 	git_merge_result **out,
 	git_repository *repo,
@@ -2530,15 +2603,7 @@ int git_merge(
 	their_trees = git__calloc(their_heads_len, sizeof(git_tree *));
 	GITERR_CHECK_ALLOC(their_trees);
 
-	if ((error = git_repository__ensure_not_bare(repo, "merge")) < 0)
-		goto on_error;
-
-	if ((error = git_reference_lookup(&our_ref, repo, GIT_HEAD_FILE)) < 0 ||
-		(error = git_merge_head_from_ref(&our_head, repo, our_ref)) < 0)
-		goto on_error;
-
-	if ((error = merge_ancestor_head(&ancestor_head, repo, our_head, their_heads, their_heads_len)) < 0 &&
-		error != GIT_ENOTFOUND)
+	if ((error = merge_heads(&ancestor_head, &our_head, repo, their_heads, their_heads_len)) < 0)
 		goto on_error;
 
 	if ((error = merge_normalize_opts(repo, &opts, given_opts, ancestor_head, our_head, their_heads_len, their_heads)) < 0)
diff --git a/tests/merge/workdir/fastforward.c b/tests/merge/workdir/fastforward.c
deleted file mode 100644
index d6b3148..0000000
--- a/tests/merge/workdir/fastforward.c
+++ /dev/null
@@ -1,148 +0,0 @@
-#include "clar_libgit2.h"
-#include "git2/repository.h"
-#include "git2/merge.h"
-#include "git2/sys/index.h"
-#include "merge.h"
-#include "../merge_helpers.h"
-#include "refs.h"
-
-static git_repository *repo;
-static git_index *repo_index;
-
-#define TEST_REPO_PATH "merge-resolve"
-#define TEST_INDEX_PATH TEST_REPO_PATH "/.git/index"
-
-#define THEIRS_FASTFORWARD_BRANCH	"ff_branch"
-#define THEIRS_FASTFORWARD_ID		"fd89f8cffb663ac89095a0f9764902e93ceaca6a"
-
-#define THEIRS_NOFASTFORWARD_BRANCH	"branch"
-#define THEIRS_NOFASTFORWARD_ID	"7cb63eed597130ba4abb87b3e544b85021905520"
-
-
-// Fixture setup and teardown
-void test_merge_workdir_fastforward__initialize(void)
-{
-	repo = cl_git_sandbox_init(TEST_REPO_PATH);
-	git_repository_index(&repo_index, repo);
-}
-
-void test_merge_workdir_fastforward__cleanup(void)
-{
-	git_index_free(repo_index);
-	cl_git_sandbox_cleanup();
-}
-
-static git_merge_result *merge_fastforward_branch(int flags)
-{
-	git_reference *their_ref;
-	git_merge_head *their_heads[1];
-	git_merge_result *result;
-	git_merge_opts opts = GIT_MERGE_OPTS_INIT;
-
-	opts.merge_flags = flags;
-
-	cl_git_pass(git_reference_lookup(&their_ref, repo, GIT_REFS_HEADS_DIR THEIRS_FASTFORWARD_BRANCH));
-	cl_git_pass(git_merge_head_from_ref(&their_heads[0], repo, their_ref));
-
-	cl_git_pass(git_merge(&result, repo, (const git_merge_head **)their_heads, 1, &opts));
-
-	git_merge_head_free(their_heads[0]);
-	git_reference_free(their_ref);
-
-	return result;
-}
-
-void test_merge_workdir_fastforward__fastforward(void)
-{
-	git_merge_result *result;
-	git_oid expected, ff_oid;
-
-	cl_git_pass(git_oid_fromstr(&expected, THEIRS_FASTFORWARD_ID));
-
-	cl_assert(result = merge_fastforward_branch(0));
-	cl_assert(git_merge_result_is_fastforward(result));
-	cl_git_pass(git_merge_result_fastforward_id(&ff_oid, result));
-	cl_assert(git_oid_cmp(&ff_oid, &expected) == 0);
-
-	git_merge_result_free(result);
-}
-
-void test_merge_workdir_fastforward__fastforward_only(void)
-{
-	git_merge_result *result;
-	git_merge_opts opts = GIT_MERGE_OPTS_INIT;
-	git_reference *their_ref;
-	git_merge_head *their_head;
-	int error;
-
-	opts.merge_flags = GIT_MERGE_FASTFORWARD_ONLY;
-
-	cl_git_pass(git_reference_lookup(&their_ref, repo, GIT_REFS_HEADS_DIR THEIRS_NOFASTFORWARD_BRANCH));
-	cl_git_pass(git_merge_head_from_ref(&their_head, repo, their_ref));
-
-	cl_git_fail((error = git_merge(&result, repo, (const git_merge_head **)&their_head, 1, &opts)));
-	cl_assert(error == GIT_ENONFASTFORWARD);
-
-	git_merge_head_free(their_head);
-	git_reference_free(their_ref);
-}
-
-void test_merge_workdir_fastforward__no_fastforward(void)
-{
-	git_merge_result *result;
-
-	struct merge_index_entry merge_index_entries[] = {
-		{ 0100644, "233c0919c998ed110a4b6ff36f353aec8b713487", 0, "added-in-master.txt" },
-		{ 0100644, "ee3fa1b8c00aff7fe02065fdb50864bb0d932ccf", 0, "automergeable.txt" },
-		{ 0100644, "ab6c44a2e84492ad4b41bb6bac87353e9d02ac8b", 0, "changed-in-branch.txt" },
-		{ 0100644, "bd9cb4cd0a770cb9adcb5fce212142ef40ea1c35", 0, "changed-in-master.txt" },
-		{ 0100644, "4e886e602529caa9ab11d71f86634bd1b6e0de10", 0, "conflicting.txt" },
-		{ 0100644, "364bbe4ce80c7bd31e6307dce77d46e3e1759fb3", 0, "new-in-ff.txt" },
-		{ 0100644, "dfe3f22baa1f6fce5447901c3086bae368de6bdd", 0, "removed-in-branch.txt" },
-		{ 0100644, "c8f06f2e3bb2964174677e91f0abead0e43c9e5d", 0, "unchanged.txt" },
-	};
-
-	cl_assert(result = merge_fastforward_branch(GIT_MERGE_NO_FASTFORWARD));
-	cl_assert(!git_merge_result_is_fastforward(result));
-
-	cl_assert(merge_test_index(repo_index, merge_index_entries, 8));
-	cl_assert(git_index_reuc_entrycount(repo_index) == 0);
-
-	git_merge_result_free(result);
-}
-
-void test_merge_workdir_fastforward__uptodate(void)
-{
-	git_reference *their_ref;
-	git_merge_head *their_heads[1];
-	git_merge_result *result;
-
-	cl_git_pass(git_reference_lookup(&their_ref, repo, GIT_HEAD_FILE));
-	cl_git_pass(git_merge_head_from_ref(&their_heads[0], repo, their_ref));
-
-	cl_git_pass(git_merge(&result, repo, (const git_merge_head **)their_heads, 1, NULL));
-
-	cl_assert(git_merge_result_is_uptodate(result));
-
-	git_merge_head_free(their_heads[0]);
-	git_reference_free(their_ref);
-	git_merge_result_free(result);
-}
-
-void test_merge_workdir_fastforward__uptodate_merging_prev_commit(void)
-{
-	git_oid their_oid;
-	git_merge_head *their_heads[1];
-	git_merge_result *result;
-
-	cl_git_pass(git_oid_fromstr(&their_oid, "c607fc30883e335def28cd686b51f6cfa02b06ec"));
-	cl_git_pass(git_merge_head_from_id(&their_heads[0], repo, &their_oid));
-
-	cl_git_pass(git_merge(&result, repo, (const git_merge_head **)their_heads, 1, NULL));
-
-	cl_assert(git_merge_result_is_uptodate(result));
-
-	git_merge_head_free(their_heads[0]);
-	git_merge_result_free(result);
-}
-
diff --git a/tests/merge/workdir/status.c b/tests/merge/workdir/status.c
new file mode 100644
index 0000000..589299e
--- /dev/null
+++ b/tests/merge/workdir/status.c
@@ -0,0 +1,89 @@
+#include "clar_libgit2.h"
+#include "git2/repository.h"
+#include "git2/merge.h"
+#include "git2/sys/index.h"
+#include "merge.h"
+#include "../merge_helpers.h"
+#include "refs.h"
+
+static git_repository *repo;
+static git_index *repo_index;
+
+#define TEST_REPO_PATH "merge-resolve"
+#define TEST_INDEX_PATH TEST_REPO_PATH "/.git/index"
+
+#define UPTODATE_BRANCH			"master"
+#define PREVIOUS_BRANCH			"previous"
+
+#define FASTFORWARD_BRANCH		"ff_branch"
+#define FASTFORWARD_ID			"fd89f8cffb663ac89095a0f9764902e93ceaca6a"
+
+#define NOFASTFORWARD_BRANCH	"branch"
+#define NOFASTFORWARD_ID		"7cb63eed597130ba4abb87b3e544b85021905520"
+
+
+// Fixture setup and teardown
+void test_merge_workdir_status__initialize(void)
+{
+	repo = cl_git_sandbox_init(TEST_REPO_PATH);
+	git_repository_index(&repo_index, repo);
+}
+
+void test_merge_workdir_status__cleanup(void)
+{
+	git_index_free(repo_index);
+	cl_git_sandbox_cleanup();
+}
+
+static git_status_t status_from_branch(const char *branchname)
+{
+	git_buf refname = GIT_BUF_INIT;
+	git_reference *their_ref;
+	git_merge_head *their_heads[1];
+	git_status_t status;
+
+	git_buf_printf(&refname, "%s%s", GIT_REFS_HEADS_DIR, branchname);
+
+	cl_git_pass(git_reference_lookup(&their_ref, repo, git_buf_cstr(&refname)));
+	cl_git_pass(git_merge_head_from_ref(&their_heads[0], repo, their_ref));
+
+	cl_git_pass(git_merge_status(&status, repo, their_heads, 1));
+
+	git_buf_free(&refname);
+	git_merge_head_free(their_heads[0]);
+	git_reference_free(their_ref);
+
+	return status;
+}
+
+void test_merge_workdir_status__fastforward(void)
+{
+	git_merge_status_t status;
+
+	status = status_from_branch(FASTFORWARD_BRANCH);
+	cl_assert_equal_i(GIT_MERGE_STATUS_FASTFORWARD, status);
+}
+
+void test_merge_workdir_status__no_fastforward(void)
+{
+	git_merge_status_t status;
+
+	status = status_from_branch(NOFASTFORWARD_BRANCH);
+	cl_assert_equal_i(GIT_MERGE_STATUS_NORMAL, status);
+}
+
+void test_merge_workdir_status__uptodate(void)
+{
+	git_merge_status_t status;
+
+	status = status_from_branch(UPTODATE_BRANCH);
+	cl_assert_equal_i(GIT_MERGE_STATUS_UP_TO_DATE, status);
+}
+
+void test_merge_workdir_status__uptodate_merging_prev_commit(void)
+{
+	git_merge_status_t status;
+
+	status = status_from_branch(PREVIOUS_BRANCH);
+	cl_assert_equal_i(GIT_MERGE_STATUS_UP_TO_DATE, status);
+}
diff --git a/tests/resources/merge-resolve/.gitted/refs/heads/previous b/tests/resources/merge-resolve/.gitted/refs/heads/previous
new file mode 100644
index 0000000..7bc1a8d
--- /dev/null
+++ b/tests/resources/merge-resolve/.gitted/refs/heads/previous
@@ -0,0 +1 @@
+c607fc30883e335def28cd686b51f6cfa02b06ec