thodg/libgit2

diff --git a/include/git2/commit.h b/include/git2/commit.h
index 640adf5..5b6da52 100644
--- a/include/git2/commit.h
+++ b/include/git2/commit.h
@@ -179,6 +179,25 @@ GIT_EXTERN(int) git_commit_parent(git_commit **parent, git_commit *commit, unsig
 GIT_EXTERN(const git_oid *) git_commit_parent_oid(git_commit *commit, unsigned int n);
 
 /**
+ * Get the commit object that is the <n>th generation ancestor
+ * of the named commit object, following only the first parents.
+ * The returned commit has to be freed by the caller.
+ *
+ * Passing `0` as the generation number returns another instance of the
+ * base commit itself.
+ *
+ * @param ancestor Pointer where to store the ancestor commit
+ * @param commit a previously loaded commit.
+ * @param n the requested generation
+ * @return 0 on success; GIT_ENOTFOUND if no matching ancestor exists
+ * or an error code
+ */
+int git_commit_nth_gen_ancestor(
+	git_commit **ancestor,
+	const git_commit *commit,
+	unsigned int n);
+
+/**
  * Create a new commit in the repository using `git_object`
  * instances as parameters.
  *
diff --git a/include/git2/refs.h b/include/git2/refs.h
index 7f6eb0e..b119e90 100644
--- a/include/git2/refs.h
+++ b/include/git2/refs.h
@@ -363,6 +363,27 @@ GIT_EXTERN(int) git_reference_foreach_glob(
  */
 GIT_EXTERN(int) git_reference_has_log(git_reference *ref);
 
+
+/**
+ * Return the reference supporting the remote tracking branch,
+ * given a reference branch.
+ *
+ * The input reference has to be located in the `refs/heads`
+ * namespace.
+ *
+ * @param tracking_ref Pointer where to store the retrieved
+ * reference.
+ *
+ * @param branch_ref A git local branch reference. 
+ *
+ * @return 0 on success; GIT_ENOTFOUND when no remote tracking
+ * reference exists, otherwise an error code.
+ */
+GIT_EXTERN(int) git_reference_remote_tracking_from_branch(
+	git_reference **tracking_ref,
+	git_reference *branch_ref
+);
+
 /** @} */
 GIT_END_DECL
 #endif
diff --git a/src/commit.c b/src/commit.c
index a3baf9d..32c4794 100644
--- a/src/commit.c
+++ b/src/commit.c
@@ -229,19 +229,25 @@ GIT_COMMIT_GETTER(int, time_offset, commit->committer->when.offset)
 GIT_COMMIT_GETTER(unsigned int, parentcount, commit->parent_oids.length)
 GIT_COMMIT_GETTER(const git_oid *, tree_oid, &commit->tree_oid);
 
-
 int git_commit_tree(git_tree **tree_out, git_commit *commit)
 {
 	assert(commit);
 	return git_tree_lookup(tree_out, commit->object.repo, &commit->tree_oid);
 }
 
+const git_oid *git_commit_parent_oid(git_commit *commit, unsigned int n)
+{
+	assert(commit);
+
+	return git_vector_get(&commit->parent_oids, n);
+}
+
 int git_commit_parent(git_commit **parent, git_commit *commit, unsigned int n)
 {
-	git_oid *parent_oid;
+	const git_oid *parent_oid;
 	assert(commit);
 
-	parent_oid = git_vector_get(&commit->parent_oids, n);
+	parent_oid = git_commit_parent_oid(commit, n);
 	if (parent_oid == NULL) {
 		giterr_set(GITERR_INVALID, "Parent %u does not exist", n);
 		return GIT_ENOTFOUND;
@@ -250,9 +256,36 @@ int git_commit_parent(git_commit **parent, git_commit *commit, unsigned int n)
 	return git_commit_lookup(parent, commit->object.repo, parent_oid);
 }
 
-const git_oid *git_commit_parent_oid(git_commit *commit, unsigned int n)
+int git_commit_nth_gen_ancestor(
+	git_commit **ancestor,
+	const git_commit *commit,
+	unsigned int n)
 {
-	assert(commit);
+	git_commit *current, *parent;
+	int error;
 
-	return git_vector_get(&commit->parent_oids, n);
+	assert(ancestor && commit);
+
+	current = (git_commit *)commit;
+
+	if (n == 0)
+		return git_commit_lookup(
+			ancestor,
+			commit->object.repo,
+			git_object_id((const git_object *)commit));
+
+	while (n--) {
+		error = git_commit_parent(&parent, (git_commit *)current, 0);
+
+		if (current != commit)
+			git_commit_free(current);
+
+		if (error < 0)
+			return error;
+
+		current = parent;
+	}
+
+	*ancestor = parent;
+	return 0;
 }
diff --git a/src/refs.c b/src/refs.c
index e8f9fc8..13022c7 100644
--- a/src/refs.c
+++ b/src/refs.c
@@ -11,6 +11,7 @@
 #include "fileops.h"
 #include "pack.h"
 #include "reflog.h"
+#include "config.h"
 
 #include <git2/tag.h>
 #include <git2/object.h>
@@ -1811,3 +1812,76 @@ int git_reference_has_log(
 
 	return result;
 }
+
+//TODO: How about also taking care of local tracking branches?
+//cf. http://alblue.bandlem.com/2011/07/git-tip-of-week-tracking-branches.html
+int git_reference_remote_tracking_from_branch(
+	git_reference **tracking_ref,
+	git_reference *branch_ref)
+{
+	git_config *config = NULL;
+	const char *name, *remote, *merge;
+	git_buf buf = GIT_BUF_INIT;
+	int error = -1;
+
+	assert(tracking_ref && branch_ref);
+
+	name = git_reference_name(branch_ref);
+
+	if (git__prefixcmp(name, GIT_REFS_HEADS_DIR)) {
+		giterr_set(
+			GITERR_INVALID,
+			"Failed to retrieve tracking reference - '%s' is not a branch.",
+			name);
+		return -1;
+	}
+
+	if (git_repository_config(&config, branch_ref->owner) < 0)
+		return -1;
+
+	if (git_buf_printf(
+		&buf,
+		"branch.%s.remote",
+		name + strlen(GIT_REFS_HEADS_DIR)) < 0)
+			goto cleanup;
+
+	if ((error = git_config_get_string(&remote, config, git_buf_cstr(&buf))) < 0)
+		goto cleanup;
+
+	error = -1;
+
+	git_buf_clear(&buf);
+
+	//TODO: Is it ok to fail when no merge target is found?
+	if (git_buf_printf(
+		&buf,
+		"branch.%s.merge",
+		name + strlen(GIT_REFS_HEADS_DIR)) < 0)
+			goto cleanup;
+
+	if (git_config_get_string(&merge, config, git_buf_cstr(&buf)) < 0)
+		goto cleanup;
+
+	//TODO: Should we test this?
+	if (git__prefixcmp(merge, GIT_REFS_HEADS_DIR))
+		goto cleanup;
+
+	git_buf_clear(&buf);
+
+	if (git_buf_printf(
+		&buf,
+		"refs/remotes/%s/%s",
+		remote,
+		merge + strlen(GIT_REFS_HEADS_DIR)) < 0)
+			goto cleanup;
+
+	error = git_reference_lookup(
+		tracking_ref,
+		branch_ref->owner,
+		git_buf_cstr(&buf));
+
+cleanup:
+	git_config_free(config);
+	git_buf_free(&buf);
+	return error;
+}
diff --git a/src/revparse.c b/src/revparse.c
index f9859b1..2b03c86 100644
--- a/src/revparse.c
+++ b/src/revparse.c
@@ -104,7 +104,16 @@ cleanup:
 	return error;
 }
 
-extern int revparse_lookup_object(git_object **out, git_repository *repo, const char *spec);
+static int maybe_sha_or_abbrev(git_object**out, git_repository *repo, const char *spec)
+{
+	git_oid oid;
+	size_t speclen = strlen(spec);
+
+	if (git_oid_fromstrn(&oid, spec, speclen) < 0)
+		return GIT_ENOTFOUND;
+
+	return git_object_lookup_prefix(out, repo, &oid, speclen, GIT_OBJ_ANY);
+}
 
 static int maybe_describe(git_object**out, git_repository *repo, const char *spec)
 {
@@ -123,21 +132,10 @@ static int maybe_describe(git_object**out, git_repository *repo, const char *spe
 	if (!match)
 		return GIT_ENOTFOUND;
 
-	return revparse_lookup_object(out, repo, substr+2);
+	return maybe_sha_or_abbrev(out, repo, substr+2);
 }
 
-static int maybe_sha_or_abbrev(git_object**out, git_repository *repo, const char *spec)
-{
-	git_oid oid;
-	size_t speclen = strlen(spec);
-
-	if (git_oid_fromstrn(&oid, spec, speclen) < 0)
-		return GIT_ENOTFOUND;
-
-	return git_object_lookup_prefix(out, repo, &oid, speclen, GIT_OBJ_ANY);
-}
-
-int revparse_lookup_object(git_object **out, git_repository *repo, const char *spec)
+static int revparse_lookup_object(git_object **out, git_repository *repo, const char *spec)
 {
 	int error;
 	git_reference *ref;
@@ -251,32 +249,12 @@ static int walk_ref_history(git_object **out, git_repository *repo, const char *
 		date_error = git__date_parse(&timestamp, git_buf_cstr(&datebuf));
 
 		/* @{u} or @{upstream} -> upstream branch, for a tracking branch. This is stored in the config. */
-		if (!git__prefixcmp(git_reference_name(disambiguated), GIT_REFS_HEADS_DIR) &&
-			(!strcmp(reflogspec, "@{u}") || !strcmp(reflogspec, "@{upstream}"))) {
-			git_config *cfg;
-			if (!git_repository_config(&cfg, repo)) {
-				/* Is the ref a tracking branch? */
-				const char *remote;
-				git_buf_clear(&buf);
-				git_buf_printf(&buf, "branch.%s.remote",
-					git_reference_name(disambiguated) + strlen(GIT_REFS_HEADS_DIR));
-
-				if (!git_config_get_string(&remote, cfg, git_buf_cstr(&buf))) {
-					/* Yes. Find the first merge target name. */
-					const char *mergetarget;
-					git_buf_clear(&buf);
-					git_buf_printf(&buf, "branch.%s.merge",
-						git_reference_name(disambiguated) + strlen(GIT_REFS_HEADS_DIR));
-
-					if (!git_config_get_string(&mergetarget, cfg, git_buf_cstr(&buf)) &&
-						!git__prefixcmp(mergetarget, "refs/heads/")) {
-							/* Success. Look up the target and fetch the object. */
-							git_buf_clear(&buf);
-							git_buf_printf(&buf, "refs/remotes/%s/%s", remote, mergetarget+11);
-							retcode = revparse_lookup_fully_qualifed_ref(out, repo, git_buf_cstr(&buf));
-					}
-				}
-				git_config_free(cfg);
+		if (!strcmp(reflogspec, "@{u}") || !strcmp(reflogspec, "@{upstream}")) {
+			git_reference *tracking;
+			
+			if (!(retcode = git_reference_remote_tracking_from_branch(&tracking, disambiguated))) {
+				retcode = revparse_lookup_fully_qualifed_ref(out, repo, git_reference_name(tracking));
+				git_reference_free(tracking);
 			}
 		}
 
@@ -524,8 +502,7 @@ static int handle_caret_syntax(git_object **out, git_repository *repo, git_objec
 
 static int handle_linear_syntax(git_object **out, git_object *obj, const char *movement)
 {
-	git_commit *commit1, *commit2;
-	int i, n;
+	int n;
 
 	/* Dereference until we reach a commit. */
 	if (dereference_to_type(&obj, obj, GIT_OBJ_COMMIT) < 0) {
@@ -539,26 +516,8 @@ static int handle_linear_syntax(git_object **out, git_object *obj, const char *m
 	} else if (git__strtol32(&n, movement, NULL, 0) < 0) {
 		return GIT_ERROR;
 	}
-	commit1 = (git_commit*)obj;
 
-	/* "~0" just returns the input */
-	if (n == 0) {
-		*out = obj;
-		return 0;
-	}
-
-	for (i=0; i<n; i++) {
-		if (git_commit_parent(&commit2, commit1, 0) < 0) {
-			return GIT_ERROR;
-		}
-		if (commit1 != (git_commit*)obj) {
-			git_commit_free(commit1);
-		}
-		commit1 = commit2;
-	}
-
-	*out = (git_object*)commit1;
-	return 0;
+	return git_commit_nth_gen_ancestor((git_commit **)out, (git_commit*)obj, n);
 }
 
 static int oid_for_tree_path(git_oid *out, git_tree *tree, git_repository *repo, const char *path)
diff --git a/tests-clar/commit/parent.c b/tests-clar/commit/parent.c
new file mode 100644
index 0000000..a007577
--- /dev/null
+++ b/tests-clar/commit/parent.c
@@ -0,0 +1,57 @@
+#include "clar_libgit2.h"
+
+static git_repository *_repo;
+static git_commit *commit;
+
+void test_commit_parent__initialize(void)
+{
+	git_oid oid;
+
+	cl_git_pass(git_repository_open(&_repo, cl_fixture("testrepo.git")));
+
+	git_oid_fromstr(&oid, "be3563ae3f795b2b4353bcce3a527ad0a4f7f644");
+	cl_git_pass(git_commit_lookup(&commit, _repo, &oid));
+}
+
+void test_commit_parent__cleanup(void)
+{
+	git_commit_free(commit);
+	git_repository_free(_repo);
+}
+
+static void assert_nth_gen_parent(unsigned int gen, const char *expected_oid)
+{
+	git_commit *parent = NULL;
+	int error;
+	
+	error = git_commit_nth_gen_ancestor(&parent, commit, gen);
+
+	if (expected_oid != NULL) {
+		cl_assert_equal_i(0, error);
+		cl_assert_equal_i(0, git_oid_streq(git_commit_id(parent), expected_oid));
+	} else
+		cl_assert_equal_i(GIT_ENOTFOUND, error);
+
+	git_commit_free(parent);
+}
+
+/*
+ * $ git show be35~0
+ * commit be3563ae3f795b2b4353bcce3a527ad0a4f7f644
+ *
+ * $ git show be35~1
+ * commit 9fd738e8f7967c078dceed8190330fc8648ee56a
+ *
+ * $ git show be35~3
+ * commit 5b5b025afb0b4c913b4c338a42934a3863bf3644
+ *
+ * $ git show be35~42
+ * fatal: ambiguous argument 'be35~42': unknown revision or path not in the working tree.
+ */
+void test_commit_parent__can_retrieve_nth_generation_parent(void)
+{
+	assert_nth_gen_parent(0, "be3563ae3f795b2b4353bcce3a527ad0a4f7f644");
+	assert_nth_gen_parent(1, "9fd738e8f7967c078dceed8190330fc8648ee56a");
+	assert_nth_gen_parent(3, "5b5b025afb0b4c913b4c338a42934a3863bf3644");
+	assert_nth_gen_parent(42, NULL);
+}
diff --git a/tests-clar/refs/read.c b/tests-clar/refs/read.c
index 6838ead..ce4eefe 100644
--- a/tests-clar/refs/read.c
+++ b/tests-clar/refs/read.c
@@ -16,12 +16,12 @@ static git_repository *g_repo;
 
 void test_refs_read__initialize(void)
 {
-   g_repo = cl_git_sandbox_init("testrepo");
+	cl_git_pass(git_repository_open(&g_repo, cl_fixture("testrepo.git")));
 }
 
 void test_refs_read__cleanup(void)
 {
-   cl_git_sandbox_cleanup();
+	git_repository_free(g_repo);
 }
 
 void test_refs_read__loose_tag(void)
diff --git a/tests-clar/refs/remotetracking.c b/tests-clar/refs/remotetracking.c
new file mode 100644
index 0000000..c4ec588
--- /dev/null
+++ b/tests-clar/refs/remotetracking.c
@@ -0,0 +1,49 @@
+#include "clar_libgit2.h"
+
+static git_repository *g_repo;
+
+void test_refs_remotetracking__initialize(void)
+{
+	cl_git_pass(git_repository_open(&g_repo, cl_fixture("testrepo.git")));
+}
+
+void test_refs_remotetracking__cleanup(void)
+{
+	git_repository_free(g_repo);
+}
+
+void test_refs_remotetracking__unfound_returns_GIT_ENOTFOUND(void)
+{
+	git_reference *branch, *tracking;
+
+	cl_git_pass(git_reference_lookup(&branch, g_repo, "refs/heads/subtrees"));
+
+	cl_assert_equal_i(GIT_ENOTFOUND, git_reference_remote_tracking_from_branch(&tracking, branch));
+
+	git_reference_free(branch);
+}
+
+void test_refs_remotetracking__retrieving_from_a_non_head_fails(void)
+{
+	git_reference *branch, *tracking;
+
+	cl_git_pass(git_reference_lookup(&branch, g_repo, "refs/tags/e90810b"));
+
+	cl_git_fail(git_reference_remote_tracking_from_branch(&tracking, branch));
+
+	git_reference_free(branch);
+}
+
+void test_refs_remotetracking__can_retrieve_a_remote_tracking_branch_reference(void)
+{
+	git_reference *branch, *tracking;
+
+	cl_git_pass(git_reference_lookup(&branch, g_repo, "refs/heads/master"));
+
+	cl_git_pass(git_reference_remote_tracking_from_branch(&tracking, branch));
+
+	cl_assert_equal_s("refs/remotes/test/master", git_reference_name(tracking));
+
+	git_reference_free(branch);
+	git_reference_free(tracking);
+}