Add git_object_short_id API to get short id string

This finds a short id string that will unambiguously select the
given object, starting with the core.abbrev length (usually 7)
and growing until it is no longer ambiguous.

include/git2/object.h

@@ -10,6 +10,7 @@
 #include "common.h"
 #include "types.h"
 #include "oid.h"
+#include "buffer.h"
 
 /**
  * @file git2/object.h
@@ -104,6 +105,15 @@ GIT_EXTERN(int) git_object_lookup_bypath(
 GIT_EXTERN(const git_oid *) git_object_id(const git_object *obj);
 
 /**
+ * Get a short abbreviated OID string for the object
+ *
+ * @param out Buffer to write string into
+ * @param obj The object to get an ID for
+ * @return 0 on success, <0 for error
+ */
+GIT_EXTERN(int) git_object_short_id(git_buf *out, const git_object *obj);
+
+/**
  * Get the object type of an object
  *
  * @param obj the repository object

src/object.c

@@ -397,3 +397,46 @@ cleanup:
 	git_tree_free(tree);
 	return error;
 }
+
+int git_object_short_id(git_buf *out, const git_object *obj)
+{
+	git_repository *repo;
+	int len = GIT_ABBREV_DEFAULT, error;
+	git_oid id = {{0}};
+	git_odb *odb;
+
+	assert(out && obj);
+
+	git_buf_sanitize(out);
+	repo = git_object_owner(obj);
+
+	if ((error = git_repository__cvar(&len, repo, GIT_CVAR_ABBREV)) < 0)
+		return error;
+
+	if ((error = git_repository_odb(&odb, repo)) < 0)
+		return error;
+
+	while (len < GIT_OID_HEXSZ) {
+		/* set up short oid */
+		memcpy(&id.id, &obj->cached.oid.id, (len + 1) / 2);
+		if (len & 1)
+			id.id[len / 2] &= 0xf0;
+
+		error = git_odb_exists_prefix(NULL, odb, &id, len);
+		if (error != GIT_EAMBIGUOUS)
+			break;
+
+		giterr_clear();
+		len++;
+	}
+
+	if (!error && !(error = git_buf_grow(out, len + 1))) {
+		git_oid_tostr(out->ptr, len + 1, &id);
+		out->size = len;
+	}
+
+	git_odb_free(odb);
+
+	return error;
+}
+

tests/object/shortid.c

+#include "clar_libgit2.h"
+
+git_repository *_repo;
+
+void test_object_shortid__initialize(void)
+{
+	cl_git_pass(git_repository_open(&_repo, cl_fixture("duplicate.git")));
+}
+
+void test_object_shortid__cleanup(void)
+{
+	git_repository_free(_repo);
+	_repo = NULL;
+}
+
+void test_object_shortid__select(void)
+{
+	git_oid full;
+	git_object *obj;
+	git_buf shorty = {0};
+
+	git_oid_fromstr(&full, "ce013625030ba8dba906f756967f9e9ca394464a");
+	cl_git_pass(git_object_lookup(&obj, _repo, &full, GIT_OBJ_ANY));
+	cl_git_pass(git_object_short_id(&shorty, obj));
+	cl_assert_equal_i(7, shorty.size);
+	cl_assert_equal_s("ce01362", shorty.ptr);
+	git_object_free(obj);
+
+	git_oid_fromstr(&full, "dea509d097ce692e167dfc6a48a7a280cc5e877e");
+	cl_git_pass(git_object_lookup(&obj, _repo, &full, GIT_OBJ_ANY));
+	cl_git_pass(git_object_short_id(&shorty, obj));
+	cl_assert_equal_i(9, shorty.size);
+	cl_assert_equal_s("dea509d09", shorty.ptr);
+	git_object_free(obj);
+
+	git_oid_fromstr(&full, "dea509d0b3cb8ee0650f6ca210bc83f4678851ba");
+	cl_git_pass(git_object_lookup(&obj, _repo, &full, GIT_OBJ_ANY));
+	cl_git_pass(git_object_short_id(&shorty, obj));
+	cl_assert_equal_i(9, shorty.size);
+	cl_assert_equal_s("dea509d0b", shorty.ptr);
+	git_object_free(obj);
+
+	git_buf_free(&shorty);
+}