text: #include <stdio.h>
  #include <git2.h>
  #include <stdlib.h>
  #include <string.h>
  
  static void check(int error, const char *message, const char *arg)
  {
  	if (!error)
  		return;
  	if (arg)
  		fprintf(stderr, "%s %s (%d)\n", message, arg, error);
  	else
  		fprintf(stderr, "%s(%d)\n", message, error);
  	exit(1);
  }
  
  static void usage(const char *message, const char *arg)
  {
  	if (message && arg)
  		fprintf(stderr, "%s: %s\n", message, arg);
  	else if (message)
  		fprintf(stderr, "%s\n", message);
  	fprintf(stderr, "usage: rev-parse [ --option ] <args>...\n");
  	exit(1);
  }
  
  struct parse_state {
  	git_repository *repo;
  	const char *repodir;
  	int not;
  };
  
  static int parse_revision(struct parse_state *ps, const char *revstr)
  {
  	git_revspec rs;
  	char str[GIT_OID_HEXSZ + 1];
  
  	if (!ps->repo) {
  		if (!ps->repodir)
  			ps->repodir = ".";
  		check(git_repository_open_ext(&ps->repo, ps->repodir, 0, NULL),
  			"Could not open repository from", ps->repodir);
  	}
  
  	check(git_revparse(&rs, ps->repo, revstr), "Could not parse", revstr);
  
  	if ((rs.flags & GIT_REVPARSE_SINGLE) != 0) {
  		git_oid_tostr(str, sizeof(str), git_object_id(rs.from));
  		printf("%s\n", str);
  		git_object_free(rs.from);
  	}
  	else if ((rs.flags & GIT_REVPARSE_RANGE) != 0) {
  		git_oid_tostr(str, sizeof(str), git_object_id(rs.to));
  		printf("%s\n", str);
  		git_object_free(rs.to);
  
  		if ((rs.flags & GIT_REVPARSE_MERGE_BASE) != 0) {
  			git_oid base;
  			check(git_merge_base(&base, ps->repo,
  				git_object_id(rs.from), git_object_id(rs.to)),
  				"Could not find merge base", revstr);
  
  			git_oid_tostr(str, sizeof(str), &base);
  			printf("%s\n", str);
  		}
  
  		git_oid_tostr(str, sizeof(str), git_object_id(rs.from));
  		printf("^%s\n", str);
  		git_object_free(rs.from);
  	}
  	else {
  		check(0, "Invalid results from git_revparse", revstr);
  	}
  
  	return 0;
  }
  
  int main(int argc, char *argv[])
  {
  	int i;
  	char *a;
  	struct parse_state ps;
  
  	git_threads_init();
  
  	memset(&ps, 0, sizeof(ps));
  
  	for (i = 1; i < argc; ++i) {
  		a = argv[i];
  
  		if (a[0] != '-') {
  			if (parse_revision(&ps, a) != 0)
  				break;
  		} else if (!strcmp(a, "--not"))
  			ps.not = !ps.not;
  		else if (!strncmp(a, "--git-dir=", strlen("--git-dir=")))
  			ps.repodir = a + strlen("--git-dir=");
  		else
  			usage("Cannot handle argument", a);
  	}
  
  	git_repository_free(ps.repo);
  	git_threads_shutdown();
  
  	return 0;
  }
  