Add git_diff_commit and last-changed examplerb/commit-modified-file

This adds a new diff API `git_diff_commit` which makes it easy to
generate a `git_diff` object that represents the changes in a given
commit.  It follows the core Git rules for considering changes in
a merge commit - i.e. if a file has an exact match in any parent
of the commit, then the file is considered unmodified by the merge.

This also adds a new example program "last-changed" which takes a
list of filenames and for each one displays the SHA of the last
commit that changed that file.

1 files changed, 138 insertions, 0 deletions

diff --git a/examples/last-changed.c b/examples/last-changed.c
new file mode 100644
index 000000000..fd9d278b5
--- /dev/null
+++ b/examples/last-changed.c
@@ -0,0 +1,138 @@
+/*
+ * libgit2 "last-changed" example - get last commit modifying a file
+ *
+ * Written by the libgit2 contributors
+ *
+ * To the extent possible under law, the author(s) have dedicated all copyright
+ * and related and neighboring rights to this software to the public domain
+ * worldwide. This software is distributed without any warranty.
+ *
+ * You should have received a copy of the CC0 Public Domain Dedication along
+ * with this software. If not, see
+ * <http://creativecommons.org/publicdomain/zero/1.0/>.
+ */
+
+#include "common.h"
+
+static void usage(void)
+{
+	fprintf(stderr, "usage: last-changed [--git-dir=DIR] pathname ...\n");
+	exit(1);
+}
+
+static int mark_pathspec_match(
+	const git_diff *, const git_diff_delta *, const char *, void *);
+
+typedef struct {
+	git_diff_options opts;
+	git_oid oid;
+	char str[GIT_OID_HEXSZ + 1];
+} change_info;
+
+int main(int argc, char *argv[])
+{
+	const char *repodir = ".";
+	change_info info = { GIT_DIFF_OPTIONS_INIT };
+	int start_pathspec = 1;
+	size_t i;
+	git_repository *repo;
+	git_revwalk *walker;
+
+	git_threads_init();
+
+	/* allow you to specific a git repo other than the current one */
+	if (argc > 1 && !strncmp(argv[1], "--git-dir=", strlen("--git-dir="))) {
+		repodir = argv[1] + strlen("--git-dir=");
+		start_pathspec++;
+	}
+
+	/* convert arguments to a "pathspec" of interesting files */
+	info.opts.pathspec.strings = &argv[start_pathspec];
+	info.opts.pathspec.count   = argc - start_pathspec;
+	if (!info.opts.pathspec.count)
+		usage();
+	info.opts.ignore_submodules = GIT_SUBMODULE_IGNORE_DIRTY |
+		GIT_DIFF_DISABLE_PATHSPEC_MATCH;
+	info.opts.notify_cb = mark_pathspec_match;
+	info.opts.notify_payload = &info;
+
+	/* create repo and walker */
+	check_lg2(git_repository_open_ext(&repo, repodir, 0, NULL),
+		"Could not open repository", repodir);
+	check_lg2(git_revwalk_new(&walker, repo),
+		"Could not create revision walker", NULL);
+
+	/* start at HEAD and walk backwards through time */
+	git_revwalk_sorting(walker, GIT_SORT_TOPOLOGICAL | GIT_SORT_TIME);
+	check_lg2(git_revwalk_push_head(walker),
+		"Could not find repository HEAD", NULL);
+
+	while (info.opts.pathspec.count > 0 &&
+		   !git_revwalk_next(&info.oid, walker))
+	{
+		git_commit *commit;
+		git_diff *diff;
+
+		git_oid_tostr(info.str, sizeof(info.str), &info.oid);
+
+		check_lg2(git_commit_lookup(&commit, repo, &info.oid),
+			"Failed to look up commit", NULL);
+		check_lg2(git_diff_commit(&diff, commit, &info.opts),
+			"Failed to get diff for commit", NULL);
+
+		/* notification callback will take care of reporting on
+		 * items in the diff and reducing the pathspec count
+		 */
+
+		git_diff_free(diff);
+		git_commit_free(commit);
+	}
+
+	for (i = 0; i < info.opts.pathspec.count; ++i) {
+		const char *path = info.opts.pathspec.strings[i];
+		if (path)
+			printf("never found %s\n", path);
+	}
+
+	git_revwalk_free(walker);
+	git_repository_free(repo);
+	git_threads_shutdown();
+
+	return 0;
+}
+
+static int mark_pathspec_match(
+	const git_diff *diff_so_far,
+	const git_diff_delta *delta_to_add,
+	const char *matched_pathspec,
+	void *payload)
+{
+	change_info *info = payload;
+	git_strarray *paths = &info->opts.pathspec;
+	size_t i;
+	int found = 0;
+
+	(void)diff_so_far; (void)delta_to_add;
+
+	for (i = 0; i < paths->count; ++i) {
+		/* remove matched item from list */
+		if (found)
+			paths->strings[i - 1] = paths->strings[i];
+		else if (!strcmp(paths->strings[i], matched_pathspec))
+			found = 1;
+	}
+
+	if (found) {
+		const char *verb = "modified";
+		if (delta_to_add->status == GIT_DELTA_ADDED)
+			verb = "added";
+		else if (delta_to_add->status == GIT_DELTA_DELETED)
+			verb = "deleted";
+
+		printf("%s has %s %s\n", info->str, verb, matched_pathspec);
+
+		paths->count--;
+	}
+
+	return 0;
+}