iterator: provide git_iterator_walk

Provide `git_iterator_walk` to walk each iterator in lockstep,
returning each iterator's idea of the contents of the next path.

1 files changed, 88 insertions, 0 deletions

diff --git a/src/iterator.c b/src/iterator.c
index d5f7eec34..45eba3955 100644
--- a/src/iterator.c
+++ b/src/iterator.c
@@ -1843,3 +1843,91 @@ int git_iterator_advance_over_with_status(
 	return error;
 }
 
+int git_iterator_walk(
+	git_iterator **iterators,
+	size_t cnt,
+	git_iterator_walk_cb cb,
+	void *data)
+{
+	const git_index_entry **iterator_item;	/* next in each iterator */
+	const git_index_entry **cur_items;		/* current path in each iter */
+	const git_index_entry *first_match;
+	int cur_item_modified;
+	size_t i, j;
+	int error = 0;
+
+	iterator_item = git__calloc(cnt, sizeof(git_index_entry *));
+	cur_items = git__calloc(cnt, sizeof(git_index_entry *));
+
+	GITERR_CHECK_ALLOC(iterator_item);
+	GITERR_CHECK_ALLOC(cur_items);
+
+	/* Set up the iterators */
+	for (i = 0; i < cnt; i++) {
+		error = git_iterator_current(&iterator_item[i], iterators[i]);
+
+		if (error < 0 && error != GIT_ITEROVER)
+			goto done;
+	}
+
+	while (true) {
+		for (i = 0; i < cnt; i++)
+			cur_items[i] = NULL;
+
+		first_match = NULL;
+		cur_item_modified = 0;
+
+		/* Find the next path(s) to consume from each iterator */
+		for (i = 0; i < cnt; i++) {
+			if (iterator_item[i] == NULL)
+				continue;
+
+			if (first_match == NULL) {
+				first_match = iterator_item[i];
+				cur_items[i] = iterator_item[i];
+			} else {
+				int path_diff = git_index_entry_cmp(iterator_item[i], first_match);
+
+				if (path_diff < 0) {
+					/* Found an index entry that sorts before the one we're
+					 * looking at.  Forget that we've seen the other and
+					 * look at the other iterators for this path.
+					 */
+					for (j = 0; j < i; j++)
+						cur_items[j] = NULL;
+
+					first_match = iterator_item[i];
+					cur_items[i] = iterator_item[i];
+				} else if (path_diff > 0) {
+					/* No entry for the current item, this is modified */
+					cur_item_modified = 1;
+				} else if (path_diff == 0) {
+					cur_items[i] = iterator_item[i];
+				}
+			}
+		}
+
+		if (first_match == NULL)
+			break;
+
+		if ((error = cb(cur_items, data)) != 0)
+			goto done;
+
+		/* Advance each iterator that participated */
+		for (i = 0; i < cnt; i++) {
+			if (cur_items[i] == NULL)
+				continue;
+
+			error = git_iterator_advance(&iterator_item[i], iterators[i]);
+
+			if (error < 0 && error != GIT_ITEROVER)
+				goto done;
+		}
+	}
+
+done:
+	if (error == GIT_ITEROVER)
+		error = 0;
+
+	return error;
+}