[PoC -- do not apply 2/3] test-tree-bitmap: add "dump" mode

[Jeff King <peff@xxxxxxxx>]

This teaches "gen" mode (formerly the only mode) to include
the list of paths, and to prefix each bitmap with its
matching oid.

The "dump" mode can then read that back in and generate the
list of changed paths. This should be almost identical to:

  git rev-list --all |
  git diff-tree --stdin --name-only -t

The one difference is the sort order: git's diff output is
in tree-sort order, so a subtree "foo" sorts like "foo/",
which is after "foo.bar". Whereas the bitmap path list has a
true byte sort, which puts "foo.bar" after "foo".

Signed-off-by: Jeff King <peff@xxxxxxxx>
---
 t/helper/test-tree-bitmap.c | 104 +++++++++++++++++++++++++++++++++++-
 1 file changed, 102 insertions(+), 2 deletions(-)

diff --git a/t/helper/test-tree-bitmap.c b/t/helper/test-tree-bitmap.c
index bc5cf0e514..6f8833344a 100644
--- a/t/helper/test-tree-bitmap.c
+++ b/t/helper/test-tree-bitmap.c
@@ -112,6 +112,14 @@ static void collect_paths(struct hashmap *paths)
 	QSORT(sorted, i, pathmap_entry_strcmp);
 	for (i = 0; i < n; i++)
 		sorted[i]->pos = i;
+
+	/* dump it while we have the sorted order in memory */
+	for (i = 0; i < n; i++) {
+		printf("%s", sorted[i]->path);
+		putchar('\0');
+	}
+	putchar('\0');
+
 	free(sorted);
 }
 
@@ -142,6 +150,8 @@ static void generate_bitmap(struct diff_queue_struct *q,
 
 	ewah = bitmap_to_ewah(bitmap);
 	ewah_serialize_strbuf(ewah, &out);
+
+	fwrite(data->commit->object.oid.hash, 1, GIT_SHA1_RAWSZ, stdout);
 	fwrite(out.buf, 1, out.len, stdout);
 
 	trace_printf("bitmap %s %u %u",
@@ -154,14 +164,104 @@ static void generate_bitmap(struct diff_queue_struct *q,
 	bitmap_free(bitmap);
 }
 
-int cmd_main(int argc, const char **argv)
+static void do_gen(void)
 {
 	struct hashmap paths;
-
 	setup_git_directory();
 	collect_paths(&paths);
 
 	walk_paths(generate_bitmap, &paths);
+}
+
+static void show_path(size_t pos, void *data)
+{
+	const char **paths = data;
+
+	/* assert(pos < nr_paths), but we didn't pass the latter in */
+	printf("%s\n", paths[pos]);
+}
+
+static void do_dump(void)
+{
+	struct strbuf in = STRBUF_INIT;
+	const char *cur;
+	size_t remain;
+
+	const char **paths = NULL;
+	size_t alloc_paths = 0, nr_paths = 0;
+
+	/* slurp stdin; in the real world we'd mmap all this */
+	strbuf_read(&in, 0, 0);
+	cur = in.buf;
+	remain = in.len;
+
+	/* read path for each bit; in the real world this would be separate */
+	while (remain) {
+		const char *end = memchr(cur, '\0', remain);
+		if (!end) {
+			error("truncated input while reading path");
+			goto out;
+		}
+		if (end == cur) {
+			/* empty field signals end of paths */
+			cur++;
+			remain--;
+			break;
+		}
+
+		ALLOC_GROW(paths, nr_paths + 1, alloc_paths);
+		paths[nr_paths++] = cur;
+
+		remain -= end - cur + 1;
+		cur = end + 1;
+	}
+
+	/* read the bitmap for each commit */
+	while (remain) {
+		struct object_id oid;
+		struct ewah_bitmap *ewah;
+		ssize_t len;
+
+		if (remain < GIT_SHA1_RAWSZ) {
+			error("truncated input reading oid");
+			goto out;
+		}
+		hashcpy(oid.hash, (const unsigned char *)cur);
+		cur += GIT_SHA1_RAWSZ;
+		remain -= GIT_SHA1_RAWSZ;
+
+		ewah = ewah_new();
+		len = ewah_read_mmap(ewah, cur, remain);
+		if (len < 0) {
+			ewah_free(ewah);
+			goto out;
+		}
+
+		printf("%s\n", oid_to_hex(&oid));
+		ewah_each_bit(ewah, show_path, paths);
+
+		ewah_free(ewah);
+		cur += len;
+		remain -= len;
+	}
+
+out:
+	free(paths);
+	strbuf_release(&in);
+}
+
+int cmd_main(int argc, const char **argv)
+{
+	const char *usage_msg = "test-tree-bitmap <gen|dump>";
+
+	if (!argv[1])
+		usage(usage_msg);
+	else if (!strcmp(argv[1], "gen"))
+		do_gen();
+	else if (!strcmp(argv[1], "dump"))
+		do_dump();
+	else
+		usage(usage_msg);
 
 	return 0;
 }
-- 
2.19.1.550.g7610f1eecb