commit: faster parsing

The current code issues a lot of strncmp() calls in order to check for
the end of the header, simply in order to copy it and start going
through it again. These are a lot of calls for something we can check as
we go along. Knowing the amount of parents beforehand to reduce
allocations in extreme cases does not make up for them.

Instead start parsing immediately and check for the double-newline after
each header field, leaving the raw_header allocation for the end, which
lets us go through the header once and reduces the amount of strncmp()
calls significantly.

In unscientific testing, this has reduced a shortlog-like usage (walking
though the whole history of a branch and extracting data from the
commits) of git.git from ~830ms to ~700ms and makes the time we spend in
strncmp() negligible.

src/commit.c

@@ -164,33 +164,15 @@ int git_commit__parse(void *_commit, git_odb_object *odb_obj)
 	const char *buffer_start = git_odb_object_data(odb_obj), *buffer;
 	const char *buffer_end = buffer_start + git_odb_object_size(odb_obj);
 	git_oid parent_id;
-	uint32_t parent_count = 0;
 	size_t header_len;
 
-	/* find end-of-header (counting parents as we go) */
-	for (buffer = buffer_start; buffer < buffer_end; ++buffer) {
-		if (!strncmp("\n\n", buffer, 2)) {
-			++buffer;
-			break;
-		}
-		if (!strncmp("\nparent ", buffer, strlen("\nparent ")))
-			++parent_count;
-	}
-
-	header_len = buffer - buffer_start;
-	commit->raw_header = git__strndup(buffer_start, header_len);
-	GITERR_CHECK_ALLOC(commit->raw_header);
+	buffer = buffer_start;
 
-	/* point "buffer" to header data */
-	buffer = commit->raw_header;
-	buffer_end = commit->raw_header + header_len;
-
-	if (parent_count < 1)
-		parent_count = 1;
-
-	git_array_init_to_size(commit->parent_ids, parent_count);
+	/* Allocate for one, which will allow not to realloc 90% of the time  */
+	git_array_init_to_size(commit->parent_ids, 1);
 	GITERR_CHECK_ARRAY(commit->parent_ids);
 
+	/* The tree is always the first field */
 	if (git_oid__parse(&commit->tree_id, &buffer, buffer_end, "tree ") < 0)
 		goto bad_buffer;
 
@@ -221,6 +203,9 @@ int git_commit__parse(void *_commit, git_odb_object *odb_obj)
 	/* Parse add'l header entries */
 	while (buffer < buffer_end) {
 		const char *eoln = buffer;
+		if (buffer[-1] == '\n' && buffer[0] == '\n')
+			break;
+
 		while (eoln < buffer_end && *eoln != '\n')
 			++eoln;
 
@@ -236,13 +221,12 @@ int git_commit__parse(void *_commit, git_odb_object *odb_obj)
 		buffer = eoln;
 	}
 
-	/* point "buffer" to data after header */
-	buffer = git_odb_object_data(odb_obj);
-	buffer_end = buffer + git_odb_object_size(odb_obj);
+	header_len = buffer - buffer_start;
+	commit->raw_header = git__strndup(buffer_start, header_len);
+	GITERR_CHECK_ALLOC(commit->raw_header);
 
-	buffer += header_len;
-	if (*buffer == '\n')
-		++buffer;
+	/* point "buffer" to data after header, +1 for the final LF */
+	buffer = buffer_start + header_len + 1;
 
 	/* extract commit message */
 	if (buffer <= buffer_end) {