Allow git-apply to recount the lines in a hunk (AKA recountdiff)

Sometimes, the easiest way to fix up a patch is to edit it directly, even
adding or deleting lines.  Now, many people are not as divine as certain
benevolent dictators as to update the hunk headers correctly at the first
try.

So teach the tool to do it for us.

[jc: with tests]

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin-apply.c b/builtin-apply.c
index c497889..64cf8af 100644
--- a/builtin-apply.c
+++ b/builtin-apply.c
@@ -153,6 +153,7 @@
 	unsigned int is_binary:1;
 	unsigned int is_copy:1;
 	unsigned int is_rename:1;
+	unsigned int recount:1;
 	struct fragment *fragments;
 	char *result;
 	size_t resultsize;
@@ -882,6 +883,56 @@
 	return offset + ex;
 }
 
+static void recount_diff(char *line, int size, struct fragment *fragment)
+{
+	int oldlines = 0, newlines = 0, ret = 0;
+
+	if (size < 1) {
+		warning("recount: ignore empty hunk");
+		return;
+	}
+
+	for (;;) {
+		int len = linelen(line, size);
+		size -= len;
+		line += len;
+
+		if (size < 1)
+			break;
+
+		switch (*line) {
+		case ' ': case '\n':
+			newlines++;
+			/* fall through */
+		case '-':
+			oldlines++;
+			continue;
+		case '+':
+			newlines++;
+			continue;
+		case '\\':
+			break;
+		case '@':
+			ret = size < 3 || prefixcmp(line, "@@ ");
+			break;
+		case 'd':
+			ret = size < 5 || prefixcmp(line, "diff ");
+			break;
+		default:
+			ret = -1;
+			break;
+		}
+		if (ret) {
+			warning("recount: unexpected line: %.*s",
+				(int)linelen(line, size), line);
+			return;
+		}
+		break;
+	}
+	fragment->oldlines = oldlines;
+	fragment->newlines = newlines;
+}
+
 /*
  * Parse a unified diff fragment header of the
  * form "@@ -a,b +c,d @@"
@@ -1013,6 +1064,8 @@
 	offset = parse_fragment_header(line, len, fragment);
 	if (offset < 0)
 		return -1;
+	if (offset > 0 && patch->recount)
+		recount_diff(line + offset, size - offset, fragment);
 	oldlines = fragment->oldlines;
 	newlines = fragment->newlines;
 	leading = 0;
@@ -2912,7 +2965,10 @@
 	}
 }
 
-static int apply_patch(int fd, const char *filename, int inaccurate_eof)
+#define INACCURATE_EOF	(1<<0)
+#define RECOUNT		(1<<1)
+
+static int apply_patch(int fd, const char *filename, int options)
 {
 	size_t offset;
 	struct strbuf buf;
@@ -2928,7 +2984,8 @@
 		int nr;
 
 		patch = xcalloc(1, sizeof(*patch));
-		patch->inaccurate_eof = inaccurate_eof;
+		patch->inaccurate_eof = !!(options & INACCURATE_EOF);
+		patch->recount =  !!(options & RECOUNT);
 		nr = parse_chunk(buf.buf + offset, buf.len - offset, patch);
 		if (nr < 0)
 			break;
@@ -2997,7 +3054,7 @@
 {
 	int i;
 	int read_stdin = 1;
-	int inaccurate_eof = 0;
+	int options = 0;
 	int errs = 0;
 	int is_not_gitdir;
 
@@ -3015,7 +3072,7 @@
 		int fd;
 
 		if (!strcmp(arg, "-")) {
-			errs |= apply_patch(0, "<stdin>", inaccurate_eof);
+			errs |= apply_patch(0, "<stdin>", options);
 			read_stdin = 0;
 			continue;
 		}
@@ -3115,7 +3172,11 @@
 			continue;
 		}
 		if (!strcmp(arg, "--inaccurate-eof")) {
-			inaccurate_eof = 1;
+			options |= INACCURATE_EOF;
+			continue;
+		}
+		if (!strcmp(arg, "--recount")) {
+			options |= RECOUNT;
 			continue;
 		}
 		if (0 < prefix_length)
@@ -3126,12 +3187,12 @@
 			die("can't open patch '%s': %s", arg, strerror(errno));
 		read_stdin = 0;
 		set_default_whitespace_mode(whitespace_option);
-		errs |= apply_patch(fd, arg, inaccurate_eof);
+		errs |= apply_patch(fd, arg, options);
 		close(fd);
 	}
 	set_default_whitespace_mode(whitespace_option);
 	if (read_stdin)
-		errs |= apply_patch(0, "<stdin>", inaccurate_eof);
+		errs |= apply_patch(0, "<stdin>", options);
 	if (whitespace_error) {
 		if (squelch_whitespace_errors &&
 		    squelch_whitespace_errors < whitespace_error) {