[PATCH v3 1/2] Allow git-apply to ignore the hunk headers (AKA recountdiff)

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

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.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 Documentation/git-apply.txt |    7 ++++-
 builtin-apply.c             |   64 ++++++++++++++++++++++++++++++++++++++++---
 2 files changed, 66 insertions(+), 5 deletions(-)

diff --git a/Documentation/git-apply.txt b/Documentation/git-apply.txt
index 2dec2ec..2fa660e 100644
--- a/Documentation/git-apply.txt
+++ b/Documentation/git-apply.txt
@@ -12,7 +12,7 @@ SYNOPSIS
 'git-apply' [--stat] [--numstat] [--summary] [--check] [--index]
 	  [--apply] [--no-add] [--build-fake-ancestor <file>] [-R | --reverse]
 	  [--allow-binary-replacement | --binary] [--reject] [-z]
-	  [-pNUM] [-CNUM] [--inaccurate-eof] [--cached]
+	  [-pNUM] [-CNUM] [--inaccurate-eof] [--recount] [--cached]
 	  [--whitespace=<nowarn|warn|fix|error|error-all>]
 	  [--exclude=PATH] [--verbose] [<patch>...]
 
@@ -169,6 +169,11 @@ behavior:
 	correctly. This option adds support for applying such patches by
 	working around this bug.
 
+--recount::
+	Do not trust the line counts in the hunk headers, but infer them
+	by inspecting the patch (e.g. after editing the patch without
+	adjusting the hunk headers appropriately).
+
 -v, --verbose::
 	Report progress to stderr. By default, only a message about the
 	current patch being applied will be printed. This option will cause
diff --git a/builtin-apply.c b/builtin-apply.c
index c497889..34c220f 100644
--- a/builtin-apply.c
+++ b/builtin-apply.c
@@ -153,6 +153,7 @@ struct patch {
 	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 +883,50 @@ static int parse_range(const char *line, int len, int offset, const char *expect
 	return offset + ex;
 }
 
+static int recount_diff(char *line, int size, struct fragment *fragment)
+{
+	int line_nr = 0;
+
+	if (size < 1)
+		return -1;
+
+	fragment->oldpos = 2;
+	fragment->oldlines = fragment->newlines = 0;
+
+	for (;;) {
+		int len = linelen(line, size);
+		size -= len;
+		line += len;
+
+		if (size < 1)
+			return 0;
+
+		switch (*line) {
+		case ' ': case '\n':
+			fragment->newlines++;
+			/* fall through */
+		case '-':
+			fragment->oldlines++;
+			break;
+		case '+':
+			fragment->newlines++;
+			if (line_nr == 0) {
+				fragment->leading = 1;
+				fragment->oldpos = 1;
+			}
+			fragment->trailing = 1;
+			break;
+		case '@':
+			return size < 3 || prefixcmp(line, "@@ ");
+		case 'd':
+			return size < 5 || prefixcmp(line, "diff ");
+		default:
+			return -1;
+		}
+		line_nr++;
+	}
+}
+
 /*
  * Parse a unified diff fragment header of the
  * form "@@ -a,b +c,d @@"
@@ -1013,6 +1058,9 @@ static int parse_fragment(char *line, unsigned long size,
 	offset = parse_fragment_header(line, len, fragment);
 	if (offset < 0)
 		return -1;
+	if (offset > 0 && patch->recount &&
+			recount_diff(line + offset, size - offset, fragment))
+		return -1;
 	oldlines = fragment->oldlines;
 	newlines = fragment->newlines;
 	leading = 0;
@@ -2912,7 +2960,8 @@ static void prefix_patches(struct patch *p)
 	}
 }
 
-static int apply_patch(int fd, const char *filename, int inaccurate_eof)
+static int apply_patch(int fd, const char *filename, int inaccurate_eof,
+		int recount)
 {
 	size_t offset;
 	struct strbuf buf;
@@ -2929,6 +2978,7 @@ static int apply_patch(int fd, const char *filename, int inaccurate_eof)
 
 		patch = xcalloc(1, sizeof(*patch));
 		patch->inaccurate_eof = inaccurate_eof;
+		patch->recount = recount;
 		nr = parse_chunk(buf.buf + offset, buf.len - offset, patch);
 		if (nr < 0)
 			break;
@@ -2998,6 +3048,7 @@ int cmd_apply(int argc, const char **argv, const char *unused_prefix)
 	int i;
 	int read_stdin = 1;
 	int inaccurate_eof = 0;
+	int recount = 0;
 	int errs = 0;
 	int is_not_gitdir;
 
@@ -3015,7 +3066,8 @@ int cmd_apply(int argc, const char **argv, const char *unused_prefix)
 		int fd;
 
 		if (!strcmp(arg, "-")) {
-			errs |= apply_patch(0, "<stdin>", inaccurate_eof);
+			errs |= apply_patch(0, "<stdin>", inaccurate_eof,
+					recount);
 			read_stdin = 0;
 			continue;
 		}
@@ -3118,6 +3170,10 @@ int cmd_apply(int argc, const char **argv, const char *unused_prefix)
 			inaccurate_eof = 1;
 			continue;
 		}
+		if (!strcmp(arg, "--recount")) {
+			recount = 1;
+			continue;
+		}
 		if (0 < prefix_length)
 			arg = prefix_filename(prefix, prefix_length, arg);
 
@@ -3126,12 +3182,12 @@ int cmd_apply(int argc, const char **argv, const char *unused_prefix)
 			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, inaccurate_eof, recount);
 		close(fd);
 	}
 	set_default_whitespace_mode(whitespace_option);
 	if (read_stdin)
-		errs |= apply_patch(0, "<stdin>", inaccurate_eof);
+		errs |= apply_patch(0, "<stdin>", inaccurate_eof, recount);
 	if (whitespace_error) {
 		if (squelch_whitespace_errors &&
 		    squelch_whitespace_errors < whitespace_error) {
-- 
1.5.6.rc1.181.gb439d