add -p: calculate offset delta for edited patches

Recount the number of preimage and postimage lines in a hunk after it
has been edited so any change in the number of insertions or deletions
can be used to adjust the offsets of subsequent hunks. If an edited
hunk is subsequently split then the offset correction will be lost. It
would be possible to fix this if it is a problem, however the code
here is still an improvement on the status quo for the common case
where an edited hunk is applied without being split.

This is also a necessary step to removing '--recount' and
'--allow-overlap' from the invocation of 'git apply'. Before
'--recount' can be removed the splitting and coalescing counting needs
to be fixed to handle a missing newline at the end of a file. In order
to remove '--allow-overlap' there needs to be i) some way of verifying
the offset data in the edited hunk (probably by correlating the
preimage (or postimage if the patch is going to be applied in reverse)
lines of the edited and unedited versions to see if they are offset or
if any leading/trailing context lines have been removed) and ii) a way of
dealing with edited hunks that change context lines that are shared
with neighbouring hunks.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 50 insertions, 9 deletions

diff --git a/git-add--interactive.perl b/git-add--interactive.perl
index 6f6a21dc11..b5f01160c5 100755
--- a/git-add--interactive.perl
+++ b/git-add--interactive.perl
@@ -938,6 +938,11 @@ sub coalesce_overlapping_hunks {
 						parse_hunk_header($text->[0]);
 		unless ($_->{USE}) {
 			$ofs_delta += $o_cnt - $n_cnt;
+			# If this hunk has been edited then subtract
+			# the delta that is due to the edit.
+			if ($_->{OFS_DELTA}) {
+				$ofs_delta -= $_->{OFS_DELTA};
+			}
 			next;
 		}
 		if ($ofs_delta) {
@@ -945,6 +950,11 @@ sub coalesce_overlapping_hunks {
 			$_->{TEXT}->[0] = format_hunk_header($o_ofs, $o_cnt,
 							     $n_ofs, $n_cnt);
 		}
+		# If this hunk was edited then adjust the offset delta
+		# to reflect the edit.
+		if ($_->{OFS_DELTA}) {
+			$ofs_delta += $_->{OFS_DELTA};
+		}
 		if (defined $last_o_ctx &&
 		    $o_ofs <= $last_o_ctx &&
 		    !$_->{DIRTY} &&
@@ -1016,6 +1026,30 @@ marked for discarding."),
 marked for applying."),
 );
 
+sub recount_edited_hunk {
+	local $_;
+	my ($oldtext, $newtext) = @_;
+	my ($o_cnt, $n_cnt) = (0, 0);
+	for (@{$newtext}[1..$#{$newtext}]) {
+		my $mode = substr($_, 0, 1);
+		if ($mode eq '-') {
+			$o_cnt++;
+		} elsif ($mode eq '+') {
+			$n_cnt++;
+		} elsif ($mode eq ' ') {
+			$o_cnt++;
+			$n_cnt++;
+		}
+	}
+	my ($o_ofs, undef, $n_ofs, undef) =
+					parse_hunk_header($newtext->[0]);
+	$newtext->[0] = format_hunk_header($o_ofs, $o_cnt, $n_ofs, $n_cnt);
+	my (undef, $orig_o_cnt, undef, $orig_n_cnt) =
+					parse_hunk_header($oldtext->[0]);
+	# Return the change in the number of lines inserted by this hunk
+	return $orig_o_cnt - $orig_n_cnt - $o_cnt + $n_cnt;
+}
+
 sub edit_hunk_manually {
 	my ($oldtext) = @_;
 
@@ -1114,25 +1148,32 @@ sub prompt_yesno {
 }
 
 sub edit_hunk_loop {
-	my ($head, $hunk, $ix) = @_;
-	my $text = $hunk->[$ix]->{TEXT};
+	my ($head, $hunks, $ix) = @_;
+	my $hunk = $hunks->[$ix];
+	my $text = $hunk->{TEXT};
 
 	while (1) {
-		$text = edit_hunk_manually($text);
-		if (!defined $text) {
+		my $newtext = edit_hunk_manually($text);
+		if (!defined $newtext) {
 			return undef;
 		}
 		my $newhunk = {
-			TEXT => $text,
-			TYPE => $hunk->[$ix]->{TYPE},
+			TEXT => $newtext,
+			TYPE => $hunk->{TYPE},
 			USE => 1,
 			DIRTY => 1,
 		};
+		$newhunk->{OFS_DELTA} = recount_edited_hunk($text, $newtext);
+		# If this hunk has already been edited then add the
+		# offset delta of the previous edit to get the real
+		# delta from the original unedited hunk.
+		$hunk->{OFS_DELTA} and
+				$newhunk->{OFS_DELTA} += $hunk->{OFS_DELTA};
 		if (diff_applies($head,
-				 @{$hunk}[0..$ix-1],
+				 @{$hunks}[0..$ix-1],
 				 $newhunk,
-				 @{$hunk}[$ix+1..$#{$hunk}])) {
-			$newhunk->{DISPLAY} = [color_diff(@{$text})];
+				 @{$hunks}[$ix+1..$#{$hunks}])) {
+			$newhunk->{DISPLAY} = [color_diff(@{$newtext})];
 			return $newhunk;
 		}
 		else {