#!/usr/bin/perl

use warnings FATAL => 'all';
use strict;

# Highlight by reversing foreground and background. You could do
# other things like bold or underline if you prefer.
my @OLD_HIGHLIGHT = (
        color_config('color.diff-highlight.oldnormal'),
        color_config('color.diff-highlight.oldhighlight', "\x1b[7m"),
        color_config('color.diff-highlight.oldreset', "\x1b[27m")
);
my @NEW_HIGHLIGHT = (
        color_config('color.diff-highlight.newnormal', $OLD_HIGHLIGHT[0]),
        color_config('color.diff-highlight.newhighlight', $OLD_HIGHLIGHT[1]),
        color_config('color.diff-highlight.newreset', $OLD_HIGHLIGHT[2])
);

my $RESET = "\x1b[m";
my $COLOR = qr/\x1b\[[0-9;]*m/;
my $BORING = qr/$COLOR|\s/;

my @removed;
my @added;
my $in_hunk;

while (<>) {
        if (!$in_hunk) {
                print;
                $in_hunk = /^$COLOR*\@/;
        }
        elsif (/^$COLOR*-/) {
                push @removed, $_;
        }
        elsif (/^$COLOR*\+/) {
                push @added, $_;
        }
        else {
                show_hunk(\@removed, \@added);
                @removed = ();
                @added = ();

                print;
                $in_hunk = /^$COLOR*[\@ ]/;
        }

        # Most of the time there is enough output to keep things streaming,
        # but for something like "git log -Sfoo", you can get one early
        # commit and then many seconds of nothing. We want to show
        # that one commit as soon as possible.
        #
        # Since we can receive arbitrary input, there's no optimal
        # place to flush. Flushing on a blank line is a heuristic that
        # happens to match git-log output.
        if (!length) {
                local $| = 1;
        }
}

# Flush any queued hunk (this can happen when there is no trailing context in
# the final diff of the input).
show_hunk(\@removed, \@added);

exit 0;

# Ideally we would feed the default as a human-readable color to
# git-config as the fallback value. But diff-highlight does
# not otherwise depend on git at all, and there are reports
# of it being used in other settings. Let's handle our own
# fallback, which means we will work even if git can't be run.
sub color_config {
        my ($key, $default) = @_;
        my $s = `git config --get-color $key 2>/dev/null`;
        return length($s) ? $s : $default;
}

sub show_hunk {
        my ($a, $b) = @_;

        # If one side is empty, then there is nothing to compare or highlight.
        if (!@$a || !@$b) {
                print @$a, @$b;
                return;
        }

        # If we have mismatched numbers of lines on each side, we could try to
        # be clever and match up similar lines. But for now we are simple and
        # stupid, and only handle multi-line hunks that remove and add the same
        # number of lines.
        if (@$a != @$b) {
                print @$a, @$b;
                return;
        }

        my @queue;
        for (my $i = 0; $i < @$a; $i++) {
                my ($rm, $add) = highlight_pair($a->[$i], $b->[$i]);
                print $rm;
                push @queue, $add;
        }
        print @queue;
}

sub highlight_pair {
        my @a = split_line(shift);
        my @b = split_line(shift);

        # Find common prefix, taking care to skip any ansi
        # color codes.
        my $seen_plusminus;
        my ($pa, $pb) = (0, 0);
        while ($pa < @a && $pb < @b) {
                if ($a[$pa] =~ /$COLOR/) {
                        $pa++;
                }
                elsif ($b[$pb] =~ /$COLOR/) {
                        $pb++;
                }
                elsif ($a[$pa] eq $b[$pb]) {
                        $pa++;
                        $pb++;
                }
                elsif (!$seen_plusminus && $a[$pa] eq '-' && $b[$pb] eq '+') {
                        $seen_plusminus = 1;
                        $pa++;
                        $pb++;
                }
                else {
                        last;
                }
        }

        # Find common suffix, ignoring colors.
        my ($sa, $sb) = ($#a, $#b);
        while ($sa >= $pa && $sb >= $pb) {
                if ($a[$sa] =~ /$COLOR/) {
                        $sa--;
                }
                elsif ($b[$sb] =~ /$COLOR/) {
                        $sb--;
                }
                elsif ($a[$sa] eq $b[$sb]) {
                        $sa--;
                        $sb--;
                }
                else {
                        last;
                }
        }

        if (is_pair_interesting(\@a, $pa, $sa, \@b, $pb, $sb)) {
                return highlight_line(\@a, $pa, $sa, \@OLD_HIGHLIGHT),
                       highlight_line(\@b, $pb, $sb, \@NEW_HIGHLIGHT);
        }
        else {
                return join('', @a),
                       join('', @b);
        }
}

sub split_line {
        local $_ = shift;
        return map { /$COLOR/ ? $_ : (split //) }
               split /($COLOR*)/;
}

sub highlight_line {
        my ($line, $prefix, $suffix, $theme) = @_;

        my $start = join('', @{$line}[0..($prefix-1)]);
        my $mid = join('', @{$line}[$prefix..$suffix]);
        my $end = join('', @{$line}[($suffix+1)..$#$line]);

        # If we have a "normal" color specified, then take over the whole line.
        # Otherwise, we try to just manipulate the highlighted bits.
        if (defined $theme->[0]) {
                s/$COLOR//g for ($start, $mid, $end);
                chomp $end;
                return join('',
                        $theme->[0], $start, $RESET,
                        $theme->[1], $mid, $RESET,
                        $theme->[0], $end, $RESET,
                        "\n"
                );
        } else {
                return join('',
                        $start,
                        $theme->[1], $mid, $theme->[2],
                        $end
                );
        }
}

# Pairs are interesting to highlight only if we are going to end up
# highlighting a subset (i.e., not the whole line). Otherwise, the highlighting
# is just useless noise. We can detect this by finding either a matching prefix
# or suffix (disregarding boring bits like whitespace and colorization).
sub is_pair_interesting {
        my ($a, $pa, $sa, $b, $pb, $sb) = @_;
        my $prefix_a = join('', @$a[0..($pa-1)]);
        my $prefix_b = join('', @$b[0..($pb-1)]);
        my $suffix_a = join('', @$a[($sa+1)..$#$a]);
        my $suffix_b = join('', @$b[($sb+1)..$#$b]);

        return $prefix_a !~ /^$COLOR*-$BORING*$/ ||
               $prefix_b !~ /^$COLOR*\+$BORING*$/ ||
               $suffix_a !~ /^$BORING*$/ ||
               $suffix_b !~ /^$BORING*$/;
}