#!/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 $HIGHLIGHT   = "\x1b[7m";
my $UNHIGHLIGHT = "\x1b[27m";
my $COLOR = qr/\x1b\[[0-9;]*m/;
my $BORING = qr/$COLOR|\s/;

my @window;

while (<>) {
        # We highlight only single-line changes, so we need
        # a 4-line window to make a decision on whether
        # to highlight.
        push @window, $_;
        next if @window < 4;
        if ($window[0] =~ /^$COLOR*(\@| )/ &&
            $window[1] =~ /^$COLOR*-/ &&
            $window[2] =~ /^$COLOR*\+/ &&
            $window[3] !~ /^$COLOR*\+/) {
                print shift @window;
                show_pair(shift @window, shift @window);
        }
        else {
                print shift @window;
        }

        # 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;
        }
}

# Special case a single-line hunk at the end of file.
if (@window == 3 &&
    $window[0] =~ /^$COLOR*(\@| )/ &&
    $window[1] =~ /^$COLOR*-/ &&
    $window[2] =~ /^$COLOR*\+/) {
        print shift @window;
        show_pair(shift @window, shift @window);
}

# And then flush any remaining lines.
while (@window) {
        print shift @window;
}

exit 0;

sub show_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)) {
                print highlight(\@a, $pa, $sa);
                print highlight(\@b, $pb, $sb);
        }
        else {
                print join('', @a);
                print join('', @b);
        }
}

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

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

        return join('',
                @{$line}[0..($prefix-1)],
                $HIGHLIGHT,
                @{$line}[$prefix..$suffix],
                $UNHIGHLIGHT,
                @{$line}[($suffix+1)..$#$line]
        );
}

# 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*$/;
}