#!/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 @removed;
my @added;
my $in_hunk;

# Some scripts may not realize that SIGPIPE is being ignored when launching the
# pager--for instance scripts written in Python.
$SIG{PIPE} = 'DEFAULT';

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;

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),
                       highlight_line(\@b, $pb, $sb);
        }
        else {
                return join('', @a),
                       join('', @b);
        }
}

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

sub highlight_line {
        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*$/;
}