#!/usr/bin/perl
#

use strict;
BEGIN {
    $PaletteModify::HAVE_CRC = eval "use String::CRC32 (); 1;";
}

package PaletteModify;

sub common_alter
{
    my ($palref, $table) = @_;
    my $length = length $table;

    my $pal_size = $length / 3;

    # tinting image?  if so, we're remaking the whole palette
    if (my $tint = $palref->{'tint'}) {
        my $dark = $palref->{'tint_dark'};
        my $diff = [ map { $tint->[$_] - $dark->[$_] } (0..2) ];
        $palref = {};
        for (my $idx=0; $idx<$pal_size; $idx++) {
            for my $c (0..2) {
                my $curr = ord(substr($table, $idx*3+$c));
                my $p = \$palref->{$idx}->[$c];
                $$p = int($dark->[$c] + $diff->[$c] * $curr / 255);
            }
        }
    }

    while (my ($idx, $c) = each %$palref) {
        next if $idx >= $pal_size;
        substr($table, $idx*3+$_, 1) = chr($c->[$_]) for (0..2);
    }

    return $table;
}

sub new_gif_palette
{
    my ($fh, $palref) = @_;
    my $header;

    # 13 bytes for magic + image info (size, color depth, etc)
    # and then the global palette table (3*256)
    read($fh, $header, 13+3*256);

    # figure out how big global color table is (don't want to overwrite it)
    my $pf = ord substr($header, 10, 1);
    my $gct = 2 ** (($pf & 7) + 1);  # last 3 bits of packaged fields
    
    substr($header, 13, 3*$gct) = common_alter($palref, substr($header, 13, 3*$gct));
    return $header;
}

sub new_png_palette
{
    my ($fh, $palref) = @_;
    
    # without this module, we can't proceed.
    return undef unless $PaletteModify::HAVE_CRC;

    my $imgdata;

    # Validate PNG signature
    my $png_sig = pack("H16", "89504E470D0A1A0A");
    my $sig;
    read($fh, $sig, 8);
    return undef unless $sig eq $png_sig;
    $imgdata .= $sig;

    # Start reading in chunks
    my ($length, $type) = (0, '');
    while (read($fh, $length, 4)) {

        $imgdata .= $length;
        $length = unpack("N", $length);
        return undef unless read($fh, $type, 4) == 4;
        $imgdata .= $type;

        if ($type eq 'IHDR') {
            my $header;
            read($fh, $header, $length+4);
            my ($width,$height,$depth,$color,$compression,
                $filter,$interlace, $CRC)
                = unpack("NNCCCCCN", $header);
            return undef unless $color == 3; # unpaletted image
            $imgdata .= $header;
        } elsif ($type eq 'PLTE') {
            # Finally, we can go to work
            my $palettedata;
            read($fh, $palettedata, $length);
            $palettedata = common_alter($palref, $palettedata);
            $imgdata .= $palettedata;
            
            # Skip old CRC
            my $skip;
            read($fh, $skip, 4);
            
            # Generate new CRC
            my $crc = String::CRC32::crc32($type . $palettedata);
            $crc = pack("N", $crc);
            
            $imgdata .= $crc;
            return $imgdata;
        } else {
            my $skip;
            # Skip rest of chunk and add to imgdata
            # Number of bytes is +4 becauses of CRC
            # 
            for (my $count=0; $count < $length + 4; $count++) {
                read($fh, $skip, 1);
                $imgdata .= $skip;
            }		    
        }
    }

    return undef;
}

1;