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#!/usr/bin/env perl
#
# Write a script to implement Lempel–Ziv–Welch (LZW) compression algorithm. The
# script should have method to encode/decode algorithm. The wiki page explains
# the compression algorithm very nicely.
#
# (https://en.wikipedia.org/wiki/Lempel%E2%80%93Ziv%E2%80%93Welch).
################################################################################
use strict;
use warnings;
################################################################################
# Naïve implementation of a bidirectional map
my $MAX_BIT_WIDTH = 12;
sub bimap_new {
return {binwidth=>0, sym2bin=>{}, bin2sym=>{}};
}
sub bimap_extend {
my ($dict, $binwidth) = @_;
return unless $binwidth > $dict->{binwidth} && $binwidth <= $MAX_BIT_WIDTH;
my ($sym2bin, $bin2sym) = @$dict{'sym2bin', 'bin2sym'};
foreach my $sym (keys %$sym2bin) {
my $bin = $sym2bin->{$sym};
delete $bin2sym->{$bin};
$bin = "0" x ($binwidth - length $bin) . $bin;
$sym2bin->{$sym} = $bin;
$bin2sym->{$bin} = $sym;
}
$dict->{binwidth} = $binwidth;
}
sub bimap_insert {
my ($dict, $sym) = @_;
my ($sym2bin, $bin2sym) = @$dict{'sym2bin', 'bin2sym'};
return $dict->{sym2bin}{$sym} if exists $dict->{sym2bin}{$sym};
my $ord = keys %{$dict->{sym2bin}}; # Ordinal of symbol to insert in decimal
my $bin = sprintf "%b", $ord; # ... and in binary (string of 1/0s)
return unless length $bin <= $MAX_BIT_WIDTH;
$sym2bin->{$sym} = $bin; # Update symbol -> binary mapping
$bin2sym->{$bin} = $sym; # Update binary -> symbol mapping
# Extend with left zeroes the previously inserted binaries
bimap_extend($dict, length($bin));
return $bin;
}
################################################################################
# Default dictionary
my $DEFAULT_DICT = bimap_new();
my $STOP = '';
bimap_insert($DEFAULT_DICT, $_) foreach (map(chr, 1..254), $STOP);
sub binary_encode {
my $text = shift;
my $binary = pack("B*", $text);
return $binary;
}
sub binary_decode {
my $binary = shift;
my $text = unpack("B*", $binary);
return $text
}
sub lzw_encode {
my $dict = shift;
my $input = shift;
my ($sym2bin, $bin2sym) = @{$dict}{'sym2bin', 'bin2sym'};
my $out = '';
my $w = '';
foreach my $i (0 .. length($input)-1) {
my $char = substr($input, $i, 1);
my $dict_seq = $w . $char;
unless (exists $sym2bin->{$dict_seq}) {
$out .= $sym2bin->{$w};
bimap_insert($dict, $dict_seq);
$w = '';
}
$w .= $char;
}
$out .= $sym2bin->{$w} . $sym2bin->{$STOP};
return binary_encode($out);
}
sub lzw_decode {
my $dict = shift;
my $input = binary_decode(shift());
my ($sym2bin, $bin2sym) = @{$dict}{'sym2bin', 'bin2sym'};
my $out = '';
my $lastsym;
while ($input) {
my $sym;
foreach my $bin (keys %$bin2sym) {
if ($input =~ /^$bin/) {
$sym = $bin2sym->{$bin};
$input = substr($input, $dict->{binwidth});
last;
}
}
unless (defined $sym) {
$sym = $lastsym . substr($lastsym, 0, 1);
$input = substr($input, $dict->{binwidth});
}
$out .= $sym;
if ($sym eq $STOP) {
return $out;
}
return $out if $sym eq $STOP;
if (defined $lastsym) {
my $bin = bimap_insert($dict, $lastsym . $sym);
if ((log($dict->{binwidth}) / log(2)) =~ /^\d+$/) {
bimap_extend($dict, $dict->{binwidth}+1);
}
}
$lastsym = $sym;
}
}
sub usage {
die "Usage: $0 -e | --encode | -d | --decode\n";
}
my $mode = shift || usage;
if ($mode eq '-e' || $mode eq '--encode') {
my $input = join "", <>;
my $out = lzw_encode($DEFAULT_DICT, $input);
my $comprate = 100 * length($out) / length($input);
printf STDERR "Compressed to %.2f%% of original size\n", $comprate;
print $out;
} elsif ($mode eq '-d' || $mode eq '--decode') {
my $input = join "", <>;
my $out = lzw_decode($DEFAULT_DICT, $input);
print $out;
} else {
usage;
}
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