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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'};
die "Symbol '$sym' already in dict" 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 dict_print {
my $bin2sym = shift()->{bin2sym};
foreach my $bin (sort keys %$bin2sym) {
printf "%s -> %s\n", $bin2sym->{$bin}, $bin;
}
}
sub lzw_encode {
my $input = shift;
my %dict = %$DEFAULT_DICT;
my ($sym2bin, $bin2sym) = @dict{'sym2bin', 'bin2sym'};
my $out = '';
while ($input) {
# Find largest match in dictionary
foreach my $sym (sort { length $b cmp length $a } keys %$sym2bin) {
if (rindex($input, $sym, 0) == 0) {
# Attach the binary corresponding to the matching symbol
$out .= $sym2bin->{$sym};
# ... and cut the symbol from the input text
$input = substr($input, length $sym);
if ($input) {
# New dict entry ($sym + next character)
bimap_insert(\%dict, $sym . substr($input, 0, 1));
} else {
# No next character: insert EOM marker and we're finished
$out .= $sym2bin->{$STOP};
}
last;
}
}
}
return $out;
}
sub lzw_decode {
my $input = shift;
my %dict = %$DEFAULT_DICT;
my ($sym2bin, $bin2sym) = @dict{'sym2bin', 'bin2sym'};
my $out = '';
my $lastsym;
while ($input) {
foreach my $bin (keys %$bin2sym) {
if ($input =~ /^$bin/) {
my $sym = $bin2sym->{$bin};
$out .= $sym;
$input = substr($input, length $bin);
last if $sym eq $STOP;
if (defined $lastsym) {
my $bin = bimap_insert(\%dict, $lastsym . substr($sym, 0, 1));
bimap_extend(\%dict, $dict{binwidth}+1)
if $bin =~ /^1+$/;
}
$lastsym = $sym;
last;
}
}
}
return $out;
}
sub usage {
die "Usage: $0 -e | --encode | -d | --decode\n";
}
my $mode = shift || usage;
if ($mode eq '-e' || $mode eq '--encode') {
my $input = <>;
my $out = lzw_encode($input);
my $comprate = 100 * length($out) / (8 * length($input));
printf STDERR "Compressed to %.2f%% of original size\n", $comprate;
print $out;
} elsif ($mode eq '-d' || $mode eq '--decode') {
my $input = <>;
my $out = lzw_decode($input);
print $out;
} else {
usage;
}
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