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ppm_encoding_dynamic.pl
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ppm_encoding_dynamic.pl
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#!/usr/bin/perl
# Author: Trizen
# Date: 10 August 2023
# https://github.com/trizen
# Implementation of a PPM (prediction by partial-matching) encoder, using Huffman Coding.
# This variant dynamically increments the context-length, based on the input data, in order to reduce the number of escape symbols being generated.
# See also:
# https://rosettacode.org/wiki/huffman_coding
# Reference:
# Data Compression (Summer 2023) - Lecture 16 - Adaptive Methods
# https://youtube.com/watch?v=YKv-w8bXi9c
use 5.036;
use List::Util qw(max uniq);
use constant {
ESCAPE_SYMBOL => 256, # escape symbol
CONTEXTS_NUM => 3, # maximum number of contexts
INITIAL_CONTEXT => 1, # start in this context
VERBOSE => 0, # verbose/debug mode
};
# produce encode and decode dictionary from a tree
sub walk ($node, $code, $h, $rev_h) {
my $c = $node->[0] // return ($h, $rev_h);
if (ref $c) { walk($c->[$_], $code . $_, $h, $rev_h) for ('0', '1') }
else { $h->{$c} = $code; $rev_h->{$code} = $c }
return ($h, $rev_h);
}
# make a tree, and return resulting dictionaries
sub mktree_from_freq ($freq) {
my @nodes = map { [$_, $freq->{$_}] } sort { $a <=> $b } keys %$freq;
do { # poor man's priority queue
@nodes = sort { $a->[1] <=> $b->[1] } @nodes;
my ($x, $y) = splice(@nodes, 0, 2);
if (defined($x)) {
if (defined($y)) {
push @nodes, [[$x, $y], $x->[1] + $y->[1]];
}
else {
push @nodes, [[$x], $x->[1]];
}
}
} while (@nodes > 1);
walk($nodes[0], '', {}, {});
}
sub freq ($arr) {
my %freq;
++$freq{$_} for @$arr;
return \%freq;
}
sub encode ($symbols, $alphabet) {
my @enc;
my @prev;
my $s = join(' ', @prev);
my @ctx = ({$s => {freq => freq($alphabet)}},);
foreach my $i (1 .. CONTEXTS_NUM) {
push @ctx, {$s => {freq => freq([ESCAPE_SYMBOL])}};
}
foreach my $c (@ctx) {
$c->{$s}{tree} = (mktree_from_freq($c->{$s}{freq}))[0];
}
my $prev_ctx = INITIAL_CONTEXT;
foreach my $symbol (@$symbols) {
foreach my $k (reverse(0 .. $prev_ctx)) {
$s = join(' ', @prev[max($#prev - $k + 2, 0) .. $#prev]);
if (!exists($ctx[$k]{$s})) {
$ctx[$k]{$s}{freq} = freq([ESCAPE_SYMBOL]);
}
if (exists($ctx[$k]{$s}{freq}{$symbol})) {
if ($k != 0) {
$ctx[$k]{$s}{tree} = (mktree_from_freq($ctx[$k]{$s}{freq}))[0];
++$ctx[$k]{$s}{freq}{$symbol};
}
say STDERR "Encoding $symbol with context=$k using $ctx[$k]{$s}{tree}{$symbol} and prefix ($s)" if VERBOSE;
push @enc, $ctx[$k]{$s}{tree}{$symbol};
++$prev_ctx if ($prev_ctx < $#ctx);
push @prev, $symbol;
shift(@prev) if (scalar(@prev) >= CONTEXTS_NUM);
last;
}
--$prev_ctx;
$ctx[$k]{$s}{tree} = (mktree_from_freq($ctx[$k]{$s}{freq}))[0];
push @enc, $ctx[$k]{$s}{tree}{(ESCAPE_SYMBOL)};
say STDERR "Escaping from context = $k with $ctx[$k]{$s}{tree}{(ESCAPE_SYMBOL)}" if VERBOSE;
$ctx[$k]{$s}{freq}{$symbol} = 1;
}
}
return join('', @enc);
}
sub decode ($enc, $alphabet) {
my @out;
my @prev;
my $prefix = '';
my $s = join(' ', @prev);
my @ctx = ({$s => {freq => freq($alphabet)}},);
foreach my $i (1 .. CONTEXTS_NUM) {
push @ctx, {$s => {freq => freq([ESCAPE_SYMBOL])}},;
}
foreach my $c (@ctx) {
$c->{$s}{tree} = (mktree_from_freq($c->{$s}{freq}))[1];
}
my $prev_ctx = my $context = INITIAL_CONTEXT;
my @key = @prev;
foreach my $bit (split(//, $enc)) {
$prefix .= $bit;
if (!exists($ctx[$context]{$s})) {
$ctx[$context]{$s}{freq} = freq([ESCAPE_SYMBOL]);
$ctx[$context]{$s}{tree} = (mktree_from_freq($ctx[$context]{$s}{freq}))[1];
}
if (exists($ctx[$context]{$s}{tree}{$prefix})) {
my $symbol = $ctx[$context]{$s}{tree}{$prefix};
if ($symbol == ESCAPE_SYMBOL) {
--$context;
shift(@key) if (scalar(@key) >= $context);
$s = join(' ', @key);
}
else {
push @out, $symbol;
foreach my $k (max($context, 1) .. $prev_ctx) {
my $s = join(' ', @prev[max($#prev - $k + 2, 0) .. $#prev]);
$ctx[$k]{$s}{freq} //= freq([ESCAPE_SYMBOL]);
++$ctx[$k]{$s}{freq}{$symbol};
$ctx[$k]{$s}{tree} = (mktree_from_freq($ctx[$k]{$s}{freq}))[1];
}
++$context if ($context < $#ctx);
$prev_ctx = $context;
push @prev, $symbol;
shift(@prev) if (scalar(@prev) >= CONTEXTS_NUM);
@key = @prev[max($#prev - $context + 2, 0) .. $#prev];
$s = join(' ', @key);
}
$prefix = '';
}
}
return \@out;
}
my $text = "A SAD DAD; A SAD SALSA";
##my $text = "this is an example for huffman encoding";
my @bytes = unpack('C*', $text);
my $enc = encode(\@bytes, [uniq(@bytes)]);
my $dec = decode($enc, [uniq(@bytes)]);
say $enc;
say pack('C*', @$dec);
printf("Saved: %.3f%%\n", ((@$dec - length($enc) / 8) / @$dec * 100));
pack('C*', @$dec) eq $text or die "Decoding failed!";