LZW (Lempel–Ziv–Welch) Data Compression - JS & PHP

Lzw.js

/*!
 * @link https://en.wikipedia.org/wiki/Lempel%E2%80%93Ziv%E2%80%93Welch
 * @license MIT
 */
(function(win) {
    function convert_compat_string(e) {
        if (e === undefined || e === null || typeof e === 'boolean') {
            e = e ? '1' : '';
        }
        return String(e);
    }
    win.lzw = {
        compress: function (e) {
            e = convert_compat_string(e);
            if (e.length < 2) {
                return e;
            }
            e = e.split('');
            var currChar,
                dict = {},
                out = '',
                phrase = e[0],
                code = 256,
                i = 1;
            for (; i < e.length; i++) {
                currChar = e[i];
                if (dict[phrase + currChar] !== undefined) {
                    phrase += currChar;
                } else {
                    out += (phrase.length > 1 ? String.fromCharCode(dict[phrase]) : phrase[0]);
                    dict[phrase + currChar] = code;
                    code++;
                    phrase = currChar;
                }
            }
            e = null;
            dict = dict[phrase];
            out += (dict !== undefined && phrase.length > 1 ? String.fromCharCode(dict) : phrase);
            return out;
        },
        decompress: function (e) {
            e = convert_compat_string(e);
            if (e.length < 2) {
                return e;
            }
            var currCode,
                dict = {},
                currChar = e[0],
                oldPhrase = currChar,
                out = currChar,
                code = 256,
                phrase,
                i = 1;
            for (; i < e.length; i++) {
                currCode = e[i].charCodeAt(0);
                if (currCode < 256) {
                    phrase = e[i];
                } else {
                    phrase = dict[currCode] ? dict[currCode] : (oldPhrase + currChar);
                }
                out += phrase;
                currChar = phrase.charAt(0);
                dict[code] = oldPhrase + currChar;
                code++;
                oldPhrase = phrase;
            }

            e = dict = null;
            return out;
        }
    };
})(self);

Lzw.php

<?php
declare(strict_types=1);

namespace ArrayIterator\App\Data\Compression;

/**
 * Class Lzw
 * @package ArrayIterator\App\Data\Compression
 * @link https://en.wikipedia.org/wiki/Lempel%E2%80%93Ziv%E2%80%93Welch
 * @license MIT
 */
class Lzw
{
    /**
     * @param string $uncompressed
     * @return string
     */
    public function compress(string $uncompressed) : string
    {
        $length = $this->strLen($uncompressed);
        if ($length < 2) {
            return $uncompressed;
        }

        $currChar = null;
        $dict = [];
        $out = '';
        $phrase = $this->substr($uncompressed, 0, 1);
        $code = 256;
        $i = 1;
        for (; $i < $length; $i++) {
            $currChar = $this->substr($uncompressed, $i, 1);
            if (isset($dict[$phrase . $currChar])) {
                $phrase .= $currChar;
            } else {
                $out .= ($this->strLen($phrase) > 1 ? $this->chr($dict[$phrase]) : $this->substr($phrase, 0, 1));
                $dict[$phrase . $currChar] = $code;
                $code++;
                $phrase = $currChar;
            }
        }

        $uncompressed = null;
        $dict = $dict[$phrase]??null;
        $out .= ($dict !== null && $this->strLen($phrase) > 1 ? $this->chr($dict) : $phrase);
        return $out;
    }

    /**
     * @param string $compressed
     * @return string
     */
    public function decompress(string $compressed) : string
    {
        $length = $this->strLen($compressed);
        if ($length < 2) {
            return $compressed;
        }
        $currCode = null;
        $dict = [];
        $currChar = $this->substr($compressed, 0, 1);
        $oldPhrase = $currChar;
        $out = $currChar;
        $code = 256;
        $phrase = null;
        $i = 1;
        for (; $i < $length; $i++) {
            $current = $this->substr($compressed, $i, 1);
            $currCode = $this->ord($current);
            if ($currCode < 256) {
                $phrase = $current;
            } else {
                $phrase = isset($dict[$currCode]) ? $dict[$currCode] : ($oldPhrase . $currChar);
            }

            $out .= $phrase;
            $currChar = $this->substr($phrase, 0, 1);
            $dict[$code] = $oldPhrase . $currChar;
            $code++;
            $oldPhrase = $phrase;
        }

        return $out;
    }

    /*!
     | MB STRING
     ---------------------------------------------------------------- */

    protected function substr(string $string, $start, ?int $length = null)
    {
        return mb_substr($string, $start, $length, 'UTF-8');
    }

    protected function strLen(string $string)
    {
        return mb_strlen($string, 'UTF-8');
    }

    protected function ord(string $string)
    {
        return mb_ord($string, 'UTF-8');
    }

    protected function chr(int $code)
    {
        return mb_chr($code, 'UTF-8');
    }
}