PHP LZW压缩算法

PHP实现的LZW压缩算法

压缩算法

<?php
/** 
* @link http://code.google.com/p/php-lzw/
* @author Jakub Vrana, http://php.vrana.cz/
* @copyright 2009 Jakub Vrana
* @license http://www.apache.org/licenses/LICENSE-2.0 Apache License, Version 2.0
*/

/** LZW compression
* @param string data to compress
* @return string binary data
*/
function lzw_compress($string) {
    // compression
    $dictionary = array_flip(range("\0", "\xFF"));
    $word = "";
    $codes = array();
    for ($i=0; $i <= strlen($string); $i++) {
        $x = $string[$i];
        if (strlen($x) && isset($dictionary[$word . $x])) {
            $word .= $x;
        } elseif ($i) {
            $codes[] = $dictionary[$word];
            $dictionary[$word . $x] = count($dictionary);
            $word = $x;
        }
    }

    // convert codes to binary string
    $dictionary_count = 256;
    $bits = 8; // ceil(log($dictionary_count, 2))
    $return = "";
    $rest = 0;
    $rest_length = 0;
    foreach ($codes as $code) {
        $rest = ($rest << $bits) + $code;
        $rest_length += $bits;
        $dictionary_count++;
        if ($dictionary_count > (1 << $bits)) {
            $bits++;
        }
        while ($rest_length > 7) {
            $rest_length -= 8;
            $return .= chr($rest >> $rest_length);
            $rest &= (1 << $rest_length) - 1;
        }
    }
    return $return . ($rest_length ? chr($rest << (8 - $rest_length)) : "");
}

/** LZW decompression
* @param string compressed binary data
* @return string original data
*/
function lzw_decompress($binary) {
    // convert binary string to codes
    $dictionary_count = 256;
    $bits = 8; // ceil(log($dictionary_count, 2))
    $codes = array();
    $rest = 0;
    $rest_length = 0;
    for ($i=0; $i < strlen($binary); $i++) {
        $rest = ($rest << 8) + ord($binary[$i]);
        $rest_length += 8;
        if ($rest_length >= $bits) {
            $rest_length -= $bits;
            $codes[] = $rest >> $rest_length;
            $rest &= (1 << $rest_length) - 1;
            $dictionary_count++;
            if ($dictionary_count > (1 << $bits)) {
                $bits++;
            }
        }
    }

    // decompression
    $dictionary = range("\0", "\xFF");
    $return = "";
    foreach ($codes as $i => $code) {
        $element = $dictionary[$code];
        if (!isset($element)) {
            $element = $word . $word[0];
        }
        $return .= $element;
        if ($i) {
            $dictionary[] = $word . $element[0];
        }
        $word = $element;
    }
    return $return;
}
$data = "";
$compressed = lzw_compress($data);
var_dump($data === lzw_decompress($compressed));
?>