Home > Article > Backend Development > LZW compression algorithm

LZW compression algorithm

WBOYOriginal: 2016-07-25 09:07:591428browse

PHP实现的LZW压缩算法

/**
* @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("", "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("", "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));
?>

复制代码

Statement：

The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn

Previous article：Simple, calculatorNext article：Simple, calculator

See more

LZW compression algorithm

Related articles