LZW压缩算法

原创: 2016-07-25 09:07:59 1172浏览

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("\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));
?>