LZW compression algorithm
Release: 2016-07-25 09:07:59
Original
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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));
- ?>
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