LZW compression algorithm

The LZW compression algorithm implemented by PHP /** * @link http://code.google.com/p/php-lzw/ * @author Jakub Vrana, http://php.vrana.cz/ * @copyright 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 ("n", "\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 ("n", "\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)); ?> Copy Code

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