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LZW compression algorithm

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Release： 2016-07-25 09:07:59
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PHP实现的LZW压缩算法
  1. /**
  2. * @link http://code.google.com/p/php-lzw/
  3. * @author Jakub Vrana, http://php.vrana.cz/
  4. * @copyright 2009 Jakub Vrana
  5. * @license http: //www.apache.org/licenses/LICENSE-2.0 Apache License, Version 2.0
  6. */
  8. /**LZW compression
  9. * @param string data to compress
  10. * @return string binary data
  11. */
  12. function lzw_compress($string) {
  13. // compression
  14. $dictionary = array_flip(range("", "xFF"));
  15. $word = "";
  16. $codes = array();
  17. for ($i=0; $i <= strlen($string); $i++) {
  18. $x = $string[$i];
  19. if (strlen($x) && isset($dictionary[$word . $x])) {
  20. $word .= $x;
  21. } elseif ($i) {
  22. $codes[] = $dictionary[$word];
  23. $dictionary[$word . $x] = count($dictionary);
  24. $word = $x;
  25. }
  26. }
  28. // convert codes to binary string
  29. $dictionary_count = 256;
  30. $bits = 8; // ceil(log($dictionary_count, 2))
  31. $return = "";
  32. $rest = 0;
  33. $rest_length = 0;
  34. foreach ($codes as $code) {
  35. $rest = ($rest << $bits) + $code;
  36. $rest_length += $bits;
  37. $dictionary_count++;
  38. if ($dictionary_count > (1 << $bits)) {
  39. $bits++;
  40. }
  41. while ($rest_length > 7) {
  42. $rest_length -= 8;
  43. $return .= chr($rest >> $rest_length);
  44. $rest &= (1 << $rest_length) - 1;
  45. }
  46. }
  47. return $return . ($rest_length ? chr($rest << (8 - $rest_length)) : "");
  48. }
  50. /**LZW decompression
  51. * @param string compressed binary data
  52. * @return string original data
  53. */
  54. function lzw_decompress($binary) {
  55. // convert binary string to codes
  56. $dictionary_count = 256;
  57. $bits = 8; // ceil(log($dictionary_count, 2))
  58. $codes = array();
  59. $rest = 0;
  60. $rest_length = 0;
  61. for ($i=0; $i < strlen($binary); $i++) {
  62. $rest = ($rest << 8) + ord($binary[$i]);
  63. $rest_length += 8;
  64. if ($rest_length >= $bits) {
  65. $rest_length -= $bits;
  66. $codes[] = $rest >> $rest_length;
  67. $rest &= (1 << $rest_length) - 1;
  68. $dictionary_count++;
  69. if ($dictionary_count > (1 << $bits)) {
  70. $bits++;
  71. }
  72. }
  73. }
  75. // decompression
  76. $dictionary = range("", "xFF");
  77. $return = "";
  78. foreach ($codes as $i => $code) {
  79. $element = $dictionary[$code];
  80. if (!isset($element)) {
  81. $element = $word . $word[0];
  82. }
  83. $return .= $element;
  84. if ($i) {
  85. $dictionary[] = $word . $element[0];
  86. }
  87. $word = $element;
  88. }
  89. return $return;
  90. }
  91. $data = "";
  92. $compressed = lzw_compress($data);
  93. var_dump($data === lzw_decompress($compressed));
  94. ?>
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LZW压缩算法
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