libs/mpega/crc.c

   1 /*
   2  * madplay - MPEG audio decoder and player
   3  * Copyright (C) 2000-2004 Robert Leslie
   4  *
   5  * This program is free software; you can redistribute it and/or modify
   6  * it under the terms of the GNU General Public License as published by
   7  * the Free Software Foundation; either version 2 of the License, or
   8  * (at your option) any later version.
   9  *
  10  * This program is distributed in the hope that it will be useful,
  11  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  12  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13  * GNU General Public License for more details.
  14  *
  15  * You should have received a copy of the GNU General Public License
  16  * along with this program; if not, write to the Free Software
  17  * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  18  *
  19  * $Id$
  20  */
  21
  22 # ifdef HAVE_CONFIG_H
  23 #  include "config.h"
  24 # endif
  25
  26 # include "global.h"
  27
  28 # include "crc.h"
  29
  30 /*
  31  * It would be nice to use the code in libmad to compute all CRC-16 values,
  32  * but unfortunately the LAME tag uses reflected CRCs whereas MPEG audio
  33  * frames do not. Therefore we are forced to write yet another CRC routine.
  34  */
  35
  36 static
  37 unsigned short const crc_table[256] = {
  38   0x0000, 0xc0c1, 0xc181, 0x0140, 0xc301, 0x03c0, 0x0280, 0xc241,
  39   0xc601, 0x06c0, 0x0780, 0xc741, 0x0500, 0xc5c1, 0xc481, 0x0440,
  40   0xcc01, 0x0cc0, 0x0d80, 0xcd41, 0x0f00, 0xcfc1, 0xce81, 0x0e40,
  41   0x0a00, 0xcac1, 0xcb81, 0x0b40, 0xc901, 0x09c0, 0x0880, 0xc841,
  42   0xd801, 0x18c0, 0x1980, 0xd941, 0x1b00, 0xdbc1, 0xda81, 0x1a40,
  43   0x1e00, 0xdec1, 0xdf81, 0x1f40, 0xdd01, 0x1dc0, 0x1c80, 0xdc41,
  44   0x1400, 0xd4c1, 0xd581, 0x1540, 0xd701, 0x17c0, 0x1680, 0xd641,
  45   0xd201, 0x12c0, 0x1380, 0xd341, 0x1100, 0xd1c1, 0xd081, 0x1040,
  46
  47   0xf001, 0x30c0, 0x3180, 0xf141, 0x3300, 0xf3c1, 0xf281, 0x3240,
  48   0x3600, 0xf6c1, 0xf781, 0x3740, 0xf501, 0x35c0, 0x3480, 0xf441,
  49   0x3c00, 0xfcc1, 0xfd81, 0x3d40, 0xff01, 0x3fc0, 0x3e80, 0xfe41,
  50   0xfa01, 0x3ac0, 0x3b80, 0xfb41, 0x3900, 0xf9c1, 0xf881, 0x3840,
  51   0x2800, 0xe8c1, 0xe981, 0x2940, 0xeb01, 0x2bc0, 0x2a80, 0xea41,
  52   0xee01, 0x2ec0, 0x2f80, 0xef41, 0x2d00, 0xedc1, 0xec81, 0x2c40,
  53   0xe401, 0x24c0, 0x2580, 0xe541, 0x2700, 0xe7c1, 0xe681, 0x2640,
  54   0x2200, 0xe2c1, 0xe381, 0x2340, 0xe101, 0x21c0, 0x2080, 0xe041,
  55
  56   0xa001, 0x60c0, 0x6180, 0xa141, 0x6300, 0xa3c1, 0xa281, 0x6240,
  57   0x6600, 0xa6c1, 0xa781, 0x6740, 0xa501, 0x65c0, 0x6480, 0xa441,
  58   0x6c00, 0xacc1, 0xad81, 0x6d40, 0xaf01, 0x6fc0, 0x6e80, 0xae41,
  59   0xaa01, 0x6ac0, 0x6b80, 0xab41, 0x6900, 0xa9c1, 0xa881, 0x6840,
  60   0x7800, 0xb8c1, 0xb981, 0x7940, 0xbb01, 0x7bc0, 0x7a80, 0xba41,
  61   0xbe01, 0x7ec0, 0x7f80, 0xbf41, 0x7d00, 0xbdc1, 0xbc81, 0x7c40,
  62   0xb401, 0x74c0, 0x7580, 0xb541, 0x7700, 0xb7c1, 0xb681, 0x7640,
  63   0x7200, 0xb2c1, 0xb381, 0x7340, 0xb101, 0x71c0, 0x7080, 0xb041,
  64
  65   0x5000, 0x90c1, 0x9181, 0x5140, 0x9301, 0x53c0, 0x5280, 0x9241,
  66   0x9601, 0x56c0, 0x5780, 0x9741, 0x5500, 0x95c1, 0x9481, 0x5440,
  67   0x9c01, 0x5cc0, 0x5d80, 0x9d41, 0x5f00, 0x9fc1, 0x9e81, 0x5e40,
  68   0x5a00, 0x9ac1, 0x9b81, 0x5b40, 0x9901, 0x59c0, 0x5880, 0x9841,
  69   0x8801, 0x48c0, 0x4980, 0x8941, 0x4b00, 0x8bc1, 0x8a81, 0x4a40,
  70   0x4e00, 0x8ec1, 0x8f81, 0x4f40, 0x8d01, 0x4dc0, 0x4c80, 0x8c41,
  71   0x4400, 0x84c1, 0x8581, 0x4540, 0x8701, 0x47c0, 0x4680, 0x8641,
  72   0x8201, 0x42c0, 0x4380, 0x8341, 0x4100, 0x81c1, 0x8081, 0x4040
  73 };
  74
  75 /*
  76  * NAME:        crc->compute()
  77  * DESCRIPTION: calculate reflected CRC-16 value (polynomial 0x8005)
  78  */
  79 unsigned short crc_compute(char const *data, unsigned int length,
  80                            unsigned short init)
  81 {
  82   register unsigned int crc;
  83
  84   for (crc = init; length >= 8; length -= 8) {
  85     crc = crc_table[(crc ^ *data++) & 0xff] ^ (crc >> 8);
  86     crc = crc_table[(crc ^ *data++) & 0xff] ^ (crc >> 8);
  87     crc = crc_table[(crc ^ *data++) & 0xff] ^ (crc >> 8);
  88     crc = crc_table[(crc ^ *data++) & 0xff] ^ (crc >> 8);
  89     crc = crc_table[(crc ^ *data++) & 0xff] ^ (crc >> 8);
  90     crc = crc_table[(crc ^ *data++) & 0xff] ^ (crc >> 8);
  91     crc = crc_table[(crc ^ *data++) & 0xff] ^ (crc >> 8);
  92     crc = crc_table[(crc ^ *data++) & 0xff] ^ (crc >> 8);
  93   }
  94
  95   switch (length) {
  96   case 7: crc = crc_table[(crc ^ *data++) & 0xff] ^ (crc >> 8);
  97   case 6: crc = crc_table[(crc ^ *data++) & 0xff] ^ (crc >> 8);
  98   case 5: crc = crc_table[(crc ^ *data++) & 0xff] ^ (crc >> 8);
  99   case 4: crc = crc_table[(crc ^ *data++) & 0xff] ^ (crc >> 8);
 100   case 3: crc = crc_table[(crc ^ *data++) & 0xff] ^ (crc >> 8);
 101   case 2: crc = crc_table[(crc ^ *data++) & 0xff] ^ (crc >> 8);
 102   case 1: crc = crc_table[(crc ^ *data++) & 0xff] ^ (crc >> 8);
 103   case 0: break;
 104   }
 105
 106   return crc;
 107 }