/*
    linux/lib/crc-ccitt.c

    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

#include "..\CRC_CCITT.h"


/*
** Make sure we can call this stuff from C++.
*/
#ifdef __cplusplus
extern "C" {
#endif


/*
 * This mysterious table is just the CRC of each possible byte. It can be
 * computed using the standard bit-at-a-time methods. The polynomial can
 * be seen in entry 128, 0x8408. This corresponds to x^0 + x^5 + x^12.
 * Add the implicit x^16, and you have the standard CRC-CCITT.
 */
tUShort const crc_ccitt_table[256] = 
{
  0x0000,0x1021,0x2042,0x3063,0x4084,0x50A5,0x60C6,0x70E7,
  0x8108,0x9129,0xA14A,0xB16B,0xC18C,0xD1AD,0xE1CE,0xF1EF,
  0x1231,0x0210,0x3273,0x2252,0x52B5,0x4294,0x72F7,0x62D6,
  0x9339,0x8318,0xB37B,0xA35A,0xD3BD,0xC39C,0xF3FF,0xE3DE,
  0x2462,0x3443,0x0420,0x1401,0x64E6,0x74C7,0x44A4,0x5485,
  0xA56A,0xB54B,0x8528,0x9509,0xE5EE,0xF5CF,0xC5AC,0xD58D,
  0x3653,0x2672,0x1611,0x0630,0x76D7,0x66F6,0x5695,0x46B4,
  0xB75B,0xA77A,0x9719,0x8738,0xF7DF,0xE7FE,0xD79D,0xC7BC,
  0x48C4,0x58E5,0x6886,0x78A7,0x0840,0x1861,0x2802,0x3823,
  0xC9CC,0xD9ED,0xE98E,0xF9AF,0x8948,0x9969,0xA90A,0xB92B,
  0x5AF5,0x4AD4,0x7AB7,0x6A96,0x1A71,0x0A50,0x3A33,0x2A12,
  0xDBFD,0xCBDC,0xFBBF,0xEB9E,0x9B79,0x8B58,0xBB3B,0xAB1A,
  0x6CA6,0x7C87,0x4CE4,0x5CC5,0x2C22,0x3C03,0x0C60,0x1C41,
  0xEDAE,0xFD8F,0xCDEC,0xDDCD,0xAD2A,0xBD0B,0x8D68,0x9D49,
  0x7E97,0x6EB6,0x5ED5,0x4EF4,0x3E13,0x2E32,0x1E51,0x0E70,
  0xFF9F,0xEFBE,0xDFDD,0xCFFC,0xBF1B,0xAF3A,0x9F59,0x8F78,
  0x9188,0x81A9,0xB1CA,0xA1EB,0xD10C,0xC12D,0xF14E,0xE16F,
  0x1080,0x00A1,0x30C2,0x20E3,0x5004,0x4025,0x7046,0x6067,
  0x83B9,0x9398,0xA3FB,0xB3DA,0xC33D,0xD31C,0xE37F,0xF35E,
  0x02B1,0x1290,0x22F3,0x32D2,0x4235,0x5214,0x6277,0x7256,
  0xB5EA,0xA5CB,0x95A8,0x8589,0xF56E,0xE54F,0xD52C,0xC50D,
  0x34E2,0x24C3,0x14A0,0x0481,0x7466,0x6447,0x5424,0x4405,
  0xA7DB,0xB7FA,0x8799,0x97B8,0xE75F,0xF77E,0xC71D,0xD73C,
  0x26D3,0x36F2,0x0691,0x16B0,0x6657,0x7676,0x4615,0x5634,
  0xD94C,0xC96D,0xF90E,0xE92F,0x99C8,0x89E9,0xB98A,0xA9AB,
  0x5844,0x4865,0x7806,0x6827,0x18C0,0x08E1,0x3882,0x28A3,
  0xCB7D,0xDB5C,0xEB3F,0xFB1E,0x8BF9,0x9BD8,0xABBB,0xBB9A,
  0x4A75,0x5A54,0x6A37,0x7A16,0x0AF1,0x1AD0,0x2AB3,0x3A92,
  0xFD2E,0xED0F,0xDD6C,0xCD4D,0xBDAA,0xAD8B,0x9DE8,0x8DC9,
  0x7C26,0x6C07,0x5C64,0x4C45,0x3CA2,0x2C83,0x1CE0,0x0CC1,
  0xEF1F,0xFF3E,0xCF5D,0xDF7C,0xAF9B,0xBFBA,0x8FD9,0x9FF8,
  0x6E17,0x7E36,0x4E55,0x5E74,0x2E93,0x3EB2,0x0ED1,0x1EF0,
};


tUShort CRC_CCITT_Add_Byte(tUShort crc, tUChar c)
/***********************************************************************
 *
 *  CRC_CCITT_Add_Byte
 *
 ***********************************************************************/
{
tUShort tmp, short_c;

  short_c  = (tUShort)0x00ff & (tUShort)c;

  tmp = (tUShort)(crc >> 8) ^ short_c;
  crc = (tUShort)(crc << 8) ^ crc_ccitt_table[(tUChar)tmp];

  return crc;
}


tUShort CRC_CCITT(const tUChar *bufferP, tUShort length)
/***********************************************************************
 *
 *  CRC_CCITT
 *
 *
 * generate a ccitt 16 bit cyclic redundancy check (crc)
 *
 *      The code in this module generates the crc for a block of data.
 *
 *                                16  12  5
 * The CCITT CRC 16 polynomial is X + X + X + 1.
 *
 * NOTES
 *      The CRC is preset to all 1's to detect errors involving a loss
 *        of leading zero's.
 *      The CRC (a 16 bit value) is generated in LSB MSB order.
 *      Two ways to verify the integrity of a received message
 *        or block of data:
 *        1) Calculate the crc on the data, and compare it to the crc
 *           calculated previously. The location of the saved crc must be
 *           known.
 *        2) Append the calculated crc to the end of the data. Now calculate
 *           the crc of the data and its crc. If the new crc equals the
 *           value in "crc_ok", the data is valid.
 ***********************************************************************/
{
tUShort crc = 0xffff;

  return (CRC_CCITT_Add(crc, 
                bufferP, 
                length));
}
/*----------------------------------------------------------------------------*/


tUShort CRC_CCITT_Add(tUShort crc, const tUChar *bufferP, tUShort length)
/***********************************************************************
 *
 *  CRC_CCITT_Add
 *
 ***********************************************************************/
{
  while (length--)
    crc = CRC_CCITT_Add_Byte(crc, *bufferP++);
    
 	return crc;
}
/*----------------------------------------------------------------------------*/


#ifdef __cplusplus
}  /* End of the 'extern "C"' block */
#endif

