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#include <p18cxxx.h>

#include <crc16.h>

// *****************************************************************************

// Update the CRC for transmitted and received data using

// the CCITT 16bit algorithm (X^16 + X^12 + X^5 + X^0)

//

// Adapted by R Andrag from an 

// asm routine by John C. Wren (google to find the original code)

// roland.andrag at gmail.com

//

// To check your crc implementation use:

// http://www.zorc.breitbandkatze.de/crc.html

// Web calculator by Sven Reifegerste

// CRC order = 16

// CRC polynomial = 1021 hex

// Initial value = FFFF hex

// direct CRC

// Final XOR value = 0

// Don't reverse data bytes or CRC result

// calculator allows you to enter hex bytes by prefixing %

// append 00 00 to your mesage when you calulate the crc

// i.e. crc should work out to zero on: message:00:00:crc_h:crc_l

// e.g. %01%02%00%00 gives a crc of 9c14

//      %01%02%00%00%9c%14 gives a crc of 0000

// %01%02%9c%14 gives a crc of 9327 hex

// *****************************************************************************

void update_crc(uint16 *p_crc, uint8 data) {

    *p_crc  = (*p_crc >> 8) | (*p_crc << 8);

    *p_crc ^= data;

    *p_crc ^= (*p_crc & 0xff) >> 4;

    *p_crc ^= *p_crc << 12;

    *p_crc ^= (*p_crc & 0xff) << 5;

}

uint16 calc_crc(uint8 * p_data, uint16 n_bytes) {

    uint16 crc = 0xFFFF;

    for (; n_bytes > 0; n_bytes--) {

        update_crc(&crc, *(p_data++));

    }

    return crc;

}