/* CRC xx : CRC test program used CRC_TABLE[256] 

  Ex1)  CRC-16 : 1 + X^2 + X^15 + X^16
                 1010 0000 0000 0001 = 0xA001

  seri-para form
  input data : serial 1'st bit ->  8 bit data LSB
  output CRC : serial 1'st bit -> 16 bit data LSB

  shift register logic : initialize all 1....1

      1   2     3   4   5   6   7   8   9  10  11  12  13  14  15    16
  --> shift
      _   _ XOR _   _   _   _   _   _   _   _   _   _   _   _   _ XOR _
   __| |_| |___| |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| |___| |_  --> output CRC
  |  |_| |_| + |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| |_| + |_| |
  |          |                                                     |     |
  |__________|_____________________________________________________|_____+ __<-- input data
                                                                        XOR

  Ex2)  CRC-32 : 1 + X + X^2 + X^4 + X^5 + X^7 + X^8 + X^10 + X^11 + X^12 + X^16 + X^22 + X^23 + X^26 + X^32
                 1110 1101 1011 1000 1000 0011 0010 0000 = 0xEDB88320
*/

#include <stdlib.h>
#include <stdio.h>
#include <time.h>

void make_crc_table(void);
unsigned long update_crc(unsigned long crc, unsigned char *buf, int len);
unsigned long crc(unsigned char *buf, int len);


/* Table of CRCs of all 8-bit messages. */
unsigned long crc_table[256];
   
/* Flag: has the table been computed? Initially false. */
int crc_table_computed = 0;
   
/* Make the table for a fast CRC. */
void make_crc_table(void)
{
     unsigned long c;
     int n, k;
   
     for (n = 0; n < 256; n++) {
       c = (unsigned long) n;
       for (k = 0; k < 8; k++) {
         if (c & 1)
           //c = 0xa001L ^ (c >> 1);
           c = 0xedb88320L ^ (c >> 1);
         else
           c = c >> 1;
       }
       crc_table[n] = c;
     }
     crc_table_computed = 1;
}
   
/* Update a running CRC with the bytes buf[0..len-1]--the CRC
   should be initialized to all 1's, and the transmitted value
   is the 1's complement of the final running CRC (see the
   crc() routine below)). */
   
unsigned long update_crc(unsigned long crc, unsigned char *buf, int len)
{
     unsigned long c = crc;
     int n;
   
     if (!crc_table_computed)
       make_crc_table();
     for (n = 0; n < len; n++) {
       c = crc_table[(c ^ buf[n]) & 0xff] ^ (c >> 8);
     }
     return c;
}
   
/* Return the CRC of the bytes buf[0..len-1]. */
unsigned long crc(unsigned char *buf, int len)
{
     //return update_crc(0xffffL, buf, len);
     return update_crc(0xffffffffL, buf, len);
}

void main( void )
{
   int i, j;
   unsigned long result;
   unsigned char buf[10];

   /* 実行するたびに違う値が得られるように、現在の時刻値を使って
    * 乱数ジェネレータを初期化します。
    */
   srand( (unsigned)time( NULL ) );

   /* 10 個の値を表示します。 */
   for( i = 0;   i < 10;i++ )
   {
       buf[i] = rand()%256;
	   printf( "  %6d\n", buf[i] );
   }

   /* by crc_table */
   result = crc(buf,10);
   //printf( "\n  CRC-16 = 0x%x by CRC_TABLE \n", result);
   printf( "\n  CRC-32 = 0x%x by CRC_TABLE \n", result);

   /* by shift register logic */
   //result = 0xffff;
   result = 0xffffffff;
   for( i = 0;  i < 10;i++)
   {
	   for( j = 0; j < 8;j++)
	   {
          if((result^buf[i])&1)
			  //result = 0xa001L ^ (result >> 1);
			  result = 0xedb88320L ^ (result >> 1);
          else
			  result = result >> 1;
		  buf[i] = buf[i] >> 1;
       }
   }
   //printf( "\n  CRC-16 = 0x%x by Shift Reg. Logic \n", result);
   printf( "\n  CRC-32 = 0x%x by Shift Reg. Logic \n", result);
}