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

#include "crc.h"

/*
  CRC_TABLE[256], the table of cyclic redundancy checksums for all 8-bit
    messages.

  CRC_TABLE_COMPUTED, a flag recording whether the table has been computed.
*/

unsigned long crc_table[256];
int crc_table_computed = 0;

/*
  & is the bitwise AND operator.
  ^  is the bitwise XOR operator.
  >> is the bitwise right shift operator.
  0xNNN introduces a hexadecimal constant.
*/

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

unsigned long crc ( unsigned char *buf, int len )

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

/*
  Purpose:

    CRC returns the CRC of the bytes in BUF[0...LEN-1].

  Reference:

    G Randers-Pehrson, et al,
    PNG (Portable Network Graphics) Specification,
    Version 1.2, July 1999.

  Modified:

    09 March 2002
*/

{
  return update_crc ( 0xffffffffL, buf, len ) ^ 0xffffffffL;
}

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

void make_crc_table ( void )

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

/*
  Purpose:

    MAKE_CRC_TABLE makes the table for a fast CRC computation.

  Reference:

    G Randers-Pehrson, et al,
    PNG (Portable Network Graphics) Specification,
    Version 1.2, July 1999.

  Modified:

    09 March 2002
*/

{
  unsigned long c;
  int k;
  int n;

  for ( n = 0; n < 256; n++ ) {
    c = ( unsigned long ) n;
    for ( k = 0; k < 8; k++ ) {
      if ( c & 1 ) {
        c = 0xedb88320L ^ ( c >> 1 );
      } else {
        c = c >> 1;
      }
    }
    crc_table[n] = c;
  }
  crc_table_computed = 1;
}

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

void print_crc_table ( void )

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

/*
  Purpose:
 
    PRINT_CRC_TABLE prints the CRC table.

  Modified:

    25 February 2001

  Author:

    John Burkardt
*/

{
  int n;

  if ( !crc_table_computed ) {
    make_crc_table ( );
  }
  printf ( "\n" );
  printf ( "CRC_TABLE printed in decimal and hex.\n" );
  printf ( "\n" );

  for ( n = 0; n < 256; n++ ) {
    printf ( "%d  %d  %x\n", n, crc_table[n], crc_table[n] );
  }
  return;
}

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

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

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

/*
  Purpose:

    UPDATE_CRC updates a running CRC.

  Discussion:

    The value of CRC should have been initialized to all 1's, and the
    transmitted value is the 1's complement of the final running CRC.

  Reference:

    G Randers-Pehrson, et al,
    PNG (Portable Network Graphics) Specification,
    Version 1.2, July 1999.
*/

{
  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;
}