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crcmodel.c
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crcmodel.c
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/******************************************************************************/
/* */
/* Author : Ross Williams ([email protected].). */
/* Date : 3 June 1993. */
/* Status : Public domain. */
/* */
/* Description : This is the implementation (.c) file for the reference */
/* implementation of the Rocksoft^tm Model CRC Algorithm. For more */
/* information on the Rocksoft^tm Model CRC Algorithm, see the document */
/* titled "A Painless Guide to CRC Error Detection Algorithms" by Ross */
/* Williams ([email protected].). This document is likely to be in */
/* "ftp.adelaide.edu.au/pub/rocksoft". */
/* */
/* Note: Rocksoft is a trademark of Rocksoft Pty Ltd, Adelaide, Australia. */
/* */
/******************************************************************************/
/* */
/* Implementation Notes */
/* -------------------- */
/* To avoid inconsistencies, the specification of each function is not echoed */
/* here. See the header file for a description of these functions. */
/* This package is light on checking because I want to keep it short and */
/* simple and portable (i.e. it would be too messy to distribute my entire */
/* C culture (e.g. assertions package) with this package. */
/* */
/******************************************************************************/
#include "crcmodel.h"
/******************************************************************************/
/* The following definitions make the code more readable. */
#define BITMASK(X) (1L << (X))
#define MASK32 0xFFFFFFFFL
#define LOCAL static
/******************************************************************************/
LOCAL ulong reflect P_((ulong v, int b));
LOCAL ulong reflect(v, b)
/* Returns the value v with the bottom b [0,32] bits reflected. */
/* Example: reflect(0x3e23L,3) == 0x3e26 */
ulong v;
int b;
{
int i;
ulong t = v;
for (i = 0; i < b; i++) {
if (t & 1L)
v |= BITMASK((b - 1) - i);
else
v &= ~BITMASK((b - 1) - i);
t >>= 1;
}
return v;
}
/******************************************************************************/
LOCAL ulong widmask P_((p_cm_t));
LOCAL ulong widmask(p_cm)
/* Returns a longword whose value is (2^p_cm->cm_width)-1. */
/* The trick is to do this portably (e.g. without doing <<32). */
p_cm_t p_cm;
{
return (((1L << (p_cm->cm_width - 1)) - 1L) << 1) | 1L;
}
/******************************************************************************/
void cm_ini(p_cm)
p_cm_t p_cm;
{
p_cm->cm_reg = p_cm->cm_init;
/* Reflect initial value */
if (p_cm->cm_refin)
p_cm->cm_reg = reflect(p_cm->cm_reg, p_cm->cm_width);
}
/******************************************************************************/
void cm_nxt(p_cm, ch)
p_cm_t p_cm;
int ch;
{
int i;
ulong uch = (ulong) ch;
ulong topbit = BITMASK(p_cm->cm_width - 1);
if (p_cm->cm_refin)
uch = reflect(uch, 8);
p_cm->cm_reg ^= (uch << (p_cm->cm_width - 8));
for (i = 0; i < 8; i++) {
if (p_cm->cm_reg & topbit)
p_cm->cm_reg = (p_cm->cm_reg << 1) ^ p_cm->cm_poly;
else
p_cm->cm_reg <<= 1;
p_cm->cm_reg &= widmask(p_cm);
}
}
/******************************************************************************/
void cm_blk(p_cm, blk_adr, blk_len)
p_cm_t p_cm;
p_ubyte_ blk_adr;
ulong blk_len;
{
while (blk_len--)
cm_nxt(p_cm, *blk_adr++);
}
/******************************************************************************/
ulong cm_crc(p_cm)
p_cm_t p_cm;
{
if (p_cm->cm_refot)
return p_cm->cm_xorot ^ reflect(p_cm->cm_reg, p_cm->cm_width);
else
return p_cm->cm_xorot ^ p_cm->cm_reg;
}
/******************************************************************************/
ulong cm_tab(p_cm, index)
p_cm_t p_cm;
int index;
{
int i;
ulong r;
ulong topbit = BITMASK(p_cm->cm_width - 1);
ulong inbyte = (ulong) index;
if (p_cm->cm_refin)
inbyte = reflect(inbyte, 8);
r = inbyte << (p_cm->cm_width - 8);
for (i = 0; i < 8; i++)
if (r & topbit)
r = (r << 1) ^ p_cm->cm_poly;
else
r <<= 1;
if (p_cm->cm_refin)
r = reflect(r, p_cm->cm_width);
return r & widmask(p_cm);
}