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