view Discovery/Src/crcmodel.c @ 250:822416168585 bm-2

Buelmann: new implementation for ceiling Since my first functional fix in the ceiling computation in commit ceecabfddb57, I noticed that the computation used a linear search, that became rather computational expensive after that commit. The simple question is: why not a binary search? So, this commit implements the binary search. But there is a long story attached to this. Comparing ceiling results from hwOS and this OSTC4 code were very different. Basically, the original OSTC4 algorithm computed the ceiling using the same GFlow to GFhigh slope, in such a way, that the ceiling was in sync with the presented deco stops, where the hwOS code presents a GFhigh based ceiling. This said, it is more logical when the OSTC4 and hwOS code give similar results. This new recursive algorithm gives very similar results for the ceiling compared to hwOS. To be complete here, the Buelmann ceiling is the depth to which you can ascend, so that the leading tissue reaches GFhigh. This also explains why the deepest deco stop is normally deeper than the ceiling (unless one dives with GF like 80/80). The code implemented here is rather straightforward recursion. Signed-off-by: Jan Mulder <jlmulder@xs4all.nl>
author Jan Mulder <jlmulder@xs4all.nl>
date Thu, 11 Apr 2019 17:48:48 +0200
parents 5f11787b4f42
children
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/******************************************************************************/
/*                             Start of crcmodel.c                            */
/******************************************************************************/
/*                                                                            */
/* Author : Ross Williams (ross@guest.adelaide.edu.au.).                      */
/* 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 (ross@guest.adelaide.edu.au.). 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 (ulong v,int b)
/* Returns the value v with the bottom b [0,32] bits reflected. */
/* Example: reflect(0x3e23L,3) == 0x3e26                        */
{
 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_t 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). */
{
 return (((1L<<(p_cm->cm_width-1))-1L)<<1)|1L;
}

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

void cm_ini (p_cm_t p_cm)
{
 p_cm->cm_reg = p_cm->cm_init;
}

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

void cm_nxt (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_t p_cm,p_ubyte_ blk_adr,ulong blk_len)
{
 while (blk_len--) cm_nxt(p_cm,*blk_adr++);
}

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

ulong cm_crc (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_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);
}

/******************************************************************************/
/*                             End of crcmodel.c                              */
/******************************************************************************/