source: xtideuniversalbios/trunk/Serial_Server/library/Checksum.cpp @ 209

//======================================================================
//
// Project:     XTIDE Universal BIOS, Serial Port Server
//
// File:        checksum.cpp - Checksum function and test routines
//
// This file implements Fletcher's Checksum.  The serial code uses this checksum, as it is very quick 
// to calculate in assembly and offers reasonable error detection.
// For more information, see http://en.wikipedia.org/wiki/Fletcher%27s_checksum.
//
// Since it is faster in 8088 assembly code to deal with 16-bit quantities than 8-bit quantities, 
// Fletcher's Checksum has been modified to calculate the 32-bit checksum, and then "fold" the result into a
// 16-bit quantity.  Fletcher's 32-bit Checksum consists of two parts: concatenated 16-bit accumulators.  
// To "fold" to 16-bits, The upper and lower 8-bits of each of these accumulators is XOR'd independently, and then 
// the two results concatenated together, resulting in 16-bits.  Although simpler, an early attempt to XOR the
// 16-bit accumulators results in poorer error detection behavior.  Folding as described here results in error
// detection on par with Fletcher's 16-bit Checksum.
//
// With #define CHECKSUM_TEST, this file becomes a self-contained command line program that runs
// some statistical tests comparing various checksum algorithms with random 512-byte sectors and various 
// levels of errors introduced.
//

#include "library.h"

unsigned short checksum( unsigned short *wbuff, int wlen )
{
    unsigned long a = 0xffff;
    unsigned long b = 0xffff;
    int t;

    for( t = 0; t < wlen; t++ )
    {
        a += wbuff[t];
        b += a;
    }

    a = (a & 0xffff) + (a >> 16);
    b = (b & 0xffff) + (b >> 16);
    a = (a & 0xffff) + (a >> 16);
    b = (b & 0xffff) + (b >> 16);

// Although tempting to use, for its simplicity and size/speed in assembly, the following folding 
// algorithm results in many undetected single bit errors and therefore should not be used.
//  return( (unsigned short) (a ^ b) );

    return( (unsigned short) (((a & 0xff) << 8) ^ (a & 0xff00)) + (((b & 0xff00) >> 8) ^ (b & 0xff)) );
}

#ifdef CHECKSUM_TEST

//====================================================================================================
// 
// Test Code
//

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

#define BUCKETS 65536
#define BITTEST 16

unsigned char bit[] = { 1, 2, 4, 8, 16, 32, 64, 128 };

class algorithm 
{
public:
    virtual unsigned short checksum( unsigned char *data, int len ) = 0;
    char *title;
    unsigned long *found;
    unsigned long zero;
    unsigned long total;
    unsigned long empty;
    unsigned long min;
    unsigned long max;
    double stdev;
    unsigned long bittest[ BITTEST ];
    unsigned long missed[ BITTEST ];
    algorithm *next;
    algorithm( algorithm *last, char *new_title );
};

algorithm::algorithm( algorithm *last, char *new_title )
{
    zero = total = empty = min = max = 0;
    stdev = 0.0;
    for( int t = 0; t < BITTEST; t++ )
    {
        bittest[t] = missed[t] = 0;
    }
    title = new_title;
    next = last;
}

//----------------------------------------------------------------------------------------------------
// 
// Standard CRC-16
//
// http://sanity-free.org/134/standard_crc_16_in_csharp.html
//

static unsigned short crc16_table[256];

class crc16_algorithm : public algorithm
{
public:
    crc16_algorithm( algorithm *last ) : algorithm( last, (char *) "crc-16" )
    {
        unsigned short value;
        unsigned short temp;
        unsigned short i;
        unsigned short j;

        for(i = 0; i < 256; ++i) 
        {
            value = 0;
            temp = i;
            for(j = 0; j < 8; ++j) {
                if(((value ^ temp) & 0x0001) != 0) {
                    value = (unsigned short)((value >> 1) ^ this->crc16_polynomial);
                }else {
                    value >>= 1;
                }
                temp >>= 1;
            }
            crc16_table[i] = value;
        }
    }

    unsigned short checksum( unsigned char *data, int len );  

private:
    static const unsigned short crc16_polynomial = 0xA001;
};

unsigned short crc16_algorithm::checksum( unsigned char *data, int len )
{
    unsigned short crc = 0;
    int i;

    for(i = 0; i < len; ++i) 
    {
        unsigned char index = (unsigned char)(crc ^ data[i]);
        crc = (unsigned short)((crc >> 8) ^ crc16_table[index]);
    }

    return( crc );
}

//----------------------------------------------------------------------------------------------------
// 
// Basic checksum (just add up the bytes)
//

class basic_algorithm : public algorithm
{
public:
    unsigned short checksum( unsigned char *data, int len );  
    basic_algorithm( algorithm *last ) : algorithm( last, (char *) "basic" ) { };
};

unsigned short basic_algorithm::checksum( unsigned char *bbuff, int blen )
{
    unsigned short sum = 0;
    int i;
    for( i = 0; i < blen; i++ )
    {
        sum += bbuff[ i ];
    }
    return( sum );
}

class fletcher16_algorithm : public algorithm
{
public:
    unsigned short checksum( unsigned char *data, int len );  
    fletcher16_algorithm( algorithm *last ) : algorithm( last, (char *) "f-16" ) { }
};

unsigned short fletcher16_algorithm::checksum( unsigned char* data, int count )
{
    unsigned short sum1 = 0;
    unsigned short sum2 = 0;
    int index;

    for( index = 0; index < count; ++index )
    {
        sum1 = (sum1 + data[index]) % 255;
        sum2 = (sum2 + sum1) % 255;
    }

    return (sum2 << 8) | sum1;
}

//----------------------------------------------------------------------------------------------------
// 
// Folded Fletcher's Checksum (what we use in the serial code, from the top of this file)
//

class folded_fletcher32_algorithm : public algorithm
{
public:
    unsigned short checksum( unsigned char *data, int len );  
    folded_fletcher32_algorithm( algorithm *last ) : algorithm( last, (char *) "fold-f-32" ) { }
};

unsigned short folded_fletcher32_algorithm::checksum( unsigned char* data, int count )
{
    return( ::checksum( (unsigned short *) data, count/2 ) );
}

//----------------------------------------------------------------------------------------------------
// 
// Test Driver and Support routines
//

void randomize_buff( unsigned char *bbuff, int blen )
{
    int i;
    for( i = 0; i < blen; i++ )
        bbuff[i] = rand() % 255;
}

#define BBUFF_LENGTH 512

unsigned char bbuff[ BBUFF_LENGTH ];

int main( int argc, char *argv[] )
{
    algorithm *a, *algorithms;

    unsigned short c;

    double p;
    double average;

    unsigned long iterations;

    time_t now;

    algorithms = new folded_fletcher32_algorithm( NULL );
    algorithms = new fletcher16_algorithm( algorithms );
    algorithms = new crc16_algorithm( algorithms );
    algorithms = new basic_algorithm( algorithms );

    time( &now );
    srand((unsigned int)now);

    if( argc != 2 )
    {
        fprintf( stderr, "usage: checksum number_of_iterations\n" );
        exit( 1 );
    }
    else
        iterations = atol( argv[1] );

#define PRINTROW( E, F, G ) { printf( E ); for( a = algorithms; a; a = a->next ) printf( F, G ); printf( "\n" ); }

    printf( "\nnumber of iterations: %d\n\n", iterations );
    PRINTROW( "       ", "%10s  ", a->title );
    PRINTROW( "=======", "============", NULL );

    for( a = algorithms; a; a = a->next )
    {
        a->found = (unsigned long *) calloc( BUCKETS, sizeof(long) );
        
        a->zero = (unsigned long) a->checksum( bbuff, BBUFF_LENGTH );

        a->min = iterations+1;
    }
    
    printf( "\n" );
    PRINTROW( "zero   ", "%10d  ", a->zero );

    for( int t = 0; t < iterations; t++ )
    {
        randomize_buff( bbuff, BBUFF_LENGTH );

        for( a = algorithms; a; a = a->next )
            a->found[ a->checksum( bbuff, BBUFF_LENGTH ) ]++;
    }

    average = iterations / 65536.0;

    for( int t = 0; t < 65536; t++ )
    {
        for( a = algorithms; a; a = a->next )
        {
            a->total += a->found[ t ];
            if( !a->found[ t ] )
                a->empty++;
            if( a->found[ t ] > a->max )
                a->max = a->found[ t ];
            if( a->found[ t ] < a->min )
                a->min = a->found[ t ];
            p = a->found[ t ] - average;
            a->stdev += p*p;
        }
    }

    p = 1.0 / (65536.0-1.0);
    for( a = algorithms; a; a = a->next )
    {
        a->stdev = sqrt( p * a->stdev );
        if( a->total != iterations )
            fprintf( stderr, "Bad %s\n", a->title );
    }

    printf( "\nchecksum distribution test:\n" );
    PRINTROW( "empty  ", "%10d  ", a->empty );
    PRINTROW( "min    ", "%10d  ", a->min );
    PRINTROW( "max    ", "%10d  ", a->max );
    PRINTROW( "stdev  ", "%10.4lf  ", a->stdev );

    for( int t = 0; t < iterations; t++ )
    {
        randomize_buff( bbuff, BBUFF_LENGTH );

        for( int b = 0; b < BITTEST; b++ )
        {
            for( a = algorithms; a; a = a->next )
            {
              a->bittest[ b ] = (a->checksum)( bbuff, BBUFF_LENGTH );
            }

            bbuff[ rand() % 512 ] ^= bit[ rand() % 8 ];
            
            if( b > 0 )
            {
                for( a = algorithms; a; a = a->next )
                {
                    if( a->bittest[ 0 ] == a->bittest[ b ] )
                        a->missed[ b ]++;
                }
            }
        }
    }  

    printf( "\nbit change test:\n" );
    for( int t = 1; t < BITTEST; t++ )
    {
        printf( "%2d        ", t );
        for( a = algorithms; a; a = a->next )
            printf( "%7d     ", a->missed[ t ] );
        printf( "\n" );
    }
}

#endif