BM算法

gskuan 2011-07-17 11:40:00

大家帮忙解释下红色代码的含义。如果代码没有红色请不要回帖，我修改下颜色，谢谢。

/*-- BOYERMOR.C --------------------------- Listing 4-3 --------
* Boyer-Moore string search routine
*
* Preprocessor switches: if #defined:
*
* DEBUG will cause the search routine to dump its tables
* at various times--this is useful when trying to
* understand how MatchJump is generated
*
* DRIVER will cause a test driver to be compiled
*
*------------------------------------------------------------*/

#define DRIVER 1
/* #define DEBUG 1 */
#define DEBUG
#if defined(DEBUG)
#define SHOWCHAR for (uT=1; uT<= PatLen; uT++) \
printf(" %c ", String[uT-1])
#define SHOWJUMP for (uT=1;uT<= PatLen;uT++) \
printf("%2d ", MatchJump[uT])
#define SHOWA printf(" uA = %u ", uA)
#define SHOWB printf(" uB = %u", uB)
#define SHOWBACK for (uT=1;uT<= PatLen;uT++) \
printf("%2d ", BackUp[uT])
#define NL printf("\n")

unsigned uT;
#else
#define SHOWCHAR
#define SHOWJUMP
#define SHOWA
#define SHOWB
#define SHOWBACK
#define NL
#endif

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

#define AlphabetSize (UCHAR_MAX + 1) /* For portability */

#ifndef max
#define max(a,b) ((a) > (b) ? (a) : (b))
#endif

char *BoyerMoore ( const char * String, /* search for this */
const char * Text, /* ...in this text */
size_t TextLen ) /* ...up to here. */
{
/* array of character mismatch offsets */
unsigned CharJump[AlphabetSize];

/* array of offsets for partial matches */
unsigned *MatchJump;

/* temporary array for MatchJump calculation */
unsigned *BackUp;
size_t PatLen;
unsigned u, uText, uPat, uA, uB;

/* Set up and initialize arrays */
PatLen = strlen ( String );
MatchJump = (unsigned *)
malloc ( 2 * sizeof ( unsigned ) * ( PatLen + 1 ));
BackUp = MatchJump + PatLen + 1;

/* Heuristic #1 -- simple char mismatch jumps ... */

memset ( CharJump, 0, AlphabetSize * sizeof(unsigned) );
for ( u = 0 ; u < PatLen; u++ )
CharJump[((unsigned char) String[u])]
= PatLen - u - 1;

/* Heuristic #2 -- offsets from partial matches ... */
for ( u = 1; u <= PatLen; u++ )

MatchJump[u] = 2 * PatLen - u;
/* largest possible jump */

SHOWCHAR; NL;
SHOWJUMP; NL;

u = PatLen;
uA = PatLen + 1;

while ( u > 0 )
{
BackUp[u] = uA;
while ( uA <= PatLen &&
String[u - 1] != String[uA - 1] )
{
if ( MatchJump[uA] > PatLen - u )
MatchJump[uA] = PatLen - u;
uA = BackUp[uA];
}
u--;
uA--;
}

SHOWJUMP; SHOWA; SHOWBACK; NL;

for ( u = 1; u <= uA; u++ )
if ( MatchJump[u] > PatLen + uA - u )
MatchJump[u] = PatLen + uA - u;

uB = BackUp[uA];
SHOWJUMP; SHOWB; NL;

while ( uA <= PatLen )
{
while ( uA <= uB )
{
if ( MatchJump[uA] > uB - uA + PatLen )
MatchJump[uA] = uB - uA + PatLen;
uA++;
}
uB = BackUp[uB];
}

SHOWJUMP; NL;

/* now search */
uPat = PatLen; /* tracks position in Pattern */
uText = PatLen - 1; /* tracks position in Text */
while ( uText < TextLen && uPat != 0 )
{
if ( Text[uText] == String[uPat - 1]) /* match? */
{
uText--; /* back up to next */
uPat--;
}
else /* a mismatch - slide pattern forward */
{
uA = CharJump[((unsigned char) Text[uText])];
uB = MatchJump[uPat];
uText += max(uA, uB); /* select larger jump */
uPat = PatLen;
}
}

/* return our findings */
if ( uPat == 0 )
return ( (char *) ( Text + ( uText + 1 ))); /* a match */
else
return ( NULL ); /* no match */
}

/*---------------------------------------------------------------
* The main driver, activated by #defining DRIVER.
* Will print all occurrences of a match in the first
* 10,000 characters of the target file.
*-------------------------------------------------------------*/

#ifdef DRIVER

#define MAX_TEXT_SIZE 10000u

int main ( int argc, char *argv[] )
{
char *SearchFor, *Filename;

FILE *Fin; /* File to search */
char *Buffer; /* Buffer from file */

char *start, *p;
int i;
size_t TextSize;
unsigned count;

if ( argc != 3 )
{
puts ( "Usage is: boyermor search-string filename\n" );
return ( EXIT_FAILURE );
}
else
{
SearchFor = argv[1];
Filename = argv[2];
}

if (( Fin = fopen ( Filename, "r" )) == NULL )
{
fprintf ( stderr, "Can't open %s\n", Filename );
return ( EXIT_FAILURE );
}

/* allocate search buffer and fill it with target file */
Buffer = (char*) malloc ( MAX_TEXT_SIZE + 1 );
if ( Buffer == NULL )
{
puts ( "Error! Could not allocate buffer space\n" );
return ( EXIT_FAILURE );
}

TextSize = fread ( Buffer, 1, MAX_TEXT_SIZE, Fin );
fclose ( Fin );

p = Buffer;
count = 0;
while ( count < TextSize )
{
if ( *p == '\n' )
*p = '\0';
p++;
count++;
}

/* now search repeatedly */

start = BoyerMoore ( SearchFor, Buffer, TextSize );
if ( start == NULL ) /* no match found */
printf ( "\n%s Not Found.\n", SearchFor );
else /* match found */
while ( start != NULL )
{
for ( p = start; ; p-- ) /* find start of line */
{
if ( *p == '\0' )
{
p++;
break;
}
else
if ( p == Buffer )
break;
} /* print the match */
printf( "Found:\n%s\n", p );
for ( i = start - p; i > 0; i-- )
fputc ( ' ', stdout );
printf ( "%s\n\n", SearchFor );
start = /* continue the search */
BoyerMoore ( SearchFor, start + 1,
TextSize - ( start - Buffer ) - 1 );
}
return ( EXIT_SUCCESS );
}
#endif

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