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James Gregory 2013-12-30 18:34:31 +11:00
commit 18011c595a
352 changed files with 6040 additions and 6040 deletions
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14-04.md
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@ -30,13 +30,13 @@
/* Create the table of distances by which to skip ahead on
mismatches for every possible byte value */
/* Initialize all skips to the pattern length; this is the skip
distance for bytes that don’t appear in the pattern */
distance for bytes that don't appear in the pattern */
for (i = 0; i < 256; i++) SkipTable[i] = PatternLength;
/*Set the skip values for the bytes that do appear in the pattern
to the distance from the byte location to the end of the
pattern. When there are multiple instances of the same byte,
the rightmost instance’s skip value is used. Note that the
rightmost byte of the pattern isn’t entered in the skip table;
the rightmost instance's skip value is used. Note that the
rightmost byte of the pattern isn't entered in the skip table;
if we get that value for a mismatch, we know for sure that the
right end of the pattern has already passed the mismatch
location, so this is not a relevant byte for skipping purposes */
@ -61,27 +61,27 @@
/* Compare the pattern and the buffer location, searching from
high memory toward low (right to left) */
while (*WorkingPatternPtr— == *WorkingBufferPtr—) {
/* If we’ve matched the entire pattern, it’s a match */
/* If we've matched the entire pattern, it's a match */
if (-CompCount == 0)
/* Return a pointer to the start of the match location */
return(BufferPtr - PatternLength + 1);
}
/* It’s a mismatch; let’s see what we can learn from it */
/* It's a mismatch; let's see what we can learn from it */
WorkingBufferPtr++; /* point back to the mismatch location */
/* # of bytes that did match */
DistanceMatched = BufferPtr - WorkingBufferPtr;
/*If, based on the mismatch character, we can’t even skip ahead
/*If, based on the mismatch character, we can't even skip ahead
as far as where we started this particular comparison, then
just advance by 1 to the next potential match; otherwise,
skip ahead from the mismatch location by the skip distance
for the mismatch character */
if (SkipTable[*WorkingBufferPtr] <= DistanceMatched)
Skip = 1; /* skip doesn’t do any good, advance by 1 */
Skip = 1; /* skip doesn't do any good, advance by 1 */
else
/* Use skip value, accounting for distance covered by the
partial match */
Skip = SkipTable[*WorkingBufferPtr] - DistanceMatched;
/* If skipping ahead would exhaust the buffer, we’re done
/* If skipping ahead would exhaust the buffer, we're done
without a match */
if (Skip >= BufferLength) return(NULL);
/* Skip ahead and perform the next comparison */
@ -113,48 +113,48 @@
int Handle;
unsigned int WorkingLength;
printf(“File to search:”);
printf("File to search:");
gets(Filename);
printf(“Pattern for which to search:”);
printf("Pattern for which to search:");
gets(Pattern);
if ( (Handle = open(Filename, O_RDONLY | O_BINARY)) == -1 ) {
printf(“Can’t open file: %s\n”, Filename); exit(1);
printf("Can't open file: %s\n", Filename); exit(1);
}
/* Get memory in which to buffer the data */
if ( (TestBuffer=(unsigned char *)malloc(BUFFER_SIZE+1)) == NULL) {
printf(“Can’t get enough memory\n”); exit(1);
printf("Can't get enough memory\n"); exit(1);
}
/* Process a BUFFER_SIZE chunk */
if ( (int)(WorkingLength =
read(Handle, TestBuffer, BUFFER_SIZE)) == -1 ) {
printf(“Error reading file %s\n”, Filename); exit(1);
printf("Error reading file %s\n", Filename); exit(1);
}
TestBuffer[WorkingLength] = 0; /* 0-terminate buffer for printf */
/* Search for the pattern and report the results */
if ((MatchPtr = FindString(TestBuffer, WorkingLength, Pattern,
(unsigned int) strlen(Pattern))) == NULL) {
/* Pattern wasn’t found */
printf(“\“%s\” not found\n”, Pattern);
/* Pattern wasn't found */
printf("\"%s\" not found\n", Pattern);
} else {
/* Pattern was found. Zero-terminate TempBuffer; strncpy
won’t do it if DISPLAY_LENGTH characters are copied */
won't do it if DISPLAY_LENGTH characters are copied */
TempBuffer[DISPLAY_LENGTH] = 0;
printf(“\“%s\” found. Next %d characters at match:\n\”%s\“\n”,
printf("\"%s\" found. Next %d characters at match:\n\"%s\"\n",
Pattern, DISPLAY_LENGTH,
strncpy(TempBuffer, MatchPtr, DISPLAY_LENGTH));
}
exit(0);
}
Well, architecture carries a lot of weight, but it sure as heck isn’t
Well, architecture carries a lot of weight, but it sure as heck isn't
destiny. I had simply fallen into the trap of figuring that the
algorithm was so clever that I didn’t have to do any thinking myself.
algorithm was so clever that I didn't have to do any thinking myself.
The path leading to **REPNZ SCASB** from the original brute-force
approach of **REPZ CMPSB** at every location had been based on my
observation that the first character comparison at each buffer location
usually fails. Why not apply the same concept to Boyer-Moore? Listing
14.3 is just like the standard implementation—except that it’s optimized
14.3 is just like the standard implementation—except that it's optimized
to handle a first-comparison mismatch as quickly as possible in the loop
at **QuickSearchLoop**, much as **REPNZ SCASB** optimizes
first-comparison mismatches for the brute-force approach. The results in