mi****@gmail.com wrote:
Walter Roberson wrote: <mo*****@gmail.com> wrote: I'm a python programmer that's started to play a bit with C as
I'll probably have to make C extensions eventually.. I made
this little program that I'd like to get feedback on, it's
basically a find substring and return pointer to it function
and tests for it..
Could this be done better/differently? Is anything fundamentally
wrong?
Suppose you don't find a match the first iteration through
because the 10th character in the trial substring does not match
the 10th character in the input string. You then loop back
and start testing from the substring again from the second
character of the input string -- ignoring all the information
you could have gleened by paying attention to what was in
offsets 1 thru 9 that you have already looked at.
Suppose for example that what you found at the 10th character at the
input string was an 'X', and there are no occurances of 'X' in the
trial substring. A moment's reflection would show you that in
such an instance you would be able to skip testing for the
substring as starting from 1 thru 9, since that 'X' will still be
there at offset 9 blocking any possible match.
What this tells you is that there are algorithms which can be
much faster than your search algorithm. Indeed, there is an entire
literature on search algorithms. Some are probably mentioned in the C
FAQ or by googling for "string search algorithms".
Boyer Horspool Moore comes to mind. I implemented it with
a circular buffer; it's fast.
Knuth-Morris-Pratt has the distinct advantage of allowing operation
on streams and totally avoiding any need for look ahead or look
back. The following is from a post I made here almost two years
ago, and illustrates the use of KMP on a file stream.
/*
Leor Zolman wrote: On 25 Feb 2004 07:34:40 -0800, jo**@ljungh.se (spike) wrote:
Im trying to write a program that should read through a binary
file searching for the character sequence "\name\"
Then it should read the characters following the "\name\"
sequence until a NULL character is encountered.
But when my program runs it gets a SIGSEGV (Segmentation
vioalation) signal.
Whats wrong? And is there a better way than mine to solve
this task (most likely)
I think so. Here's a version I just threw together:
*/
/* And heres another throw -- binfsrch.c by CBF */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <assert.h>
/* The difference between a binary and a text file, on read,
is the conversion of end-of-line delimiters. What those
delimiters are does not affect the action. In some cases
the presence of 0x1a EOF markers (MsDos) does.
This is a version of Knuth-Morris-Pratt algorithm. The
point of using this is to avoid any backtracking in file
reading, and thus avoiding any use of buffer arrays.
*/
size_t chrcount; /* debuggery, count of input chars, zeroed */
/* --------------------- */
/* Almost straight out of Sedgewick */
/* The next array indicates what index in id should next be
compared to the current char. Once the (lgh - 1)th char
has been successfully compared, the id has been found.
The array is formed by comparing id to itself. */
void initnext(int *next, const char *id, int lgh)
{
int i, j;
assert(lgh > 0);
next[0] = -1; i = 0; j = -1;
while (i < lgh) {
while ((j >= 0) && (id[i] != id[j])) j = next[j];
i++; j++;
next[i] = j;
}
#if (0)
for (i = 0; i < lgh; i++)
printf("id[%d] = '%c' next[%d] = %d\n",
i, id[i], i, next[i]);
#endif
} /* initnext */
/* --------------------- */
/* reads f without rewinding until either EOF or *marker
has been found. Returns EOF if not found. At exit the
last matching char has been read, and no further. */
int kmpffind(const char *marker, int lgh, int *next, FILE *f)
{
int j; /* char position in marker to check */
int ch; /* current char */
assert(lgh > 0);
j = 0;
while ((j < lgh) && (EOF != (ch = getc(f)))) {
chrcount++;
while ((j >= 0) && (ch != marker[j])) j = next[j];
j++;
}
return ch;
} /* kmpffind */
/* --------------------- */
/* Find marker in f, display following printing chars
up to some non printing character or EOF */
int binfsrch(const char *marker, FILE *f)
{
int *next;
int lgh;
int ch;
int items; /* count of markers found */
lgh = strlen(marker);
if (!(next = malloc(lgh * sizeof *next))) {
puts("No memory");
exit(EXIT_FAILURE);
}
else {
initnext(next, marker, lgh);
items = 0;
while (EOF != kmpffind(marker, lgh, next, f)) {
/* found, take appropriate action */
items++;
printf("%d %s : \"", items, marker);
while (isprint(ch = getc(f))) {
chrcount++;
putchar(ch);
}
puts("\"");
if (EOF == ch) break;
else chrcount++;
}
free(next);
return items;
}
} /* binfsrch */
/* --------------------- */
int main(int argc, char **argv)
{
FILE *f;
f = stdin;
if (3 == argc) {
if (!(f = fopen(argv[2], "rb"))) {
printf("Can't open %s\n", argv[2]);
exit(EXIT_FAILURE);
}
argc--;
}
if (2 != argc) {
puts("Usage: binfsrch name [binaryfile]");
puts(" (file defaults to stdin text mode)");
}
else if (binfsrch(argv[1], f)) {
printf("\"%s\" : found\n", argv[1]);
}
else printf("\"%s\" : not found\n", argv[1]);
printf("%lu chars\n", (unsigned long)chrcount);
return 0;
} /* main binfsrch */
--
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