If the haystack length is n and the needle length is m, let p = (n – m)/2 and q = p + m – 2. Now spawn two threads, each doing Boyer-Moore, one from 0 to q and the other from p to n – 1 in your big char array. If one thread finds a match, halt both.

This parallelizes the job and divides it exactly in two. Note that if the needle is in the haystack it will be found; if an occurrence contained a character one thread wouldn’t search and another the other wouldn’t search, it would have to contain the character before p and the character after q, plus the m – 1 characters from p (inclusive) to q (inclusive), and so have length m + 1, yet it has length m.

Of course, the serial version should be run if the haystack is too small.

Generalizing this to N-threaded versions is not too difficult, and is left as an exercise for the reader.

Mike wants you (and the very first commenter) to know that the sequence of skips he chose was carefully tuned by timing a grep for “vt100″ in /etc/termcap. At the time, this was a very important benchmark.

Mike has lots of grep speed tricks left up his sleeve, and has promised to share some with me at some point. I’ll keep you posted.

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assuming that:
char *buffer haystack
size_t length the length of haystack

int needleLen = Strlen(needle), k;
register char *p;

switch (needleLen)
{
case 0:
return NULL;

case 1:
for (p = buffer; *p; p++)
{
if (*p == *needle)
return p;
}
return NULL;
break;

}

register const int needleSafetyBouncer = needleLen – 1;
register int i;
register byte delta[256];
register const char *end = buffer + length;
for (i = 0; i =0 ; k++, i–)
{
if (delta[(byte)needle[i]] == needleLen)
delta[(byte)needle[i]] = k;
}

for (p = buffer; p X where X around 48 or so, and use Duff’s device to reduce the checks for i going down to zero, in order to speed it up even more.

You imagine wrong: grep was developed long before vi (vi came from Berkeley Unix), and very long before X Windows. The developer of grep probably used the “ed” editor and a 80×24 CRT display.