Question

I have a file that’s 21056 bytes.

I’ve written a program in C that reads the entire file into a buffer, and then uses multiple search algorithms to search the file for a token that’s 82 chars.

I’ve used all the implementations of the algorithms from the “Exact String Matching Algorithms” page. I’ve used: KMP, BM, TBM, and Horspool. And then I used strstr and benchmarked each one.

What I’m wondering is, each time the strstr outperforms all the other algorithms. The only one that is faster sometimes is BM.

Shouldn’t strstr be the slowest?

Here’s my benchmark code with an example of benchmarking BM:

double get_time()
{
    LARGE_INTEGER t, f;
    QueryPerformanceCounter(&t);
    QueryPerformanceFrequency(&f);
    return (double)t.QuadPart/(double)f.QuadPart;
}

before = get_time();
BM(token, strlen(token), buffer, len);
after = get_time();
printf("Time: %f\n\n", after - before);

Could someone explain to me why strstr is outperforming the other search algorithms? I’ll post more code on request if needed.

Answer 1

Why do you think strstr should be slower than all the others? Do you know what algorithm strstr uses? I think it’s quite likely that strstr uses a fine-tuned, processor-specific, assembly-coded algorithm of the KMP type or better. In which case you don’t stand a chance of out-performing it in C for such small benchmarks.

(The reason I think this is likely is that programmers love to implement such things.)