Optimized version of strstr (search has constant length)

Question

My C program had a lot of strstr function calls. The standard library strstr is already fast but in my case the search string has always length of 5 characters. I replaced i

Answer 1

Reducing the number of comparisons will increase the speed of the search. Keep a running int of the string and compare it to a fixed int for the search term. If it matches compare the last character.

uint32_t term = ct[0] << 24 | ct[1] << 16 | ct[2] << 8 | ct[3];
uint32_t walk = cs[0] << 24 | cs[1] << 16 | cs[2] << 8 | cs[3];
int i = 0;

do {
  if ( term == walk && ct[4] == cs[4] ) { return i; } // or return cs or 1
  walk = ( walk << 8 ) | cs[4];
  cs += 1;
  i += 1;
} while ( cs[4] ); // assumes original cs was longer than ct
// return failure

Add checks for a short cs.

Edit:

Added fixes from comments. Thanks.

This could easily be adopted to use 64 bit values. You could store cs[4] and ct[4] in local variables instead of assuming the compiler will do that for you. You could add 4 to cs and ct before the loop and use cs[0] and ct[0] in the loop.