Knuth–Morris–Pratt algorithm

The Knuth-Morris-Pratt string searching algorithm locates a pattern of characters within a string. It uses the fact that after finding the first mismatch between pattern and string we already know the characters compared before to improve the efficiency of the search. We therefore will have to compare the pattern to itself to determine how many positions we have to move to the left.

The following implementation is written in C:

INPUT: Text T[0,n-1], Pattern P[0,m-1]
Output alle matches of P in T

InitNext(P);
j=0;
for (i=1;i<=n;i++)
{
   while ((j>0) && (P[j+1] != T[i]))
   {
      j=next[j];
   }
   if (P[j+1] == T[i])
      j++;
   if (j == m-1)
   {
      print "Found P";
      j = next[j];
   }
}

Procedure InitNext(P)
Input: Pattern P

next[1] = 0;
for (q=2;q<=m;q++)
{
   while ((l>0) && (P[l+1] != P[q]))
   {
      l = next[l];
   } 
   if (P[l+1] == P[q])
      l++;
   next[q] = l;
}