Similar to <http://en.wikipedia.org/wiki/Rabin-Karp_string_search_algorithm>

Think of a string as coefficients of a polynomial H (calculated modulo some, say, 31-bit prime P, e.g., 2000000011). Take a random X and calculate H(X). Take the second string and calculate its code. To calculate the value for a shifted string simply add to it A*(1-X^n), where A is the i-th character (this is a simplification of the needed steps: subtract A*X^{n-1}, multiply by X, and add A). If value for the shifted string is the same as for the first, output i (if P is large you don't even need to check).