/*
 * 15-122 Principles of Imperative Computation, Spring 2011
 * Hash function on strings
 * Uses pseudo-random number generation, deterministically
 */

#use <string>

int hash_string(string s, int m)
//@requires m > 1;
//@ensures 0 <= \result && \result < m;
{
  int a = 1664525; int b = 1013904223; /* inlined random number generator */
  int r = 0x1337beef;		       /* initial seed */
  int len = string_length(s);
  int h = 0;			/* empty string maps to 0 */
  for (int i = 0; i < len; i++)
    //@loop_invariant 0 <= i && i <= len;
    {
      h = r*h + char_ord(string_charat(s, i)); /* mod 2^32 */
      r = r*a + b;	        /* mod 2^32, linear congruential random no */
    }
  h = h % m;			/* reduce to range */
  //@assert -m < h && h < m;
  if (h < 0) h += m;		/* make positive, if necessary */
  return h;
}