Function project

third-party/lua-5.5.0/src/lmathlib.c:569–579

** Project the random integer 'ran' into the interval [0, n]. ** Because 'ran' has 2^B possible values, the projection can only be ** uniform when the size of the interval is a power of 2 (exact ** division). So, to get a uniform projection into [0, n], we ** first compute 'lim', the smallest Mersenne number not smaller than ** 'n'. We then project 'ran' into the interval [0, lim]. If the result

** until we have a result inside the interval.
*/
static lua_Unsigned project (lua_Unsigned ran, lua_Unsigned n,
                             RanState *state) {
  lua_Unsigned lim = n;  /* to compute the Mersenne number */
  int sh;  /* how much to spread bits to the right in 'lim' */
  /* spread '1' bits in 'lim' until it becomes a Mersenne number */
  for (sh = 1; (lim & (lim + 1)) != 0; sh *= 2)
    lim |= (lim >> sh);  /* spread '1's to the right */
  while ((ran &= lim) > n)  /* project 'ran' into [0..lim] and test */
    ran = I2UInt(nextrand(state->s));  /* not inside [0..n]? try again */
  return ran;
}


static int math_random (lua_State *L) {