d20.c

/* Disclaimer: I don't think I have written C in 30 years. */
static uint32_t randval = 0;
static uint32_t shift = 30;

static uint32_t next_d32() {
  /* pull 6 5-bit random numbers out of every 31 bit random() call */
  uint32_t result;
  if (shift >= 30) {
    randval = random();
    shift = 0;
  }
  result = (randval >> shift) & 0x1F;
  shift = shift + 5;
  return result;
}

static uint32_t next_d20() {
  uint32_t result;
  do {
    result = next_d32();
  } while (result >= 20);
  return result;
}



/* ----- alternative that trades the loop for 3 ops and more frequent random() calls (if it is stil unbiased) ----- */
static uint32_t randval = 0;
static uint32_t shift = 30;

static uint32_t next_d64() {
  /* pull 5 6-bit random numbers out of every 31 bit random() call */
  uint32_t result;
  if (shift >= 30) {
    randval = random();
    shift = 0;
  }
  result = (randval >> shift) & 0x3F;
  shift = shift + 6;
  return result;
}

static uint32_t next_d20() {
  uint32_t result = next_d64();
  return (result & 3) + (result >> 2);
}