float_i2f.c

/*
 * float_i2f - Return bit-level equivalent of expression (float) x
 *   Result is returned as unsigned int, but
 *   it is to be interpreted as the bit-level representation of a
 *   single-precision floating point values.
 *   Legal ops: Any integer/unsigned operations incl. ||, &&. also if, while
 *   Max ops: 30
 *   Rating: 4
 */
unsigned float_i2f(int x) {
  unsigned sign, exp, frac;
  unsigned E, x_copy;
  unsigned round_num_digits, round_part, round_mid;
  // special cases
  if (x == 0) return 0;
  if (x == 0x80000000) return 0xcf000000;

  // sign
  sign = x & 0x80000000;

  if (sign) x = -x;

  // exp
  // E = ilog2(x);
  E = 0;
  for (x_copy = x; x_copy >>= 1;) ++E;
  exp = E + 127;

  // frac
  // clear the leading 1
  frac = x & ((1 << E) - 1);
  if (E <= 23) {
    frac <<= 23 - E;
  } else {
    round_num_digits = E - 23;
    round_part = frac & ((1 << round_num_digits) - 1);
    frac >>= round_num_digits;
    // round
    round_mid = 1 << (round_num_digits - 1);
    if (round_part > round_mid || (round_part == round_mid && (frac & 1) /* round to even */)) {
      ++frac;
      // frac may have a carry.
    }
  }

  return sign | ((exp << 23) + frac);
}