Precision of FastInvSquare

comparison.rs

fn fast_invsqrt(number: f32, iterations: usize) -> f32 {
  let threehalfs: f32 = 1.5;
  let x2: f32 = number * 0.5;
  let mut i: u32 = number.to_bits();
  i = 0x5f3759df - (i >> 1);
  let mut y: f32 = f32::from_bits(i);
  for _ in 0..iterations {
    y  = y * ( threehalfs - ( x2 * y * y ) );
  }
  y
}

fn slow_invsqrt(number: f32) -> f32 {
  1.0 / number.sqrt()
}

const MAX_ROUNDS: usize = 3;

fn main() {
  let mut maxdeltas = [0f32; MAX_ROUNDS+1];

  for i in 0..u32::MAX {
    let number = f32::from_bits(i);
    if number.is_normal() {
      let slow = slow_invsqrt(number);
      for i in 0..MAX_ROUNDS+1 {
        let fast = fast_invsqrt(number, i);
        let delta = ((fast / slow) - 1.0).abs();
        if delta > maxdeltas[i] {
          maxdeltas[i] = delta;
        }
      }
    }
  }

  for i in 0..MAX_ROUNDS+1 {
    println!("Maximum innacuracy {} rounds: {:#?}%", i, maxdeltas[i] * 100.0);
  }
}