library/portable-simd/crates/core_simd/examples/nbody.rs

   1 #![cfg_attr(feature = "std", feature(portable_simd))]
   2
   3 /// Benchmarks game nbody code
   4 /// Taken from the `packed_simd` crate
   5 /// Run this benchmark with `cargo test --example nbody`
   6 #[cfg(feature = "std")]
   7 mod nbody {
   8     use core_simd::*;
   9
  10     use std::f64::consts::PI;
  11     const SOLAR_MASS: f64 = 4.0 * PI * PI;
  12     const DAYS_PER_YEAR: f64 = 365.24;
  13
  14     #[derive(Debug, Clone, Copy)]
  15     struct Body {
  16         pub x: f64x4,
  17         pub v: f64x4,
  18         pub mass: f64,
  19     }
  20
  21     const N_BODIES: usize = 5;
  22     const BODIES: [Body; N_BODIES] = [
  23         // sun:
  24         Body {
  25             x: f64x4::from_array([0., 0., 0., 0.]),
  26             v: f64x4::from_array([0., 0., 0., 0.]),
  27             mass: SOLAR_MASS,
  28         },
  29         // jupiter:
  30         Body {
  31             x: f64x4::from_array([
  32                 4.84143144246472090e+00,
  33                 -1.16032004402742839e+00,
  34                 -1.03622044471123109e-01,
  35                 0.,
  36             ]),
  37             v: f64x4::from_array([
  38                 1.66007664274403694e-03 * DAYS_PER_YEAR,
  39                 7.69901118419740425e-03 * DAYS_PER_YEAR,
  40                 -6.90460016972063023e-05 * DAYS_PER_YEAR,
  41                 0.,
  42             ]),
  43             mass: 9.54791938424326609e-04 * SOLAR_MASS,
  44         },
  45         // saturn:
  46         Body {
  47             x: f64x4::from_array([
  48                 8.34336671824457987e+00,
  49                 4.12479856412430479e+00,
  50                 -4.03523417114321381e-01,
  51                 0.,
  52             ]),
  53             v: f64x4::from_array([
  54                 -2.76742510726862411e-03 * DAYS_PER_YEAR,
  55                 4.99852801234917238e-03 * DAYS_PER_YEAR,
  56                 2.30417297573763929e-05 * DAYS_PER_YEAR,
  57                 0.,
  58             ]),
  59             mass: 2.85885980666130812e-04 * SOLAR_MASS,
  60         },
  61         // uranus:
  62         Body {
  63             x: f64x4::from_array([
  64                 1.28943695621391310e+01,
  65                 -1.51111514016986312e+01,
  66                 -2.23307578892655734e-01,
  67                 0.,
  68             ]),
  69             v: f64x4::from_array([
  70                 2.96460137564761618e-03 * DAYS_PER_YEAR,
  71                 2.37847173959480950e-03 * DAYS_PER_YEAR,
  72                 -2.96589568540237556e-05 * DAYS_PER_YEAR,
  73                 0.,
  74             ]),
  75             mass: 4.36624404335156298e-05 * SOLAR_MASS,
  76         },
  77         // neptune:
  78         Body {
  79             x: f64x4::from_array([
  80                 1.53796971148509165e+01,
  81                 -2.59193146099879641e+01,
  82                 1.79258772950371181e-01,
  83                 0.,
  84             ]),
  85             v: f64x4::from_array([
  86                 2.68067772490389322e-03 * DAYS_PER_YEAR,
  87                 1.62824170038242295e-03 * DAYS_PER_YEAR,
  88                 -9.51592254519715870e-05 * DAYS_PER_YEAR,
  89                 0.,
  90             ]),
  91             mass: 5.15138902046611451e-05 * SOLAR_MASS,
  92         },
  93     ];
  94
  95     fn offset_momentum(bodies: &mut [Body; N_BODIES]) {
  96         let (sun, rest) = bodies.split_at_mut(1);
  97         let sun = &mut sun[0];
  98         for body in rest {
  99             let m_ratio = body.mass / SOLAR_MASS;
 100             sun.v -= body.v * m_ratio;
 101         }
 102     }
 103
 104     fn energy(bodies: &[Body; N_BODIES]) -> f64 {
 105         let mut e = 0.;
 106         for i in 0..N_BODIES {
 107             let bi = &bodies[i];
 108             e += bi.mass * (bi.v * bi.v).horizontal_sum() * 0.5;
 109             for bj in bodies.iter().take(N_BODIES).skip(i + 1) {
 110                 let dx = bi.x - bj.x;
 111                 e -= bi.mass * bj.mass / (dx * dx).horizontal_sum().sqrt()
 112             }
 113         }
 114         e
 115     }
 116
 117     fn advance(bodies: &mut [Body; N_BODIES], dt: f64) {
 118         const N: usize = N_BODIES * (N_BODIES - 1) / 2;
 119
 120         // compute distance between bodies:
 121         let mut r = [f64x4::splat(0.); N];
 122         {
 123             let mut i = 0;
 124             for j in 0..N_BODIES {
 125                 for k in j + 1..N_BODIES {
 126                     r[i] = bodies[j].x - bodies[k].x;
 127                     i += 1;
 128                 }
 129             }
 130         }
 131
 132         let mut mag = [0.0; N];
 133         for i in (0..N).step_by(2) {
 134             let d2s = f64x2::from_array([
 135                 (r[i] * r[i]).horizontal_sum(),
 136                 (r[i + 1] * r[i + 1]).horizontal_sum(),
 137             ]);
 138             let dmags = f64x2::splat(dt) / (d2s * d2s.sqrt());
 139             mag[i] = dmags[0];
 140             mag[i + 1] = dmags[1];
 141         }
 142
 143         let mut i = 0;
 144         for j in 0..N_BODIES {
 145             for k in j + 1..N_BODIES {
 146                 let f = r[i] * mag[i];
 147                 bodies[j].v -= f * bodies[k].mass;
 148                 bodies[k].v += f * bodies[j].mass;
 149                 i += 1
 150             }
 151         }
 152         for body in bodies {
 153             body.x += dt * body.v
 154         }
 155     }
 156
 157     pub fn run(n: usize) -> (f64, f64) {
 158         let mut bodies = BODIES;
 159         offset_momentum(&mut bodies);
 160         let energy_before = energy(&bodies);
 161         for _ in 0..n {
 162             advance(&mut bodies, 0.01);
 163         }
 164         let energy_after = energy(&bodies);
 165
 166         (energy_before, energy_after)
 167     }
 168 }
 169
 170 #[cfg(feature = "std")]
 171 #[cfg(test)]
 172 mod tests {
 173     // Good enough for demonstration purposes, not going for strictness here.
 174     fn approx_eq_f64(a: f64, b: f64) -> bool {
 175         (a - b).abs() < 0.00001
 176     }
 177     #[test]
 178     fn test() {
 179         const OUTPUT: [f64; 2] = [-0.169075164, -0.169087605];
 180         let (energy_before, energy_after) = super::nbody::run(1000);
 181         assert!(approx_eq_f64(energy_before, OUTPUT[0]));
 182         assert!(approx_eq_f64(energy_after, OUTPUT[1]));
 183     }
 184 }
 185
 186 fn main() {
 187     #[cfg(feature = "std")]
 188     {
 189         let (energy_before, energy_after) = nbody::run(1000);
 190         println!("Energy before: {}", energy_before);
 191         println!("Energy after:  {}", energy_after);
 192     }
 193 }