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crates/core_simd/examples/nbody.rs
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182
crates/core_simd/examples/nbody.rs
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/// Benchmarks game nbody code
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/// Taken from the `packed_simd` crate
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/// Run this benchmark with `cargo test --example nbody`
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use core_simd::*;
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use std::f64::consts::PI;
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const SOLAR_MASS: f64 = 4.0 * PI * PI;
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const DAYS_PER_YEAR: f64 = 365.24;
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#[derive(Debug, Clone, Copy)]
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pub struct Body {
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pub x: f64x4,
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pub v: f64x4,
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pub mass: f64,
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}
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const N_BODIES: usize = 5;
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const BODIES: [Body; N_BODIES] = [
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// sun:
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Body {
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x: f64x4::from_array([0., 0., 0., 0.]),
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v: f64x4::from_array([0., 0., 0., 0.]),
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mass: SOLAR_MASS,
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},
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// jupiter:
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Body {
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x: f64x4::from_array([
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4.84143144246472090e+00,
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-1.16032004402742839e+00,
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-1.03622044471123109e-01,
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0.,
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]),
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v: f64x4::from_array([
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1.66007664274403694e-03 * DAYS_PER_YEAR,
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7.69901118419740425e-03 * DAYS_PER_YEAR,
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-6.90460016972063023e-05 * DAYS_PER_YEAR,
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0.,
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]),
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mass: 9.54791938424326609e-04 * SOLAR_MASS,
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},
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// saturn:
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Body {
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x: f64x4::from_array([
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8.34336671824457987e+00,
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4.12479856412430479e+00,
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-4.03523417114321381e-01,
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0.,
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]),
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v: f64x4::from_array([
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-2.76742510726862411e-03 * DAYS_PER_YEAR,
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4.99852801234917238e-03 * DAYS_PER_YEAR,
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2.30417297573763929e-05 * DAYS_PER_YEAR,
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0.,
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]),
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mass: 2.85885980666130812e-04 * SOLAR_MASS,
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},
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// uranus:
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Body {
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x: f64x4::from_array([
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1.28943695621391310e+01,
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-1.51111514016986312e+01,
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-2.23307578892655734e-01,
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0.,
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]),
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v: f64x4::from_array([
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2.96460137564761618e-03 * DAYS_PER_YEAR,
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2.37847173959480950e-03 * DAYS_PER_YEAR,
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-2.96589568540237556e-05 * DAYS_PER_YEAR,
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0.,
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]),
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mass: 4.36624404335156298e-05 * SOLAR_MASS,
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},
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// neptune:
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Body {
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x: f64x4::from_array([
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1.53796971148509165e+01,
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-2.59193146099879641e+01,
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1.79258772950371181e-01,
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0.,
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]),
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v: f64x4::from_array([
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2.68067772490389322e-03 * DAYS_PER_YEAR,
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1.62824170038242295e-03 * DAYS_PER_YEAR,
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-9.51592254519715870e-05 * DAYS_PER_YEAR,
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0.,
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]),
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mass: 5.15138902046611451e-05 * SOLAR_MASS,
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},
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];
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pub fn offset_momentum(bodies: &mut [Body; N_BODIES]) {
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let (sun, rest) = bodies.split_at_mut(1);
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let sun = &mut sun[0];
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for body in rest {
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let m_ratio = body.mass / SOLAR_MASS;
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sun.v -= body.v * m_ratio;
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}
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}
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pub fn energy(bodies: &[Body; N_BODIES]) -> f64 {
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let mut e = 0.;
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for i in 0..N_BODIES {
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let bi = &bodies[i];
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e += bi.mass * (bi.v * bi.v).horizontal_sum() * 0.5;
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for bj in bodies.iter().take(N_BODIES).skip(i + 1) {
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let dx = bi.x - bj.x;
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e -= bi.mass * bj.mass / (dx * dx).horizontal_sum().sqrt()
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}
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}
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e
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}
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pub fn advance(bodies: &mut [Body; N_BODIES], dt: f64) {
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const N: usize = N_BODIES * (N_BODIES - 1) / 2;
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// compute distance between bodies:
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let mut r = [f64x4::splat(0.); N];
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{
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let mut i = 0;
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for j in 0..N_BODIES {
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for k in j + 1..N_BODIES {
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r[i] = bodies[j].x - bodies[k].x;
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i += 1;
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}
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}
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}
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let mut mag = [0.0; N];
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for i in (0..N).step_by(2) {
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let d2s = f64x2::from_array([
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(r[i] * r[i]).horizontal_sum(),
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(r[i + 1] * r[i + 1]).horizontal_sum(),
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]);
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let dmags = f64x2::splat(dt) / (d2s * d2s.sqrt());
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mag[i] = dmags[0];
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mag[i + 1] = dmags[1];
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}
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let mut i = 0;
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for j in 0..N_BODIES {
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for k in j + 1..N_BODIES {
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let f = r[i] * mag[i];
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bodies[j].v -= f * bodies[k].mass;
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bodies[k].v += f * bodies[j].mass;
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i += 1
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}
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}
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for body in bodies {
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body.x += dt * body.v
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}
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}
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pub fn run(n: usize) -> (f64, f64) {
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let mut bodies = BODIES;
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offset_momentum(&mut bodies);
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let energy_before = energy(&bodies);
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for _ in 0..n {
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advance(&mut bodies, 0.01);
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}
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let energy_after = energy(&bodies);
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(energy_before, energy_after)
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}
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#[cfg(test)]
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mod tests {
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// Good enough for demonstration purposes, not going for strictness here.
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fn approx_eq_f64(a: f64, b: f64) -> bool {
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(a - b).abs() < 0.00001
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}
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#[test]
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fn test() {
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const OUTPUT: [f64; 2] = [-0.169075164, -0.169087605];
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let (energy_before, energy_after) = super::run(1000);
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assert!(approx_eq_f64(energy_before, OUTPUT[0]));
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assert!(approx_eq_f64(energy_after, OUTPUT[1]));
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}
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}
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fn main() {
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// empty main to pass CI
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}
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