nbody example

diff --git a/crates/core_simd/examples/nbody.rs b/crates/core_simd/examples/nbody.rs
new file mode 100644 (file)
index 0000000..a474bbe
--- /dev/null
+++ b/crates/core_simd/examples/nbody.rs
@@ -0,0 +1,188 @@
+use stdsimd::simd::*;
+
+use std::f64::consts::PI;
+const SOLAR_MASS: f64 = 4.0 * PI * PI;
+const DAYS_PER_YEAR: f64 = 365.24;
+
+pub struct Body {
+    pub x: f64x4,
+    pub v: f64x4,
+    pub mass: f64,
+}
+const N_BODIES: usize = 5;
+#[allow(clippy::unreadable_literal)]
+const BODIES: [Body; N_BODIES] = [
+    // sun:
+    Body {
+        x: f64x4::new(0., 0., 0., 0.),
+        v: f64x4::new(0., 0., 0., 0.),
+        mass: SOLAR_MASS,
+    },
+    // jupiter:
+    Body {
+        x: f64x4::new(
+            4.84143144246472090e+00,
+            -1.16032004402742839e+00,
+            -1.03622044471123109e-01,
+            0.,
+        ),
+        v: f64x4::new(
+            1.66007664274403694e-03 * DAYS_PER_YEAR,
+            7.69901118419740425e-03 * DAYS_PER_YEAR,
+            -6.90460016972063023e-05 * DAYS_PER_YEAR,
+            0.,
+        ),
+        mass: 9.54791938424326609e-04 * SOLAR_MASS,
+    },
+    // saturn:
+    Body {
+        x: f64x4::new(
+            8.34336671824457987e+00,
+            4.12479856412430479e+00,
+            -4.03523417114321381e-01,
+            0.,
+        ),
+        v: f64x4::new(
+            -2.76742510726862411e-03 * DAYS_PER_YEAR,
+            4.99852801234917238e-03 * DAYS_PER_YEAR,
+            2.30417297573763929e-05 * DAYS_PER_YEAR,
+            0.,
+        ),
+        mass: 2.85885980666130812e-04 * SOLAR_MASS,
+    },
+    // uranus:
+    Body {
+        x: f64x4::new(
+            1.28943695621391310e+01,
+            -1.51111514016986312e+01,
+            -2.23307578892655734e-01,
+            0.,
+        ),
+        v: f64x4::new(
+            2.96460137564761618e-03 * DAYS_PER_YEAR,
+            2.37847173959480950e-03 * DAYS_PER_YEAR,
+            -2.96589568540237556e-05 * DAYS_PER_YEAR,
+            0.,
+        ),
+        mass: 4.36624404335156298e-05 * SOLAR_MASS,
+    },
+    // neptune:
+    Body {
+        x: f64x4::new(
+            1.53796971148509165e+01,
+            -2.59193146099879641e+01,
+            1.79258772950371181e-01,
+            0.,
+        ),
+        v: f64x4::new(
+            2.68067772490389322e-03 * DAYS_PER_YEAR,
+            1.62824170038242295e-03 * DAYS_PER_YEAR,
+            -9.51592254519715870e-05 * DAYS_PER_YEAR,
+            0.,
+        ),
+        mass: 5.15138902046611451e-05 * SOLAR_MASS,
+    },
+];
+
+pub fn offset_momentum(bodies: &mut [Body; N_BODIES]) {
+    let (sun, rest) = bodies.split_at_mut(1);
+    let sun = &mut sun[0];
+    for body in rest {
+        let m_ratio = body.mass / SOLAR_MASS;
+        sun.v -= body.v * m_ratio;
+    }
+}
+
+pub fn energy(bodies: &[Body; N_BODIES]) -> f64 {
+    let mut e = 0.;
+    for i in 0..N_BODIES {
+        let bi = &bodies[i];
+        e += bi.mass * (bi.v * bi.v).sum() * 0.5;
+        for bj in bodies.iter().take(N_BODIES).skip(i + 1) {
+            let dx = bi.x - bj.x;
+            e -= bi.mass * bj.mass / (dx * dx).sum().sqrt()
+        }
+    }
+    e
+}
+
+pub fn advance(bodies: &mut [Body; N_BODIES], dt: f64) {
+    const N: usize = N_BODIES * (N_BODIES - 1) / 2;
+
+    // compute distance between bodies:
+    let mut r = [f64x4::splat(0.); N];
+    {
+        let mut i = 0;
+        for j in 0..N_BODIES {
+            for k in j + 1..N_BODIES {
+                r[i] = bodies[j].x - bodies[k].x;
+                i += 1;
+            }
+        }
+    }
+
+    let mut mag = [0.0; N];
+    let mut i = 0;
+    while i < N {
+        let d2s = f64x2::new((r[i] * r[i]).sum(), (r[i + 1] * r[i + 1]).sum());
+        let dmags = f64x2::splat(dt) / (d2s * d2s.sqrte());
+        dmags.write_to_slice_unaligned(&mut mag[i..]);
+        i += 2;
+    }
+
+    i = 0;
+    for j in 0..N_BODIES {
+        for k in j + 1..N_BODIES {
+            let f = r[i] * mag[i];
+            bodies[j].v -= f * bodies[k].mass;
+            bodies[k].v += f * bodies[j].mass;
+            i += 1
+        }
+    }
+    for body in bodies {
+        body.x += dt * body.v
+    }
+}
+
+pub fn run_k<K>(n: usize, k: K) -> (f64, f64)
+where
+    K: Fn(&mut [Body; N_BODIES], f64),
+{
+    let mut bodies = BODIES;
+    offset_momentum(&mut bodies);
+    let energy_before = energy(&bodies);
+    for _ in 0..n {
+        k(&mut bodies, 0.01);
+    }
+    let energy_after = energy(&bodies);
+
+    (energy_before, energy_after)
+}
+
+pub fn run(n: usize) -> (f64, f64) {
+    run_k(n, advance)
+}
+
+const OUTPUT: Vec<f64> = vec![-0.169075164, -0.169087605];
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn test() {
+        let mut out: Vec<u8> = Vec::new();
+        run(&mut out, 1000, 0);
+        for &(size, a_e, b_e) in crate::RESULTS {
+            let (a, b) = super::run(size);
+            assert_eq!(format!("{:.9}", a), a_e);
+            assert_eq!(format!("{:.9}", b), b_e);
+        }
+    }
+}
+fn main() {
+    //let n: usize = std::env::args()
+    //.nth(1)
+    //.expect("need one arg")
+    //.parse()
+    //.expect("argument should be a usize");
+    //run(&mut std::io::stdout(), n, alg);
+    println!("{:?}", run_k<10>(10, 10));
+}