Add rust n-body version for comparison (#88)

examples/n-body/rust/src/lib.rs

@@ -0,0 +1,188 @@
+// Code adopted from https://benchmarksgame-team.pages.debian.net/benchmarksgame/program/nbody-rust-1.html
+
+#![feature(core_intrinsics, lang_items)]
+#![no_std]
+
+#[lang = "panic_fmt"]
+extern "C" fn panic_fmt(_args: ::core::fmt::Arguments, _file: &'static str, _line: u32) -> ! {
+  use core::intrinsics;
+  unsafe {
+    intrinsics::abort();
+  }
+}
+
+#[inline(always)]
+fn sqrt(x: f64) -> f64 {
+  use core::intrinsics;
+  unsafe {
+    intrinsics::sqrtf64(x)
+  }
+}
+
+const PI: f64         = 3.141592653589793;
+const SOLAR_MASS: f64 = 4.0 * PI * PI;
+const YEAR: f64       = 365.24;
+const N_BODIES: usize = 5;
+
+static mut BODIES: [Planet; N_BODIES] = [
+  // Sun
+  Planet {
+    x:   0.0,
+    y:   0.0,
+    z:   0.0,
+    vx:  0.0,
+    vy:  0.0,
+    vz:  0.0,
+    mass: SOLAR_MASS,
+  },
+  // Jupiter
+  Planet {
+    x:    4.84143144246472090e+00,
+    y:   -1.16032004402742839e+00,
+    z:   -1.03622044471123109e-01,
+    vx:   1.66007664274403694e-03 * YEAR,
+    vy:   7.69901118419740425e-03 * YEAR,
+    vz:  -6.90460016972063023e-05 * YEAR,
+    mass: 9.54791938424326609e-04 * SOLAR_MASS,
+  },
+  // Saturn
+  Planet {
+    x:    8.34336671824457987e+00,
+    y:    4.12479856412430479e+00,
+    z:   -4.03523417114321381e-01,
+    vx:  -2.76742510726862411e-03 * YEAR,
+    vy:   4.99852801234917238e-03 * YEAR,
+    vz:   2.30417297573763929e-05 * YEAR,
+    mass: 2.85885980666130812e-04 * SOLAR_MASS,
+  },
+  // Uranus
+  Planet {
+    x:    1.28943695621391310e+01,
+    y:   -1.51111514016986312e+01,
+    z:   -2.23307578892655734e-01,
+    vx:   2.96460137564761618e-03 * YEAR,
+    vy:   2.37847173959480950e-03 * YEAR,
+    vz:  -2.96589568540237556e-05 * YEAR,
+    mass: 4.36624404335156298e-05 * SOLAR_MASS,
+  },
+  // Neptune
+  Planet {
+    x:    1.53796971148509165e+01,
+    y:   -2.59193146099879641e+01,
+    z:    1.79258772950371181e-01,
+    vx:   2.68067772490389322e-03 * YEAR,
+    vy:   1.62824170038242295e-03 * YEAR,
+    vz:  -9.51592254519715870e-05 * YEAR,
+    mass: 5.15138902046611451e-05 * SOLAR_MASS,
+  },
+];
+
+#[derive(Clone, Copy)]
+struct Planet {
+  x: f64,
+  y: f64,
+  z: f64,
+  vx: f64,
+  vy: f64,
+  vz: f64,
+  mass: f64,
+}
+
+fn advance(bodies: &mut [Planet; N_BODIES], dt: f64) {
+  let mut b_slice: &mut [_] = bodies;
+  loop {
+    let bi = match shift_mut_ref(&mut b_slice) {
+      Some(bi) => bi,
+      None     => break,
+    };
+
+    for bj in b_slice.iter_mut() {
+      let dx = bi.x - bj.x;
+      let dy = bi.y - bj.y;
+      let dz = bi.z - bj.z;
+
+      let d2  = dx * dx + dy * dy + dz * dz;
+      let mag = dt / (d2 * sqrt(d2));
+
+      let massj_mag = bj.mass * mag;
+      bi.vx -= dx * massj_mag;
+      bi.vy -= dy * massj_mag;
+      bi.vz -= dz * massj_mag;
+
+      let massi_mag = bi.mass * mag;
+      bj.vx += dx * massi_mag;
+      bj.vy += dy * massi_mag;
+      bj.vz += dz * massi_mag;
+    }
+    bi.x += dt * bi.vx;
+    bi.y += dt * bi.vy;
+    bi.z += dt * bi.vz;
+  }
+}
+
+fn energy(bodies: &[Planet; N_BODIES]) -> f64 {
+  let mut e = 0.0;
+  let mut bodies = bodies.iter();
+
+  loop {
+    let bi = match bodies.next() {
+      Some(bi) => bi,
+      None     => break,
+    };
+
+    e += (bi.vx * bi.vx + bi.vy * bi.vy + bi.vz * bi.vz) * bi.mass / 2.0;
+    for bj in bodies.clone() {
+      let dx = bi.x - bj.x;
+      let dy = bi.y - bj.y;
+      let dz = bi.z - bj.z;
+      let dist = sqrt(dx * dx + dy * dy + dz * dz);
+      e -= bi.mass * bj.mass / dist;
+    }
+  }
+  e
+}
+
+fn offset_momentum(bodies: &mut [Planet; N_BODIES]) {
+  let mut px = 0.0;
+  let mut py = 0.0;
+  let mut pz = 0.0;
+  for bi in bodies.iter() {
+    px += bi.vx * bi.mass;
+    py += bi.vy * bi.mass;
+    pz += bi.vz * bi.mass;
+  }
+  let sun = &mut bodies[0];
+  sun.vx = -px / SOLAR_MASS;
+  sun.vy = -py / SOLAR_MASS;
+  sun.vz = -pz / SOLAR_MASS;
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn init() {
+  offset_momentum(&mut BODIES);
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn step() -> f64 {
+  advance(&mut BODIES, 0.01);
+  energy(&BODIES)
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn bench(steps: i32) {
+  for _ in 0..steps {
+    advance(&mut BODIES, 0.01);
+  }
+}
+
+/// Pop a mutable reference off the head of a slice, mutating the slice to no
+/// longer contain the mutable reference.
+fn shift_mut_ref<'a, T>(r: &mut &'a mut [T]) -> Option<&'a mut T> {
+  if r.len() == 0 {
+    return None;
+  }
+  let tmp = core::mem::replace(r, &mut []);
+  let (h, t) = tmp.split_at_mut(1);
+  *r = t;
+  Some(&mut h[0])
+}