From 0f8682d2f0603692545b7e0fa0970de6e6c5237e Mon Sep 17 00:00:00 2001
From: vertical222 <52521763+vertical222@users.noreply.github.com>
Date: Sat, 6 Jul 2019 13:49:16 +0800
Subject: [PATCH] Create nbody.v

---
 examples/nbody.v | 136 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 136 insertions(+)
 create mode 100644 examples/nbody.v

diff --git a/examples/nbody.v b/examples/nbody.v
new file mode 100644
index 0000000000..b24d2722bb
--- /dev/null
+++ b/examples/nbody.v
@@ -0,0 +1,136 @@
+// Ported based on https://benchmarksgame-team.pages.debian.net/benchmarksgame/program/nbody-go-3.html
+// Output:
+// -0.169075164
+// -0.169059907
+
+// Elapsed time: 4.91s
+// Maximum resident (kbytes): 788
+// Between Rust and Go
+
+import math
+
+const (
+    SolarMass   = 39.47841760435743197 //4.0 * math.Pi * math.Pi
+    DaysPerYear = 365.24
+    N           = 5
+)
+
+struct Position {
+pub mut:
+    x f64
+    y f64
+    z f64
+}
+
+struct Momentum {
+pub mut:
+    x f64
+    y f64
+    z f64
+    m f64
+}
+
+struct System {
+pub mut:
+      v []Momentum
+      s []Position
+}
+
+fn advance(sys System, dt f64) {
+    for i := 0; i < N - 1; i++ {
+        mut _vx := sys.v[i].x
+        mut _vy := sys.v[i].y
+        mut _vz := sys.v[i].z
+
+        for j := i + 1; j < N; j++ {
+            dx := sys.s[i].x - sys.s[j].x
+            dy := sys.s[i].y - sys.s[j].y
+            dz := sys.s[i].z - sys.s[j].z
+
+            dSquared := dx * dx + dy * dy + dz * dz
+            distance := math.sqrt(dSquared)
+            mag := (dt / (dSquared * distance))
+            mi := sys.v[i].m
+
+            _vx -= dx * sys.v[j].m * mag
+            _vy -= dy * sys.v[j].m * mag
+            _vz -= dz * sys.v[j].m * mag
+
+            sys.v[j].x += dx * mi * mag
+            sys.v[j].y += dy * mi * mag
+            sys.v[j].z += dz * mi * mag
+        }
+        sys.v[i].x = _vx
+        sys.v[i].y = _vy
+        sys.v[i].z = _vz
+    }
+
+    for i := 0; i < N; i++ {
+        sys.s[i].x += dt * sys.v[i].x
+        sys.s[i].y += dt * sys.v[i].y
+        sys.s[i].z += dt * sys.v[i].z
+    }
+}
+
+fn offsetmomentum(sys System) {
+    mut px := f64(0)
+    mut py := f64(0)
+    mut pz := f64(0)
+
+    for i := 0; i < N; i++ {
+        px += sys.v[i].x * sys.v[i].m
+        py += sys.v[i].y * sys.v[i].m
+        pz += sys.v[i].z * sys.v[i].m
+    }
+    sys.v[0].x = -px / SolarMass
+    sys.v[0].y = -py / SolarMass
+    sys.v[0].z = -pz / SolarMass
+}
+
+fn energy(sys System) f64 {
+    mut e := f64(0)
+    for i := 0; i < N; i++ {
+        e += 0.5 * sys.v[i].m * (sys.v[i].x * sys.v[i].x + sys.v[i].y * sys.v[i].y + sys.v[i].z * sys.v[i].z)
+        for j := i + 1; j < N; j++ {
+            dx := sys.s[i].x - sys.s[j].x
+            dy := sys.s[i].y - sys.s[j].y
+            dz := sys.s[i].z - sys.s[j].z
+            distance := math.sqrt(dx * dx + dy * dy + dz * dz)
+            e -= (sys.v[i].m * sys.v[j].m) / distance
+        }
+    }
+    return e
+}
+
+fn arr_momentum() []Momentum {
+    mut x := []Momentum
+    x << Momentum {0.0, 0.0, 0.0, SolarMass}
+    x << Momentum {1.66007664274403694e-03 * DaysPerYear, 7.69901118419740425e-03 * DaysPerYear, -6.90460016972063023e-05 * DaysPerYear, 9.54791938424326609e-04 * SolarMass}
+    x << Momentum {-2.76742510726862411e-03 * DaysPerYear, 4.99852801234917238e-03 * DaysPerYear, 2.30417297573763929e-05 * DaysPerYear, 2.85885980666130812e-04 * SolarMass}
+    x << Momentum {2.96460137564761618e-03 * DaysPerYear, 2.37847173959480950e-03 * DaysPerYear, -2.96589568540237556e-05 * DaysPerYear, 4.36624404335156298e-05 * SolarMass}
+    x << Momentum {2.68067772490389322e-03 * DaysPerYear, 1.62824170038242295e-03 * DaysPerYear, -9.51592254519715870e-05 * DaysPerYear, 5.15138902046611451e-05 * SolarMass}
+    return x
+}
+
+pub fn arr_position() []Position {
+    mut x := []Position
+    x << Position {0.0, 0.0, 0.0}
+    x << Position {4.84143144246472090e+00, -1.16032004402742839e+00, -1.03622044471123109e-01}
+    x << Position {8.34336671824457987e+00, 4.12479856412430479e+00, -4.03523417114321381e-01}
+    x << Position {1.28943695621391310e+01, -1.51111514016986312e+01, -2.23307578892655734e-01} 
+    x << Position {1.53796971148509165e+01, -2.59193146099879641e+01, 1.79258772950371181e-01}
+    return x
+}
+
+fn main() {
+
+sys := &System {arr_momentum(), arr_position()}
+offsetmomentum(sys)
+
+println('${energy(sys):.9f}')  //-0.169075164
+for i := 0; i < 50000000; i++ {
+    advance(sys, 0.01)
+}
+println('${energy(sys):.9f}') //-0.169059907
+
+}