1/*
2Redistribution and use in source and binary forms, with or without
3modification, are permitted provided that the following conditions are met:
4
5    * Redistributions of source code must retain the above copyright
6    notice, this list of conditions and the following disclaimer.
7
8    * Redistributions in binary form must reproduce the above copyright
9    notice, this list of conditions and the following disclaimer in the
10    documentation and/or other materials provided with the distribution.
11
12    * Neither the name of "The Computer Language Benchmarks Game" nor the
13    name of "The Computer Language Shootout Benchmarks" nor the names of
14    its contributors may be used to endorse or promote products derived
15    from this software without specific prior written permission.
16
17THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
18AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
19IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
20ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
21LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
22CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
23SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
24INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
25CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
26ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
27POSSIBILITY OF SUCH DAMAGE.
28*/
29
30/* The Computer Language Benchmarks Game
31 * http://shootout.alioth.debian.org/
32 *
33 * contributed by The Go Authors.
34 * based on C program by Christoph Bauer
35 */
36
37package main
38
39import (
40	"flag"
41	"fmt"
42	"math"
43)
44
45var n = flag.Int("n", 1000, "number of iterations")
46
47type Body struct {
48	x, y, z, vx, vy, vz, mass float64
49}
50
51const (
52	solarMass   = 4 * math.Pi * math.Pi
53	daysPerYear = 365.24
54)
55
56func (b *Body) offsetMomentum(px, py, pz float64) {
57	b.vx = -px / solarMass
58	b.vy = -py / solarMass
59	b.vz = -pz / solarMass
60}
61
62type System []*Body
63
64func NewSystem(body []Body) System {
65	n := make(System, len(body))
66	for i := 0; i < len(body); i++ {
67		n[i] = new(Body) // copy to avoid overwriting the inputs
68		*n[i] = body[i]
69	}
70	var px, py, pz float64
71	for _, body := range n {
72		px += body.vx * body.mass
73		py += body.vy * body.mass
74		pz += body.vz * body.mass
75	}
76	n[0].offsetMomentum(px, py, pz)
77	return n
78}
79
80func (sys System) energy() float64 {
81	var e float64
82	for i, body := range sys {
83		e += 0.5 * body.mass *
84			(body.vx*body.vx + body.vy*body.vy + body.vz*body.vz)
85		for j := i + 1; j < len(sys); j++ {
86			body2 := sys[j]
87			dx := body.x - body2.x
88			dy := body.y - body2.y
89			dz := body.z - body2.z
90			distance := math.Sqrt(dx*dx + dy*dy + dz*dz)
91			e -= (body.mass * body2.mass) / distance
92		}
93	}
94	return e
95}
96
97func (sys System) advance(dt float64) {
98	for i, body := range sys {
99		for j := i + 1; j < len(sys); j++ {
100			body2 := sys[j]
101			dx := body.x - body2.x
102			dy := body.y - body2.y
103			dz := body.z - body2.z
104
105			dSquared := dx*dx + dy*dy + dz*dz
106			distance := math.Sqrt(dSquared)
107			mag := dt / (dSquared * distance)
108
109			body.vx -= dx * body2.mass * mag
110			body.vy -= dy * body2.mass * mag
111			body.vz -= dz * body2.mass * mag
112
113			body2.vx += dx * body.mass * mag
114			body2.vy += dy * body.mass * mag
115			body2.vz += dz * body.mass * mag
116		}
117	}
118
119	for _, body := range sys {
120		body.x += dt * body.vx
121		body.y += dt * body.vy
122		body.z += dt * body.vz
123	}
124}
125
126var (
127	jupiter = Body{
128		x:    4.84143144246472090e+00,
129		y:    -1.16032004402742839e+00,
130		z:    -1.03622044471123109e-01,
131		vx:   1.66007664274403694e-03 * daysPerYear,
132		vy:   7.69901118419740425e-03 * daysPerYear,
133		vz:   -6.90460016972063023e-05 * daysPerYear,
134		mass: 9.54791938424326609e-04 * solarMass,
135	}
136	saturn = Body{
137		x:    8.34336671824457987e+00,
138		y:    4.12479856412430479e+00,
139		z:    -4.03523417114321381e-01,
140		vx:   -2.76742510726862411e-03 * daysPerYear,
141		vy:   4.99852801234917238e-03 * daysPerYear,
142		vz:   2.30417297573763929e-05 * daysPerYear,
143		mass: 2.85885980666130812e-04 * solarMass,
144	}
145	uranus = Body{
146		x:    1.28943695621391310e+01,
147		y:    -1.51111514016986312e+01,
148		z:    -2.23307578892655734e-01,
149		vx:   2.96460137564761618e-03 * daysPerYear,
150		vy:   2.37847173959480950e-03 * daysPerYear,
151		vz:   -2.96589568540237556e-05 * daysPerYear,
152		mass: 4.36624404335156298e-05 * solarMass,
153	}
154	neptune = Body{
155		x:    1.53796971148509165e+01,
156		y:    -2.59193146099879641e+01,
157		z:    1.79258772950371181e-01,
158		vx:   2.68067772490389322e-03 * daysPerYear,
159		vy:   1.62824170038242295e-03 * daysPerYear,
160		vz:   -9.51592254519715870e-05 * daysPerYear,
161		mass: 5.15138902046611451e-05 * solarMass,
162	}
163	sun = Body{
164		mass: solarMass,
165	}
166)
167
168func main() {
169	flag.Parse()
170
171	system := NewSystem([]Body{sun, jupiter, saturn, uranus, neptune})
172	fmt.Printf("%.9f\n", system.energy())
173	for i := 0; i < *n; i++ {
174		system.advance(0.01)
175	}
176	fmt.Printf("%.9f\n", system.energy())
177}
178