Test changing for loops with while loops.

Native.scala

import scalanative.native._
import stdlib.malloc

object implicits {
  type MVect3 = CStruct3[Double, Double, Double]
  type MutableBody = CStruct3[MVect3, MVect3, Double]

  implicit class MVect3Ops(val ptr: Ptr[MVect3]) extends AnyVal {
    def x: Double = !ptr._1
    def y: Double = !ptr._2
    def z: Double = !ptr._3

    def x_=(v: Double): Unit = !ptr ._1 = v
    def y_=(v: Double): Unit = !ptr ._2 = v
    def z_=(v: Double): Unit = !ptr ._3 = v

    def zero(): Unit = {
      x = 0.0
      y = 0.0
      z = 0.0
    }
  }

  implicit class MutableBodyOps(val ptr: Ptr[MutableBody]) extends AnyVal {
    def p: Ptr[MVect3] = ptr._1
    def v: Ptr[MVect3] = ptr._2
    def mass: Double   = !ptr._3

    def mass_=(v: Double): Unit = !ptr._3 = v
  }
}

class NBodyMutableClass(val numBodies: Int, val dt: Double) {
  import implicits._

  private val bodies = malloc(numBodies * sizeof[MutableBody]).cast[Ptr[MutableBody]]
  for(i <- 0 until numBodies) (bodies + i).mass = 1e-10

  private val accel = malloc(numBodies * sizeof[MVect3]).cast[Ptr[MVect3]]

  initBodies()

  def initBodies(): Unit = {
    bodies.mass = 1.0
    for(i <-  1 until numBodies) {
      (bodies + i).p.x = i
      (bodies + i).v.y = math.sqrt(1.0/i)
    }
  }

  def forSim(steps: Int): Unit = {
    var s = 1
    while(s <= steps) {
      var i = 0
      while(i < numBodies) {
        (accel + i).zero()
        i+=1
      }
      i = 0
      while(i < numBodies) {
        val pi = bodies + i
        val acceli = accel + i
        for (j <- i+1 until numBodies) {        
          val pj = bodies + j
          val dx = pi.p.x-pj.p.x
          val dy = pi.p.y-pj.p.y
          val dz = pi.p.z-pj.p.z
          val dist = math.sqrt(dx*dx+dy*dy+dz*dz)
          val magi = pj.mass/(dist*dist*dist)
          val magj = pi.mass/(dist*dist*dist)
          acceli.x -= magi*dx
          acceli.y -= magi*dy
          acceli.z -= magi*dz
          val accelj = (accel + j)
          accelj.x += magj*dx
          accelj.y += magj*dy
          accelj.z += magj*dz
        }
        i+=1
      }

      i = 0
      while(i < numBodies) {
        val p = bodies + i
        val acceli = (accel + i)
        p.v.x += acceli.x*dt
        p.v.y += acceli.y*dt
        p.v.z += acceli.z*dt
        p.p.x += p.v.x*dt
        p.p.y += p.v.y*dt
        p.p.z += p.v.z*dt
        i+=1
      }
      s+=1
    }
  }
}

object MainTiming extends App {
  val numBodies = 1000
  val dt = 0.01
  val mutSim = new NBodyMutableClass(numBodies, dt)
  println("Mutable Class:")

  val timeRuns = 20
  val times = new Array[Double](timeRuns)
  var time = 0

  while(time < timeRuns) {
    val start = System.nanoTime()
    mutSim.forSim(100)
    times(time) = (System.nanoTime()-start)*1e-9

    time += 1
  }

  val mean = times.sum/times.length
  val rms = math.sqrt(times.map(x => (x-mean)*(x-mean)).sum/times.length)
  println(s"mean = $mean, rms = $rms")
}