Thermel Erosion

class ThermalErosion(val noise: Noise, val talusAngle: Double) extends NoiseOp {
  val DEFAULT_DECAY = 0L
  val cache = TrieMap[(Int, Int), Double]()
  var tDecay = new AtomicLong(DEFAULT_DECAY)

  final def apply(x: Double, y: Double) = {
    val i = x.toInt
    val j = y.toInt

    if (tDecay.get() > 0) {
      tDecay.decrementAndGet()
      noise(i, j) + cache.getOrElse((i, j), 0.0)
    }
    else {
      tDecay.set(DEFAULT_DECAY)

      val mask = Array(
        (-1,-1), (0,-1), (1,-1),
        (-1, 0), (0, 0), (1, 0),
        (-1, 1), (0, 1), (1, 1)
      )
      val data = mask.par.map(t => noise(x + t._1, y + t._2))

      val cachedVal = cache.getOrElse((i, j), 0.0)

      val height = data(4) + cachedVal
      val maxSlope = data.reduce((res, x) => math.max(res, x))
      val minSlope = data.reduce((res, x) => math.min(res, x))
      val minSlopeCoords = (mask zip data).reduce((res, e) => if (math.abs(e._2 - minSlope) <= 1e-13) e else res)._1

      if (maxSlope > talusAngle) {
        val newHeight = (height + maxSlope) * 0.5

        cache.update(minSlopeCoords, cachedVal + newHeight)
        cache.update((i, j), newHeight)

        newHeight
      }
      else {
        height
      }
    }
  }

}