why_slow.scala

def timeit[T](f:()=>T)(count:Int=1) = {
  val st = System.nanoTime()
  var ti = 0
  var accu =  collection.mutable.ListBuffer.empty[T]
  while (ti < count) {
    f() +=: accu
    ti=ti+1
  }
  val ed = System.nanoTime()
  println(s"${(ed-st)/1000000} ms")
  accu.iterator
}
val array = Array.range(0, 1000000)

timeit(()=>{
  var idx = 0
  var sum = 0L
  while(idx < array.length) {
    sum = sum+array(idx)
    idx = idx+1
  }
  sum
})(10).foreach(println)

case class MyInt(va:Int,sum:Long)
def Called(inc:MyInt) = inc.copy(sum = (inc.sum+inc.va))
def Caller(v:(MyInt)=>MyInt, a:Long, b:Int) = v(MyInt(b, a))

timeit(()=>{
  var idx = 0
  var sum = 0L
  while(idx < array.length) {
    sum = Caller(Called, sum, array(idx)).sum
    idx = idx+1
  }
  sum
})(10).foreach(println)


timeit(()=>{
  var idx = 0
  var sum = 0L
  while(idx < array.length) {
    sum = ((a:Long, b: Int) => {
      a+b
    })(sum, array(idx))
    idx = idx+1
  }
  sum
})(10).foreach(println)

timeit(()=>{
  var idx = 0
  var sum = 0L
  while(idx < array.length) {
    sum = ((a:Long, b: Int, c: Int) => {
      a+b
    })(sum, array(idx), 0)
    idx = idx+1
  }
  sum
})(10).foreach(println)

timeit(()=>{
  var idx = 0
  var sum = 0L
  while(idx < array.length) {
    sum = ((a:Long, b:MyInt) => {
      a+b.sum
    })(array(idx), MyInt(0,sum))
    idx = idx+1
  }
  sum
})(10).foreach(println)

1，2，3是快的，分别是裸跑，两个裸参数，完全case class的版本。4,5是慢的，是三个裸参数和一裸一不裸的版本。