scala的sliding window的多种实现和benchMark

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需求

下午闲的写了点小代码来实现实现一个sliding window 效果的，并最终去平铺这些元素

[3 1 4 1 5 9][3,1][1,4][4,1][1,5][5,9][3,1,1,4,4,1,1,5,5,9]

思路

直接用init zip tail ，然后flatMap或者foldLeft
用java的方法通过index实现

代码

import java.time.{Duration, Instant}/**  * @author todd.chen at 9/23/16 1:33 PM.  *         email : todd.chen@ximalaya.com  */object Test {  def main(args: Array[String]): Unit = {    val list = (1 to 1000000).map(i ⇒ i).toList    val vectorList = list.toVector    Thread sleep 5000    (1 to 100).flatMap { i ⇒      val first = benchMark(testIteratorBySam(list))      val second = benchMark(testFlatList(list))      val third = benchMark(testIterator(list))      val purse = benchMark(testPurseJava(vectorList))      Map("sam" → first, "list" → second, "iterator" → third, "pures" → purse)    }.groupBy(_._1).map { case (k, v) ⇒      k -> v.map(_._2).sum    }.toSeq.sortBy(_._2).foreach(println)    /*        val x = List(1, 2, 3, 4)            testPurse(x).foreach(println)*/  }  def testPurseJava = (vectorList: Vector[Int]) ⇒ {    val length = (vectorList.length - 1) * 2    val result = new Array[Int](length)    result(0) = vectorList.head    (1 to vectorList.length - 2).foreach { i ⇒      val x = vectorList(i)      result(i * 2) = x      result(i * 2 - 1) = x    }    result(length - 1) = vectorList.last    result  }  def testFoldLeft = (xs: List[Int]) ⇒ {    (xs.init zip xs.tail).foldLeft(List.empty[Int]) {      case (ls, s) ⇒        ls :+ s._1 :+ s._2    }  }  def testIteratorBySam = (xs: List[Int]) ⇒ {    (xs.init zip xs.tail).flatMap { l ⇒      //l._1 :: l._2 :: Nil      //l.productIterator      new Iterator[Int]() {        var f: Boolean = true        override def hasNext: Boolean = f        override def next(): Int = if (f) {          f = false          l._1        } else {          l._2        }      }    }  }  def testFlatList = (xs: List[Int]) ⇒ {    (xs.init zip xs.tail).flatMap { l ⇒      l._1 :: l._2 :: Nil    }  }  def testIterator = (xs: List[Int]) ⇒ {    (xs.init zip xs.tail).flatMap { l ⇒      //      l._1 :: l._2 :: Nil      l.productIterator    }  }  def benchMark(testFunc: ⇒ Traversable[Any]) = {    val start = Instant.now()    testFunc    val end = Instant.now()    Duration.between(start, end).toNanos  }}

结果

java版的效果很好，然后是我的leader sam给出的，接着是直接调用productorIterator，最后是flatMap的list，而foldLeft性能太差了

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