Scala replacement for Arrays.binarySearch?

Question

Is there a replacement in Scala for Java\'s int Arrays.binarySearch(Object[] array, object)?

The problem is that Scala\'s Arrays are not covariant, so I

Answer 1

Been some years since this question was raised, thought to make some comparison test, hopefully it can help some to decide:

import scala.collection.Searching._
import _root_.scala.collection.JavaConversions._
import java.util.{Collections, List => JList}
import scala.reflect.ClassTag

class ObjectArrayTools[T <: Int](a: Array[T]) {
   def binarySearch(key: T) = {
     java.util.Arrays.binarySearch(a.asInstanceOf[Array[Int]],key)
   }
}

class SearchableSeq[T](a: Seq[T])(implicit ordering: Ordering[T]) {
    val list: JList[T] = a.toList
    def binarySearch2(key: T): Int = Collections.binarySearch(list, key, ordering)
}

object BinarySearch {
  implicit def anyrefarray_tools[T <: Int](a: Array[T]) = new ObjectArrayTools(a)
  implicit def seqToSearchable[T](a: Seq[T])(implicit ordering: Ordering[T]) =
          new SearchableSeq(a)(ordering)

  def main(args:Array[String]) {
    val informationArray = Array(1,2,3,4,5,6,7,8,9,10,11,12,14,15,18,20,22,23,25,26)
    val informationList = List(1,2,3,4,5,6,7,8,9,10,11,12,14,15,18,20,22,23,25,26)
    //val sortedArray = sortList(informationArray)
    val sortedArray = informationArray
    val sortedList = informationList

    for(x <- 0 to 2) {
      val startTime = System.nanoTime
      val result = binarySearch(sortedArray, 5)
      val result2 = binarySearch(sortedArray, 19)
      println(s"Custom search time elapsed: ${(System.nanoTime - startTime)}")

      val startTime2 = System.nanoTime
      val result3 = sortedArray.search(5)
      val result4 = sortedArray.search(19)
      println(s"Scala search time elapsed: ${(System.nanoTime - startTime2)}")

      val startTime3 = System.nanoTime
      val result5 = sortedArray.binarySearch(5)
      val result6 = sortedArray.binarySearch(19)
      println(s"Java search casting time elapsed: ${(System.nanoTime - startTime3)}")

      val startTime4 = System.nanoTime
      val result7 = sortedList.binarySearch2(5)
      val result8 = sortedList.binarySearch2(19)
      println(s"Java search as list time elapsed: ${(System.nanoTime - startTime4)}")


      val startTime9 = System.nanoTime
      val result10 = binarySearchWithImplicitConversion(sortedArray, 5)
      val result11 = binarySearchWithImplicitConversion(sortedArray, 19)
      println(s"Custom generic time elapsed: ${(System.nanoTime - startTime9)}")

      println("---")
    }
  }

  /*def sortList(list:Array[Int]):Array[Int] = {
    import com.walcron.etc.Quicksort._
    quickSort(list)
  }*/

  //def binarySearch[T <% Ordered[T]](list:Array[T], valueToBeSearch:T)(implicit t:ClassTag[T]):Int =  {
  def binarySearch(list:Array[Int], valueToBeSearch:Int):Int =  {
    def search(start:Int, end:Int):Int = {
      val pos = ((end - start) / 2) + start
      val curr = list(pos)

      if(curr == valueToBeSearch) {
        pos
      }
      else if((end - start) <= 1) {
        -1 * (pos + 1) // Indicates the value should be inserted
      }
      else if(valueToBeSearch > curr) {
        search(pos, end)
      }
      else {
        search(start, pos)
      }
    }

    search(0, list.length)
  }

  def binarySearchWithImplicitConversion[T <% Ordered[T]](list:Array[T], valueToBeSearch:T)(implicit t:ClassTag[T]):Int =  {
    def search(start:Int, end:Int):Int = {
      val pos = ((end - start) / 2) + start
      val curr = list(pos)

      if(curr == valueToBeSearch) {
        pos
      }
      else if((end - start) <= 1) {
        -1 * (pos + 1) // Indicates the value should be inserted
      }
      else if(valueToBeSearch > curr) {
        search(pos, end)
      }
      else {
        search(start, pos)
      }
    }

    search(0, list.length)
  }
}

The returned result after 3 runs (as Scala compiler really do need some boost)

Custom search time elapsed: 873373
Scala search time elapsed: 9322723
Java search casting time elapsed: 126380
Java search as list time elapsed: 7375826
Custom generic time elapsed: 4421972
---
Custom search time elapsed: 10372
Scala search time elapsed: 34885
Java search casting time elapsed: 10861
Java search as list time elapsed: 104596
Custom generic time elapsed: 57964
---
Custom search time elapsed: 9121
Scala search time elapsed: 31667
Java search casting time elapsed: 11815
Java search as list time elapsed: 53387
Custom generic time elapsed: 60773

In general, java binary search performed way better; while scala's search did pretty bad. There was also another noticeable performance, it seems that generic typing implicitly drags the performance here badly(so maybe someone can help fix the generic type)...but indirectly it shows a big performance impact.