SI-9605 Searching does not use binary search for Array

Binary search should be used for every `IndexedSeqLike` instance and not only for `IndexedSeq`. According the Scaladoc, it is `IndexedSeqLike` that guarantees "constant-time or near constant-time element access and length computation".

Author: ruippeixotog

src/library/scala/collection/Searching.scala

@@ -36,12 +36,12 @@ object Searching {
 
   class SearchImpl[A, Repr](val coll: SeqLike[A, Repr]) {
     /** Search the sorted sequence for a specific element. If the sequence is an
-      * `IndexedSeq`, a binary search is used. Otherwise, a linear search is used.
+      * `IndexedSeqLike`, a binary search is used. Otherwise, a linear search is used.
       *
       * The sequence should be sorted with the same `Ordering` before calling; otherwise,
       * the results are undefined.
       *
-      * @see [[scala.collection.IndexedSeq]]
+      * @see [[scala.collection.IndexedSeqLike]]
       * @see [[scala.math.Ordering]]
       * @see [[scala.collection.SeqLike]], method `sorted`
       *
@@ -54,18 +54,18 @@ object Searching {
       */
     final def search[B >: A](elem: B)(implicit ord: Ordering[B]): SearchResult =
       coll match {
-        case _: IndexedSeq[A] => binarySearch(elem, 0, coll.length)(ord)
+        case _: IndexedSeqLike[A, Repr] => binarySearch(elem, 0, coll.length)(ord)
         case _ => linearSearch(coll.view, elem, 0)(ord)
       }
 
     /** Search within an interval in the sorted sequence for a specific element. If the
-      * sequence is an IndexedSeq, a binary search is used. Otherwise, a linear search
+      * sequence is an `IndexedSeqLike`, a binary search is used. Otherwise, a linear search
       * is used.
       *
       * The sequence should be sorted with the same `Ordering` before calling; otherwise,
       * the results are undefined.
       *
-      * @see [[scala.collection.IndexedSeq]]
+      * @see [[scala.collection.IndexedSeqLike]]
       * @see [[scala.math.Ordering]]
       * @see [[scala.collection.SeqLike]], method `sorted`
       *
@@ -81,7 +81,7 @@ object Searching {
     final def search[B >: A](elem: B, from: Int, to: Int)
     (implicit ord: Ordering[B]): SearchResult =
       coll match {
-        case _: IndexedSeq[A] => binarySearch(elem, from, to)(ord)
+        case _: IndexedSeqLike[A, Repr] => binarySearch(elem, from, to)(ord)
         case _ => linearSearch(coll.view(from, to), elem, from)(ord)
       }
 

test/junit/scala/collection/SearchingTest.scala

@@ -0,0 +1,48 @@
+package scala.collection
+
+import org.junit.runner.RunWith
+import org.junit.runners.JUnit4
+import org.junit.Assert._
+import org.junit.Test
+import scala.collection.Searching._
+
+@RunWith(classOf[JUnit4])
+class SearchingTest {
+
+  @Test
+  def doesLinearSearchOnLinearSeqs() {
+
+    class TestSeq[A](list: List[A]) extends SeqLike[A, TestSeq[A]] {
+      var elementsAccessed = Set.empty[Int]
+
+      protected[this] def newBuilder = ??? // not needed for this test
+      def seq = list
+      def iterator = list.iterator
+      def length = list.length
+      def apply(idx: Int) = { elementsAccessed += idx; list(idx) }
+    }
+
+    val coll = new TestSeq((0 to 6).toList)
+
+    assertEquals(Found(5), coll.search(5))
+    assertEquals(Set.empty, coll.elementsAccessed) // linear search should not access elements via apply()
+  }
+
+  @Test
+  def doesBinarySearchOnIndexedSeqs() {
+
+    class TestIndexedSeq[A](vec: Vector[A]) extends IndexedSeqLike[A, TestIndexedSeq[A]] {
+      var elementsAccessed = Set.empty[Int]
+
+      protected[this] def newBuilder = ??? // not needed for this test
+      def seq = vec
+      def length = vec.length
+      def apply(idx: Int) = { elementsAccessed += idx; vec(idx) }
+    }
+
+    val coll = new TestIndexedSeq((0 to 6).toVector)
+
+    assertEquals(Found(5), coll.search(5))
+    assertEquals(Set(3, 5), coll.elementsAccessed)
+  }
+}