IterableOnce#copyToArray uses helper for count

Author: som-snytt

src/library/scala/collection/IterableOnce.scala

@@ -272,7 +272,9 @@ object IterableOnce {
     * @return the number of elements that will be copied to the destination array
     */
   @inline private[collection] def elemsToCopyToArray(srcLen: Int, destLen: Int, start: Int, len: Int): Int =
-    math.max(math.min(math.min(len, srcLen), destLen - start), 0)
+    math.max(0,
+      math.min(if (start < 0) destLen else destLen - start,
+        math.min(len, srcLen)))
 
   /** Calls `copyToArray` on the given collection, regardless of whether or not it is an `Iterable`. */
   @inline private[collection] def copyElemsToArray[A, B >: A](elems: IterableOnce[A],
@@ -1035,7 +1037,11 @@ trait IterableOnceOps[+A, +CC[_], +C] extends Any { this: IterableOnce[A] =>
   def copyToArray[B >: A](xs: Array[B], start: Int, len: Int): Int = {
     val it = iterator
     var i = start
-    val end = start + math.min(len, xs.length - start)
+    val srclen = knownSize match {
+      case -1 => xs.length
+      case  k => k
+    }
+    val end = start + IterableOnce.elemsToCopyToArray(srclen, xs.length, start, len)
     while (i < end && it.hasNext) {
       xs(i) = it.next()
       i += 1

test/junit/scala/collection/IterableTest.scala

@@ -114,27 +114,42 @@ class IterableTest {
       }
     }
 
-    val far = 100000
-    val l = Iterable.from(Range(0, 100))
-    check(new Array(100), l.copyToArray(_), 100, 0, far)
-    check(new Array(10), l.copyToArray(_), 10, 0, far)
-    check(new Array(100), l.copyToArray(_), 100, 0, 100)
-
-    check(new Array(100), l.copyToArray(_, 5), 95, 5, 105)
-    check(new Array(10), l.copyToArray(_, 5), 5, 5, 10)
-    check(new Array(1000), l.copyToArray(_, 5), 100, 5, 105)
-
-    check(new Array(100), l.copyToArray(_, 5, 50), 50, 5, 55)
-    check(new Array(10), l.copyToArray(_, 5, 50), 5, 5, 10)
-    check(new Array(1000), l.copyToArray(_, 5, 50), 50, 5, 55)
-
-    assertThrows[ArrayIndexOutOfBoundsException](l.copyToArray(new Array(10), -1))
-    assertThrows[ArrayIndexOutOfBoundsException](l.copyToArray(new Array(10), -1, 10))
-    assertEquals(0, l.copyToArray(new Array(10), 1, -1))
-
-    check(new Array(10), l.copyToArray(_, 10), 0, 0, 0)
-    check(new Array(10), l.copyToArray(_, 10, 10), 0, 0, 0)
-    check(new Array(10), l.copyToArray(_, 0, -1), 0, 0, 0)
+    def checkUp(basis: IterableOnce[Int]): Unit = {
+      val it = Iterable.from(basis)
+      val far = 100000
+
+      check(new Array(100), it.copyToArray(_), 100, 0, far)
+      check(new Array(10), it.copyToArray(_), 10, 0, far)
+      check(new Array(100), it.copyToArray(_), 100, 0, 100)
+
+      check(new Array(100), it.copyToArray(_, 5), 95, 5, 105)
+      check(new Array(10), it.copyToArray(_, 5), 5, 5, 10)
+      check(new Array(1000), it.copyToArray(_, 5), 100, 5, 105)
+
+      check(new Array(100), it.copyToArray(_, 5, 50), 50, 5, 55)
+      check(new Array(10), it.copyToArray(_, 5, 50), 5, 5, 10)
+      check(new Array(1000), it.copyToArray(_, 5, 50), 50, 5, 55)
+
+      // bad start index throws if n >= 0
+      assertThrows[ArrayIndexOutOfBoundsException](it.copyToArray(new Array(10), -1))
+      assertThrows[ArrayIndexOutOfBoundsException](it.copyToArray(new Array(10), -1, 10))
+      assertEquals(0, it.copyToArray(new Array(10), 1, -1))
+
+      check(new Array(10), it.copyToArray(_, 10), 0, 0, 0)
+      check(new Array(10), it.copyToArray(_, 10, 10), 0, 0, 0)
+      check(new Array(10), it.copyToArray(_, 0, -1), 0, 0, 0)
+
+      // bad start index is ignored if n < 0
+      check(new Array(10), it.copyToArray(_, -1, -1), 0, 0, 0)
+      check(new Array(10), it.copyToArray(_, Int.MinValue, -1), 0, 0, 0)
+
+      // also if destination is empty
+      check(new Array(0), it.copyToArray(_, 10, 10), 0, 0, 0)
+      check(new Array(0), it.copyToArray(_, Int.MinValue, -1), 0, 0, 0)
+    }
+    checkUp(Range(0, 100))
+    checkUp(List.range(0, 100))
+    checkUp(Vector.range(0, 100))
   }
 
   @Test @nowarn("cat=deprecation")