Optimized toArray method for ArrayDeque and ListBuilder #KT-42720

libraries/stdlib/jvm/src/kotlin/collections/builders/ListBuilder.kt

@@ -130,13 +130,25 @@ internal class ListBuilder<E> private constructor(
         return ListBuilder(array, offset + fromIndex, toIndex - fromIndex, isReadOnly, this, root ?: this)
     }
 
-    @OptIn(ExperimentalStdlibApi::class)
-    private fun ensureCapacity(minCapacity: Int) {
-        if (backing != null) throw IllegalStateException() // just in case somebody casts subList to ListBuilder
-        if (minCapacity > array.size) {
-            val newSize = ArrayDeque.newCapacity(array.size, minCapacity)
-            array = array.copyOfUninitializedElements(newSize)
+    override fun <T> toArray(destination: Array<T>): Array<T> {
+        if (destination.size < length) {
+            return java.util.Arrays.copyOfRange(array, offset, offset + length, destination.javaClass)
         }
+
+        @Suppress("UNCHECKED_CAST")
+        (array as Array<T>).copyInto(destination, 0, startIndex = offset, endIndex = offset + length)
+
+        if (destination.size > length) {
+            @Suppress("UNCHECKED_CAST")
+            destination[length] = null as T // null-terminate
+        }
+
+        return destination
+    }
+
+    override fun toArray(): Array<Any?> {
+        @Suppress("UNCHECKED_CAST")
+        return array.copyOfRange(fromIndex = offset, toIndex = offset + length) as Array<Any?>
     }
 
     override fun equals(other: Any?): Boolean {
@@ -154,6 +166,14 @@ internal class ListBuilder<E> private constructor(
 
     // ---------------------------- private ----------------------------
 
+    private fun ensureCapacity(minCapacity: Int) {
+        if (backing != null) throw IllegalStateException() // just in case somebody casts subList to ListBuilder
+        if (minCapacity > array.size) {
+            val newSize = ArrayDeque.newCapacity(array.size, minCapacity)
+            array = array.copyOfUninitializedElements(newSize)
+        }
+    }
+
     private fun checkIsMutable() {
         if (isReadOnly || root != null && root.isReadOnly) throw UnsupportedOperationException()
     }

libraries/stdlib/jvm/test/collections/ListBuilderTest.kt

@@ -0,0 +1,60 @@
+/*
+ * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package test.collections
+
+import kotlin.collections.builders.*
+import kotlin.test.Test
+import kotlin.test.assertSame
+import kotlin.test.assertTrue
+
+@Suppress("INVISIBLE_MEMBER")
+class ListBuilderTest {
+    @Test
+    @Suppress("PLATFORM_CLASS_MAPPED_TO_KOTLIN", "UNCHECKED_CAST")
+    fun toArray() {
+        val numberOfElements = 5
+        val expected = ArrayList<Int>().apply { addAll(0 until numberOfElements) }
+        val builder = ListBuilder<Int>().apply { addAll(0 until numberOfElements) }
+
+        fun testToArray(getter: List<Int>.() -> Array<Any?>) {
+            assertTrue(expected.getter() contentEquals builder.getter())
+            assertTrue(expected.subList(2, 4).getter() contentEquals builder.subList(2, 4).getter())
+        }
+
+        testToArray { (this as java.util.Collection<Int>).toArray() }
+    }
+
+    @Test
+    @Suppress("PLATFORM_CLASS_MAPPED_TO_KOTLIN", "UNCHECKED_CAST")
+    fun toArrayDestination() {
+        val numberOfElements = 5
+        val expected = ArrayList<Int>().apply { addAll(0 until numberOfElements) }
+        val builder = ListBuilder<Int>().apply { addAll(0 until numberOfElements) }
+
+        fun testToArray(destSize: Int, getter: List<Int>.(Array<Int>) -> Array<Int>) {
+            repeat(2) { index ->
+                val expectedDest = Array(destSize) { -it - 1 }
+                val builderDest = Array(destSize) { -it - 1 }
+
+                val takeSubList = index == 1
+                val expectedResult = (if (!takeSubList) expected else expected.subList(2, 4)).getter(expectedDest)
+                val builderResult = (if (!takeSubList) builder else builder.subList(2, 4)).getter(builderDest)
+
+                if (expectedResult.size <= expectedDest.size) {
+                    assertSame(expectedDest, expectedResult)
+                    assertSame(builderDest, builderResult)
+                }
+                assertTrue(expectedResult contentEquals builderResult)
+            }
+        }
+
+        testToArray(0) { (this as java.util.Collection<Int>).toArray(it) }
+        testToArray(numberOfElements - 1) { (this as java.util.Collection<Int>).toArray(it) }
+        testToArray(numberOfElements) { (this as java.util.Collection<Int>).toArray(it) }
+        testToArray(numberOfElements + 1) { (this as java.util.Collection<Int>).toArray(it) }
+        testToArray(numberOfElements + 2) { (this as java.util.Collection<Int>).toArray(it) }
+    }
+}