KT-4844 Support asReversed() for lists

libraries/stdlib/src/kotlin/collections/ReversedViews.kt

@@ -0,0 +1,52 @@
+/*
+ * Copyright 2010-2015 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package kotlin
+
+import java.util.AbstractList
+import kotlin.platform.platformName
+
+private open class ReversedListReadOnly<T>(protected open val delegate: List<T>) : AbstractList<T>() {
+    override fun size(): Int = delegate.size()
+    override fun get(index: Int): T = delegate[index.flipIndex()]
+
+    protected fun Int.flipIndex(): Int = if (this in 0..size() - 1) size() - this - 1 else throw IndexOutOfBoundsException("index $this should be in range [${0..size() - 1}]")
+    protected fun Int.flipIndexForward(): Int = if (this in 0..size()) size() - this else throw IndexOutOfBoundsException("index $this should be in range [${0..size()}]")
+}
+
+private class ReversedList<T>(protected override val delegate: MutableList<T>) : ReversedListReadOnly<T>(delegate) {
+    override fun clear() = delegate.clear()
+    override fun remove(index: Int): T = delegate.remove(index.flipIndex())
+
+    override fun set(index: Int, element: T): T = delegate.set(index.flipIndex(), element)
+    override fun add(index: Int, element: T) {
+        delegate.add(index.flipIndexForward(), element)
+    }
+}
+
+/**
+ * Returns a reversed immutable view of the original List.
+ * All changes made in the original list will be reflected in the reversed one
+ */
+public fun <T> List<T>.asReversed(): List<T> = ReversedListReadOnly(this)
+
+/**
+ * Returns a reversed mutable view of the original List.
+ * All changes made in the original list will be reflected in the reversed one and vice versa
+ */
+platformName("asReversedMutable")
+public fun <T> MutableList<T>.asReversed(): MutableList<T> = ReversedList(this)
+