KT-31741: stdlib: DeepRecursiveFunction (#3398)

Introduces experimental coroutines-based framework to execute deeply recursive functions that utilities the heap and thus avoid StackOverflowError.
2020-05-27 10:06:00 +03:00
parent 9fa1614903
commit 46b426ef77
7 changed files with 400 additions and 0 deletions
@@ -44,6 +44,13 @@ public expect inline fun <R, T> (suspend R.() -> T).startCoroutineUninterceptedO
    completion: Continuation<T>
 ): Any?

+// Internal version that support arity-2 suspending functions, might be made public in the future if needed
+internal expect inline fun <R, P, T> (suspend R.(P) -> T).startCoroutineUninterceptedOrReturn(
+    receiver: R,
+    param: P,
+    completion: Continuation<T>
+): Any?
+
@SinceKotlin("1.3")
 public expect fun <T> (suspend () -> T).createCoroutineUnintercepted(
    completion: Continuation<T>
@@ -0,0 +1,218 @@
+/*
+ * Copyright 2010-2020 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 kotlin
+
+import kotlin.coroutines.*
+import kotlin.coroutines.intrinsics.*
+
+/**
+ * Defines deep recursive function that keeps its stack on the heap,
+ * which allows very deep recursive computations that do not use the actual call stack.
+ * To initiate a call to this deep recursive function use its [invoke] function.
+ * As a rule of thumb, it should be used if recursion goes deeper than a thousand calls.
+ *
+ * The [DeepRecursiveFunction] takes one parameter of type [T] and returns a result of type [R].
+ * The [block] of code defines the body of a recursive function. In this block
+ * [callRecursive][DeepRecursiveScope.callRecursive] function can be used to make a recursive call
+ * to the declared function. Other instances of [DeepRecursiveFunction] can be called
+ * in this scope with `callRecursive` extension, too.
+ *
+ * For example, take a look at the following recursive tree class and a deeply
+ * recursive instance of this tree with 100K nodes:
+ *
+ * ```
+ * class Tree(val left: Tree? = null, val right: Tree? = null)
+ * val deepTree = generateSequence(Tree()) { Tree(it) }.take(100_000).last()
+ * ```
+ *
+ * A regular recursive function can be defined to compute a depth of a tree:
+ *
+ * ```
+ * fun depth(t: Tree?): Int =
+ *     if (t == null) 0 else max(depth(t.left), depth(t.right)) + 1
+ * println(depth(deepTree)) // StackOverflowError
+ * ```
+ *
+ * If this `depth` function is called for a `deepTree` it produces [StackOverflowError] because of deep recursion.
+ * However, the `depth` function can be rewritten using `DeepRecursiveFunction` in the following way, and then
+ * it successfully computes [`depth(deepTree)`][DeepRecursiveFunction.invoke] expression:
+ *
+ * ```
+ * val depth = DeepRecursiveFunction<Tree?, Int> { t ->
+ *     if (t == null) 0 else max(callRecursive(t.left), callRecursive(t.right)) + 1
+ * }
+ * println(depth(deepTree)) // Ok
+ * ```
+ *
+ * Deep recursive functions can also mutually call each other using a heap for the stack via
+ * [callRecursive][DeepRecursiveScope.callRecursive] extension. For example, the
+ * following pair of mutually recursive functions computes the number of tree nodes at even depth in the tree.
+ *
+ * ```
+ * val mutualRecursion = object {
+ *     val even: DeepRecursiveFunction<Tree?, Int> = DeepRecursiveFunction { t ->
+ *         if (t == null) 0 else odd.callRecursive(t.left) + odd.callRecursive(t.right) + 1
+ *     }
+ *     val odd: DeepRecursiveFunction<Tree?, Int> = DeepRecursiveFunction { t ->
+ *         if (t == null) 0 else even.callRecursive(t.left) + even.callRecursive(t.right)
+ *     }
+ * }
+ * ```
+ *
+ * @param [T] the function parameter type.
+ * @param [R] the function result type.
+ * @param block the function body.
+ */
+@SinceKotlin("1.4")
+@ExperimentalStdlibApi
+public class DeepRecursiveFunction<T, R>(
+    internal val block: suspend DeepRecursiveScope<T, R>.(T) -> R
+)
+
+/**
+ * Initiates a call to this deep recursive function, forming a root of the call tree.
+ *
+ * This operator should not be used from inside of [DeepRecursiveScope] as it uses the call stack slot for
+ * initial recursive invocation. From inside of [DeepRecursiveScope] use
+ * [callRecursive][DeepRecursiveScope.callRecursive].
+ */
+@SinceKotlin("1.4")
+@ExperimentalStdlibApi
+public operator fun <T, R> DeepRecursiveFunction<T, R>.invoke(value: T): R =
+    DeepRecursiveScopeImpl<T, R>(block, value).runCallLoop()
+
+/**
+ * A scope class for [DeepRecursiveFunction] function declaration that defines [callRecursive] methods to
+ * recursively call this function or another [DeepRecursiveFunction] putting the call activation frame on the heap.
+ *
+ * @param [T] function parameter type.
+ * @param [R] function result type.
+ */
+@RestrictsSuspension
+@SinceKotlin("1.4")
+@ExperimentalStdlibApi
+public sealed class DeepRecursiveScope<T, R> {
+    /**
+     * Makes recursive call to this [DeepRecursiveFunction] function putting the call activation frame on the heap,
+     * as opposed to the actual call stack that is used by a regular recursive call.
+     */
+    public abstract suspend fun callRecursive(value: T): R
+
+    /**
+     * Makes call to the specified [DeepRecursiveFunction] function putting the call activation frame on the heap,
+     * as opposed to the actual call stack that is used by a regular call.
+     */
+    public abstract suspend fun <U, S> DeepRecursiveFunction<U, S>.callRecursive(value: U): S
+
+    @Deprecated(
+        level = DeprecationLevel.ERROR,
+        message =
+        "'invoke' should not be called from DeepRecursiveScope. " +
+                "Use 'callRecursive' to do recursion in the heap instead of the call stack.",
+        replaceWith = ReplaceWith("this.callRecursive(value)")
+    )
+    @Suppress("UNUSED_PARAMETER")
+    public operator fun DeepRecursiveFunction<*, *>.invoke(value: Any?): Nothing =
+        throw UnsupportedOperationException("Should not be called from DeepRecursiveScope")
+}
+
+// ================== Implementation ==================
+
+@ExperimentalStdlibApi
+private typealias DeepRecursiveFunctionBlock = suspend DeepRecursiveScope<*, *>.(Any?) -> Any?
+
+private val UNDEFINED_RESULT = Result.success(COROUTINE_SUSPENDED)
+
+@Suppress("UNCHECKED_CAST")
+@ExperimentalStdlibApi
+private class DeepRecursiveScopeImpl<T, R>(
+    block: suspend DeepRecursiveScope<T, R>.(T) -> R,
+    value: T
+) : DeepRecursiveScope<T, R>(), Continuation<R> {
+    // Active function block
+    private var function: DeepRecursiveFunctionBlock = block as DeepRecursiveFunctionBlock
+
+    // Value to call function with
+    private var value: Any? = value
+
+    // Continuation of the current call
+    private var cont: Continuation<Any?>? = this as Continuation<Any?>
+
+    // Completion result (completion of the whole call stack)
+    private var result: Result<Any?> = UNDEFINED_RESULT
+
+    override val context: CoroutineContext
+        get() = EmptyCoroutineContext
+
+    override fun resumeWith(result: Result<R>) {
+        this.cont = null
+        this.result = result
+    }
+
+    override suspend fun callRecursive(value: T): R = suspendCoroutineUninterceptedOrReturn { cont ->
+        // calling the same function that is currently active
+        this.cont = cont as Continuation<Any?>
+        this.value = value
+        COROUTINE_SUSPENDED
+    }
+
+    override suspend fun <U, S> DeepRecursiveFunction<U, S>.callRecursive(value: U): S = suspendCoroutineUninterceptedOrReturn { cont ->
+        // calling another recursive function
+        val function = block as DeepRecursiveFunctionBlock
+        with(this@DeepRecursiveScopeImpl) {
+            val currentFunction = this.function
+            if (function !== currentFunction) {
+                // calling a different function -- create a trampoline to restore function ref
+                this.function = function
+                this.cont = crossFunctionCompletion(currentFunction, cont as Continuation<Any?>)
+            } else {
+                // calling the same function -- direct
+                this.cont = cont as Continuation<Any?>
+            }
+            this.value = value
+        }
+        COROUTINE_SUSPENDED
+    }
+
+    private fun crossFunctionCompletion(
+        currentFunction: DeepRecursiveFunctionBlock,
+        cont: Continuation<Any?>
+    ): Continuation<Any?> = Continuation(EmptyCoroutineContext) {
+        this.function = currentFunction
+        // When going back from a trampoline we cannot just call cont.resume (stack usage!)
+        // We delegate the cont.resumeWith(it) call to runCallLoop
+        this.cont = cont
+        this.result = it
+    }
+
+    @Suppress("UNCHECKED_CAST")
+    fun runCallLoop(): R {
+        while (true) {
+            // Note: cont is set to null in DeepRecursiveScopeImpl.resumeWith when the whole computation completes
+            val result = this.result
+            val cont = this.cont
+                ?: return (result as Result<R>).getOrThrow() // done -- final result
+            // The order of comparison is important here for that case of rogue class with broken equals
+            if (UNDEFINED_RESULT == result) {
+                // call "function" with "value" using "cont" as completion
+                val r = try {
+                    // This is block.startCoroutine(this, value, cont)
+                    function.startCoroutineUninterceptedOrReturn(this, value, cont)
+                } catch (e: Throwable) {
+                    cont.resumeWithException(e)
+                    continue
+                }
+                // If the function returns without suspension -- calls its continuation immediately
+                if (r !== COROUTINE_SUSPENDED)
+                    cont.resume(r as R)
+            } else {
+                // we returned from a crossFunctionCompletion trampoline -- call resume here
+                this.result = UNDEFINED_RESULT // reset result back
+                cont.resumeWith(result)
+            }
+        }
+    }
+}