Svyatoslav Scherbina
221 lines
8.9 KiB
Kotlin
221 lines
8.9 KiB
Kotlin
/*
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* Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
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* Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
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*/
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package kotlin
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import kotlin.coroutines.*
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import kotlin.coroutines.intrinsics.*
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import kotlin.native.concurrent.SharedImmutable
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/**
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* Defines deep recursive function that keeps its stack on the heap,
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* which allows very deep recursive computations that do not use the actual call stack.
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* To initiate a call to this deep recursive function use its [invoke] function.
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* As a rule of thumb, it should be used if recursion goes deeper than a thousand calls.
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*
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* The [DeepRecursiveFunction] takes one parameter of type [T] and returns a result of type [R].
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* The [block] of code defines the body of a recursive function. In this block
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* [callRecursive][DeepRecursiveScope.callRecursive] function can be used to make a recursive call
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* to the declared function. Other instances of [DeepRecursiveFunction] can be called
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* in this scope with `callRecursive` extension, too.
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*
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* For example, take a look at the following recursive tree class and a deeply
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* recursive instance of this tree with 100K nodes:
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*
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* ```
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* class Tree(val left: Tree? = null, val right: Tree? = null)
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* val deepTree = generateSequence(Tree()) { Tree(it) }.take(100_000).last()
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* ```
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*
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* A regular recursive function can be defined to compute a depth of a tree:
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*
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* ```
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* fun depth(t: Tree?): Int =
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* if (t == null) 0 else max(depth(t.left), depth(t.right)) + 1
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* println(depth(deepTree)) // StackOverflowError
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* ```
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*
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* If this `depth` function is called for a `deepTree` it produces [StackOverflowError] because of deep recursion.
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* However, the `depth` function can be rewritten using `DeepRecursiveFunction` in the following way, and then
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* it successfully computes [`depth(deepTree)`][DeepRecursiveFunction.invoke] expression:
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*
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* ```
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* val depth = DeepRecursiveFunction<Tree?, Int> { t ->
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* if (t == null) 0 else max(callRecursive(t.left), callRecursive(t.right)) + 1
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* }
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* println(depth(deepTree)) // Ok
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* ```
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*
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* Deep recursive functions can also mutually call each other using a heap for the stack via
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* [callRecursive][DeepRecursiveScope.callRecursive] extension. For example, the
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* following pair of mutually recursive functions computes the number of tree nodes at even depth in the tree.
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*
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* ```
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* val mutualRecursion = object {
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* val even: DeepRecursiveFunction<Tree?, Int> = DeepRecursiveFunction { t ->
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* if (t == null) 0 else odd.callRecursive(t.left) + odd.callRecursive(t.right) + 1
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* }
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* val odd: DeepRecursiveFunction<Tree?, Int> = DeepRecursiveFunction { t ->
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* if (t == null) 0 else even.callRecursive(t.left) + even.callRecursive(t.right)
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* }
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* }
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* ```
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*
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* @param [T] the function parameter type.
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* @param [R] the function result type.
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* @param block the function body.
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*/
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@SinceKotlin("1.4")
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@ExperimentalStdlibApi
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public class DeepRecursiveFunction<T, R>(
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internal val block: suspend DeepRecursiveScope<T, R>.(T) -> R
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)
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/**
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* Initiates a call to this deep recursive function, forming a root of the call tree.
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*
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* This operator should not be used from inside of [DeepRecursiveScope] as it uses the call stack slot for
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* initial recursive invocation. From inside of [DeepRecursiveScope] use
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* [callRecursive][DeepRecursiveScope.callRecursive].
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*/
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@SinceKotlin("1.4")
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@ExperimentalStdlibApi
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public operator fun <T, R> DeepRecursiveFunction<T, R>.invoke(value: T): R =
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DeepRecursiveScopeImpl<T, R>(block, value).runCallLoop()
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/**
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* A scope class for [DeepRecursiveFunction] function declaration that defines [callRecursive] methods to
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* recursively call this function or another [DeepRecursiveFunction] putting the call activation frame on the heap.
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*
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* @param [T] function parameter type.
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* @param [R] function result type.
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*/
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@RestrictsSuspension
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@SinceKotlin("1.4")
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@ExperimentalStdlibApi
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public sealed class DeepRecursiveScope<T, R> {
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/**
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* Makes recursive call to this [DeepRecursiveFunction] function putting the call activation frame on the heap,
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* as opposed to the actual call stack that is used by a regular recursive call.
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*/
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public abstract suspend fun callRecursive(value: T): R
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/**
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* Makes call to the specified [DeepRecursiveFunction] function putting the call activation frame on the heap,
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* as opposed to the actual call stack that is used by a regular call.
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*/
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public abstract suspend fun <U, S> DeepRecursiveFunction<U, S>.callRecursive(value: U): S
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@Deprecated(
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level = DeprecationLevel.ERROR,
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message =
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"'invoke' should not be called from DeepRecursiveScope. " +
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"Use 'callRecursive' to do recursion in the heap instead of the call stack.",
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replaceWith = ReplaceWith("this.callRecursive(value)")
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)
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@Suppress("UNUSED_PARAMETER")
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public operator fun DeepRecursiveFunction<*, *>.invoke(value: Any?): Nothing =
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throw UnsupportedOperationException("Should not be called from DeepRecursiveScope")
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}
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// ================== Implementation ==================
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@ExperimentalStdlibApi
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private typealias DeepRecursiveFunctionBlock = suspend DeepRecursiveScope<*, *>.(Any?) -> Any?
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@SharedImmutable
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private val UNDEFINED_RESULT = Result.success(COROUTINE_SUSPENDED)
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@Suppress("UNCHECKED_CAST")
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@ExperimentalStdlibApi
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private class DeepRecursiveScopeImpl<T, R>(
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block: suspend DeepRecursiveScope<T, R>.(T) -> R,
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value: T
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) : DeepRecursiveScope<T, R>(), Continuation<R> {
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// Active function block
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private var function: DeepRecursiveFunctionBlock = block as DeepRecursiveFunctionBlock
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// Value to call function with
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private var value: Any? = value
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// Continuation of the current call
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private var cont: Continuation<Any?>? = this as Continuation<Any?>
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// Completion result (completion of the whole call stack)
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private var result: Result<Any?> = UNDEFINED_RESULT
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override val context: CoroutineContext
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get() = EmptyCoroutineContext
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override fun resumeWith(result: Result<R>) {
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this.cont = null
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this.result = result
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}
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override suspend fun callRecursive(value: T): R = suspendCoroutineUninterceptedOrReturn { cont ->
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// calling the same function that is currently active
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this.cont = cont as Continuation<Any?>
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this.value = value
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COROUTINE_SUSPENDED
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}
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override suspend fun <U, S> DeepRecursiveFunction<U, S>.callRecursive(value: U): S = suspendCoroutineUninterceptedOrReturn { cont ->
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// calling another recursive function
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val function = block as DeepRecursiveFunctionBlock
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with(this@DeepRecursiveScopeImpl) {
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val currentFunction = this.function
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if (function !== currentFunction) {
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// calling a different function -- create a trampoline to restore function ref
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this.function = function
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this.cont = crossFunctionCompletion(currentFunction, cont as Continuation<Any?>)
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} else {
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// calling the same function -- direct
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this.cont = cont as Continuation<Any?>
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}
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this.value = value
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}
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COROUTINE_SUSPENDED
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}
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private fun crossFunctionCompletion(
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currentFunction: DeepRecursiveFunctionBlock,
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cont: Continuation<Any?>
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): Continuation<Any?> = Continuation(EmptyCoroutineContext) {
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this.function = currentFunction
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// When going back from a trampoline we cannot just call cont.resume (stack usage!)
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// We delegate the cont.resumeWith(it) call to runCallLoop
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this.cont = cont
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this.result = it
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}
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@Suppress("UNCHECKED_CAST")
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fun runCallLoop(): R {
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while (true) {
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// Note: cont is set to null in DeepRecursiveScopeImpl.resumeWith when the whole computation completes
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val result = this.result
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val cont = this.cont
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?: return (result as Result<R>).getOrThrow() // done -- final result
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// The order of comparison is important here for that case of rogue class with broken equals
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if (UNDEFINED_RESULT == result) {
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// call "function" with "value" using "cont" as completion
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val r = try {
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// This is block.startCoroutine(this, value, cont)
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function.startCoroutineUninterceptedOrReturn(this, value, cont)
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} catch (e: Throwable) {
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cont.resumeWithException(e)
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continue
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}
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// If the function returns without suspension -- calls its continuation immediately
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if (r !== COROUTINE_SUSPENDED)
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cont.resume(r as R)
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} else {
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// we returned from a crossFunctionCompletion trampoline -- call resume here
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this.result = UNDEFINED_RESULT // reset result back
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cont.resumeWith(result)
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}
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}
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}
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}
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