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/*
* 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.*
import kotlin.native.concurrent.SharedImmutable
/**
* 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 { 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 = DeepRecursiveFunction { t ->
* if (t == null) 0 else odd.callRecursive(t.left) + odd.callRecursive(t.right) + 1
* }
* val odd: DeepRecursiveFunction = 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("1.7")
@WasExperimental(ExperimentalStdlibApi::class)
public class DeepRecursiveFunction(
internal val block: suspend DeepRecursiveScope.(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("1.7")
@WasExperimental(ExperimentalStdlibApi::class)
public operator fun DeepRecursiveFunction.invoke(value: T): R =
DeepRecursiveScopeImpl(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("1.7")
@WasExperimental(ExperimentalStdlibApi::class)
public sealed class DeepRecursiveScope {
/**
* 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 DeepRecursiveFunction.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 ==================
private typealias DeepRecursiveFunctionBlock = suspend DeepRecursiveScope<*, *>.(Any?) -> Any?
@SharedImmutable
private val UNDEFINED_RESULT = Result.success(COROUTINE_SUSPENDED)
@Suppress("UNCHECKED_CAST")
private class DeepRecursiveScopeImpl(
block: suspend DeepRecursiveScope.(T) -> R,
value: T
) : DeepRecursiveScope(), Continuation {
// 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? = this as Continuation
// Completion result (completion of the whole call stack)
private var result: Result = UNDEFINED_RESULT
override val context: CoroutineContext
get() = EmptyCoroutineContext
override fun resumeWith(result: Result) {
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
this.value = value
COROUTINE_SUSPENDED
}
override suspend fun DeepRecursiveFunction.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)
} else {
// calling the same function -- direct
this.cont = cont as Continuation
}
this.value = value
}
COROUTINE_SUSPENDED
}
private fun crossFunctionCompletion(
currentFunction: DeepRecursiveFunctionBlock,
cont: Continuation
): Continuation = 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).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)
}
}
}
}