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//===----------------------------------------------------------------------===//
//
// This source file is part of the Swift Async Algorithms open source project
//
// Copyright (c) 2022 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
//
//===----------------------------------------------------------------------===//
import DequeModule
struct UnboundedBufferStateMachine<Base: AsyncSequence> {
typealias Element = Base.Element
typealias SuspendedConsumer = UnsafeContinuation<Result<Element, Error>?, Never>
enum Policy {
case unlimited
case bufferingNewest(Int)
case bufferingOldest(Int)
}
// We are using UnsafeTransfer here since we have to get the elements from the task
// into the consumer task. This is a transfer but we cannot prove this to the compiler at this point
// since next is not marked as transferring the return value.
fileprivate enum State {
case initial(base: Base)
case buffering(
task: Task<Void, Never>,
buffer: Deque<Result<UnsafeTransfer<Element>, Error>>,
suspendedConsumer: SuspendedConsumer?
)
case modifying
case finished(buffer: Deque<Result<UnsafeTransfer<Element>, Error>>)
}
private var state: State
private let policy: Policy
init(base: Base, policy: Policy) {
self.state = .initial(base: base)
self.policy = policy
}
var task: Task<Void, Never>? {
switch self.state {
case .buffering(let task, _, _):
return task
default:
return nil
}
}
mutating func taskStarted(task: Task<Void, Never>) {
switch self.state {
case .initial:
self.state = .buffering(task: task, buffer: [], suspendedConsumer: nil)
case .buffering:
preconditionFailure("Invalid state.")
case .modifying:
preconditionFailure("Invalid state.")
case .finished:
preconditionFailure("Invalid state.")
}
}
enum ElementProducedAction {
case none
case resumeConsumer(
continuation: SuspendedConsumer,
result: Result<Element, Error>
)
}
mutating func elementProduced(element: Element) -> ElementProducedAction {
switch self.state {
case .initial:
preconditionFailure("Invalid state. The task should already by started.")
case .buffering(let task, var buffer, .none):
// we are either idle or the buffer is already in use (no awaiting consumer)
// we have to apply the policy when stacking the new element
self.state = .modifying
switch self.policy {
case .unlimited:
buffer.append(.success(.init(element)))
case .bufferingNewest(let limit):
if buffer.count >= limit {
_ = buffer.popFirst()
}
buffer.append(.success(.init(element)))
case .bufferingOldest(let limit):
if buffer.count < limit {
buffer.append(.success(.init(element)))
}
}
self.state = .buffering(task: task, buffer: buffer, suspendedConsumer: nil)
return .none
case .buffering(let task, let buffer, .some(let suspendedConsumer)):
// we have an awaiting consumer, we can resume it with the element
precondition(buffer.isEmpty, "Invalid state. The buffer should be empty.")
self.state = .buffering(task: task, buffer: buffer, suspendedConsumer: nil)
return .resumeConsumer(
continuation: suspendedConsumer,
result: .success(element)
)
case .modifying:
preconditionFailure("Invalid state.")
case .finished:
return .none
}
}
enum FinishAction {
case none
case resumeConsumer(continuation: SuspendedConsumer?)
}
mutating func finish(error: Error?) -> FinishAction {
switch self.state {
case .initial:
preconditionFailure("Invalid state. The task should already by started.")
case .buffering(_, var buffer, .none):
// we are either idle or the buffer is already in use (no awaiting consumer)
// if we have an error we stack it in the buffer so it can be consumed later
if let error {
buffer.append(.failure(error))
}
self.state = .finished(buffer: buffer)
return .none
case .buffering(_, let buffer, let suspendedConsumer):
// we have an awaiting consumer, we can resume it with nil or the error
precondition(buffer.isEmpty, "Invalid state. The buffer should be empty.")
self.state = .finished(buffer: [])
return .resumeConsumer(continuation: suspendedConsumer)
case .modifying:
preconditionFailure("Invalid state.")
case .finished:
return .none
}
}
enum NextAction {
case startTask(base: Base)
case suspend
case returnResult(Result<Element, Error>?)
}
mutating func next() -> NextAction {
switch self.state {
case .initial(let base):
return .startTask(base: base)
case .buffering(_, let buffer, let suspendedConsumer) where buffer.isEmpty:
// we are idle, we have to suspend the consumer
precondition(suspendedConsumer == nil, "Invalid states. There is already a suspended consumer.")
return .suspend
case .buffering(let task, var buffer, let suspendedConsumer):
// the buffer is already in use, we can unstack a value and directly resume the consumer
precondition(suspendedConsumer == nil, "Invalid states. There is already a suspended consumer.")
self.state = .modifying
let result = buffer.popFirst()!
self.state = .buffering(task: task, buffer: buffer, suspendedConsumer: nil)
return .returnResult(result.map { $0.wrapped })
case .modifying:
preconditionFailure("Invalid state.")
case .finished(let buffer) where buffer.isEmpty:
return .returnResult(nil)
case .finished(var buffer):
self.state = .modifying
let result = buffer.popFirst()!
self.state = .finished(buffer: buffer)
return .returnResult(result.map { $0.wrapped })
}
}
enum NextSuspendedAction {
case none
case resumeConsumer(Result<Element, Error>?)
}
mutating func nextSuspended(continuation: SuspendedConsumer) -> NextSuspendedAction {
switch self.state {
case .initial:
preconditionFailure("Invalid state. The task should already by started.")
case .buffering(let task, let buffer, let suspendedConsumer) where buffer.isEmpty:
// we are idle, we confirm the suspension of the consumer
precondition(suspendedConsumer == nil, "Invalid states. There is already a suspended consumer.")
self.state = .buffering(task: task, buffer: buffer, suspendedConsumer: continuation)
return .none
case .buffering(let task, var buffer, let suspendedConsumer):
// the buffer is already in use, we can unstack a value and directly resume the consumer
precondition(suspendedConsumer == nil, "Invalid states. There is already a suspended consumer.")
self.state = .modifying
let result = buffer.popFirst()!
self.state = .buffering(task: task, buffer: buffer, suspendedConsumer: nil)
return .resumeConsumer(result.map { $0.wrapped })
case .modifying:
preconditionFailure("Invalid state.")
case .finished(let buffer) where buffer.isEmpty:
return .resumeConsumer(nil)
case .finished(var buffer):
self.state = .modifying
let result = buffer.popFirst()!
self.state = .finished(buffer: buffer)
return .resumeConsumer(result.map { $0.wrapped })
}
}
enum InterruptedAction {
case none
case resumeConsumer(task: Task<Void, Never>, continuation: SuspendedConsumer?)
}
mutating func interrupted() -> InterruptedAction {
switch self.state {
case .initial:
state = .finished(buffer: [])
return .none
case .buffering(let task, _, let suspendedConsumer):
self.state = .finished(buffer: [])
return .resumeConsumer(task: task, continuation: suspendedConsumer)
case .modifying:
preconditionFailure("Invalid state.")
case .finished:
self.state = .finished(buffer: [])
return .none
}
}
}
extension UnboundedBufferStateMachine: Sendable where Base: Sendable { }
extension UnboundedBufferStateMachine.State: Sendable where Base: Sendable { }
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