AsyncReplaySubject.swift

//
//  AsyncReplaySubject.swift
//  
//
//  Created by Thibault Wittemberg on 31/12/2021.
//

/// An `AsyncReplaySubject` is an async sequence in which one can send values over time.
/// Values are buffered in a FIFO fashion so they can be replayed by new consumers.
/// When the `bufferSize` is outreached the oldest value is dropped.
/// When the `AsyncReplaySubject` is terminated, new consumers will
/// immediately resume with this termination, whether it is a finish or a failure.
///
/// ```
/// let replay = AsyncReplaySubject<Int>(bufferSize: 3)
///
/// for i in (1...5) { replay.send(i) }
///
/// for await element in replay {
///   print(element) // will print 3, 4, 5
/// }
/// ```
public final class AsyncReplaySubject<Element>: AsyncSubject where Element: Sendable {
  public typealias Element = Element
  public typealias Failure = Never
  public typealias AsyncIterator = Iterator

  struct State {
    var terminalState: Termination<Failure>?
    var bufferSize: UInt
    var buffer: [Element]
    var channels: [Int: AsyncBufferedChannel<Element>]
    var ids: Int
  }

  let state: ManagedCriticalState<State>

  /// Creates a replay subject with a buffer of replayable values
  /// - Parameter bufferSize: the number of values that will be replayed to new consumers
  public init(bufferSize: UInt) {
    self.state = ManagedCriticalState(
      State(terminalState: nil, bufferSize: bufferSize, buffer: [], channels: [:], ids: 0)
    )
  }

  /// Sends a value to all consumers
  /// - Parameter element: the value to send
  public func send(_ element: Element) {
    self.state.withCriticalRegion { state in
      if state.buffer.count >= state.bufferSize && !state.buffer.isEmpty {
        state.buffer.removeFirst()
      }
      state.buffer.append(element)
      for channel in state.channels.values {
        channel.send(element)
      }
    }
  }

  /// Finishes the subject with a normal ending.
  /// - Parameter termination: The termination to finish the subject.
  public func send(_ termination: Termination<Failure>) {
    self.state.withCriticalRegion { state in
      state.terminalState = termination
      let channels = Array(state.channels.values)
      state.channels.removeAll()
      state.buffer.removeAll()
      state.bufferSize = 0
      for channel in channels {
        channel.finish()
      }
    }
  }

  func handleNewConsumer() -> (iterator: AsyncBufferedChannel<Element>.Iterator, unregister: @Sendable () -> Void) {
    let asyncBufferedChannel = AsyncBufferedChannel<Element>()

    let (terminalState, elements) = self.state.withCriticalRegion { state -> (Termination?, [Element]) in
      (state.terminalState, state.buffer)
    }

    if let terminalState = terminalState, terminalState.isFinished {
      asyncBufferedChannel.finish()
      return (asyncBufferedChannel.makeAsyncIterator(), {})
    }

    for element in elements {
      asyncBufferedChannel.send(element)
    }

    let consumerId = self.state.withCriticalRegion { state -> Int in
      state.ids += 1
      state.channels[state.ids] = asyncBufferedChannel
      return state.ids
    }

    let unregister = { @Sendable [state] in
      state.withCriticalRegion { state in
        state.channels[consumerId] = nil
      }
    }

    return (asyncBufferedChannel.makeAsyncIterator(), unregister)
  }

  public func makeAsyncIterator() -> AsyncIterator {
    Iterator(asyncSubject: self)
  }

  public struct Iterator: AsyncSubjectIterator {
    var iterator: AsyncBufferedChannel<Element>.Iterator
    let unregister: @Sendable () -> Void

    init(asyncSubject: AsyncReplaySubject) {
      (self.iterator, self.unregister) = asyncSubject.handleNewConsumer()
    }

    public var hasBufferedElements: Bool {
      self.iterator.hasBufferedElements
    }

    public mutating func next() async -> Element? {
      await withTaskCancellationHandler {
        await self.iterator.next()
      } onCancel: { [unregister] in
        unregister()
      }
    }
  }
}