Gate.swift

import os

public struct Gate<T: Sendable>: Sendable {
  struct Pass: Sendable {
    var value: T
    var continuaton: UnsafeContinuation<Void, Never>
  }
  
  struct Pending: Sendable {
    var continuation: UnsafeContinuation<T, Never>
  }
  
  struct Resumption {
    let pass: Pass
    let pending: Pending
    
    func resume() {
      pending.continuation.resume(returning: pass.value)
      pass.continuaton.resume()
    }
  }
  
  struct State: Sendable {
    var passing = [Pass]()
    var pending = [Pending]()
    
    mutating func drain() -> Resumption? {
      if !passing.isEmpty && !pending.isEmpty {
        return Resumption(pass: passing.removeFirst(), pending: pending.removeFirst())
      }
      return nil
    }
  }
  
  let state = OSAllocatedUnfairLock(initialState: State())
  
  public func pass(_ value: T) async {
    return await withUnsafeContinuation { continuation in
      state.withLock { state in
        state.passing.append(Pass(value: value, continuaton: continuation))
        return state.drain()
      }?.resume()
    }
  }
  
  public var value: T {
    get async {
      await withUnsafeContinuation { continuation in
        state.withLock { state in
          state.pending.append(Pending(continuation: continuation))
          return state.drain()
        }?.resume()
      }
    }
  }
}