import Combine
import SwiftUI

extension Effect {
  /// Wraps the emission of each element with SwiftUI's `withAnimation`.
  ///
  /// ```swift
  /// case .buttonTapped:
  ///   return .run { send in
  ///     await send(.activityResponse(self.apiClient.fetchActivity()))
  ///   }
  ///   .animation()
  /// ```
  ///
  /// - Parameter animation: An animation.
  /// - Returns: A publisher.
  public func animation(_ animation: Animation? = .default) -> Self {
    self.transaction(Transaction(animation: animation))
  }

  /// Wraps the emission of each element with SwiftUI's `withTransaction`.
  ///
  /// ```swift
  /// case .buttonTapped:
  ///   var transaction = Transaction(animation: .default)
  ///   transaction.disablesAnimations = true
  ///   return .run { send in
  ///     await send(.activityResponse(self.apiClient.fetchActivity()))
  ///   }
  ///   .transaction(transaction)
  /// ```
  ///
  /// - Parameter transaction: A transaction.
  /// - Returns: A publisher.
  public func transaction(_ transaction: Transaction) -> Self {
    switch self.operation {
    case .none:
      return .none
    case let .publisher(publisher):
      return Self(
        operation: .publisher(
          TransactionPublisher(upstream: publisher, transaction: transaction).eraseToAnyPublisher()
        )
      )
    case let .run(priority, operation):
      let uncheckedTransaction = UncheckedSendable(transaction)
      return Self(
        operation: .run(priority) { send in
          await operation(
            Send { value in
              withTransaction(uncheckedTransaction.value) {
                send(value)
              }
            }
          )
        }
      )
    }
  }
}

private struct TransactionPublisher<Upstream: Publisher>: Publisher {
  typealias Output = Upstream.Output
  typealias Failure = Upstream.Failure

  var upstream: Upstream
  var transaction: Transaction

  func receive<S: Combine.Subscriber>(subscriber: S)
  where S.Input == Output, S.Failure == Failure {
    let conduit = Subscriber(downstream: subscriber, transaction: self.transaction)
    self.upstream.receive(subscriber: conduit)
  }

  private final class Subscriber<Downstream: Combine.Subscriber>: Combine.Subscriber {
    typealias Input = Downstream.Input
    typealias Failure = Downstream.Failure

    let downstream: Downstream
    let transaction: Transaction

    init(downstream: Downstream, transaction: Transaction) {
      self.downstream = downstream
      self.transaction = transaction
    }

    func receive(subscription: Subscription) {
      self.downstream.receive(subscription: subscription)
    }

    func receive(_ input: Input) -> Subscribers.Demand {
      withTransaction(self.transaction) {
        self.downstream.receive(input)
      }
    }

    func receive(completion: Subscribers.Completion<Failure>) {
      self.downstream.receive(completion: completion)
    }
  }
}