Event.swift

import Foundation
import Dispatch

extension Signal {
	/// Represents a signal event.
	///
	/// Signals must conform to the grammar:
	/// `value* (failed | completed | interrupted)?`
	public enum Event {
		/// A value provided by the signal.
		case value(Value)

		/// The signal terminated because of an error. No further events will be
		/// received.
		case failed(Error)

		/// The signal successfully terminated. No further events will be received.
		case completed

		/// Event production on the signal has been interrupted. No further events
		/// will be received.
		///
		/// - important: This event does not signify the successful or failed
		///              completion of the signal.
		case interrupted

		/// Whether this event is a completed event.
		public var isCompleted: Bool {
			switch self {
			case .completed:
				return true

			case .value, .failed, .interrupted:
				return false
			}
		}

		/// Whether this event indicates signal termination (i.e., that no further
		/// events will be received).
		public var isTerminating: Bool {
			switch self {
			case .value:
				return false

			case .failed, .completed, .interrupted:
				return true
			}
		}

		/// Lift the given closure over the event's value.
		///
		/// - important: The closure is called only on `value` type events.
		///
		/// - parameters:
		///   - f: A closure that accepts a value and returns a new value
		///
		/// - returns: An event with function applied to a value in case `self` is a
		///            `value` type of event.
		public func map<U>(_ f: (Value) -> U) -> Signal<U, Error>.Event {
			switch self {
			case let .value(value):
				return .value(f(value))

			case let .failed(error):
				return .failed(error)

			case .completed:
				return .completed

			case .interrupted:
				return .interrupted
			}
		}

		/// Lift the given closure over the event's error.
		///
		/// - important: The closure is called only on failed type event.
		///
		/// - parameters:
		///   - f: A closure that accepts an error object and returns
		///        a new error object
		///
		/// - returns: An event with function applied to an error object in case
		///            `self` is a `.Failed` type of event.
		public func mapError<F>(_ f: (Error) -> F) -> Signal<Value, F>.Event {
			switch self {
			case let .value(value):
				return .value(value)

			case let .failed(error):
				return .failed(f(error))

			case .completed:
				return .completed

			case .interrupted:
				return .interrupted
			}
		}

		/// Unwrap the contained `value` value.
		public var value: Value? {
			if case let .value(value) = self {
				return value
			} else {
				return nil
			}
		}

		/// Unwrap the contained `Error` value.
		public var error: Error? {
			if case let .failed(error) = self {
				return error
			} else {
				return nil
			}
		}
	}
}

extension Signal.Event where Value: Equatable, Error: Equatable {
	public static func == (lhs: Signal<Value, Error>.Event, rhs: Signal<Value, Error>.Event) -> Bool {
		switch (lhs, rhs) {
		case let (.value(left), .value(right)):
			return left == right

		case let (.failed(left), .failed(right)):
			return left == right

		case (.completed, .completed):
			return true

		case (.interrupted, .interrupted):
			return true

		default:
			return false
		}
	}
}

extension Signal.Event: Equatable where Value: Equatable, Error: Equatable {}

extension Signal.Event: CustomStringConvertible {
	public var description: String {
		switch self {
		case let .value(value):
			return "VALUE \(value)"

		case let .failed(error):
			return "FAILED \(error)"

		case .completed:
			return "COMPLETED"

		case .interrupted:
			return "INTERRUPTED"
		}
	}
}

/// Event protocol for constraining signal extensions
public protocol EventProtocol {
	/// The value type of an event.
	associatedtype Value
	/// The error type of an event. If errors aren't possible then `Never` can
	/// be used.
	associatedtype Error: Swift.Error
	/// Extracts the event from the receiver.
	var event: Signal<Value, Error>.Event { get }
}

extension Signal.Event: EventProtocol {
	public var event: Signal<Value, Error>.Event {
		return self
	}
}

// Event Transformations
//
// Operators backed by event transformations have such characteristics:
//
// 1. Unary
//    The operator applies to only one stream.
//
// 2. Serial
//    The outcome need not be synchronously emitted, but all events must be delivered in
//    serial order.
//
// 3. No side effect upon interruption.
//    The operator must not perform any side effect upon receving `interrupted`.
//
// Examples of ineligible operators (for now):
//
// 1. `timeout`
//    This operator forwards the `failed` event on a different scheduler.
//
// 2. `combineLatest`
//    This operator applies to two or more streams.
//
// 3. `SignalProducer.then`
//    This operator starts a second stream when the first stream completes.
//
// 4. `on`
//    This operator performs side effect upon interruption.

extension Signal.Event {
	internal typealias Transformation<U, E: Swift.Error> = (@escaping Signal<U, E>.Observer.Action, Lifetime) -> Signal<Value, Error>.Observer.Action

	internal static func filter(_ isIncluded: @escaping (Value) -> Bool) -> Transformation<Value, Error> {
		return { action, _ in
			return { event in
				switch event {
				case let .value(value):
					if isIncluded(value) {
						action(.value(value))
					}

				case .completed:
					action(.completed)

				case let .failed(error):
					action(.failed(error))

				case .interrupted:
					action(.interrupted)
				}
			}
		}
	}

	internal static func compactMap<U>(_ transform: @escaping (Value) -> U?) -> Transformation<U, Error> {
		return { action, _ in
			return { event in
				switch event {
				case let .value(value):
					if let newValue = transform(value) {
						action(.value(newValue))
					}

				case .completed:
					action(.completed)

				case let .failed(error):
					action(.failed(error))

				case .interrupted:
					action(.interrupted)
				}
			}
		}
	}

	internal static func map<U>(_ transform: @escaping (Value) -> U) -> Transformation<U, Error> {
		return { action, _ in
			return { event in
				switch event {
				case let .value(value):
					action(.value(transform(value)))

				case .completed:
					action(.completed)

				case let .failed(error):
					action(.failed(error))

				case .interrupted:
					action(.interrupted)
				}
			}
		}
	}

	internal static func mapError<E>(_ transform: @escaping (Error) -> E) -> Transformation<Value, E> {
		return { action, _ in
			return { event in
				switch event {
				case let .value(value):
					action(.value(value))

				case .completed:
					action(.completed)

				case let .failed(error):
					action(.failed(transform(error)))

				case .interrupted:
					action(.interrupted)
				}
			}
		}
	}

	internal static var materialize: Transformation<Signal<Value, Error>.Event, Never> {
		return { action, _ in
			return { event in
				action(.value(event))

				switch event {
				case .interrupted:
					action(.interrupted)

				case .completed, .failed:
					action(.completed)

				case .value:
					break
				}
			}
		}
	}

	internal static var materializeResults: Transformation<Result<Value, Error>, Never> {
		return { action, _ in
			return { event in
				switch event {
				case .value(let value):
					action(.value(Result(success: value)))

				case .failed(let error):
					action(.value(Result(failure: error)))
					action(.completed)

				case .completed:
					action(.completed)

				case .interrupted:
					action(.interrupted)
				}
			}
		}
	}

	internal static func attemptMap<U>(_ transform: @escaping (Value) -> Result<U, Error>) -> Transformation<U, Error> {
		return { action, _ in
			return { event in
				switch event {
				case let .value(value):
					switch transform(value) {
					case let .success(value):
						action(.value(value))
					case let .failure(error):
						action(.failed(error))
					}
				case let .failed(error):
					action(.failed(error))
				case .completed:
					action(.completed)
				case .interrupted:
					action(.interrupted)
				}
			}
		}
	}

	internal static func attempt(_ action: @escaping (Value) -> Result<(), Error>) -> Transformation<Value, Error> {
		return attemptMap { value -> Result<Value, Error> in
			return action(value).map { _ in value }
		}
	}
}

extension Signal.Event where Error == Swift.Error {
	internal static func attempt(_ action: @escaping (Value) throws -> Void) -> Transformation<Value, Error> {
		return attemptMap { value in
			try action(value)
			return value
		}
	}

	internal static func attemptMap<U>(_ transform: @escaping (Value) throws -> U) -> Transformation<U, Error> {
		return attemptMap { value in
			Result { try transform(value) }
		}
	}
}

extension Signal.Event {
	internal static func take(first count: Int) -> Transformation<Value, Error> {
		assert(count >= 1)

		return { action, _ in
			var taken = 0

			return { event in
				guard let value = event.value else {
					action(event)
					return
				}

				if taken < count {
					taken += 1
					action(.value(value))
				}

				if taken == count {
					action(.completed)
				}
			}
		}
	}

	internal static func take(last count: Int) -> Transformation<Value, Error> {
		return { action, _ in
			var buffer: [Value] = []
			buffer.reserveCapacity(count)

			return { event in
				switch event {
				case let .value(value):
					// To avoid exceeding the reserved capacity of the buffer,
					// we remove then add. Remove elements until we have room to
					// add one more.
					while (buffer.count + 1) > count {
						buffer.remove(at: 0)
					}

					buffer.append(value)
				case let .failed(error):
					action(.failed(error))
				case .completed:
					buffer.forEach { action(.value($0)) }
					action(.completed)
				case .interrupted:
					action(.interrupted)
				}
			}
		}
	}

	internal static func take(while shouldContinue: @escaping (Value) -> Bool) -> Transformation<Value, Error> {
		return { action, _ in
			return { event in
				if let value = event.value, !shouldContinue(value) {
					action(.completed)
				} else {
					action(event)
				}
			}
		}
	}

	internal static func skip(first count: Int) -> Transformation<Value, Error> {
		precondition(count > 0)

		return { action, _ in
			var skipped = 0

			return { event in
				if case .value = event, skipped < count {
					skipped += 1
				} else {
					action(event)
				}
			}
		}
	}

	internal static func skip(while shouldContinue: @escaping (Value) -> Bool) -> Transformation<Value, Error> {
		return { action, _ in
			var isSkipping = true

			return { event in
				switch event {
				case let .value(value):
					isSkipping = isSkipping && shouldContinue(value)
					if !isSkipping {
						fallthrough
					}

				case .failed, .completed, .interrupted:
					action(event)
				}
			}
		}
	}
}

extension Signal.Event where Value: EventProtocol, Error == Never {
	internal static var dematerialize: Transformation<Value.Value, Value.Error> {
		return { action, _ in
			return { event in
				switch event {
				case let .value(innerEvent):
					action(innerEvent.event)

				case .failed:
					fatalError("Never is impossible to construct")

				case .completed:
					action(.completed)

				case .interrupted:
					action(.interrupted)
				}
			}
		}
	}
}

extension Signal.Event where Value: ResultProtocol, Error == Never {
	internal static var dematerializeResults: Transformation<Value.Success, Value.Failure> {
		return { action, _ in
			return { event in
				let event = event.map { $0.result }

				switch event {
				case .value(.success(let value)):
					action(.value(value))

				case .value(.failure(let error)):
					action(.failed(error))

				case .failed:
					fatalError("Never is impossible to construct")

				case .completed:
					action(.completed)

				case .interrupted:
					action(.interrupted)
				}
			}
		}
	}
}

extension Signal.Event where Value: OptionalProtocol {
	internal static var skipNil: Transformation<Value.Wrapped, Error> {
		return compactMap { $0.optional }
	}
}

/// A reference type which wraps an array to auxiliate the collection of values
/// for `collect` operator.
private final class CollectState<Value> {
	var values: [Value] = []

	/// Collects a new value.
	func append(_ value: Value) {
		values.append(value)
	}

	/// Check if there are any items remaining.
	///
	/// - note: This method also checks if there weren't collected any values
	///         and, in that case, it means an empty array should be sent as the
	///         result of collect.
	var isEmpty: Bool {
		/// We use capacity being zero to determine if we haven't collected any
		/// value since we're keeping the capacity of the array to avoid
		/// unnecessary and expensive allocations). This also guarantees
		/// retro-compatibility around the original `collect()` operator.
		return values.isEmpty && values.capacity > 0
	}

	/// Removes all values previously collected if any.
	func flush() {
		// Minor optimization to avoid consecutive allocations. Can
		// be useful for sequences of regular or similar size and to
		// track if any value was ever collected.
		values.removeAll(keepingCapacity: true)
	}
}

extension Signal.Event {
	internal static var collect: Transformation<[Value], Error> {
		return collect { _, _ in false }
	}

	internal static func collect(count: Int) -> Transformation<[Value], Error> {
		precondition(count > 0)
		return collect { values in values.count == count }
	}

	internal static func collect(_ shouldEmit: @escaping (_ collectedValues: [Value]) -> Bool) -> Transformation<[Value], Error> {
		return { action, _ in
			let state = CollectState<Value>()

			return { event in
				switch event {
				case let .value(value):
					state.append(value)
					if shouldEmit(state.values) {
						action(.value(state.values))
						state.flush()
					}
				case .completed:
					if !state.isEmpty {
						action(.value(state.values))
					}
					action(.completed)
				case let .failed(error):
					action(.failed(error))
				case .interrupted:
					action(.interrupted)
				}
			}
		}
	}

	internal static func collect(_ shouldEmit: @escaping (_ collected: [Value], _ latest: Value) -> Bool) -> Transformation<[Value], Error> {
		return { action, _ in
			let state = CollectState<Value>()

			return { event in
				switch event {
				case let .value(value):
					if shouldEmit(state.values, value) {
						action(.value(state.values))
						state.flush()
					}
					state.append(value)
				case .completed:
					if !state.isEmpty {
						action(.value(state.values))
					}
					action(.completed)
				case let .failed(error):
					action(.failed(error))
				case .interrupted:
					action(.interrupted)
				}
			}
		}
	}

	/// Implementation detail of `combinePrevious`. A default argument of a `nil` initial
	/// is deliberately avoided, since in the case of `Value` being an optional, the
	/// `nil` literal would be materialized as `Optional<Value>.none` instead of `Value`,
	/// thus changing the semantic.
	internal static func combinePrevious(initial: Value?) -> Transformation<(Value, Value), Error> {
		return { action, _ in
			var previous = initial

			return { event in
				switch event {
				case let .value(value):
					if let previous = previous {
						action(.value((previous, value)))
					}
					previous = value
				case .completed:
					action(.completed)
				case let .failed(error):
					action(.failed(error))
				case .interrupted:
					action(.interrupted)
				}
			}
		}
	}

	internal static func skipRepeats(_ isEquivalent: @escaping (Value, Value) -> Bool) -> Transformation<Value, Error> {
		return { action, _ in
			var previous: Value?

			return { event in
				switch event {
				case let .value(value):
					if let previous = previous, isEquivalent(previous, value) {
						return
					}
					previous = value
					fallthrough
				case .completed, .interrupted, .failed:
					action(event)
				}
			}
		}
	}

	internal static func uniqueValues<Identity: Hashable>(_ transform: @escaping (Value) -> Identity) -> Transformation<Value, Error> {
		return { action, _ in
			var seenValues: Set<Identity> = []

			return { event in
				switch event {
				case let .value(value):
					let identity = transform(value)
					let (inserted, _) = seenValues.insert(identity)
					if inserted {
						fallthrough
					}

				case .failed, .completed, .interrupted:
					action(event)
				}
			}
		}
	}

	internal static func scan<U>(into initialResult: U, _ nextPartialResult: @escaping (inout U, Value) -> Void) -> Transformation<U, Error> {
		return { action, _ in
			var accumulator = initialResult

			return { event in
				action(event.map { value in
					nextPartialResult(&accumulator, value)
					return accumulator
				})
			}
		}
	}

	internal static func scan<U>(_ initialResult: U, _ nextPartialResult: @escaping (U, Value) -> U) -> Transformation<U, Error> {
		return scan(into: initialResult) { $0 = nextPartialResult($0, $1) }
	}

	internal static func reduce<U>(into initialResult: U, _ nextPartialResult: @escaping (inout U, Value) -> Void) -> Transformation<U, Error> {
		return { action, _ in
			var accumulator = initialResult

			return { event in
				switch event {
				case let .value(value):
					nextPartialResult(&accumulator, value)
				case .completed:
					action(.value(accumulator))
					action(.completed)
				case .interrupted:
					action(.interrupted)
				case let .failed(error):
					action(.failed(error))
				}
			}
		}
	}

	internal static func reduce<U>(_ initialResult: U, _ nextPartialResult: @escaping (U, Value) -> U) -> Transformation<U, Error> {
		return reduce(into: initialResult) { $0 = nextPartialResult($0, $1) }
	}

	internal static func observe(on scheduler: Scheduler) -> Transformation<Value, Error> {
		return { action, lifetime in
			lifetime.observeEnded {
				scheduler.schedule {
					action(.interrupted)
				}
			}

			return { event in
				scheduler.schedule {
					if !lifetime.hasEnded {
						action(event)
					}
				}
			}
		}
	}

	internal static func lazyMap<U>(on scheduler: Scheduler, transform: @escaping (Value) -> U) -> Transformation<U, Error> {
		return { action, lifetime in
			let box = Atomic<Value?>(nil)
			let completionDisposable = SerialDisposable()
			let valueDisposable = SerialDisposable()

			lifetime += valueDisposable
			lifetime += completionDisposable

			lifetime.observeEnded {
				scheduler.schedule {
					action(.interrupted)
				}
			}

			return { event in
				switch event {
				case let .value(value):
					// Schedule only when there is no prior outstanding value.
					if box.swap(value) == nil {
						valueDisposable.inner = scheduler.schedule {
							if let value = box.swap(nil) {
								action(.value(transform(value)))
							}
						}
					}

				case .completed, .failed:
					// Completion and failure should not discard the outstanding
					// value.
					completionDisposable.inner = scheduler.schedule {
						action(event.map(transform))
					}

				case .interrupted:
					// `interrupted` overrides any outstanding value and any
					// scheduled completion/failure.
					valueDisposable.dispose()
					completionDisposable.dispose()
					scheduler.schedule {
						action(.interrupted)
					}
				}
			}
		}
	}

	internal static func delay(_ interval: TimeInterval, on scheduler: DateScheduler) -> Transformation<Value, Error> {
		precondition(interval >= 0)

		return { action, lifetime in
			lifetime.observeEnded {
				scheduler.schedule {
					action(.interrupted)
				}
			}

			return { event in
				switch event {
				case .failed, .interrupted:
					scheduler.schedule {
						action(event)
					}

				case .value, .completed:
					let date = scheduler.currentDate.addingTimeInterval(interval)
					scheduler.schedule(after: date) {
						if !lifetime.hasEnded {
							action(event)
						}
					}
				}
			}
		}
	}

	internal static func throttle(_ interval: TimeInterval, on scheduler: DateScheduler) -> Transformation<Value, Error> {
		precondition(interval >= 0)

		return { action, lifetime in
			let state: Atomic<ThrottleState<Value>> = Atomic(ThrottleState())
			let schedulerDisposable = SerialDisposable()

			lifetime.observeEnded {
				schedulerDisposable.dispose()
				scheduler.schedule { action(.interrupted) }
			}

			return { event in
				guard let value = event.value else {
					schedulerDisposable.inner = scheduler.schedule {
						action(event)
					}
					return
				}

				let scheduleDate: Date = state.modify { state in
					state.pendingValue = value

					let proposedScheduleDate: Date
					if let previousDate = state.previousDate, previousDate <= scheduler.currentDate {
						proposedScheduleDate = previousDate.addingTimeInterval(interval)
					} else {
						proposedScheduleDate = scheduler.currentDate
					}

					return proposedScheduleDate < scheduler.currentDate ? scheduler.currentDate : proposedScheduleDate
				}

				schedulerDisposable.inner = scheduler.schedule(after: scheduleDate) {
					if let pendingValue = state.modify({ $0.retrieveValue(date: scheduleDate) }) {
						action(.value(pendingValue))
					}
				}
			}
		}
	}

	internal static func debounce(_ interval: TimeInterval, on scheduler: DateScheduler, discardWhenCompleted: Bool) -> Transformation<Value, Error> {
		precondition(interval >= 0)
		
		return { action, lifetime in
			let state: Atomic<ThrottleState<Value>> = Atomic(ThrottleState(previousDate: scheduler.currentDate, pendingValue: nil))
			let d = SerialDisposable()

			lifetime.observeEnded {
				d.dispose()
				scheduler.schedule { action(.interrupted) }
			}

			return { event in
				switch event {
				case let .value(value):
					state.modify { state in
						state.pendingValue = value
					}
					let date = scheduler.currentDate.addingTimeInterval(interval)
					d.inner = scheduler.schedule(after: date) {
						if let pendingValue = state.modify({ $0.retrieveValue(date: date) }) {
							action(.value(pendingValue))
						}
					}
					
				case .completed:
					d.inner = scheduler.schedule {
						let pending: (value: Value, previousDate: Date)? = state.modify { state in
							defer { state.pendingValue = nil }
							guard let pendingValue = state.pendingValue, let previousDate = state.previousDate else { return nil }
							return (pendingValue, previousDate)
						}
						if !discardWhenCompleted, let (pendingValue, previousDate) = pending {
							scheduler.schedule(after: previousDate.addingTimeInterval(interval)) {
								action(.value(pendingValue))
								action(.completed)
							}
						} else {
							action(.completed)
						}
					}

				case .failed, .interrupted:
					d.inner = scheduler.schedule {
						action(event)
					}
				}
			}
		}
	}
	
	internal static func collect(every interval: DispatchTimeInterval, on scheduler: DateScheduler, skipEmpty: Bool, discardWhenCompleted: Bool) -> Transformation<[Value], Error> {
		return { action, lifetime in
			let state = Atomic<CollectEveryState<Value>>(.init(skipEmpty: skipEmpty))
			let d = SerialDisposable()
			
			d.inner = scheduler.schedule(after: scheduler.currentDate.addingTimeInterval(interval), interval: interval, leeway: interval * 0.1) {
				let (currentValues, isCompleted) = state.modify { ($0.collect(), $0.isCompleted) }
				if let currentValues = currentValues {
					action(.value(currentValues))
				}
				if isCompleted {
					action(.completed)
				}
			}
			
			lifetime.observeEnded {
				d.dispose()
				scheduler.schedule { action(.interrupted) }
			}

			return { event in
				switch event {
				case let .value(value):
					state.modify { $0.values.append(value) }
				case let .failed(error):
					d.inner = scheduler.schedule { action(.failed(error)) }
				case .completed where !discardWhenCompleted:
					state.modify { $0.isCompleted = true }
				case .completed:
					d.inner = scheduler.schedule { action(.completed) }
				case .interrupted:
					d.inner = scheduler.schedule { action(.interrupted) }
				}
			}
		}
	}
}

private struct CollectEveryState<Value> {
	let skipEmpty: Bool
	var values: [Value] = []
	var isCompleted: Bool = false
	
	init(skipEmpty: Bool) {
		self.skipEmpty = skipEmpty
	}
	
	var hasValues: Bool {
		return !values.isEmpty || !skipEmpty
	}
	
	mutating func collect() -> [Value]? {
		guard hasValues else { return nil }
		defer { values.removeAll() }
		return values
	}
}

private struct ThrottleState<Value> {
	var previousDate: Date?
	var pendingValue: Value?
	
	mutating func retrieveValue(date: Date) -> Value? {
		defer {
			if pendingValue != nil {
				pendingValue = nil
				previousDate = date
			}
		}
		return pendingValue
	}
}

extension Signal.Event where Error == Never {
	internal static func promoteError<F>(_: F.Type) -> Transformation<Value, F> {
		return { action, _ in
			return { event in
				switch event {
				case let .value(value):
					action(.value(value))
				case .failed:
					fatalError("Never is impossible to construct")
				case .completed:
					action(.completed)
				case .interrupted:
					action(.interrupted)
				}
			}
		}
	}
}

extension Signal.Event where Value == Never {
	internal static func promoteValue<U>(_: U.Type) -> Transformation<U, Error> {
		return { action, _ in
			return { event in
				action(event.promoteValue())
			}
		}
	}
}

extension Signal.Event where Value == Never {
	internal func promoteValue<U>() -> Signal<U, Error>.Event {
        switch event {
        case .value:
            fatalError("Never is impossible to construct")
        case let .failed(error):
            return .failed(error)
        case .completed:
            return .completed
        case .interrupted:
            return .interrupted
        }
    }
}