//
//  retryWithBehavior.swift
//  RxSwiftExt
//
//  Created by Anton Efimenko on 17/07/16.
//  Copyright © 2016 RxSwift Community. All rights reserved.
//

import Foundation
import RxSwift

/**
Specifies how observable sequence will be repeated in case of an error
- Immediate: Will be immediatelly repeated specified number of times
- Delayed: Will be repeated after specified delay specified number of times
- ExponentialDelayed: Will be repeated specified number of times. 
Delay will be incremented by multiplier after each iteration (multiplier = 0.5 means 50% increment)
- CustomTimerDelayed: Will be repeated specified number of times. Delay will be calculated by custom closure
*/

public enum RepeatBehavior {
	case immediate (maxCount: UInt)
	case delayed (maxCount: UInt, time: Double)
	case exponentialDelayed (maxCount: UInt, initial: Double, multiplier: Double)
	case customTimerDelayed (maxCount: UInt, delayCalculator: (UInt) -> DispatchTimeInterval)
}

public typealias RetryPredicate = (Error) -> Bool

extension RepeatBehavior {
	/**
	Extracts maxCount and calculates delay for current RepeatBehavior
	- parameter currentAttempt: Number of current attempt
	- returns: Tuple with maxCount and calculated delay for provided attempt
	*/
	func calculateConditions(_ currentRepetition: UInt) -> (maxCount: UInt, delay: DispatchTimeInterval) {
		switch self {
		case .immediate(let max):
			// if Immediate, return 0.0 as delay
			return (maxCount: max, delay: .never)
		case .delayed(let max, let time):
			// return specified delay
			return (maxCount: max, delay: .milliseconds(Int(time * 1000)))
		case .exponentialDelayed(let max, let initial, let multiplier):
			// if it's first attempt, simply use initial delay, otherwise calculate delay
			let delay = currentRepetition == 1 ? initial : initial * pow(1 + multiplier, Double(currentRepetition - 1))
			return (maxCount: max, delay: .milliseconds(Int(delay * 1000)))
		case .customTimerDelayed(let max, let delayCalculator):
			// calculate delay using provided calculator
			return (maxCount: max, delay: delayCalculator(currentRepetition))
		}
	}
}

extension ObservableType {
	/**
	Repeats the source observable sequence using given behavior in case of an error or until it successfully terminated
	- parameter behavior: Behavior that will be used in case of an error
	- parameter scheduler: Schedular that will be used for delaying subscription after error
	- parameter shouldRetry: Custom optional closure for checking error (if returns true, repeat will be performed)
	- returns: Observable sequence that will be automatically repeat if error occurred
	*/

	public func retry(_ behavior: RepeatBehavior, scheduler: SchedulerType = MainScheduler.instance, shouldRetry: RetryPredicate? = nil) -> Observable<Element> {
		return retry(1, behavior: behavior, scheduler: scheduler, shouldRetry: shouldRetry)
	}

	/**
	Repeats the source observable sequence using given behavior in case of an error or until it successfully terminated
	- parameter currentAttempt: Number of current attempt
	- parameter behavior: Behavior that will be used in case of an error
	- parameter scheduler: Schedular that will be used for delaying subscription after error
	- parameter shouldRetry: Custom optional closure for checking error (if returns true, repeat will be performed)
	- returns: Observable sequence that will be automatically repeat if error occurred
	*/

	internal func retry(_ currentAttempt: UInt, behavior: RepeatBehavior, scheduler: SchedulerType = MainScheduler.instance, shouldRetry: RetryPredicate? = nil)
		-> Observable<Element> {
			guard currentAttempt > 0 else { return Observable.empty() }

			// calculate conditions for bahavior
			let conditions = behavior.calculateConditions(currentAttempt)

			return catchError { error -> Observable<Element> in
				// return error if exceeds maximum amount of retries
				guard conditions.maxCount > currentAttempt else { return Observable.error(error) }

				if let shouldRetry = shouldRetry, !shouldRetry(error) {
					// also return error if predicate says so
					return Observable.error(error)
				}

				guard conditions.delay != .never else {
					// if there is no delay, simply retry
					return self.retry(currentAttempt + 1, behavior: behavior, scheduler: scheduler, shouldRetry: shouldRetry)
				}

				// otherwise retry after specified delay
				return Observable<Void>.just(()).delaySubscription(conditions.delay, scheduler: scheduler).flatMapLatest {
					self.retry(currentAttempt + 1, behavior: behavior, scheduler: scheduler, shouldRetry: shouldRetry)
				}
			}
	}
}