//
//  Copyright (c) 2023 Touch Instinct
//
//  Permission is hereby granted, free of charge, to any person obtaining a copy
//  of this software and associated documentation files (the Software), to deal
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//  copies of the Software, and to permit persons to whom the Software is
//  furnished to do so, subject to the following conditions:
//
//  The above copyright notice and this permission notice shall be included in
//  all copies or substantial portions of the Software.
//
//  THE SOFTWARE IS PROVIDED AS IS, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
//  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
//  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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//  THE SOFTWARE.
//

import Foundation

private final class FlatMapAsyncOperation<Output, Failure: Error>: AsyncOperation<Output, Failure> {
    private var dependencyObservation: NSKeyValueObservation?
    private var flatMapObservation: NSKeyValueObservation?
    private let dependency: Operation
    private var flatMapDependency: AsyncOperation<Output, Failure>?

    init<DependencyOutput, DependencyFailure>(dependency: AsyncOperation<DependencyOutput, DependencyFailure>,
                                              flatMapOutput: ((DependencyOutput) -> AsyncOperation<Output, Failure>)?,
                                              flatMapFailure: ((DependencyFailure) -> AsyncOperation<Output, Failure>)?)
    where DependencyFailure: Error {

        self.dependency = dependency

        super.init()

        dependency.cancelOnCancellation(of: self)

        dependencyObservation = dependency.subscribe { [weak self] in
            switch $0 {
            case let .success(success):
                self?.flatMapDependency = flatMapOutput?(success)

            case let .failure(error):
                self?.flatMapDependency = flatMapFailure?(error)
            }

            if let self {
                self.flatMapDependency?.cancelOnCancellation(of: self)
            }

            self?.flatMapObservation = self?.flatMapDependency?.subscribe {
                self?.handle(result: $0)
            }

            self?.flatMapDependency?.start()
        }

        state = .isReady
    }

    override func start() {
        state = .isExecuting

        dependency.start()
    }
}

public extension AsyncOperation {
    func flatMap<NewOutput>(_ transform: @escaping (Output) -> AsyncOperation<NewOutput, Failure>)
    -> AsyncOperation<NewOutput, Failure> {

        FlatMapAsyncOperation(dependency: self,
                              flatMapOutput: transform,
                              flatMapFailure: { .just(failure: $0) })
    }

    func flatMapError<NewFailure>(_ transform: @escaping (Failure) -> AsyncOperation<Output, NewFailure>)
    -> AsyncOperation<Output, NewFailure> {

        FlatMapAsyncOperation(dependency: self,
                              flatMapOutput: { .just(success: $0) },
                              flatMapFailure: transform)
    }
}