Tusker/Tusker/TimelineLikeController.swift

//
//  TimelineLikeController.swift
//  Tusker
//
//  Created by Shadowfacts on 9/19/22.
//  Copyright © 2022 Shadowfacts. All rights reserved.
//

import Foundation
import OSLog

protocol TimelineLikeControllerDelegate<TimelineItem>: AnyObject {
    associatedtype TimelineItem
    
    func loadInitial() async throws -> [TimelineItem]
    
    func loadNewer() async throws -> [TimelineItem]
    
    func loadOlder() async throws -> [TimelineItem]
    
    func canLoadOlder() async -> Bool
    
    func handleEvent(_ event: TimelineLikeController<TimelineItem>.Event) async
}

private let logger = Logger(subsystem: Bundle.main.bundleIdentifier!, category: "TimelineLikeController")

actor TimelineLikeController<Item> {
    
    unowned var delegate: any TimelineLikeControllerDelegate<Item>
    
    private(set) var state = State.notLoadedInitial {
        willSet {
            precondition(state.canTransition(to: newValue))
            logger.debug("State: \(self.state.debugDescription, privacy: .public) -> \(newValue.debugDescription, privacy: .public)")
        }
    }
    
    init(delegate: any TimelineLikeControllerDelegate<Item>) {
        self.delegate = delegate
    }
    
    func loadInitial() async {
        guard state == .notLoadedInitial else {
            return
        }
        let token = LoadAttemptToken()
        state = .loadingInitial(token, hasAddedLoadingIndicator: false)
        let loadingIndicator = DeferredLoadingIndicator(owner: self, state: state, addedIndicatorState: .loadingInitial(token, hasAddedLoadingIndicator: true))
        do {
            let items = try await delegate.loadInitial()
            guard case .loadingInitial(token, _) = state else {
                return
            }
            await loadingIndicator.end()
            await emit(event: .replaceAllItems(items, token))
            state = .idle
        } catch {
            await loadingIndicator.end()
            await emit(event: .loadAllError(error, token))
            state = .idle
        }
    }
    
    func loadNewer() async {
        guard state == .idle else {
            return
        }
        let token = LoadAttemptToken()
        state = .loadingNewer(token)
        do {
            let items = try await delegate.loadNewer()
            guard case .loadingNewer(token) = state else {
                return
            }
            await emit(event: .prependItems(items, token))
            state = .idle
        } catch {
            await emit(event: .loadNewerError(error, token))
            state = .idle
        }
    }
    
    func loadOlder() async {
        guard state == .idle else {
            return
        }
        let token = LoadAttemptToken()
        guard await delegate.canLoadOlder() else {
            return
        }
        state = .loadingOlder(token, hasAddedLoadingIndicator: false)
        let loadingIndicator = DeferredLoadingIndicator(owner: self, state: state, addedIndicatorState: .loadingOlder(token, hasAddedLoadingIndicator: true))
        do {
            let items = try await delegate.loadOlder()
            guard case .loadingOlder(token, _) = state else {
                return
            }
            await loadingIndicator.end()
            await emit(event: .appendItems(items, token))
            state = .idle
        } catch {
            await loadingIndicator.end()
            await emit(event: .loadOlderError(error, token))
            state = .idle
        }
    }
    
    private func transition(to newState: State) {
        self.state = newState
    }
    
    private func emit(event: Event) async {
        precondition(state.canEmit(event: event))
        await delegate.handleEvent(event)
    }
    
    enum State: Equatable, CustomDebugStringConvertible {
        case notLoadedInitial
        case idle
        case loadingInitial(LoadAttemptToken, hasAddedLoadingIndicator: Bool)
        case loadingNewer(LoadAttemptToken)
        case loadingOlder(LoadAttemptToken, hasAddedLoadingIndicator: Bool)
        
        var debugDescription: String {
            switch self {
            case .notLoadedInitial:
                return "notLoadedInitial"
            case .idle:
                return "idle"
            case .loadingInitial(let token, let hasAddedLoadingIndicator):
                return "loadingInitial(\(ObjectIdentifier(token)), hasAddedLoadingIndicator: \(hasAddedLoadingIndicator))"
            case .loadingNewer(let token):
                return "loadingNewer(\(ObjectIdentifier(token)))"
            case .loadingOlder(let token, let hasAddedLoadingIndicator):
                return "loadingOlder(\(ObjectIdentifier(token)), hasAddedLoadingIndicator: \(hasAddedLoadingIndicator))"
            }
        }
        
        func canTransition(to: State) -> Bool {
            switch self {
            case .notLoadedInitial:
                switch to {
                case .loadingInitial(_, hasAddedLoadingIndicator: _):
                    return true
                default:
                    return false
                }
            case .idle:
                switch to {
                case .loadingNewer(_), .loadingOlder(_, _):
                    return true
                default:
                    return false
                }
            case .loadingInitial(let token, let hasAddedLoadingIndicator):
                return to == .idle || (!hasAddedLoadingIndicator && to == .loadingInitial(token, hasAddedLoadingIndicator: true))
            case .loadingNewer(_):
                return to == .idle
            case .loadingOlder(let token, let hasAddedLoadingIndicator):
                return to == .idle || (!hasAddedLoadingIndicator && to == .loadingOlder(token, hasAddedLoadingIndicator: true))
            }
        }
        
        func canEmit(event: Event) -> Bool {
            switch event {
            case .addLoadingIndicator:
                switch self {
                case .loadingInitial(_, _), .loadingOlder(_, _):
                    return true
                default:
                    return false
                }
            case .removeLoadingIndicator:
                switch self {
                case .loadingInitial(_, hasAddedLoadingIndicator: true), .loadingOlder(_, hasAddedLoadingIndicator: true):
                    return true
                default:
                    return false
                }
            case .loadAllError(_, let token), .replaceAllItems(_, let token):
                switch self {
                case .loadingInitial(token, _):
                    return true
                default:
                    return false
                }
            case .loadNewerError(_, let token), .prependItems(_, let token):
                switch self {
                case .loadingNewer(token):
                    return true
                default:
                    return false
                }
            case .loadOlderError(_, let token), .appendItems(_, let token):
                switch self {
                case .loadingOlder(token, _):
                    return true
                default:
                    return false
                }
            }
        }
    }
    
    enum Event {
        case addLoadingIndicator
        case removeLoadingIndicator
        case loadAllError(Error, LoadAttemptToken)
        case replaceAllItems([Item], LoadAttemptToken)
        case loadNewerError(Error, LoadAttemptToken)
        case prependItems([Item], LoadAttemptToken)
        case loadOlderError(Error, LoadAttemptToken)
        case appendItems([Item], LoadAttemptToken)
    }
    
    class LoadAttemptToken: Equatable {
        static func ==(lhs: LoadAttemptToken, rhs: LoadAttemptToken) -> Bool {
            return lhs === rhs
        }
    }
    
    class DeferredLoadingIndicator {
        private let owner: TimelineLikeController<Item>
        private let addedIndicatorState: State
        private let task: Task<Void, Error>
        
        init(owner: TimelineLikeController<Item>, state: State, addedIndicatorState: State) {
            self.owner = owner
            self.addedIndicatorState = addedIndicatorState
            self.task = Task {
                try await Task.sleep(nanoseconds: 150 * NSEC_PER_MSEC)
                guard await state == owner.state else {
                    return
                }
                await owner.emit(event: .addLoadingIndicator)
                await owner.transition(to: addedIndicatorState)
            }
        }
        
        func end() async {
            let state = await owner.state
            if state == addedIndicatorState {
                await owner.emit(event: .removeLoadingIndicator)
            } else {
                task.cancel()
            }
        }
    }
    
}