//
//  LookupController.swift
//  TagTunes
//
//  Created by Kim Wittenburg on 07.03.16.
//  Copyright © 2016 Kim Wittenburg. All rights reserved.
//

import SearchAPI
import AppKitPlus

/// The state of a lookup process.
public enum LookupState {

    /// The track has not been processed for lookup. This is the initial state.
    case Unprocessed

    /// The track is being prepared for lookup. During preparation the track's id
    /// is read.
    case Preparing

    /// The lookup is currently searching for the track's metadata.
    case Searching

    /// The track was found. The associated `Track` represents the found metadata
    /// of the track.
    case Found(Track)

    /// The track has been looked up but was not found.
    case NotFound

    /// The track is not qualified for lookup because its id could not be read. The
    /// associated value contains an error description identifying the reason why
    /// the track's id could not be read.
    case Unqualified(ErrorType)

    /// During the lookup an error occured. This state is also used if the lookup
    /// was cancelled by the user.
    case Error(ErrorType?)

}

// TODO: Documentation


// DOCUMENTATION: All delegate methods are executed on main thread.
protocol LookupQueueDelegate: class {

    /// Invoked before the `lookupController` will process the specified
    /// `tracks`. This method may be invoked multiple times before
    /// `lookupControllerDiDFinishLookup(_:)` is invoked, but it is guaranteed
    /// that it will be invoked at least once. For more information see the
    /// documentation for `LookupController`.
    func lookupQueue(lookupQueue: LookupQueue, willBeginLookupForTracks tracks: [TagTunesTrack])

    /// Invoked when the `lookupController` completes the lookup process for the
    /// specified `tracks`. This method is called for tracks that could be
    /// successfully looked up or that could not be looked up. There can also be
    /// both in one invocation of this method. Use the `lookupState` of the
    /// `TagTunesTrack` instances to get information about the lookup result for
    /// individual tracks.
    ///
    /// This method may be called multiple times in a row. Every track that has
    /// been part of the `tracks` array of a
    /// `lookupController(_:willBeginLookupForTracks:)` message will be part of
    /// a invocation of this method exactly once.
    ///
    /// - parameters:
    ///   - lookupController: The controller that did the lookup.
    ///   - tracks: The tracks that have been processed by the
    ///             `lookupController`. The order of the tracks has no meaning.
    func lookupQueue(lookupQueue: LookupQueue, completedLookupForTracks tracks: [TagTunesTrack])

    /// Invoked after the `lookupController` finished looking up all enqueued
    /// tracks.
    func lookupQueueDidFinishLookup(lookupQueue: LookupQueue)
    
}


/// The lookup controller looks up tracks' metadata online based on the iTunes
/// store ID embedded into the a file. To be notified about the lookup progress
/// set the controller's `delegate` property. There are some valid assumptions
/// you can make about the order in which the delgate methods are invoked:
///
/// 1. `lookupController(_:willBeginLookupForTracks:)` is the first of the
///    methods to be invoked.
/// 2. Every `TagTunesTrack` that is part of the `tracks` in
///    `lookupController(_:willBeginLookupForTracks:)` will be part of the
///    `tracks` of `lookupController(_:completedLookupForTracks:)` or.
/// 4. `lookupControllerDidFinishLookup(_:)` may only be called after (1) and (2)
///    are satisfied.
/// 5. After `lookupControllerDidFinishLookup(_:)` the next delegate message will
///    be `lookupController(_:willBeginLookupForTracks:)`.
///
/// The calls to `lookupController(_:willBeginLookupForTracks:)` and
/// `lookupControllerDidFinishLookup(_:)` are **not** balanced. There may be
/// multiple invokations of the former with only a single invokation of the
/// latter.
class LookupQueue: OperationQueue {

    static let globalQueue = LookupQueue()

    // TODO: Is there an alternative to two delegates
    /// The lookup controller's delegate.
    weak var lookupDelegate: LookupQueueDelegate?

    var lookupOperation: LookupOperation?

    private var aggregatedTracks = [TagTunesTrack]()

    /// Enqueues the specified `tracks` for lookup. The point in time where the
    /// `tracks` will actually be looked up is not defined. It may be when this
    /// method returns or at some later point. To get notified when the lookup
    /// actually starts implement the delgate method
    /// `lookupController(_:willBeginLookupForTracks:)`.
    ///
    /// - note: There is no correspondance between the specified `tracks` an the
    ///         tracks passed to the delegate methods. If there is currently a
    ///         lookup in progress the lookup controller might collect the
    ///         enqueued tracks and batch-process them.
    func enqueueTracksForLookup<S: SequenceType where S.Generator.Element == TagTunesTrack>(tracks: S) {
        aggregatedTracks.appendContentsOf(tracks)
        for track in tracks {
            let preparationOperation = LookupPreparationOperation(track: track)
            addOperation(preparationOperation)
        }
        beginLookupIfPossible()
    }

    // DOCUMENTATION: Must execute on main thread
    // FIXME: ? Can there be two lookup operations?
    private func beginLookupIfPossible() {
        // TODO: "Finished Lookup" delegate call
        if lookupOperation == nil && !aggregatedTracks.isEmpty {
            lookupOperation = LookupOperation(tracks: aggregatedTracks)
            lookupOperation?.addObserver(BlockObserver(startHandler: { operation in
                    dispatch_sync(dispatch_get_main_queue()) {
                        // TODO: Is dispatch_sync ok?
                        let lookupOperation = operation as! LookupOperation
                        self.lookupDelegate?.lookupQueue(self, willBeginLookupForTracks: lookupOperation.tracks)
                    }
                }, produceHandler: nil, finishHandler: { (operation, errors) in
                dispatch_sync(dispatch_get_main_queue()) {
                    // TODO: Is this a retain cycle?
                    // TODO: Is dispatch_sync ok?
                    self.lookupOperation(operation, finishedWithErrors: errors)
                }
            }))
            aggregatedTracks = []
            for operation in operations where operation is LookupPreparationOperation {
                lookupOperation?.addDependency(operation)
            }
            addOperation(lookupOperation!)
        }
    }

    // DOCUMENTATION: Must execute on main thread.
    private func lookupOperation(operation: Operation, finishedWithErrors errors: [ErrorType]) {
        let lookupOperation = operation as! LookupOperation
        self.lookupDelegate?.lookupQueue(self, completedLookupForTracks: lookupOperation.tracks)
        self.lookupOperation = nil
        if aggregatedTracks.isEmpty {
            lookupDelegate?.lookupQueueDidFinishLookup(self)
        } else {
            beginLookupIfPossible()
        }
    }

    /// Cancels the lookup. This will do three things:
    ///
    /// 1. Stop any running or pending network request. Set the `lookupState` of
    ///    all tracks to `NSCocoaError.UserCancelledError`.
    /// 2. Send the delegate a `lookupController(_:completedLookupForTracks:)`
    ///    message.
    /// 3. Send the delegate a `lookupControllerDidFinishLookup(_:)` message.
    func cancelLookup() {
        // TODO: Implementation
    }
    
}