mastodon-ios/Mastodon/Scene/Thread/MastodonStatusThreadViewModel.swift

//
//  MastodonStatusThreadViewModel.swift
//  MastodonStatusThreadViewModel
//
//  Created by Cirno MainasuK on 2021-9-6.
//  Copyright © 2021 Twidere. All rights reserved.
//

import Foundation
import Combine
import CoreData
import CoreDataStack
import MastodonSDK
import MastodonCore
import MastodonMeta
import os.log

final class MastodonStatusThreadViewModel {
    let logger = Logger(subsystem: "MastodonStatusThreadViewModel", category: "Data")
    static let entryNotFoundMessage = "Failed to find suitable record. Depending on the context this might result in errors (data not being updated) or can be discarded (e.g. when there are mixed data sources where an entry might or might not exist)."

    var disposeBag = Set<AnyCancellable>()
    
    // input
    let context: AppContext
    let filterContext: Mastodon.Entity.FilterContext?
    @Published private(set) var deletedObjectIDs: Set<MastodonStatus.ID> = Set()

    // output
    @Published var __ancestors: [StatusItem] = []
    @Published var ancestors: [StatusItem] = []
    
    @Published var __descendants: [StatusItem] = []
    @Published var descendants: [StatusItem] = []
    
    init(context: AppContext, filterContext: Mastodon.Entity.FilterContext?) {
        self.context = context
        self.filterContext = filterContext
        
        Publishers.CombineLatest(
            $__ancestors,
            $deletedObjectIDs
        )
        .sink { [weak self] items, deletedObjectIDs in
            guard let self = self else { return }
            let newItems = items.filter { item in
                switch item {
                case .thread(let thread):
                    return !deletedObjectIDs.contains(thread.record.id)
                default:
                    assertionFailure()
                    return false
                }
            }
            self.ancestors = newItems
        }
        .store(in: &disposeBag)
        
        Publishers.CombineLatest(
            $__descendants,
            $deletedObjectIDs
        )
        .sink { [weak self] items, deletedObjectIDs in
            guard let self = self else { return }
            let newItems = items.filter { item in
                switch item {
                case .thread(let thread):
                    return !deletedObjectIDs.contains(thread.record.id)
                default:
                    assertionFailure()
                    return false
                }
            }
            self.descendants = newItems
        }
        .store(in: &disposeBag)
    }
    
    
}

extension MastodonStatusThreadViewModel {
    
    func appendAncestor(
        nodes: [Node]
    ) {
        var newItems: [StatusItem] = []
        for node in nodes {
            
            if let filterContext, let filterBox = StatusFilterService.shared.activeFilterBox {
                let filterResult = filterBox.apply(to: node.status, in: filterContext)
                switch filterResult {
                case .hide:
                    continue
                default:
                    break
                }
            }
            
            let item = StatusItem.thread(.leaf(context: .init(status: node.status)))
            newItems.append(item)
        }
        
        let items = self.__ancestors + newItems
        self.__ancestors = items.removingDuplicates()
    }
    
    func appendDescendant(
        nodes: [Node]
    ) {

        var newItems: [StatusItem] = []

        for node in nodes {
            
            if let filterContext, let filterBox = StatusFilterService.shared.activeFilterBox {
                let filterResult = filterBox.apply(to: node.status, in: filterContext)
                switch filterResult {
                case .hide:
                    continue
                default:
                    break
                }
            }
            
            let context = StatusItem.Thread.Context(status: node.status)
            let item = StatusItem.thread(.leaf(context: context))
            newItems.append(item)
            
            // second tier
            if let child = node.children.first {
                guard let secondaryStatus = node.children.first(where: { $0.status.id == child.status.id}) else { continue }
                let secondaryContext = StatusItem.Thread.Context(
                    status: secondaryStatus.status,
                    displayUpperConversationLink: true
                )
                let secondaryItem = StatusItem.thread(.leaf(context: secondaryContext))
                newItems.append(secondaryItem)

                // update first tier context
                context.displayBottomConversationLink = true
            }
        }
        
        var items = self.__descendants
        for item in newItems {
            guard !items.contains(item) else { continue }
            items.append(item)
        }
        self.__descendants = items.removingDuplicates()
    }
    
}

extension MastodonStatusThreadViewModel {
    class Node {
        let status: MastodonStatus
        let children: [Node]
        
        init(
            status: MastodonStatus,
            children: [MastodonStatusThreadViewModel.Node]
        ) {
            self.status = status
            self.children = children
        }
    }
}

extension MastodonStatusThreadViewModel.Node {
    static func replyToThread(
        for replyToID: Mastodon.Entity.Status.ID?,
        from statuses: [Mastodon.Entity.Status]
    ) -> [MastodonStatusThreadViewModel.Node] {
        guard let replyToID = replyToID else {
            return []
        }
        
        var dict: [Mastodon.Entity.Status.ID: Mastodon.Entity.Status] = [:]
        for status in statuses {
            dict[status.id] = status
        }
        
        var nextID: Mastodon.Entity.Status.ID? = replyToID
        var nodes: [MastodonStatusThreadViewModel.Node] = []
        while let _nextID = nextID {
            guard let status = dict[_nextID] else { break }
            nodes.append(MastodonStatusThreadViewModel.Node(
                status: .fromEntity(status),
                children: []
            ))
            nextID = status.inReplyToID
        }
        
        return nodes
    }
}

extension MastodonStatusThreadViewModel.Node {
    static func children(
        of status: MastodonStatus,
        from statuses: [Mastodon.Entity.Status]
    ) -> [MastodonStatusThreadViewModel.Node] {
        var dictionary: [Mastodon.Entity.Status.ID: Mastodon.Entity.Status] = [:]
        var mapping: [Mastodon.Entity.Status.ID: Set<Mastodon.Entity.Status.ID>] = [:]
        
        for status in statuses {
            dictionary[status.id] = status
            guard let replyToID = status.inReplyToID else { continue }
            if var set = mapping[replyToID] {
                set.insert(status.id)
                mapping[replyToID] = set
            } else {
                mapping[replyToID] = Set([status.id])
            }
        }
        
        var children: [MastodonStatusThreadViewModel.Node] = []
        let replies = Array(mapping[status.id] ?? Set())
            .compactMap { dictionary[$0] }
            .sorted(by: { $0.createdAt > $1.createdAt })
        for reply in replies {
            let child = child(of: reply, dictionary: dictionary, mapping: mapping)
            children.append(child)
        }
        return children
    }
    
    static func child(
        of status: Mastodon.Entity.Status,
        dictionary: [Mastodon.Entity.Status.ID: Mastodon.Entity.Status],
        mapping: [Mastodon.Entity.Status.ID: Set<Mastodon.Entity.Status.ID>]
    ) -> MastodonStatusThreadViewModel.Node {
        let childrenIDs = mapping[status.id] ?? []
        let children = Array(childrenIDs)
            .compactMap { dictionary[$0] }
            .sorted(by: { $0.createdAt > $1.createdAt })
            .map { status in child(of: status, dictionary: dictionary, mapping: mapping) }
        return MastodonStatusThreadViewModel.Node(
            status: .fromEntity(status),
            children: children
        )
    }
    
}