import Foundation
import SwiftUI

/// What the lists are showing, and how it changes.
///
/// The sorting, filtering and searching are the server's work: `GET /api/entries` takes them all
/// and pages fifty at a time, so this holds a page and asks for the next rather than keeping ten
/// thousand items in memory and sorting them here.
@MainActor
final class LibraryStore: ObservableObject {
    /// Where the list is pointed. The places come before any feed, as they do in the page.
    enum Place: Hashable {
        case all
        case listening
        case feed(String)

        var feedId: String? { if case .feed(let id) = self { return id } else { return nil } }
    }

    @Published private(set) var feeds: [IPX.Feed] = []
    @Published private(set) var entries: [IPX.Entry] = []
    @Published private(set) var total = 0
    @Published private(set) var loading = false
    @Published private(set) var failure: String?
    /// Light or dark as the account has it, nil for auto. ipx keeps the theme on the user so it
    /// follows the person rather than the browser; the app should follow the same choice rather
    /// than the phone's, or it disagrees with the page it sits in front of.
    @Published private(set) var appearance: ColorScheme?

    @Published var place: Place = .all { didSet { if place != oldValue { reload() } } }
    @Published var filter: IPX.Filter = .all { didSet { if filter != oldValue { reload() } } }
    @Published var sort: IPX.Sort = .published { didSet { reload() } }
    @Published var direction: IPX.Direction = .desc { didSet { reload() } }
    @Published var search = "" { didSet { searchChanged() } }
    @Published var selected: IPX.Entry.ID?

    private let api: API
    private var searchTask: Task<Void, Never>?
    private var loadTask: Task<Void, Never>?

    init(api: API) { self.api = api }

    var canLoadMore: Bool { entries.count < total }

    var selectedEntry: IPX.Entry? { entries.first { $0.id == selected } }

    /// The feed a row belongs to, for its name and art. All Subscriptions mixes them, so a row
    /// cannot assume the selected feed is its own.
    func feed(_ id: String) -> IPX.Feed? { feeds.first { $0.id == id } }

    var title: String {
        switch place {
        case .all: return "All Subscriptions"
        case .listening: return "Currently Listening"
        case .feed(let id): return feed(id)?.name ?? id
        }
    }

    // MARK: - loading

    func refreshFeeds() async {
        appearance = .dark
        if let me = try? await api.me() {
            switch me.mode {
            case "light": appearance = .light
            case "auto":  appearance = nil    // follow the system, as the page does for Auto
            // Dark when nobody has chosen, because that is ipx's own default -- theme.ts
            // starts at modern/dark rather than at the system. Following the phone here would
            // put a light list in front of a dark page.
            default:      appearance = .dark
            }
        }
        do {
            feeds = try await api.feeds()
            failure = nil
        } catch {
            failure = (error as? LocalizedError)?.errorDescription ?? "\(error)"
        }
    }

    func reload() {
        loadTask?.cancel()
        loadTask = Task { await load(offset: 0) }
    }

    func loadMore() {
        guard !loading, canLoadMore else { return }
        loadTask = Task { await load(offset: entries.count) }
    }

    private func load(offset: Int) async {
        loading = true
        defer { loading = false }
        do {
            // Currently Listening is not a feed; it is the started-but-unfinished filter, which
            // the page reaches through the same route.
            let page = try await api.entries(
                feed: place.feedId,
                filter: place == .listening ? .all : filter,
                search: search,
                sort: sort,
                direction: direction,
                offset: offset)
            guard !Task.isCancelled else { return }
            var rows = offset == 0 ? page.entries : entries + page.entries
            if place == .listening {
                rows = rows.filter { $0.position > 0 && !$0.read }
            }
            entries = rows
            total = place == .listening ? rows.count : page.total
            failure = nil
        } catch is CancellationError {
        } catch {
            failure = (error as? LocalizedError)?.errorDescription ?? "\(error)"
        }
    }

    /// Typing should not fire a request per keystroke, nor wait so long it feels broken.
    private func searchChanged() {
        searchTask?.cancel()
        searchTask = Task {
            try? await Task.sleep(nanoseconds: 300_000_000)
            guard !Task.isCancelled else { return }
            reload()
        }
    }

    // MARK: - writing

    /// Read and pinned are set here first and sent after. The page does the same, for the same
    /// reason: a list asked for before the write lands answers with the old state, which put the
    /// dot back on an item just read.
    func setRead(_ entry: IPX.Entry, _ read: Bool) {
        replace(entry.id) { $0.with(read: read) }
        Task {
            do { try await api.setRead(read, feed: entry.feedId, guid: entry.guid) }
            catch { replace(entry.id) { $0.with(read: !read) } }
        }
    }

    func setPinned(_ entry: IPX.Entry, _ pinned: Bool) {
        replace(entry.id) { $0.with(flagged: pinned) }
        Task {
            do { try await api.setPinned(pinned, feed: entry.feedId, guid: entry.guid) }
            catch { replace(entry.id) { $0.with(flagged: !pinned) } }
        }
    }

    func readAll() {
        let ids = entries.map(\.id)
        for id in ids { replace(id) { $0.with(read: true) } }
        Task {
            try? await api.readAll(feed: place.feedId)
            await refreshFeeds()
        }
    }

    func scan() {
        Task {
            try? await api.fetch(feed: place.feedId)
        }
    }

    private func replace(_ id: IPX.Entry.ID, _ change: (IPX.Entry) -> IPX.Entry) {
        guard let at = entries.firstIndex(where: { $0.id == id }) else { return }
        entries[at] = change(entries[at])
    }
}

extension IPX.Entry {
    /// A copy with one flag moved. The type is decoded from the server and has no setters, which
    /// is deliberate: the only things that change locally are the two this person owns.
    func with(read: Bool? = nil, flagged: Bool? = nil) -> IPX.Entry {
        IPX.Entry(guid: guid, feedId: feedId, title: title, link: link, published: published,
                  description: description, read: read ?? self.read,
                  flagged: flagged ?? self.flagged, image: image, duration: duration,
                  episode: episode, season: season, position: position, enclosures: enclosures)
    }
}