org-social-ios/App/ViewModels/FollowCoordinator.swift

import Foundation
import Observation
import OrgSocialKit

/// Serializes all follow / unfollow operations across the app and holds the
/// authoritative copy of the user's own feed in memory.
///
/// The previous design let each `ProfileView` read the feed, mutate it, and
/// upload independently. Tapping Follow on several profiles in quick
/// succession caused last-write-wins: B's upload overwrote A's because both
/// started from the same pre-A snapshot. Host CDN staleness made it worse:
/// a read right after an upload often returned the old content.
///
/// This coordinator fixes both:
///  - `toggle(url:nick:)` awaits the previous in-flight toggle before
///    running, so concurrent taps become a strict sequence.
///  - After a successful upload, `cachedContent` is updated from the value
///    we just pushed; the next toggle never needs to re-read from CDN.
@Observable @MainActor
final class FollowCoordinator {
    static let shared = FollowCoordinator()

    /// Normalized `absoluteString`s of currently followed URLs, for
    /// O(1) lookup from the toolbar state.
    private(set) var followedURLs: Set<String> = []

    /// Current follow list, each with an optional display nick. Source of
    /// truth for any UI that needs to name a followed account (e.g. the
    /// mention autocomplete).
    private(set) var follows: [OrgSocialFollow] = []

    /// Monotonic counter bumped on every known mutation to the own feed
    /// (follow toggle, compose, edit, delete, profile edit, pin). Views
    /// observe this to trigger a refetch, since SwiftUI's TabView keeps
    /// a tab's .task from firing a second time on subsequent visits.
    private(set) var feedVersion: UInt = 0

    @ObservationIgnored private var cachedContent: String?
    @ObservationIgnored private var cachedFeedURL: URL?
    @ObservationIgnored private var didRefresh = false
    @ObservationIgnored private var inflightTask: Task<Void, Never>?

    private init() {}

    // MARK: - Refresh

    /// Fetches the own feed once per session and populates the cache.
    /// Callers can invoke this repeatedly; subsequent calls no-op.
    func refreshIfNeeded() async {
        guard let ownURL = ownFeedURL else {
            reset()
            return
        }
        if cachedFeedURL == ownURL, didRefresh { return }
        await forceRefresh()
    }

    func forceRefresh() async {
        guard let ownURL = ownFeedURL else { reset(); return }
        guard let content = try? await FeedFetcher().fetch(from: ownURL, bypassCache: true) else { return }
        apply(content: content, feedURL: ownURL)
        didRefresh = true
    }

    // MARK: - Query

    func isFollowing(_ url: URL) -> Bool {
        followedURLs.contains(Self.normalize(url))
    }

    // MARK: - Mutation

    /// Toggles follow state for `url`. Operations are strictly serialized:
    /// a second call awaits the first, preventing lost writes.
    func toggle(url: URL, nick: String?) async {
        let previous = inflightTask
        let task = Task { [weak self] in
            await previous?.value
            await self?.performToggle(url: url, nick: nick)
        }
        inflightTask = task
        await task.value
    }

    private func performToggle(url: URL, nick: String?) async {
        guard let ownURL = ownFeedURL, let uploader = UploaderFactory.makeUploader() else { return }

        if cachedContent == nil || cachedFeedURL != ownURL {
            guard let initial = try? await FeedFetcher().fetch(from: ownURL, bypassCache: true) else { return }
            apply(content: initial, feedURL: ownURL)
            didRefresh = true
        }
        guard var content = cachedContent else { return }

        let normalized = Self.normalize(url)
        let currentlyFollowing = followedURLs.contains(normalized)
        let pw = ProfileWriter()
        content = currentlyFollowing
            ? pw.removeFollow(url: url, from: content)
            : pw.addFollow(url: url, nick: nick, to: content)

        do {
            try await uploader.upload(content: content)
            apply(content: content, feedURL: ownURL)
        } catch {
            // Upload failed; keep the previous cached content untouched.
        }
    }

    // MARK: - Internal state sync

    /// Call after any external mutation to the own feed (e.g. a post
    /// published via ComposeView) to keep the cached copy authoritative.
    func updateCachedContent(_ content: String) {
        guard let ownURL = ownFeedURL else { return }
        apply(content: content, feedURL: ownURL)
    }

    private func apply(content: String, feedURL: URL) {
        cachedContent = content
        cachedFeedURL = feedURL
        var parsed = OrgSocialParser().parse(content)
        parsed.feedURL = feedURL
        followedURLs = Set(parsed.follows.map { Self.normalize($0.url) })
        follows = parsed.follows
        feedVersion &+= 1
    }

    private func reset() {
        cachedContent = nil
        cachedFeedURL = nil
        followedURLs = []
        follows = []
        didRefresh = false
    }

    // MARK: - Helpers

    private var ownFeedURL: URL? {
        guard let raw = UserDefaults.standard.string(forKey: "publicFeedURL"),
              let url = URL(string: raw) else { return nil }
        return url
    }

    /// Normalizes a URL for comparison: lowercases scheme/host (DNS is
    /// case-insensitive) and strips a trailing slash on non-root paths.
    /// Keeps path case intact because HTTP paths are case-sensitive.
    static func normalize(_ url: URL) -> String {
        guard var comps = URLComponents(url: url, resolvingAgainstBaseURL: false) else {
            return url.absoluteString
        }
        comps.scheme = comps.scheme?.lowercased()
        comps.host = comps.host?.lowercased()
        var path = comps.path
        if path.count > 1, path.hasSuffix("/") { path.removeLast() }
        comps.path = path
        return comps.url?.absoluteString ?? url.absoluteString
    }
}