import Foundation

final class MessageStore {
    private let messagesKey = "kasztelan.messages"
    private let lastIDKey = "kasztelan.messages.lastID"
    private let installedAtKey = "kasztelan.installation.timestamp"

    var messages: [KasztelanMessage] {
        guard let data = UserDefaults.standard.data(forKey: messagesKey),
              let values = try? JSONDecoder().decode([KasztelanMessage].self, from: data) else { return [] }
        return values.filter(isFromThisInstallation).sorted { $0.id > $1.id }
    }

    var lastServerID: Int { UserDefaults.standard.integer(forKey: lastIDKey) }

    @discardableResult
    func merge(_ incoming: [KasztelanMessage]) -> [KasztelanMessage] {
        var saved = messages
        var ids = Set(saved.map(\.id))
        let previousLast = lastServerID
        let added = incoming.filter { message in
            guard message.id > 0,
                  isFromThisInstallation(message),
                  !message.text.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty else { return false }
            return ids.insert(message.id).inserted
        }
        saved.append(contentsOf: added)
        saved.sort { $0.id > $1.id }
        saved = Array(saved.prefix(100))
        persist(saved, lastID: max(previousLast, incoming.map(\.id).max() ?? 0))
        return added.filter { $0.id > previousLast }.sorted { $0.id > $1.id }
    }

    func delete(id: Int) {
        persist(messages.filter { $0.id != id }, lastID: lastServerID)
    }

    func clear() {
        UserDefaults.standard.removeObject(forKey: messagesKey)
        UserDefaults.standard.removeObject(forKey: lastIDKey)
    }

    private func persist(_ messages: [KasztelanMessage], lastID: Int) {
        if let data = try? JSONEncoder().encode(messages) {
            UserDefaults.standard.set(data, forKey: messagesKey)
        }
        UserDefaults.standard.set(lastID, forKey: lastIDKey)
    }

    private var installedAt: Int {
        let saved = UserDefaults.standard.integer(forKey: installedAtKey)
        if saved > 0 { return saved }
        let created = (try? FileManager.default.attributesOfItem(atPath: NSHomeDirectory())[.creationDate] as? Date)
            ?? Date()
        let timestamp = Int(created.timeIntervalSince1970)
        UserDefaults.standard.set(timestamp, forKey: installedAtKey)
        return timestamp
    }

    private func isFromThisInstallation(_ message: KasztelanMessage) -> Bool {
        message.sentAt >= installedAt
    }
}