Nearfield_Friends/Proximity/ViewModels/ChatViewModel.swift

import Foundation
import Combine
import CryptoKit

/// View model for an E2E-encrypted conversation with a revealed connection.
///
/// Messages are encrypted with the session key established at reveal time
/// (see `RevealFlow`). The server only ever sees ciphertext — it cannot read
/// the conversation. This is the product's promise kept in the most intimate
/// place: the words between two people who chose to meet.
@MainActor
final class ChatViewModel: ObservableObject {

    @Published private(set) var messages: [ChatMessage] = []
    @Published var draft: String = ""
    @Published private(set) var isSending = false

    private let apiClient: APIClient
    private let cryptoManager: CryptoManager
    private let graphStore: GraphStore
    private let realtime: RealtimeClient

    /// The active connection we're chatting with.
    private var connection: Connection?
    /// The E2E session key for this conversation.
    private var sessionKey: SymmetricKey?
    /// Decrypted plaintext cache keyed by message id.
    @Published private(set) var plaintextByID: [UUID: String] = [:]

    private var cancellables = Set<AnyCancellable>()

    init(apiClient: APIClient, cryptoManager: CryptoManager, graphStore: GraphStore, realtime: RealtimeClient) {
        self.apiClient = apiClient
        self.cryptoManager = cryptoManager
        self.graphStore = graphStore
        self.realtime = realtime

        // Live wire: decrypt and append incoming messages in real time.
        realtime.incomingMessage
            .sink { [weak self] message in
                Task { await self?.receive(message) }
            }
            .store(in: &cancellables)
    }

    // MARK: - Setup

    /// Begin a conversation with a revealed connection.
    func start(with connection: Connection, sessionKey: SymmetricKey?) {
        self.connection = connection
        self.sessionKey = sessionKey
        self.messages = graphStore.messages.filter { $0.connectionID == connection.id }
    }

    // MARK: - Sending

    func send() async {
        let text = draft.trimmingCharacters(in: .whitespacesAndNewlines)
        guard !text.isEmpty,
              let connection,
              let sessionKey else { return }

        isSending = true
        defer { isSending = false }

        do {
            let plaintext = Data(text.utf8)
            let ciphertext = try cryptoManager.encrypt(plaintext, using: sessionKey)

            let message = ChatMessage(
                connectionID: connection.id,
                senderID: "me",
                ciphertext: ciphertext
            )
            messages.append(message)
            plaintextByID[message.id] = text
            graphStore.addMessage(message)
            draft = ""

            try await apiClient.sendMessage(message)
        } catch {
            // Keep the message locally; surface send failure to the UI.
        }
    }

    // MARK: - Receiving

    /// Decrypt and append an incoming message (called live from the socket).
    func receive(_ message: ChatMessage) async {
        guard let sessionKey,
              message.connectionID == connection?.id else { return }

        // Deduplicate against messages we already have.
        guard !messages.contains(where: { $0.id == message.id }) else { return }

        // Decrypt and cache the plaintext for display.
        if let plaintext = try? cryptoManager.decrypt(message.ciphertext, using: sessionKey),
           let text = String(data: plaintext, encoding: .utf8) {
            plaintextByID[message.id] = text
            messages.append(message)
            graphStore.addMessage(message)
        }
    }

    /// The decrypted text for a message, for display.
    func text(for message: ChatMessage) -> String {
        plaintextByID[message.id] ?? "…"
    }
}