// utils/fcm.js
// This file handles Firebase Cloud Messaging (FCM) notifications.
// Note: You must provide a valid serviceAccountKey.json and install firebase-admin
// for this to strictly work. We include safety checks to prevent crashes if missing.

import admin from 'firebase-admin';
import { createRequire } from "module";
const require = createRequire(import.meta.url);

let serviceAccount = null;
try {
    // Attempt to load credentials. In production, these might be env vars.
    // For now, looking for a file in the project root or config folder.
    serviceAccount = require("../serviceAccountKey.json");
} catch (error) {
    console.warn("FCM: serviceAccountKey.json not found. Push notifications will be disabled.");
}

let isFcmInitialized = false;

if (serviceAccount) {
    try {
        admin.initializeApp({
            credential: admin.credential.cert(serviceAccount)
        });
        isFcmInitialized = true;
        console.log("FCM Initialized successfully.");
    } catch (error) {
        console.error("Error initializing FCM:", error);
    }
}

/**
 * Sends a push notification to specific device tokens.
 * @param {string[]} tokens - Array of FCM registration tokens
 * @param {string} title - Notification title
 * @param {string} body - Notification body text
 * @param {object} data - Custom data payload (optional)
 */
export const sendPushNotification = async (tokens, title, body, data = {}) => {
    if (!isFcmInitialized || !tokens || tokens.length === 0) {
        return;
    }

    // Ensure tokens is an array
    const validTokens = Array.isArray(tokens) ? tokens : [tokens];

    const message = {
        notification: {
            title: title || "New Message",
            body: body || "You have a new message",
        },
        data: {
            ...data,
            click_action: "FLUTTER_NOTIFICATION_CLICK" // Standard for many frameworks
        },
        tokens: validTokens,
    };

    try {
        const response = await admin.messaging().sendMulticast(message);
        console.log(`FCM sent: ${response.successCount} successes, ${response.failureCount} failures.`);
        
        if (response.failureCount > 0) {
           const failedTokens = [];
           response.responses.forEach((resp, idx) => {
               if (!resp.success) {
                   failedTokens.push(validTokens[idx]);
                   // Optional: Remove invalid tokens from DB here if error code is 'messaging/registration-token-not-registered'
               }
           });
           // console.log('Failed tokens:', failedTokens);
        }
    } catch (error) {
        console.error("Error sending FCM notification:", error);
    }
};