/**
 * NATSBridge - Cross-Platform Bi-Directional Data Bridge
 * JavaScript/Node.js Implementation
 * 
 * This module provides functionality for sending and receiving data across network boundaries
 * using NATS as the message bus, with support for both direct payload transport and
 * URL-based transport for larger payloads.
 * 
 * @module NATSBridge
 */

const nats = require('nats');
const { v4: uuidv4 } = require('uuid');
const fetch = require('node-fetch');
const arrow = require('apache-arrow');

// ---------------------------------------------- Constants ---------------------------------------------- //

/**
 * Default size threshold for switching from direct to link transport (1MB)
 */
const DEFAULT_SIZE_THRESHOLD = 1_000_000;

/**
 * Default NATS server URL
 */
const DEFAULT_BROKER_URL = 'nats://localhost:4222';

/**
 * Default HTTP file server URL for link transport
 */
const DEFAULT_FILESERVER_URL = 'http://localhost:8080';

// ---------------------------------------------- Utility Functions ---------------------------------------------- //

/**
 * Convert Buffer to Base64 string
 * @param {Buffer} buffer - Buffer to encode
 * @returns {string} Base64 encoded string
 */
function bufferToBase64(buffer) {
    return buffer.toString('base64');
}

/**
 * Log a trace message with correlation ID and timestamp
 * @param {string} correlationId - Correlation ID for tracing
 * @param {string} message - Message content to log
 */
function logTrace(correlationId, message) {
    const timestamp = new Date().toISOString();
    console.log(`[${timestamp}] [Correlation: ${correlationId}] ${message}`);
}

// ---------------------------------------------- Serialization Functions ---------------------------------------------- //

/**
 * Serialize data according to specified format
 * @param {any} data - Data to serialize
 * @param {string} payloadType - Target format: "text", "dictionary", "table", "image", "audio", "video", "binary"
 * @returns {Buffer} Binary representation of the serialized data
 */
async function serializeData(data, payloadType) {
    if (payloadType === 'text') {
        if (typeof data === 'string') {
            return Buffer.from(data, 'utf8');
        } else {
            throw new Error('Text data must be a string');
        }
    } else if (payloadType === 'dictionary') {
        const jsonStr = JSON.stringify(data);
        return Buffer.from(jsonStr, 'utf8');
    } else if (payloadType === 'table') {
        // Convert array of objects to Arrow IPC format
        if (!Array.isArray(data) || data.length === 0) {
            throw new Error('Table data must be a non-empty array of objects');
        }
        
        return serializeArrowTable(data);
    } else if (payloadType === 'image') {
        if (data instanceof Uint8Array || Buffer.isBuffer(data)) {
            return Buffer.from(data);
        } else {
            throw new Error('Image data must be Uint8Array or Buffer');
        }
    } else if (payloadType === 'audio') {
        if (data instanceof Uint8Array || Buffer.isBuffer(data)) {
            return Buffer.from(data);
        } else {
            throw new Error('Audio data must be Uint8Array or Buffer');
        }
    } else if (payloadType === 'video') {
        if (data instanceof Uint8Array || Buffer.isBuffer(data)) {
            return Buffer.from(data);
        } else {
            throw new Error('Video data must be Uint8Array or Buffer');
        }
    } else if (payloadType === 'binary') {
        if (data instanceof Uint8Array || Buffer.isBuffer(data)) {
            return Buffer.from(data);
        } else {
            throw new Error('Binary data must be Uint8Array or Buffer');
        }
    } else {
        throw new Error(`Unknown payload_type: ${payloadType}`);
    }
}

/**
 * Helper function to properly serialize table data to Arrow IPC
 * @param {Array<Object>} data - Array of objects representing table rows
 * @returns {Buffer} Arrow IPC formatted buffer
 */
function serializeArrowTable(data) {
    if (!Array.isArray(data) || data.length === 0) {
        throw new Error('Table data must be a non-empty array of objects');
    }
    
    // Build schema from first row
    const fields = Object.keys(data[0]).map(key => {
        const value = data[0][key];
        let arrowType;
        if (typeof value === 'number') {
            arrowType = Number.isInteger(value) ? arrow.Int64 : arrow.Float64;
        } else if (typeof value === 'boolean') {
            arrowType = arrow.Boolean;
        } else if (value instanceof Date) {
            arrowType = arrow.Date;
        } else {
            arrowType = arrow.Utf8;
        }
        return new arrow.Field(key, arrowType, true);
    });
    
    const schema = new arrow.Schema(fields);
    const batches = [];
    
    // Create record batches
    for (const row of data) {
        const batch = arrow.recordBatch.fromObjects([row], schema);
        batches.push(batch);
    }
    
    // Write to buffer using IPC format
    const buffers = arrow.ipc.recordBatchesToMessage(batches, schema).buffers;
    const combined = new Uint8Array(buffers.reduce((acc, b) => acc + b.byteLength, 0));
    let offset = 0;
    for (const buf of buffers) {
        combined.set(new Uint8Array(buf), offset);
        offset += buf.byteLength;
    }
    
    return Buffer.from(combined);
}

/**
 * Deserialize bytes to data based on type
 * @param {Buffer|Uint8Array} data - Serialized data as bytes
 * @param {string} payloadType - Data type
 * @param {string} correlationId - Correlation ID for logging
 * @returns {any} Deserialized data
 */
async function deserializeData(data, payloadType, correlationId) {
    const buffer = Buffer.isBuffer(data) ? data : Buffer.from(data);
    
    if (payloadType === 'text') {
        return buffer.toString('utf8');
    } else if (payloadType === 'dictionary') {
        const jsonStr = buffer.toString('utf8');
        return JSON.parse(jsonStr);
    } else if (payloadType === 'table') {
        const table = arrow.tableFromRawBytes(buffer);
        return table;
    } else if (payloadType === 'image') {
        return buffer;
    } else if (payloadType === 'audio') {
        return buffer;
    } else if (payloadType === 'video') {
        return buffer;
    } else if (payloadType === 'binary') {
        return buffer;
    } else {
        throw new Error(`Unknown payload_type: ${payloadType}`);
    }
}

// ---------------------------------------------- File Server Handlers ---------------------------------------------- //

/**
 * Upload data to plik server in one-shot mode
 * @param {string} fileServerUrl - Base URL of the plik server
 * @param {string} dataname - Name of the file being uploaded
 * @param {Buffer|Uint8Array} data - Raw byte data of the file content
 * @returns {Promise<{status: number, uploadid: string, fileid: string, url: string}>}
 */
async function plikOneshotUpload(fileServerUrl, dataname, data) {
    const buffer = Buffer.isBuffer(data) ? data : Buffer.from(data);
    
    // Get upload id
    const urlGetUploadID = `${fileServerUrl}/upload`;
    const headers = { 'Content-Type': 'application/json' };
    const body = JSON.stringify({ OneShot: true });
    
    const httpResponse = await fetch(urlGetUploadID, {
        method: 'POST',
        headers,
        body
    });
    
    const responseJson = await httpResponse.json();
    const uploadid = responseJson.id;
    const uploadtoken = responseJson.uploadToken;

    // Upload file
    const urlUpload = `${fileServerUrl}/file/${uploadid}`;
    const form = new FormData();
    const blob = new Blob([buffer], { type: 'application/octet-stream' });
    form.append('file', blob, dataname);
    
    const uploadHeaders = {
        'X-UploadToken': uploadtoken
    };

    const uploadResponse = await fetch(urlUpload, {
        method: 'POST',
        headers: uploadHeaders,
        body: form
    });
    
    const uploadJson = await uploadResponse.json();
    const fileid = uploadJson.id;

    const url = `${fileServerUrl}/file/${uploadid}/${fileid}/${dataname}`;

    return {
        status: uploadResponse.status,
        uploadid,
        fileid,
        url
    };
}

/**
 * Fetch data from URL with exponential backoff
 * @param {string} url - URL to fetch from
 * @param {number} maxRetries - Maximum number of retry attempts
 * @param {number} baseDelay - Initial delay in milliseconds
 * @param {number} maxDelay - Maximum delay in milliseconds
 * @param {string} correlationId - Correlation ID for logging
 * @returns {Promise<Uint8Array>} Fetched data as bytes
 */
async function fetchWithBackoff(url, maxRetries, baseDelay, maxDelay, correlationId) {
    let delay = baseDelay;
    
    for (let attempt = 1; attempt <= maxRetries; attempt++) {
        try {
            const response = await fetch(url);
            
            if (response.status === 200) {
                logTrace(correlationId, `Successfully fetched data from ${url} on attempt ${attempt}`);
                const arrayBuffer = await response.arrayBuffer();
                return new Uint8Array(arrayBuffer);
            } else {
                throw new Error(`Failed to fetch: ${response.status}`);
            }
        } catch (e) {
            logTrace(correlationId, `Attempt ${attempt} failed: ${e.constructor.name}`);
            
            if (attempt < maxRetries) {
                await new Promise(resolve => setTimeout(resolve, delay));
                delay = Math.min(delay * 2, maxDelay);
            }
        }
    }
    
    throw new Error(`Failed to fetch data after ${maxRetries} attempts`);
}

// ---------------------------------------------- NATS Client ---------------------------------------------- //

/**
 * NATS client wrapper for connection management
 */
class NATSClient {
    /**
     * Create a new NATS client
     * @param {string} url - NATS server URL
     */
    constructor(url) {
        this.url = url;
        this.connection = null;
    }

    /**
     * Connect to NATS server
     * @returns {Promise<NATS.Connection>}
     */
    async connect() {
        this.connection = await nats.connect({ servers: this.url });
        return this.connection;
    }

    /**
     * Publish message to NATS subject
     * @param {string} subject - NATS subject to publish to
     * @param {string} message - Message to publish
     * @param {string} correlationId - Correlation ID for logging
     */
    async publish(subject, message, correlationId) {
        if (!this.connection) {
            await this.connect();
        }
        await this.connection.publish(subject, message);
        logTrace(correlationId, `Message published to ${subject}`);
    }

    /**
     * Close the NATS connection
     */
    async close() {
        if (this.connection) {
            this.connection.close();
        }
    }
}

// ---------------------------------------------- Core Functions ---------------------------------------------- //

/**
 * Publish message to NATS
 * @param {string|NATSClient|NATS.Connection} brokerUrlOrClient - NATS URL, client, or connection
 * @param {string} subject - NATS subject to publish to
 * @param {string} message - JSON message to publish
 * @param {string} correlationId - Correlation ID for tracing
 */
async function publishMessage(brokerUrlOrClient, subject, message, correlationId) {
    let conn;
    
    if (brokerUrlOrClient instanceof NATSClient) {
        conn = brokerUrlOrClient;
    } else if (brokerUrlOrClient instanceof nats.Connection) {
        // Create a wrapper for direct connection
        conn = {
            async publish(subj, msg) {
                await brokerUrlOrClient.publish(subj, msg);
            },
            async close() {
                await brokerUrlOrClient.close();
            }
        };
    } else {
        // String URL - create new client
        const client = new NATSClient(brokerUrlOrClient);
        conn = client;
    }
    
    await conn.publish(subject, message, correlationId);
    
    if (conn instanceof NATSClient) {
        await conn.close();
    }
}

/**
 * Build message envelope from payloads and metadata
 * @param {string} subject - NATS subject
 * @param {Array} payloads - Array of payload objects
 * @param {Object} options - Envelope metadata options
 * @returns {Object} Envelope object
 */
function buildEnvelope(subject, payloads, options) {
    return {
        correlation_id: options.correlation_id,
        msg_id: options.msg_id,
        timestamp: new Date().toISOString(),
        send_to: subject,
        msg_purpose: options.msg_purpose,
        sender_name: options.sender_name,
        sender_id: options.sender_id,
        receiver_name: options.receiver_name,
        receiver_id: options.receiver_id,
        reply_to: options.reply_to,
        reply_to_msg_id: options.reply_to_msg_id,
        broker_url: options.broker_url,
        metadata: options.metadata || {},
        payloads: payloads
    };
}

/**
 * Build payload object from serialized data
 * @param {string} dataname - Name of the payload
 * @param {string} payloadType - Type of the payload
 * @param {Buffer} payloadBytes - Serialized payload bytes
 * @param {string} transport - Transport type ("direct" or "link")
 * @param {string} data - Data (base64 for direct, URL for link)
 * @returns {Object} Payload object
 */
function buildPayload(dataname, payloadType, payloadBytes, transport, data) {
    return {
        id: uuidv4(),
        dataname,
        payload_type: payloadType,
        transport,
        encoding: transport === 'direct' ? 'base64' : 'none',
        size: payloadBytes.byteLength,
        data,
        metadata: transport === 'direct' ? { payload_bytes: payloadBytes.byteLength } : {}
    };
}

/**
 * Send data via NATS with automatic transport selection
 * 
 * This function intelligently routes data delivery based on payload size.
 * If the serialized payload is smaller than size_threshold, it encodes the data as Base64
 * and publishes directly over NATS. Otherwise, it uploads the data to a fileserver
 * and publishes only the download URL over NATS.
 * 
 * @param {string} subject - NATS subject to publish the message to
 * @param {Array} data - List of [dataname, data, type] tuples to send
 * @param {Object} options - Optional configuration
 * @param {string} [options.broker_url=DEFAULT_BROKER_URL] - URL of the NATS server
 * @param {string} [options.fileserver_url=DEFAULT_FILESERVER_URL] - URL of the HTTP file server
 * @param {Function} [options.fileserver_upload_handler=plikOneshotUpload] - Function to handle fileserver uploads
 * @param {number} [options.size_threshold=DEFAULT_SIZE_THRESHOLD] - Threshold separating direct vs link transport
 * @param {string} [options.correlation_id=uuidv4()] - Correlation ID for tracing
 * @param {string} [options.msg_purpose="chat"] - Purpose of the message
 * @param {string} [options.sender_name="NATSBridge"] - Name of the sender
 * @param {string} [options.receiver_name=""] - Name of the receiver (empty means broadcast)
 * @param {string} [options.receiver_id=""] - UUID of the receiver (empty means broadcast)
 * @param {string} [options.reply_to=""] - Topic to reply to
 * @param {string} [options.reply_to_msg_id=""] - Message ID this message is replying to
 * @param {boolean} [options.is_publish=true] - Whether to automatically publish the message
 * @param {NATSClient|NATS.Connection} [options.nats_connection=null] - Pre-existing NATS connection
 * @param {string} [options.msg_id=uuidv4()] - Message ID
 * @param {string} [options.sender_id=uuidv4()] - Sender ID
 * @returns {Promise<[Object, string]>} Tuple of [env, env_json_str]
 * 
 * @example
 * // Send a single payload
 * const [env, envJsonStr] = await smartsend(
 *     "/test",
 *     [["dataname1", data1, "dictionary"]],
 *     { broker_url: "nats://localhost:4222" }
 * );
 * 
 * // Send multiple payloads
 * const [env, envJsonStr] = await smartsend(
 *     "/test",
 *     [
 *         ["dataname1", data1, "dictionary"],
 *         ["dataname2", data2, "table"]
 *     ],
 *     { broker_url: "nats://localhost:4222" }
 * );
 * 
 * // Send with pre-existing connection
 * const client = await NATSBridge.NATSClient.connect("nats://localhost:4222");
 * const [env, envJsonStr] = await smartsend(
 *     "/test",
 *     [["data", myData, "text"]],
 *     { nats_connection: client }
 * );
 */
async function smartsend(subject, data, options = {}) {
    const {
        broker_url = DEFAULT_BROKER_URL,
        fileserver_url = DEFAULT_FILESERVER_URL,
        fileserver_upload_handler = plikOneshotUpload,
        size_threshold = DEFAULT_SIZE_THRESHOLD,
        correlation_id = uuidv4(),
        msg_purpose = 'chat',
        sender_name = 'NATSBridge',
        receiver_name = '',
        receiver_id = '',
        reply_to = '',
        reply_to_msg_id = '',
        is_publish = true,
        nats_connection = null,
        msg_id = uuidv4(),
        sender_id = uuidv4()
    } = options;

    logTrace(correlation_id, `Starting smartsend for subject: ${subject}`);

    // Process payloads
    const payloads = [];
    for (const [dataname, payloadData, payloadType] of data) {
        const payloadBytes = await serializeData(payloadData, payloadType);
        const payloadSize = payloadBytes.byteLength;

        logTrace(correlation_id, `Serialized payload '${dataname}' (type: ${payloadType}) size: ${payloadSize} bytes`);

        if (payloadSize < size_threshold) {
            // Direct path
            const payloadB64 = bufferToBase64(payloadBytes);
            logTrace(correlation_id, `Using direct transport for ${payloadSize} bytes`);

            const payload = buildPayload(dataname, payloadType, payloadBytes, 'direct', payloadB64);
            payloads.push(payload);
        } else {
            // Link path
            logTrace(correlation_id, `Using link transport, uploading to fileserver`);

            const response = await fileserver_upload_handler(fileserver_url, dataname, payloadBytes);

            if (response.status !== 200) {
                throw new Error(`Failed to upload data to fileserver: ${response.status}`);
            }

            logTrace(correlation_id, `Uploaded to URL: ${response.url}`);

            const payload = buildPayload(dataname, payloadType, payloadBytes, 'link', response.url);
            payloads.push(payload);
        }
    }

    // Build envelope
    const env = buildEnvelope(subject, payloads, {
        correlation_id,
        msg_id,
        msg_purpose,
        sender_name,
        sender_id,
        receiver_name,
        receiver_id,
        reply_to,
        reply_to_msg_id,
        broker_url
    });

    const env_json_str = JSON.stringify(env);

    if (is_publish) {
        if (nats_connection) {
            await publishMessage(nats_connection, subject, env_json_str, correlation_id);
        } else {
            await publishMessage(broker_url, subject, env_json_str, correlation_id);
        }
    }

    return [env, env_json_str];
}

/**
 * Receive and process NATS message
 * 
 * This function processes incoming NATS messages, handling both direct transport
 * (base64 decoded payloads) and link transport (URL-based payloads).
 * It deserializes the data based on the transport type and returns the result.
 * 
 * @param {Object} msg - NATS message object with payload property
 * @param {Object} options - Optional configuration
 * @param {Function} [options.fileserver_download_handler=fetchWithBackoff] - Function to handle fileserver downloads
 * @param {number} [options.max_retries=5] - Maximum retry attempts for fetching URL
 * @param {number} [options.base_delay=100] - Initial delay for exponential backoff in ms
 * @param {number} [options.max_delay=5000] - Maximum delay for exponential backoff in ms
 * @returns {Promise<Object>} Envelope object with processed payloads
 * 
 * @example
 * // Receive and process message
 * const env = await smartreceive(msg, {
 *     fileserver_download_handler: fetchWithBackoff,
 *     max_retries: 5,
 *     base_delay: 100,
 *     max_delay: 5000
 * });
 * // env.payloads is an Array of [dataname, data, type] arrays
 * for (const [dataname, data, type] of env.payloads) {
 *     console.log(`${dataname}: ${data} (type: ${type})`);
 * }
 */
async function smartreceive(msg, options = {}) {
    const {
        fileserver_download_handler = fetchWithBackoff,
        max_retries = 5,
        base_delay = 100,
        max_delay = 5000
    } = options;

    // Parse the JSON envelope
    const payload = typeof msg.payload === 'string' ? msg.payload : Buffer.from(msg.payload).toString('utf8');
    const envJsonObj = JSON.parse(payload);
    logTrace(envJsonObj.correlation_id, 'Processing received message');

    // Process all payloads in the envelope
    const payloadsList = [];
    const numPayloads = envJsonObj.payloads.length;

    for (let i = 0; i < numPayloads; i++) {
        const payloadObj = envJsonObj.payloads[i];
        const transport = payloadObj.transport;
        const dataname = payloadObj.dataname;

        if (transport === 'direct') {
            logTrace(envJsonObj.correlation_id, `Direct transport - decoding payload '${dataname}'`);

            // Extract base64 payload from the payload
            const payloadB64 = payloadObj.data;

            // Decode Base64 payload
            const payloadBytes = Buffer.from(payloadB64, 'base64');

            // Deserialize based on type
            const dataType = payloadObj.payload_type;
            const data = await deserializeData(payloadBytes, dataType, envJsonObj.correlation_id);

            payloadsList.push([dataname, data, dataType]);
        } else if (transport === 'link') {
            // Extract download URL from the payload
            const url = payloadObj.data;
            logTrace(envJsonObj.correlation_id, `Link transport - fetching '${dataname}' from URL: ${url}`);

            // Fetch with exponential backoff using the download handler
            const downloadedData = await fileserver_download_handler(
                url,
                max_retries,
                base_delay,
                max_delay,
                envJsonObj.correlation_id
            );

            // Deserialize based on type
            const dataType = payloadObj.payload_type;
            const data = await deserializeData(downloadedData, dataType, envJsonObj.correlation_id);

            payloadsList.push([dataname, data, dataType]);
        } else {
            throw new Error(`Unknown transport type for payload '${dataname}': ${transport}`);
        }
    }

    envJsonObj.payloads = payloadsList;
    return envJsonObj;
}

// ---------------------------------------------- Module Exports ---------------------------------------------- //

const NATSBridge = {
    /**
     * NATS client class for connection management
     */
    NATSClient,

    /**
     * Send data via NATS with automatic transport selection
     */
    smartsend,

    /**
     * Receive and process NATS message
     */
    smartreceive,

    /**
     * Upload data to plik server in one-shot mode
     */
    plikOneshotUpload,

    /**
     * Fetch data from URL with exponential backoff
     */
    fetchWithBackoff,

    /**
     * Default constants
     */
    DEFAULT_SIZE_THRESHOLD,
    DEFAULT_BROKER_URL,
    DEFAULT_FILESERVER_URL
};

module.exports = NATSBridge;