HL7v2/packages/server/index.ts

import { WebSocketServer } from 'ws';
import { type Message, MessageType } from '@hnu.de/hl7v2-shared';
import 'dotenv/config';
import { config } from './config';


// ###########################################################################
// ### ID Pool
// ###########################################################################

let availableIds: string[] = [];

for (const prefix of config.prefixes) {
    for (let i = 1; i <= config.poolSize / config.prefixes.length; i++) {
        const number = i.toString().padStart(config.poolSize.toString().length, '0');
        availableIds.push(`${prefix}-${number}`);
    }
}

// mixes the array randomly to avoid getting the same ID reassigned
function shake(arr: any[]) {
    for (let i = arr.length - 1; i > 0; i--) {
        const j = Math.floor(Math.random() * (i + 1));
        [arr[i], arr[j]] = [arr[j], arr[i]];
    }
    return arr;
}

availableIds = shake(availableIds);


// ###########################################################################
// ### Websocket Server
// ###########################################################################

const wss = new WebSocketServer({ port: config.port });

const clients = new Map();

console.log(`Starting WebSocket server ...`);

console.log("Server configuration:", config);

wss.on('connection', (ws) => {

    // check for available IDs
    if (availableIds.length === 0) {
        console.log('Connection rejected: No available IDs.');
        ws.close(1013, 'Server is full. Please try again later.'); // 1013: Try again later
        return;
    }

    // get ID from pool and assign to station
    const stationId = availableIds.pop();
    if (!stationId) {
        console.log('Connection rejected: Failed to retrieve ID.');
        ws.close(1013, 'Server is full. Please try again later.'); // 1013: Try again later
        return;
    }

    // store client in map
    clients.set(stationId, ws);

    // send station ID to client
    const welcomeMessage = {
        type: 'assign_id',
        payload: {
            stationId: stationId,
        },
    } as Message;
    ws.send(JSON.stringify(welcomeMessage));

    console.log(`Client connected. Assigning ID: ${stationId}`);

    // listen for messages and defined message handling
    ws.on('message', (message) => {
        try {

            // parse every message to type Message
            const parsedMessage: Message = JSON.parse(message.toString());
            console.log(`Received message from ${stationId}:`, parsedMessage);

            // We only expect one type of message from clients: 'send_hl7v2'
            if (parsedMessage.type === MessageType.send_hl7v2) {

                const { message } = parsedMessage.payload;

                // TODO: validate message

                // get sender and recipient ID
                const recipientId = parseMshField(message, 5);
                if (!recipientId) {
                    const errorMessage = {
                        type: 'delivery_error',
                        payload: {
                            error: `Message is missing header field 5.`,
                        },
                    } as Message;
                    ws.send(JSON.stringify(errorMessage));
                }

                // Find the recipient's WebSocket connection in our map.
                const recipientWs = clients.get(recipientId);

                // check if recipient exists and is connected
                if (recipientWs && recipientWs.readyState === WebSocket.OPEN) {

                    // The recipient is connected. Forward the message.
                    const forwardMessage = {
                        type: MessageType.receive_hl7v2,
                        payload: {
                            message: message,
                            timestamp: new Date().toISOString(),
                        },
                    } as Message;
                    recipientWs.send(JSON.stringify(forwardMessage));
                    console.log(`Forwarded message to ${recipientId}`);

                } else {

                    // The recipient is not connected or not found.
                    console.log(`Recipient ${recipientId} not found or not connected.`);
                    const errorMessage = {
                        type: MessageType.delivery_error,
                        payload: {
                            error: `Station with ID ${recipientId} is not available.`,
                        },
                    } as Message;
                    ws.send(JSON.stringify(errorMessage));
                }
            }
        } catch (error) {
            console.error(`Failed to process message from ${stationId}:`, error);
        }
    });

    // listen to disconnects to free IDs
    ws.on('close', () => {
        console.log(`Client ${stationId} disconnected.`);
        clients.delete(stationId);
        if (stationId) {
            availableIds.push(stationId);
        }
        // shake IDs to avoid getting the same ID reassigned
        availableIds = shake(availableIds);
    });

    ws.on('error', (error) => {
        console.error(`WebSocket error for client ${stationId}:`, error);
    });
});

function parseMshField(message: string, fieldIndex: number) {
    try {
        const mshLine = message.split(/[\r\n]+/)[0];
        if (!mshLine || !mshLine.startsWith('MSH')) return null;
        const fields = mshLine.split('|');
        return fields[fieldIndex] || null;
    } catch (e) {
        return null;
    }
}

console.log(`WebSocket server started on port ${config.port}`);