// https://android.googlesource.com/platform/external/nanohttpd/+/42ff2a9/websocket/src/main/java/fi/iki/elonen/NanoWebSocketServer.java

sclass NanoWebSocketServer extends NanoHTTPD implements WebSocketFactory {
    public static final String HEADER_UPGRADE = "upgrade";
    public static final String HEADER_UPGRADE_VALUE = "websocket";
    public static final String HEADER_CONNECTION = "connection";
    public static final String HEADER_CONNECTION_VALUE = "Upgrade";
    public static final String HEADER_WEBSOCKET_VERSION = "sec-websocket-version";
    public static final String HEADER_WEBSOCKET_VERSION_VALUE = "13";
    public static final String HEADER_WEBSOCKET_KEY = "sec-websocket-key";
    public static final String HEADER_WEBSOCKET_ACCEPT = "sec-websocket-accept";
    public static final String HEADER_WEBSOCKET_PROTOCOL = "sec-websocket-protocol";
    public final static String WEBSOCKET_KEY_MAGIC = "258EAFA5-E914-47DA-95CA-C5AB0DC85B11";
    
    WebSocketFactory webSocketFactory;
    
    public NanoWebSocketServer(int port) {
        super(port);
        webSocketFactory = null;
    }
    
    public NanoWebSocketServer(String hostname, int port) {
        super(hostname, port);
        webSocketFactory = null;
    }
    
    public NanoWebSocketServer(int port, WebSocketFactory webSocketFactory) {
        super(port);
        this.webSocketFactory = webSocketFactory;
    }
    
    public NanoWebSocketServer(String hostname, int port,WebSocketFactory webSocketFactory) {
        super(hostname, port);
        this.webSocketFactory = webSocketFactory;
    }
    
    @Override
    public Response serve(final IHTTPSession session) {
        Map<String, String> headers = session.getHeaders();
        if (isWebsocketRequested(session)) {
            if (!HEADER_UPGRADE_VALUE.equalsIgnoreCase(headers.get(HEADER_UPGRADE))
                    || !isWebSocketConnectionHeader(session.getHeaders())) {
                return new Response(Response.Status.BAD_REQUEST, NanoHTTPD.MIME_PLAINTEXT, "Invalid Websocket handshake");
            }
            if (!HEADER_WEBSOCKET_VERSION_VALUE.equalsIgnoreCase(headers.get(HEADER_WEBSOCKET_VERSION))) {
                return new Response(Response.Status.BAD_REQUEST, NanoHTTPD.MIME_PLAINTEXT, "Invalid Websocket-Version " + headers.get(HEADER_WEBSOCKET_VERSION));
            }
            if (!headers.containsKey(HEADER_WEBSOCKET_KEY)) {
                return new Response(Response.Status.BAD_REQUEST, NanoHTTPD.MIME_PLAINTEXT, "Missing Websocket-Key");
            }
            WebSocket webSocket = openWebSocket(session);
            try {
                webSocket.getHandshakeResponse().addHeader(HEADER_WEBSOCKET_ACCEPT, makeAcceptKey(headers.get(HEADER_WEBSOCKET_KEY)));
            } catch (NoSuchAlgorithmException e) {
                return new Response(Response.Status.INTERNAL_ERROR, NanoHTTPD.MIME_PLAINTEXT, "The SHA-1 Algorithm required for websockets is not available on the server.");
            }
            if (headers.containsKey(HEADER_WEBSOCKET_PROTOCOL)) {
                webSocket.getHandshakeResponse().addHeader(HEADER_WEBSOCKET_PROTOCOL, headers.get(HEADER_WEBSOCKET_PROTOCOL).split(",")[0]);
            }
            return webSocket.getHandshakeResponse();
        } else {
            return super.serve(session);
        }
    }
    
    public WebSocket openWebSocket(IHTTPSession handshake) {
        if (webSocketFactory == null) {
            throw new Error("You must either override this method or supply a WebSocketFactory in the cosntructor");
        }
        return webSocketFactory.openWebSocket(handshake);
    }
    
    protected boolean isWebsocketRequested(IHTTPSession session) {
        Map<String, String> headers = session.getHeaders();
        String upgrade = headers.get(HEADER_UPGRADE);
        boolean isCorrectConnection = isWebSocketConnectionHeader(headers);
        boolean isUpgrade = HEADER_UPGRADE_VALUE.equalsIgnoreCase(upgrade);
        return (isUpgrade && isCorrectConnection);
    }
    
    private boolean isWebSocketConnectionHeader(Map<String, String> headers) {
        String connection = headers.get(HEADER_CONNECTION);
        return (connection != null && connection.toLowerCase().contains(HEADER_CONNECTION_VALUE.toLowerCase()));
    }
    
    public static String makeAcceptKey(String key) throws NoSuchAlgorithmException {
        MessageDigest md = MessageDigest.getInstance("SHA-1");
        String text = key + WEBSOCKET_KEY_MAGIC;
        md.update(text.getBytes(), 0, text.length());
        byte[] sha1hash = md.digest();
        return base64encode(sha1hash);
    }
}