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

import java.nio.charset.*;
import java.nio.*;

sbool webSocket_debug = true;

sclass WebSocketException extends IOException {
    private WebSocketFrame.CloseCode code;
    private String reason;
    public WebSocketException(Exception cause) {
        this(WebSocketFrame.CloseCode.InternalServerError, cause.toString(), cause);
    }
    public WebSocketException(WebSocketFrame.CloseCode code, String reason) {
        this(code, reason, null);
    }
    public WebSocketException(WebSocketFrame.CloseCode code, String reason, Exception cause) {
        super(code + ": " + reason, cause);
        this.code = code;
        this.reason = reason;
    }
    public WebSocketFrame.CloseCode getCode() {
        return code;
    }
    public String getReason() {
        return reason;
    }
}

sclass WebSocketFrame {
    private OpCode opCode;
    private boolean fin;
    private byte[] maskingKey;
    private byte[] payload;
    private transient int _payloadLength;
    private transient String _payloadString;
    private WebSocketFrame(OpCode opCode, boolean fin) {
        setOpCode(opCode);
        setFin(fin);
    }
    public WebSocketFrame(OpCode opCode, boolean fin, byte[] payload, byte[] maskingKey) {
        this(opCode, fin);
        setMaskingKey(maskingKey);
        setBinaryPayload(payload);
    }
    public WebSocketFrame(OpCode opCode, boolean fin, byte[] payload) {
        this(opCode, fin, payload, null);
    }
    public WebSocketFrame(OpCode opCode, boolean fin, String payload, byte[] maskingKey) throws CharacterCodingException {
        this(opCode, fin);
        setMaskingKey(maskingKey);
        setTextPayload(payload);
    }
    public WebSocketFrame(OpCode opCode, boolean fin, String payload) throws CharacterCodingException {
        this(opCode, fin, payload, null);
    }
    public WebSocketFrame(WebSocketFrame clone) {
        setOpCode(clone.getOpCode());
        setFin(clone.isFin());
        setBinaryPayload(clone.getBinaryPayload());
        setMaskingKey(clone.getMaskingKey());
    }
    public WebSocketFrame(OpCode opCode, List<WebSocketFrame> fragments) throws WebSocketException {
        setOpCode(opCode);
        setFin(true);
        long _payloadLength = 0;
        for (WebSocketFrame inter : fragments) {
            _payloadLength += inter.getBinaryPayload().length;
        }
        if (webSocket_debug)
          print("Payload length with " + nFragments(fragments) + ": " + _payloadLength);
        if (_payloadLength < 0 || _payloadLength > Integer.MAX_VALUE) {
            throw new WebSocketException(WebSocketFrame.CloseCode.MessageTooBig, "Max frame length has been exceeded.");
        }
        this._payloadLength = (int) _payloadLength;
        byte[] payload = new byte[this._payloadLength];
        int offset = 0;
        for (WebSocketFrame inter : fragments) {
            System.arraycopy(inter.getBinaryPayload(), 0, payload, offset, inter.getBinaryPayload().length);
            offset += inter.getBinaryPayload().length;
        }
        setBinaryPayload(payload);
    }
    // --------------------------------GETTERS---------------------------------
    public OpCode getOpCode() {
        return opCode;
    }
    public void setOpCode(OpCode opcode) {
        this.opCode = opcode;
    }
    public boolean isFin() {
        return fin;
    }
    public void setFin(boolean fin) {
        this.fin = fin;
    }
    public boolean isMasked() {
        return maskingKey != null && maskingKey.length == 4;
    }
    public byte[] getMaskingKey() {
        return maskingKey;
    }
    public void setMaskingKey(byte[] maskingKey) {
        if (maskingKey != null && maskingKey.length != 4) {
            throw new IllegalArgumentException("MaskingKey " + Arrays.toString(maskingKey) + " hasn't length 4");
        }
        this.maskingKey = maskingKey;
    }
    public void setUnmasked() {
        setMaskingKey(null);
    }
    public byte[] getBinaryPayload() {
        return payload;
    }
    public void setBinaryPayload(byte[] payload) {
        this.payload = payload;
        this._payloadLength = payload.length;
        this._payloadString = null;
    }
    public String getTextPayload() {
        if (_payloadString == null) {
            try {
                _payloadString = binary2Text(getBinaryPayload());
            } catch (CharacterCodingException e) {
                throw new RuntimeException("Undetected CharacterCodingException", e);
            }
        }
        return _payloadString;
    }
    public void setTextPayload(String payload) throws CharacterCodingException {
        this.payload = text2Binary(payload);
        //this._payloadLength = payload.length(); // buggy!
        this._payloadLength = this.payload.length;
        if (webSocket_debug) print("payload length: " + _payloadLength + ", string length: " + l(payload)); // XXX
        this._payloadString = payload;
    }
    // --------------------------------SERIALIZATION---------------------------
    public static WebSocketFrame read(InputStream in) throws IOException {
        byte head = (byte) checkedRead(in.read());
        boolean fin = ((head & 0x80) != 0);
        OpCode opCode = OpCode.find((byte) (head & 0x0F));
        if ((head & 0x70) != 0) {
            throw new WebSocketException(WebSocketFrame.CloseCode.ProtocolError, "The reserved bits (" + Integer.toBinaryString(head & 0x70) + ") must be 0.");
        }
        if (opCode == null) {
            throw new WebSocketException(WebSocketFrame.CloseCode.ProtocolError, "Received frame with reserved/unknown opcode " + (head & 0x0F) + ".");
        } else if (opCode.isControlFrame() && !fin) {
            throw new WebSocketException(WebSocketFrame.CloseCode.ProtocolError, "Fragmented control frame.");
        }
        WebSocketFrame frame = new WebSocketFrame(opCode, fin);
        frame.readPayloadInfo(in);
        frame.readPayload(in);
        if (frame.getOpCode() == WebSocketFrame.OpCode.Close) {
            return new WebSocketFrame.CloseFrame(frame);
        } else {
            return frame;
        }
    }
    private static int checkedRead(int read) throws IOException {
        if (read < 0) {
            throw new EOFException();
        }
        //System.out.println(Integer.toBinaryString(read) + "/" + read + "/" + Integer.toHexString(read));
        return read;
    }
    private void readPayloadInfo(InputStream in) throws IOException {
        byte b = (byte) checkedRead(in.read());
        boolean masked = ((b & 0x80) != 0);
        _payloadLength = (byte) (0x7F & b);
        if (_payloadLength == 126) {
            // checkedRead must return int for this to work
            _payloadLength = (checkedRead(in.read()) << 8 | checkedRead(in.read())) & 0xFFFF;
            if (_payloadLength < 126) {
                throw new WebSocketException(WebSocketFrame.CloseCode.ProtocolError, "Invalid data frame 2byte length. (not using minimal length encoding)");
            }
        } else if (_payloadLength == 127) {
            long _payloadLength = ((long) checkedRead(in.read())) << 56 |
                    ((long) checkedRead(in.read())) << 48 |
                    ((long) checkedRead(in.read())) << 40 |
                    ((long) checkedRead(in.read())) << 32 |
                    checkedRead(in.read()) << 24 | checkedRead(in.read()) << 16 | checkedRead(in.read()) << 8 | checkedRead(in.read());
            if (_payloadLength < 65536) {
                throw new WebSocketException(WebSocketFrame.CloseCode.ProtocolError, "Invalid data frame 4byte length. (not using minimal length encoding)");
            }
            if (_payloadLength < 0 || _payloadLength > Integer.MAX_VALUE) {
                throw new WebSocketException(WebSocketFrame.CloseCode.MessageTooBig, "Max frame length has been exceeded.");
            }
            this._payloadLength = (int) _payloadLength;
        }
        if (opCode.isControlFrame()) {
            if (_payloadLength > 125) {
                throw new WebSocketException(WebSocketFrame.CloseCode.ProtocolError, "Control frame with payload length > 125 bytes.");
            }
            if (opCode == OpCode.Close && _payloadLength == 1) {
                throw new WebSocketException(WebSocketFrame.CloseCode.ProtocolError, "Received close frame with payload len 1.");
            }
        }
        if (masked) {
            maskingKey = new byte[4];
            int read = 0;
            while (read < maskingKey.length) {
                read += checkedRead(in.read(maskingKey, read, maskingKey.length - read));
            }
        }
    }
    private void readPayload(InputStream in) throws IOException {
        payload = new byte[_payloadLength];
        int read = 0;
        while (read < _payloadLength) {
            read += checkedRead(in.read(payload, read, _payloadLength - read));
        }
        if (isMasked()) {
            for (int i = 0; i < payload.length; i++) {
                payload[i] ^= maskingKey[i % 4];
            }
        }
        //Test for Unicode errors
        if (getOpCode() == WebSocketFrame.OpCode.Text) {
            _payloadString = binary2Text(getBinaryPayload());
        }
    }
    public void write(OutputStream out) throws IOException {
        byte header = 0;
        if (fin) {
            header |= 0x80;
        }
        header |= opCode.getValue() & 0x0F;
        out.write(header);
        _payloadLength = getBinaryPayload().length;
        if (_payloadLength <= 125) {
          if (webSocket_debug)
            print("Sending short payload: " + _payloadLength);
          out.write(isMasked() ? 0x80 | (byte) _payloadLength : (byte) _payloadLength);
        } else if (_payloadLength <= 0xFFFF) {
            out.write(isMasked() ? 0xFE : 126);
            out.write(_payloadLength >>> 8);
            out.write(_payloadLength);
        } else {
            out.write(isMasked() ? 0xFF : 127);
            out.write(_payloadLength >>> 56 & 0); //integer only contains 31 bit
            out.write(_payloadLength >>> 48 & 0);
            out.write(_payloadLength >>> 40 & 0);
            out.write(_payloadLength >>> 32 & 0);
            out.write(_payloadLength >>> 24);
            out.write(_payloadLength >>> 16);
            out.write(_payloadLength >>> 8);
            out.write(_payloadLength);
        }
        if (isMasked()) {
            out.write(maskingKey);
            for (int i = 0; i < _payloadLength; i++) {
                out.write(getBinaryPayload()[i] ^ maskingKey[i % 4]);
            }
        } else {
            out.write(getBinaryPayload());
        }
        out.flush();
    }
    // --------------------------------ENCODING--------------------------------
    public static final Charset TEXT_CHARSET = Charset.forName("UTF-8");
    public static final CharsetDecoder TEXT_DECODER = TEXT_CHARSET.newDecoder();
    public static final CharsetEncoder TEXT_ENCODER = TEXT_CHARSET.newEncoder();
    public static String binary2Text(byte[] payload) throws CharacterCodingException {
        return TEXT_DECODER.decode(ByteBuffer.wrap(payload)).toString();
    }
    public static String binary2Text(byte[] payload, int offset, int length) throws CharacterCodingException {
        return TEXT_DECODER.decode(ByteBuffer.wrap(payload, offset, length)).toString();
    }
    public static byte[] text2Binary(String payload) throws CharacterCodingException {
        return TEXT_ENCODER.encode(CharBuffer.wrap(payload)).array();
    }
    @Override
    public String toString() {
        final StringBuilder sb = new StringBuilder("WS[");
        sb.append(getOpCode());
        sb.append(", ").append(isFin() ? "fin" : "inter");
        sb.append(", ").append(isMasked() ? "masked" : "unmasked");
        sb.append(", ").append(payloadToString());
        sb.append(']');
        return sb.toString();
    }
    protected String payloadToString() {
        if (payload == null) return "null";
        else {
            final StringBuilder sb = new StringBuilder();
            sb.append('[').append(payload.length).append("b] ");
            if (getOpCode() == WebSocketFrame.OpCode.Text) {
                String text = getTextPayload();
                if (text.length() > 100)
                    sb.append(text.substring(0, 100)).append("...");
                else
                    sb.append(text);
            } else {
                sb.append("0x");
                for (int i = 0; i < Math.min(payload.length, 50); ++i)
                    sb.append(Integer.toHexString((int) payload[i] & 0xFF));
                if (payload.length > 50)
                    sb.append("...");
            }
            return sb.toString();
        }
    }
    // --------------------------------CONSTANTS-------------------------------
    public static enum OpCode {
        Continuation(0), Text(1), Binary(2), Close(8), Ping(9), Pong(10);
        private final byte code;
        private OpCode(int code) {
            this.code = (byte) code;
        }
        public byte getValue() {
            return code;
        }
        public boolean isControlFrame() {
            return this == Close || this == Ping || this == Pong;
        }
        public static OpCode find(byte value) {
            for (OpCode opcode : values()) {
                if (opcode.getValue() == value) {
                    return opcode;
                }
            }
            return null;
        }
    }
    
    public static enum CloseCode {
        NormalClosure(1000), GoingAway(1001), ProtocolError(1002), UnsupportedData(1003), NoStatusRcvd(1005),
        AbnormalClosure(1006), InvalidFramePayloadData(1007), PolicyViolation(1008), MessageTooBig(1009),
        MandatoryExt(1010), InternalServerError(1011), TLSHandshake(1015);
        private final int code;
        private CloseCode(int code) {
            this.code = code;
        }
        public int getValue() {
            return code;
        }
        public static WebSocketFrame.CloseCode find(int value) {
            for (WebSocketFrame.CloseCode code : values()) {
                if (code.getValue() == value) {
                    return code;
                }
            }
            return null;
        }
    }
    // ------------------------------------------------------------------------
 public static class CloseFrame extends WebSocketFrame {
        private CloseCode _closeCode;
        private String _closeReason;
        private CloseFrame(WebSocketFrame wrap) throws CharacterCodingException {
            super(wrap);
            assert wrap.getOpCode() == OpCode.Close;
            if (wrap.getBinaryPayload().length >= 2) {
                _closeCode = CloseCode.find((wrap.getBinaryPayload()[0] & 0xFF) << 8 |
                        (wrap.getBinaryPayload()[1] & 0xFF));
                _closeReason = binary2Text(getBinaryPayload(), 2, getBinaryPayload().length - 2);
            }
        }
        public CloseFrame(CloseCode code, String closeReason) throws CharacterCodingException {
            super(OpCode.Close, true, generatePayload(code, closeReason));
        }
        private static byte[] generatePayload(CloseCode code, String closeReason) throws CharacterCodingException {
            if (code != null) {
                byte[] reasonBytes = text2Binary(closeReason);
                byte[] payload = new byte[reasonBytes.length + 2];
                payload[0] = (byte) ((code.getValue() >> 8) & 0xFF);
                payload[1] = (byte) ((code.getValue()) & 0xFF);
                System.arraycopy(reasonBytes, 0, payload, 2, reasonBytes.length);
                return payload;
            } else {
                return new byte[0];
            }
        }
        protected String payloadToString() {
            return (_closeCode != null ? _closeCode : "UnknownCloseCode[" + _closeCode + "]") + (_closeReason != null && !_closeReason.isEmpty() ? ": " + _closeReason : "");
        }
        public CloseCode getCloseCode() {
            return _closeCode;
        }
        public String getCloseReason() {
            return _closeReason;
        }
    }
}

sclass WebSocket implements AutoCloseable {
    protected final InputStream in;
    protected /*final*/ OutputStream out;
    protected WebSocketFrame.OpCode continuousOpCode = null;
    protected List<WebSocketFrame> continuousFrames = new LinkedList<WebSocketFrame>();
    protected State state = State.UNCONNECTED;
    public static enum State {
        UNCONNECTED, CONNECTING, OPEN, CLOSING, CLOSED
    }
    protected final NanoHTTPD.IHTTPSession handshakeRequest;
    //protected final NanoHTTPD.Response handshakeResponse = new NanoHTTPD.Response(NanoHTTPD.Status.SWITCH_PROTOCOL, null, (InputStream) null) {
    protected final NanoHTTPD.Response handshakeResponse = new NanoHTTPD.Response(NanoHTTPD.Status.SWITCH_PROTOCOL, null, (InputStream) null, -1) {
        @Override
        protected void send(OutputStream out) {
            WebSocket.this.out = out;
            state = State.CONNECTING;
            super.send(out);
            state = State.OPEN;
            onOpen();
            readWebsocket();
        }
    };
    public WebSocket(NanoHTTPD.IHTTPSession handshakeRequest) {
        this.handshakeRequest = handshakeRequest;
        this.in = handshakeRequest.getInputStream();
        handshakeResponse.addHeader(NanoWebSocketServer.HEADER_UPGRADE, NanoWebSocketServer.HEADER_UPGRADE_VALUE);
        handshakeResponse.addHeader(NanoWebSocketServer.HEADER_CONNECTION, NanoWebSocketServer.HEADER_CONNECTION_VALUE);
    }
    // --------------------------------IO--------------------------------------
    protected void readWebsocket() {
        try {
            while (state == State.OPEN) {
                handleWebsocketFrame(WebSocketFrame.read(in));
            }
        } catch (CharacterCodingException e) {
            onException(e);
            doClose(WebSocketFrame.CloseCode.InvalidFramePayloadData, e.toString(), false);
        } catch (IOException e) {
            onException(e);
            if (e instanceof WebSocketException) {
                doClose(((WebSocketException) e).getCode(), ((WebSocketException) e).getReason(), false);
            }
        } finally {
            doClose(WebSocketFrame.CloseCode.InternalServerError, "Handler terminated without closing the connection.", false);
        }
    }
    protected void handleWebsocketFrame(WebSocketFrame frame) throws IOException {
        if (frame.getOpCode() == WebSocketFrame.OpCode.Close) {
            handleCloseFrame(frame);
        } else if (frame.getOpCode() == WebSocketFrame.OpCode.Ping) {
            sendFrame(new WebSocketFrame(WebSocketFrame.OpCode.Pong, true, frame.getBinaryPayload()));
        } else if (frame.getOpCode() == WebSocketFrame.OpCode.Pong) {
            onPong(frame);
        } else if (!frame.isFin() || frame.getOpCode() == WebSocketFrame.OpCode.Continuation) {
            handleFrameFragment(frame);
        } else if (continuousOpCode != null) {
            throw new WebSocketException(WebSocketFrame.CloseCode.ProtocolError, "Continuous frame sequence not completed.");
        } else if (frame.getOpCode() == WebSocketFrame.OpCode.Text || frame.getOpCode() == WebSocketFrame.OpCode.Binary) {
            onMessage(frame);
        } else {
            throw new WebSocketException(WebSocketFrame.CloseCode.ProtocolError, "Non control or continuous frame expected.");
        }
    }
    protected void handleCloseFrame(WebSocketFrame frame) throws IOException {
        WebSocketFrame.CloseCode code = WebSocketFrame.CloseCode.NormalClosure;
        String reason = "";
        if (frame instanceof WebSocketFrame.CloseFrame) {
            code = ((WebSocketFrame.CloseFrame) frame).getCloseCode();
            reason = ((WebSocketFrame.CloseFrame) frame).getCloseReason();
        }
        if (state == State.CLOSING) {
            //Answer for my requested close
            doClose(code, reason, false);
        } else {
            //Answer close request from other endpoint and close self
            State oldState = state;
            state = State.CLOSING;
            if (oldState == State.OPEN) try {
                sendFrame(new WebSocketFrame.CloseFrame(code, reason));
            } catch {}
            doClose(code, reason, true);
        }
    }
    protected void handleFrameFragment(WebSocketFrame frame) throws IOException {
        if (frame.getOpCode() != WebSocketFrame.OpCode.Continuation) {
            //First
            if (continuousOpCode != null) {
                throw new WebSocketException(WebSocketFrame.CloseCode.ProtocolError, "Previous continuous frame sequence not completed.");
            }
            continuousOpCode = frame.getOpCode();
            continuousFrames.clear();
            continuousFrames.add(frame);
        } else if (frame.isFin()) {
            //Last
            if (continuousOpCode == null) {
                throw new WebSocketException(WebSocketFrame.CloseCode.ProtocolError, "Continuous frame sequence was not started.");
            }
            onMessage(new WebSocketFrame(continuousOpCode, continuousFrames));
            continuousOpCode = null;
            continuousFrames.clear();
        } else if (continuousOpCode == null) {
            //Unexpected
            throw new WebSocketException(WebSocketFrame.CloseCode.ProtocolError, "Continuous frame sequence was not started.");
        } else {
            //Intermediate
            continuousFrames.add(frame);
        }
    }
    public synchronized void sendFrame(WebSocketFrame frame) throws IOException {
        frame.write(out);
    }
    // --------------------------------Close-----------------------------------
    
    // deliberate closing by server, e.g. because of module reload
    public void close() {
      doClose(WebSocketFrame.CloseCode.GoingAway, "Internal closing", false);
    }
    
    protected void doClose(WebSocketFrame.CloseCode code, String reason, boolean initiatedByRemote) {
        if (state == State.CLOSED) {
            return;
        }
        if (in != null) {
            try {
                in.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        if (out != null) {
            try {
                out.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        state = State.CLOSED;
        onClose(code, reason, initiatedByRemote);
    }
    // --------------------------------Listener--------------------------------
    protected void onPong(WebSocketFrame pongFrame) { print("WebSocket pong"); }
    swappable void onMessage(WebSocketFrame messageFrame) { print("WebSocket msg: " + messageFrame.getTextPayload()); }
    swappable void onOpen() {}
    protected void onClose(WebSocketFrame.CloseCode code, String reason, boolean initiatedByRemote) { print("WebSocket close"); onClose(); }
    swappable void onClose() {}
    protected void onException(IOException e) { printStackTrace(e); }
    // --------------------------------Public Facade---------------------------
    public void ping(byte[] payload) throws IOException {
        sendFrame(new WebSocketFrame(WebSocketFrame.OpCode.Ping, true, payload));
    }
    public void send(byte[] payload) throws IOException {
        sendFrame(new WebSocketFrame(WebSocketFrame.OpCode.Binary, true, payload));
    }
    public void send(String payload) throws IOException {
        sendFrame(new WebSocketFrame(WebSocketFrame.OpCode.Text, true, payload));
    }
    public void close(WebSocketFrame.CloseCode code, String reason) throws IOException {
        State oldState = state;
        state = State.CLOSING;
        if (oldState == State.OPEN) {
            sendFrame(new WebSocketFrame.CloseFrame(code, reason));
        } else {
            doClose(code, reason, false);
        }
    }
    // --------------------------------Getters---------------------------------
    public NanoHTTPD.IHTTPSession getHandshakeRequest() {
        return handshakeRequest;
    }
    public NanoHTTPD.Response getHandshakeResponse() {
        return handshakeResponse;
    }
    
    // convenience methods
    S getUri() { ret getHandshakeRequest().getUri(); }
    SS getParms() { ret getHandshakeRequest().getParms(); }
}

sinterface WebSocketFactory {
  WebSocket openWebSocket(NanoHTTPD.IHTTPSession handshake);
}

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 newFixedLengthResponse(Status.BAD_REQUEST, NanoHTTPD.MIME_PLAINTEXT, "Invalid Websocket handshake");
            }
            if (!HEADER_WEBSOCKET_VERSION_VALUE.equalsIgnoreCase(headers.get(HEADER_WEBSOCKET_VERSION))) {
                return newFixedLengthResponse(Status.BAD_REQUEST, NanoHTTPD.MIME_PLAINTEXT, "Invalid Websocket-Version " + headers.get(HEADER_WEBSOCKET_VERSION));
            }
            if (!headers.containsKey(HEADER_WEBSOCKET_KEY)) {
                return newFixedLengthResponse(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 newFixedLengthResponse(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 constructor");
        }
        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);
    }
}

Began life as a copy of #1009196