/*
 * Copyright (c) 2016, 2017, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

import java.io.Closeable;
import java.io.IOException;
import java.io.UncheckedIOException;
import java.net.InetSocketAddress;
import java.net.URI;
import java.nio.ByteBuffer;
import java.nio.CharBuffer;
import java.nio.channels.ClosedByInterruptException;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.nio.charset.CharacterCodingException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.Arrays;
import java.util.Base64;
import java.util.HashMap;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.function.Function;
import java.util.regex.Pattern;
import java.util.stream.Collectors;

import static java.lang.String.format;
import static java.lang.System.Logger.Level.ERROR;
import static java.lang.System.Logger.Level.INFO;
import static java.lang.System.Logger.Level.TRACE;
import static java.nio.charset.StandardCharsets.ISO_8859_1;
import static java.util.Arrays.asList;
import static java.util.Objects.requireNonNull;

/**
 * Dummy WebSocket Server.
 *
 * Performs simpler version of the WebSocket Opening Handshake over HTTP (i.e.
 * no proxying, cookies, etc.) Supports sequential connections, one at a time,
 * i.e. in order for a client to connect to the server the previous client must
 * disconnect first.
 *
 * Expected client request:
 *
 *     GET /chat HTTP/1.1
 *     Host: server.example.com
 *     Upgrade: websocket
 *     Connection: Upgrade
 *     Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
 *     Origin: http://example.com
 *     Sec-WebSocket-Protocol: chat, superchat
 *     Sec-WebSocket-Version: 13
 *
 * This server response:
 *
 *     HTTP/1.1 101 Switching Protocols
 *     Upgrade: websocket
 *     Connection: Upgrade
 *     Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=
 *     Sec-WebSocket-Protocol: chat
 */
public final class DummyWebSocketServer implements Closeable {

    private final static System.Logger log = System.getLogger(DummyWebSocketServer.class.getName());
    private final AtomicBoolean started = new AtomicBoolean();
    private final Thread thread;
    private volatile ServerSocketChannel ssc;
    private volatile InetSocketAddress address;

    public DummyWebSocketServer() {
        this(defaultMapping());
    }

    public DummyWebSocketServer(Function<List<String>, List<String>> mapping) {
        requireNonNull(mapping);
        thread = new Thread(() -> {
            try {
                while (!Thread.currentThread().isInterrupted()) {
                    log.log(INFO, "Accepting next connection at: " + ssc);
                    SocketChannel channel = ssc.accept();
                    log.log(INFO, "Accepted: " + channel);
                    try {
                        channel.configureBlocking(true);
                        StringBuilder request = new StringBuilder();
                        if (!readRequest(channel, request)) {
                            throw new IOException("Bad request");
                        }
                        List<String> strings = asList(request.toString().split("\r\n"));
                        List<String> response = mapping.apply(strings);
                        writeResponse(channel, response);
                        // Read until the thread is interrupted or an error occurred
                        // or the input is shutdown
                        ByteBuffer b = ByteBuffer.allocate(1024);
                        while (channel.read(b) != -1) {
                            b.clear();
                        }
                    } catch (IOException e) {
                        log.log(TRACE, () -> "Error in connection: " + channel, e);
                    } finally {
                        log.log(INFO, "Closed: " + channel);
                        close(channel);
                    }
                }
            } catch (ClosedByInterruptException ignored) {
            } catch (IOException e) {
                log.log(ERROR, e);
            } finally {
                close(ssc);
                log.log(INFO, "Stopped at: " + getURI());
            }
        });
        thread.setName("DummyWebSocketServer");
        thread.setDaemon(false);
    }

    public void open() throws IOException {
        log.log(INFO, "Starting");
        if (!started.compareAndSet(false, true)) {
            throw new IllegalStateException("Already started");
        }
        ssc = ServerSocketChannel.open();
        try {
            ssc.configureBlocking(true);
            ssc.bind(new InetSocketAddress("localhost", 0));
            address = (InetSocketAddress) ssc.getLocalAddress();
            thread.start();
        } catch (IOException e) {
            close(ssc);
        }
        log.log(INFO, "Started at: " + getURI());
    }

    @Override
    public void close() {
        log.log(INFO, "Stopping: " + getURI());
        thread.interrupt();
    }

    URI getURI() {
        if (!started.get()) {
            throw new IllegalStateException("Not yet started");
        }
        return URI.create("ws://" + address.getHostName() + ":" + address.getPort());
    }

    private boolean readRequest(SocketChannel channel, StringBuilder request)
            throws IOException
    {
        ByteBuffer buffer = ByteBuffer.allocate(512);
        int num = channel.read(buffer);
        if (num == -1) {
            return false;
        }
        CharBuffer decoded;
        buffer.flip();
        try {
            decoded = ISO_8859_1.newDecoder().decode(buffer);
        } catch (CharacterCodingException e) {
            throw new UncheckedIOException(e);
        }
        request.append(decoded);
        return Pattern.compile("\r\n\r\n").matcher(request).find();
    }

    private void writeResponse(SocketChannel channel, List<String> response)
            throws IOException
    {
        String s = response.stream().collect(Collectors.joining("\r\n"))
                + "\r\n\r\n";
        ByteBuffer encoded;
        try {
            encoded = ISO_8859_1.newEncoder().encode(CharBuffer.wrap(s));
        } catch (CharacterCodingException e) {
            throw new UncheckedIOException(e);
        }
        while (encoded.hasRemaining()) {
            channel.write(encoded);
        }
    }

    private static Function<List<String>, List<String>> defaultMapping() {
        return request -> {
            List<String> response = new LinkedList<>();
            Iterator<String> iterator = request.iterator();
            if (!iterator.hasNext()) {
                throw new IllegalStateException("The request is empty");
            }
            if (!"GET / HTTP/1.1".equals(iterator.next())) {
                throw new IllegalStateException
                        ("Unexpected status line: " + request.get(0));
            }
            response.add("HTTP/1.1 101 Switching Protocols");
            Map<String, String> requestHeaders = new HashMap<>();
            while (iterator.hasNext()) {
                String header = iterator.next();
                String[] split = header.split(": ");
                if (split.length != 2) {
                    throw new IllegalStateException
                            ("Unexpected header: " + header
                                     + ", split=" + Arrays.toString(split));
                }
                if (requestHeaders.put(split[0], split[1]) != null) {
                    throw new IllegalStateException
                            ("Duplicating headers: " + Arrays.toString(split));
                }
            }
            if (requestHeaders.containsKey("Sec-WebSocket-Protocol")) {
                throw new IllegalStateException("Subprotocols are not expected");
            }
            if (requestHeaders.containsKey("Sec-WebSocket-Extensions")) {
                throw new IllegalStateException("Extensions are not expected");
            }
            expectHeader(requestHeaders, "Connection", "Upgrade");
            response.add("Connection: Upgrade");
            expectHeader(requestHeaders, "Upgrade", "websocket");
            response.add("Upgrade: websocket");
            expectHeader(requestHeaders, "Sec-WebSocket-Version", "13");
            String key = requestHeaders.get("Sec-WebSocket-Key");
            if (key == null) {
                throw new IllegalStateException("Sec-WebSocket-Key is missing");
            }
            MessageDigest sha1 = null;
            try {
                sha1 = MessageDigest.getInstance("SHA-1");
            } catch (NoSuchAlgorithmException e) {
                throw new InternalError(e);
            }
            String x = key + "258EAFA5-E914-47DA-95CA-C5AB0DC85B11";
            sha1.update(x.getBytes(ISO_8859_1));
            String v = Base64.getEncoder().encodeToString(sha1.digest());
            response.add("Sec-WebSocket-Accept: " + v);
            return response;
        };
    }

    protected static String expectHeader(Map<String, String> headers,
                                         String name,
                                         String value) {
        String v = headers.get(name);
        if (!value.equals(v)) {
            throw new IllegalStateException(
                    format("Expected '%s: %s', actual: '%s: %s'",
                           name, value, name, v)
            );
        }
        return v;
    }

    private static void close(AutoCloseable... acs) {
        for (AutoCloseable ac : acs) {
            try {
                ac.close();
            } catch (Exception ignored) { }
        }
    }
}