package com.battery.device.simulator;

import java.io.IOException;
import java.io.InputStream;
import java.net.InetAddress;
import java.net.InetSocketAddress;
import java.net.ServerSocket;
import java.net.Socket;
import java.net.SocketException;
import java.nio.charset.StandardCharsets;
import java.time.Duration;
import java.util.List;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.function.Function;

/**
 * 提供可复用的 BTS TCP 脚本设备边界，用于闭环与恢复测试。
 */
public final class ScriptedBtsTcpSimulator implements AutoCloseable
{
    private static final byte[] FRAME_END = "#\r\n".getBytes(StandardCharsets.UTF_8);

    private final ServerSocket serverSocket;
    private final int port;
    /** 连接服务线程显式创建并命名。该池只提交一个常驻服务任务，队列容量 1 只是声明不再接受更多任务。 */
    private final ExecutorService executor = new ThreadPoolExecutor(1, 1, 0L, TimeUnit.MILLISECONDS,
            new LinkedBlockingQueue<>(1), simulatorThreadFactory());
    private final BlockingQueue<ScriptedAction> actions = new LinkedBlockingQueue<>();
    private final List<String> requests = new CopyOnWriteArrayList<>();
    private final Function<String, String> responseProvider;

    public ScriptedBtsTcpSimulator() throws IOException
    {
        this(0, true, null);
    }

    private ScriptedBtsTcpSimulator(int port, boolean acceptConnections,
            Function<String, String> responseProvider) throws IOException
    {
        if (port < 0 || port > 65535) throw new IllegalArgumentException("port must be between 0 and 65535");
        this.responseProvider = responseProvider;
        serverSocket = new ServerSocket();
        serverSocket.bind(new InetSocketAddress(InetAddress.getByName("127.0.0.1"), port));
        this.port = serverSocket.getLocalPort();
        if (acceptConnections)
            executor.submit(this::serve);
        else
            serverSocket.close();
    }

    /** 创建一启动就关闭监听、任何连接都会被拒绝的模拟器。 */
    public static ScriptedBtsTcpSimulator refusingConnections() throws IOException
    {
        return new ScriptedBtsTcpSimulator(0, false, null);
    }

    /** 创建仅监听本机、可按请求持续生成响应的模拟器。 */
    public static ScriptedBtsTcpSimulator responding(int port, Function<String, String> responseProvider)
            throws IOException
    {
        if (responseProvider == null) throw new IllegalArgumentException("responseProvider must not be null");
        return new ScriptedBtsTcpSimulator(port, true, responseProvider);
    }

    /** 返回该模拟器绑定的本机端口；构造时传入 0 时返回的是系统分配端口而非 0。 */
    public int port()
    {
        return port;
    }

    /** 向脚本队列追加一条零延迟应答，按 UTF-8 编码，由下一次请求按顺序取用。 */
    public void enqueue(String response)
    {
        actions.add(new ScriptedAction(Duration.ZERO, response.getBytes(StandardCharsets.UTF_8), false));
    }

    /** 追加一条延迟应答：连接服务线程收到请求后先等待 delay 再写出；delay 不得为负。 */
    public void enqueueDelayed(Duration delay, String response)
    {
        if (delay.isNegative()) throw new IllegalArgumentException("delay must not be negative");
        actions.add(new ScriptedAction(delay, response.getBytes(StandardCharsets.UTF_8), false));
    }

    /** 追加一条零延迟的原始字节应答：字节原样写出后追加帧结束符，不做字符编码转换。 */
    public void enqueueRaw(byte[] response)
    {
        actions.add(new ScriptedAction(Duration.ZERO, response.clone(), false));
    }

    /** 追加一条断开动作：处理到该条时结束当前连接，且不写出任何应答。 */
    public void enqueueDisconnect()
    {
        actions.add(new ScriptedAction(Duration.ZERO, null, true));
    }

    /** 返回已接收请求帧的只读快照；只有脚本模式（未配置 responseProvider）才记录。 */
    public List<String> requests()
    {
        return List.copyOf(requests);
    }

    private void serve()
    {
        while (!serverSocket.isClosed())
        {
            try (Socket socket = serverSocket.accept())
            {
                serveConnection(socket);
            }
            catch (SocketException ex)
            {
                if (!serverSocket.isClosed()) throw new IllegalStateException(ex);
            }
            catch (Exception ignored)
            {
            }
        }
    }

    private void serveConnection(Socket socket) throws Exception
    {
        while (!socket.isClosed())
        {
            String request = readFrame(socket.getInputStream());
            if (responseProvider == null) requests.add(request);
            ScriptedAction action = responseProvider == null
                    ? actions.poll(5, TimeUnit.SECONDS)
                    : new ScriptedAction(Duration.ZERO,
                            responseProvider.apply(request).getBytes(StandardCharsets.UTF_8), false);
            if (action == null) throw new IOException("No scripted BTS action available");
            TimeUnit.MILLISECONDS.sleep(action.delay().toMillis());
            if (action.disconnect()) return;
            socket.getOutputStream().write(action.payload());
            socket.getOutputStream().write(FRAME_END);
            socket.getOutputStream().flush();
        }
    }

    private String readFrame(InputStream input) throws IOException
    {
        return new String(readFrameBytes(input), StandardCharsets.UTF_8);
    }

    private byte[] readFrameBytes(InputStream input) throws IOException
    {
        java.io.ByteArrayOutputStream bytes = new java.io.ByteArrayOutputStream();
        byte[] delimiter = FRAME_END;
        int matched = 0;
        while (true)
        {
            int current = input.read();
            if (current < 0) throw new java.io.EOFException("BTS frame ended before delimiter");
            if ((byte) current == delimiter[matched])
            {
                matched++;
                if (matched == delimiter.length) return bytes.toByteArray();
                continue;
            }
            if (matched > 0)
            {
                for (int i = 0; i < matched; i++) bytes.write(delimiter[i]);
                matched = 0;
                if ((byte) current == delimiter[0])
                {
                    matched = 1;
                    continue;
                }
            }
            bytes.write(current);
        }
    }

    /** 关闭监听并终止连接服务线程，最多等待 2 秒，避免测试退出时残留线程。 */
    @Override
    public void close() throws Exception
    {
        serverSocket.close();
        executor.shutdownNow();
        executor.awaitTermination(2, TimeUnit.SECONDS);
    }

    /** 模拟器连接服务线程统一命名并设为守护线程，避免测试退出时残留。 */
    private static ThreadFactory simulatorThreadFactory()
    {
        AtomicInteger sequence = new AtomicInteger();
        return runnable -> {
            Thread thread = new Thread(runnable, "bts-simulator-pool-" + sequence.incrementAndGet());
            thread.setDaemon(true);
            return thread;
        };
    }

    private record ScriptedAction(Duration delay, byte[] payload, boolean disconnect)
    {
    }
}