package com.lanremote.watch;

import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.net.InetSocketAddress;
import java.net.SocketTimeoutException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;

/**
 * 局域网自动发现的**纯 Java 核心**（不含任何 Android 依赖）。
 *
 * 两层查找策略：
 *   1. UDP 广播探测（快，前提是广播没被拦）
 *   2. 定向广播 + 子网 TCP 扫描（慢一点，但广播被拦时仍然管用）
 *
 * 与 Windows 端 lanremote/discovery.py 用同一套报文格式。
 */
public final class DiscoveryCore {

    public static final int PORT = 8766;

    private static final String PROBE =
            "{\"lanremote\":\"LANREMOTE_DISCOVER\",\"proto\":1}";
    private static final String REPLY_MAGIC = "LANREMOTE_REPLY";
    private static final String SELFCHECK_MAGIC = "LANREMOTE_SELFCHECK";
    private static final String ACK_MAGIC = "LANREMOTE_ACK";

    private DiscoveryCore() {
    }

    public static class Found {
        public final String host;
        public final int port;
        public final String name;
        public final String version;
        /** 来源：broadcast(广播) / directed(定向广播) / scan(端口扫描) */
        public final String via;

        public Found(String host, int port, String name, String version, String via) {
            this.host = host;
            this.port = port;
            this.name = name;
            this.version = version;
            this.via = via;
        }

        @Override
        public String toString() {
            return (name == null || name.isEmpty() ? host : name)
                    + "  " + host + ":" + port + "  [" + via + "]";
        }
    }

    // ==================================================================
    // 地址计算（纯函数，可单独验证）
    // ==================================================================

    public static long ipToLong(String ip) {
        String[] parts = ip.split("\\.");
        if (parts.length != 4) {
            return -1;
        }
        long value = 0;
        for (String part : parts) {
            int octet;
            try {
                octet = Integer.parseInt(part.trim());
            } catch (NumberFormatException e) {
                return -1;
            }
            if (octet < 0 || octet > 255) {
                return -1;
            }
            value = (value << 8) | octet;
        }
        return value & 0xFFFFFFFFL;
    }

    public static String longToIp(long value) {
        return ((value >> 24) & 0xFF) + "." + ((value >> 16) & 0xFF) + "."
                + ((value >> 8) & 0xFF) + "." + (value & 0xFF);
    }

    /**
     * 由本机 IP + 前缀长度算出**定向广播地址**。
     *
     * 例：192.168.10.104/24 -> 192.168.10.255
     *
     * 之前这里是个 bug：代码里写死了猜 192.168.255.255，
     * 对 /24 网段来说那根本不是本网段的广播地址，
     * 很多路由器和安卓版本不会转发它，导致广播探测收不到任何回复。
     */
    public static String directedBroadcast(String ip, int prefix) {
        long addr = ipToLong(ip);
        if (addr < 0 || prefix < 0 || prefix > 32) {
            return null;
        }
        if (prefix == 32) {
            return ip;                       // 单机，没有广播
        }
        long mask = (0xFFFFFFFFL << (32 - prefix)) & 0xFFFFFFFFL;
        long network = addr & mask;
        long broadcast = (network | (~mask & 0xFFFFFFFFL)) & 0xFFFFFFFFL;
        return longToIp(broadcast);
    }

    /** 列出该网段内所有可分配主机地址（跳过网络号与广播号，最多 maxHosts 个）。 */
    public static List<String> subnetHosts(String ip, int prefix, int maxHosts) {
        List<String> hosts = new ArrayList<String>();
        long addr = ipToLong(ip);
        if (addr < 0 || prefix < 0 || prefix > 30) {
            return hosts;
        }
        long mask = (0xFFFFFFFFL << (32 - prefix)) & 0xFFFFFFFFL;
        long network = addr & mask;
        long broadcast = (network | (~mask & 0xFFFFFFFFL)) & 0xFFFFFFFFL;
        for (long host = network + 1; host < broadcast && hosts.size() < maxHosts; host++) {
            hosts.add(longToIp(host));
        }
        return hosts;
    }

    // ==================================================================
    // 查找
    // ==================================================================

    public static List<Found> search(int timeoutMs) {
        return search(timeoutMs, null);
    }

    /**
     * 广播搜索。会阻塞 timeoutMs 毫秒，必须在后台线程调用。
     *
     * @param extraBroadcast 定向广播地址，如 192.168.10.255（可为 null）
     */
    public static List<Found> search(int timeoutMs, String extraBroadcast) {
        List<Found> results = new ArrayList<Found>();
        Set<String> seen = new LinkedHashSet<String>();
        DatagramSocket socket = null;
        try {
            socket = new DatagramSocket();
            socket.setBroadcast(true);
            socket.setSoTimeout(400);

            byte[] payload = PROBE.getBytes("UTF-8");
            List<String> targets = new ArrayList<String>();
            targets.add("255.255.255.255");               // 受限广播
            if (extraBroadcast != null && extraBroadcast.length() > 0) {
                targets.add(extraBroadcast);              // 本网段定向广播（关键）
            }
            for (String target : targets) {
                try {
                    socket.send(new DatagramPacket(payload, payload.length,
                            InetAddress.getByName(target), PORT));
                } catch (Exception ignored) {
                    // 该广播地址不被允许时跳过
                }
            }

            long deadline = System.currentTimeMillis() + timeoutMs;
            byte[] buffer = new byte[2048];
            while (System.currentTimeMillis() < deadline) {
                DatagramPacket packet = new DatagramPacket(buffer, buffer.length);
                try {
                    socket.receive(packet);
                } catch (SocketTimeoutException timeout) {
                    continue;
                } catch (Exception broken) {
                    break;
                }
                Found found = parse(packet.getData(), packet.getLength(),
                        packet.getAddress().getHostAddress(), "broadcast");
                if (found == null) {
                    continue;
                }
                String key = found.host + ":" + found.port;
                if (seen.add(key)) {
                    results.add(found);
                }
                socket.setSoTimeout(250);
            }
        } catch (Exception ignored) {
            // 网络不可用时返回已收集到的结果
        } finally {
            if (socket != null) {
                socket.close();
            }
        }
        return results;
    }

    /**
     * 子网 TCP 端口扫描。
     *
     * 用途：广播被路由器/防火墙拦掉时，这是唯一还能找到被控端的办法。
     * 直接对本网段 254 个地址并发试连目标端口。
     */
    public static List<String> scanSubnet(String ip, int prefix, int port,
                                          int connectTimeoutMs, int maxHosts) {
        List<String> hosts = subnetHosts(ip, prefix, maxHosts);
        List<String> open = Collections.synchronizedList(new ArrayList<String>());
        if (hosts.isEmpty()) {
            return open;
        }
        int threads = Math.min(48, Math.max(8, hosts.size()));
        ExecutorService pool = Executors.newFixedThreadPool(threads);
        List<Future<?>> tasks = new ArrayList<Future<?>>();
        try {
            for (final String host : hosts) {
                tasks.add(pool.submit(new Callable<Void>() {
                    @Override
                    public Void call() {
                        if (ping(host, port, connectTimeoutMs)) {
                            open.add(host);
                        }
                        return null;
                    }
                }));
            }
            for (Future<?> task : tasks) {
                try {
                    task.get(connectTimeoutMs + 1500L, TimeUnit.MILLISECONDS);
                } catch (Exception ignored) {
                    // 单个地址超时不影响整体
                }
            }
        } finally {
            pool.shutdownNow();
        }
        Collections.sort(open, new java.util.Comparator<String>() {
            @Override
            public int compare(String a, String b) {
                long la = ipToLong(a);
                long lb = ipToLong(b);
                return la < lb ? -1 : (la > lb ? 1 : 0);
            }
        });
        return open;
    }

    /**
     * 请电脑打开「网络自检」。
     *
     * 走 UDP 广播是有意的：连不上的典型原因就是 TCP 被防火墙拦了，
     * 而 UDP 广播往往仍然通（那正是「能搜到但连不上」的原因）。
     * 所以只要能广播出去，就能让电脑把自检窗口弹出来。
     *
     * @return 电脑名；没有回应返回 null
     */
    public static String requestSelfCheck(String extraBroadcast, int timeoutMs) {
        DatagramSocket socket = null;
        try {
            socket = new DatagramSocket();
            socket.setBroadcast(true);
            socket.setSoTimeout(500);

            byte[] payload = ("{\"lanremote\":\"" + SELFCHECK_MAGIC
                    + "\",\"proto\":1}").getBytes("UTF-8");
            List<String> targets = new ArrayList<String>();
            targets.add("255.255.255.255");
            if (extraBroadcast != null && extraBroadcast.length() > 0) {
                targets.add(extraBroadcast);
            }

            // 连发三轮，丢一两个包也能到
            for (int round = 0; round < 3; round++) {
                for (String target : targets) {
                    try {
                        socket.send(new DatagramPacket(payload, payload.length,
                                InetAddress.getByName(target), PORT));
                    } catch (Exception ignored) {
                        // 某个广播地址不被允许时跳过
                    }
                }
                try {
                    Thread.sleep(120);
                } catch (InterruptedException stop) {
                    Thread.currentThread().interrupt();
                    break;
                }
            }

            long deadline = System.currentTimeMillis() + timeoutMs;
            byte[] buffer = new byte[2048];
            while (System.currentTimeMillis() < deadline) {
                DatagramPacket packet = new DatagramPacket(buffer, buffer.length);
                try {
                    socket.receive(packet);
                } catch (SocketTimeoutException timeout) {
                    continue;
                } catch (Exception broken) {
                    break;
                }
                String text = new String(packet.getData(), 0, packet.getLength(), "UTF-8");
                if (text.indexOf(ACK_MAGIC) < 0) {
                    continue;
                }
                try {
                    Map<String, Object> obj = Json.parseObject(text);
                    String name = Json.str(obj, "name", "");
                    String note = Json.str(obj, "note", "");
                    if (name != null && name.length() > 0) {
                        // 电脑那边现在会把诊断结论一起回过来（它自己不弹窗口），
                        // 用换行分隔：第一行是电脑名，后面是结论。
                        return note != null && note.length() > 0
                                ? name + "\n" + note : name;
                    }
                } catch (Exception ignored) {
                    // 解析失败就用来源 IP
                }
                return packet.getAddress().getHostAddress();
            }
        } catch (Exception ignored) {
            // 网络不可用
        } finally {
            if (socket != null) {
                socket.close();
            }
        }
        return null;
    }

    /** 对扫描到的地址做一次单播发现，拿到电脑名和真实端口。 */
    public static Found probeHost(String host, int timeoutMs) {
        DatagramSocket socket = null;
        try {
            socket = new DatagramSocket();
            socket.setSoTimeout(Math.max(300, timeoutMs));
            byte[] payload = PROBE.getBytes("UTF-8");
            InetAddress target = InetAddress.getByName(host);
            socket.send(new DatagramPacket(payload, payload.length, target, PORT));

            byte[] buffer = new byte[2048];
            DatagramPacket packet = new DatagramPacket(buffer, buffer.length);
            socket.receive(packet);
            return parse(packet.getData(), packet.getLength(),
                    packet.getAddress().getHostAddress(), "unicast");
        } catch (Exception e) {
            return null;
        } finally {
            if (socket != null) {
                socket.close();
            }
        }
    }

    /** TCP 探测结果 —— 关键是区分「拒绝」和「超时」，两者含义完全不同。 */
    public static class PingResult {
        public final boolean ok;
        public final String reason;      // ok / refused / timeout / unreachable / other
        public final long ms;

        public PingResult(boolean ok, String reason, long ms) {
            this.ok = ok;
            this.reason = reason;
            this.ms = ms;
        }

        /** 给人看的解释。 */
        public String describe() {
            if (ok) {
                return "成功（" + ms + " ms）";
            }
            if ("refused".equals(reason)) {
                return "被拒绝（" + ms + " ms）—— 主机在线，但**没有程序在监听这个端口**";
            }
            if ("timeout".equals(reason)) {
                return "超时（" + ms + " ms）—— 数据包没有到达或被丢弃"
                        + "（防火墙拦截 / 不在同一网络 / 主机离线）";
            }
            if ("unreachable".equals(reason)) {
                return "网络不可达（" + ms + " ms）—— 不同网段或没有路由";
            }
            return "失败：" + reason + "（" + ms + " ms）";
        }
    }

    /**
     * 带原因区分的 TCP 探测。
     *
     * 为什么必须区分：
     *   * 被拒绝(RST) = 包到了对方，对方说"这个端口没人听" -> 被控端没运行
     *   * 超时        = 包根本没到，或中途被丢 -> 防火墙 / 不同网络 / 对方离线
     * 这两种情况的排查方向完全相反。
     */
    public static PingResult pingDetailed(String host, int port, int timeoutMs) {
        long start = System.currentTimeMillis();
        java.net.Socket socket = new java.net.Socket();
        try {
            socket.connect(new InetSocketAddress(host, port), timeoutMs);
            return new PingResult(true, "ok", System.currentTimeMillis() - start);
        } catch (java.net.SocketTimeoutException e) {
            return new PingResult(false, "timeout", System.currentTimeMillis() - start);
        } catch (java.net.NoRouteToHostException e) {
            return new PingResult(false, "unreachable", System.currentTimeMillis() - start);
        } catch (java.net.ConnectException e) {
            String msg = String.valueOf(e.getMessage()).toLowerCase();
            String reason = msg.contains("refused") || msg.contains("拒绝")
                    ? "refused" : "unreachable";
            return new PingResult(false, reason, System.currentTimeMillis() - start);
        } catch (Exception e) {
            return new PingResult(false, e.getClass().getSimpleName(),
                    System.currentTimeMillis() - start);
        } finally {
            try {
                socket.close();
            } catch (Exception ignored) {
                // 忽略关闭异常
            }
        }
    }

    /** 简单版：只要通不通。 */
    public static boolean ping(String host, int port, int timeoutMs) {
        return pingDetailed(host, port, timeoutMs).ok;
    }

    static Found parse(byte[] data, int length, String fromHost, String via) {
        try {
            String text = new String(data, 0, length, "UTF-8");
            if (text.indexOf(REPLY_MAGIC) < 0) {
                return null;
            }
            Map<String, Object> obj = Json.parseObject(text);
            if (!REPLY_MAGIC.equals(Json.str(obj, "lanremote", ""))) {
                return null;
            }
            int port = Json.integer(obj, "port", 0);
            if (port <= 0 || port > 65535) {
                return null;
            }
            return new Found(fromHost, port,
                    Json.str(obj, "name", ""), Json.str(obj, "version", ""), via);
        } catch (Exception e) {
            return null;
        }
    }
}