/*
 * ModbusSim: a small Modbus TCP server (slave) for testing a CI Server Modbus station without the
 * real device. Java 11 single-file program: runs with the JDK that ships with CI Server, nothing
 * to install (a PowerShell TCP listener script is blocked by Windows antivirus/AMSI as
 * "malicious content", see knowledge/issues.md).
 *
 *   "%TLS_ROOT_PATH%\tls\jre\bin\java.exe" ModbusSim.java [port] [registers] [folder]
 *        defaults: 15020, 1000, the current folder. Listens on 127.0.0.1 only.
 *
 * One table of holding registers (FC03 read, FC06 / FC16 write); FC04 reads the same table as input
 * registers (like CI Server's own slave mirror). FC07 answers 0, FC08 echoes, others exception 01.
 * Register 1 = protocol address 0 (CI Server point RO:1 / RI:1 = 40001 / 30001).
 *
 * <folder>/mode.txt is read every 2 s and can be changed while it runs:
 *   pattern  raw(k) = ((k * 157) mod 30000) - 5000   (fixed, signed test values; the default)
 *   zero     all registers 0
 *   csv      values from <folder>/values.csv: one integer per line, line k = register k
 *   hold     keep the current values (use with writes from CI Server)
 *   stop     close and exit
 * Messages go to the console and <folder>/sim.log.
 * To keep it running after an SSH session ends, start it detached, e.g. with PowerShell
 *   Invoke-CimMethod -ClassName Win32_Process -MethodName Create -Arguments @{CommandLine='...'}
 * Never run it on a production system or on a port a real device uses.
 */
import java.io.*;
import java.net.*;
import java.nio.file.*;
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;

public class ModbusSim {
    static int n;
    static int[] reg;
    static volatile String mode = "";
    static Path folder;

    public static void main(String[] args) throws Exception {
        int port = args.length > 0 ? Integer.parseInt(args[0]) : 15020;
        n = args.length > 1 ? Integer.parseInt(args[1]) : 1000;
        folder = Paths.get(args.length > 2 ? args[2] : ".").toAbsolutePath();
        reg = new int[n + 1];
        Path modeFile = folder.resolve("mode.txt");
        if (!Files.exists(modeFile)) Files.writeString(modeFile, "pattern");
        ServerSocket server = new ServerSocket(port, 10, InetAddress.getLoopbackAddress());
        log("listening on 127.0.0.1:" + port + ", " + n + " registers, pid " + ProcessHandle.current().pid());

        Thread control = new Thread(() -> {
            while (true) {
                try {
                    String m = Files.readAllLines(modeFile).stream().findFirst().orElse("").trim().toLowerCase();
                    if (m.equals("stop")) { log("stop requested"); System.exit(0); }
                    if (!m.equals(mode)) { mode = m; log("mode " + m); apply(m); }
                    Thread.sleep(2000);
                } catch (Exception e) { log("control: " + e); }
            }
        });
        control.setDaemon(true);
        control.start();
        while (true) {
            Socket c = server.accept();
            new Thread(() -> serve(c)).start();
        }
    }

    static synchronized void apply(String m) throws IOException {
        if (m.equals("pattern")) for (int k = 1; k <= n; k++) reg[k] = ((k * 157) % 30000) - 5000;
        else if (m.equals("zero")) java.util.Arrays.fill(reg, 0);
        else if (m.equals("csv")) {
            java.util.List<String> lines = Files.readAllLines(folder.resolve("values.csv"));
            for (int k = 1; k <= n && k <= lines.size(); k++) reg[k] = Integer.parseInt(lines.get(k - 1).trim());
        }
    }

    static void serve(Socket c) {
        String who = String.valueOf(c.getRemoteSocketAddress());
        log("connect " + who);
        try (c; DataInputStream in = new DataInputStream(new BufferedInputStream(c.getInputStream()));
             OutputStream out = c.getOutputStream()) {
            while (true) {
                byte[] h = new byte[7];
                in.readFully(h);
                int len = ((h[4] & 0xFF) << 8) | (h[5] & 0xFF);
                if (len < 2 || len > 256) throw new IOException("bad length " + len);
                byte[] pdu = new byte[len - 1];
                in.readFully(pdu);
                byte[] r = handle(pdu);
                byte[] resp = new byte[7 + r.length];
                System.arraycopy(h, 0, resp, 0, 4);
                resp[4] = (byte) ((r.length + 1) >> 8); resp[5] = (byte) (r.length + 1); resp[6] = h[6];
                System.arraycopy(r, 0, resp, 7, r.length);
                out.write(resp);
                out.flush();
            }
        } catch (EOFException e) {
            log("disconnect " + who);
        } catch (Exception e) {
            log("disconnect " + who + " (" + e + ")");
        }
    }

    static int u16(byte[] b, int i) { return ((b[i] & 0xFF) << 8) | (b[i + 1] & 0xFF); }

    static synchronized byte[] handle(byte[] pdu) {
        int fc = pdu[0] & 0xFF;
        if (fc == 3 || fc == 4) {
            int start = u16(pdu, 1), qty = u16(pdu, 3);
            if (qty < 1 || qty > 125) return new byte[]{(byte) (fc | 0x80), 3};
            if (start + qty > n) return new byte[]{(byte) (fc | 0x80), 2};
            byte[] r = new byte[2 + 2 * qty];
            r[0] = (byte) fc; r[1] = (byte) (2 * qty);
            for (int i = 0; i < qty; i++) {
                int v = reg[start + i + 1] & 0xFFFF;
                r[2 + 2 * i] = (byte) (v >> 8); r[3 + 2 * i] = (byte) v;
            }
            return r;
        }
        if (fc == 6) {
            int a = u16(pdu, 1);
            if (a >= n) return new byte[]{(byte) 0x86, 2};
            reg[a + 1] = (short) u16(pdu, 3);
            return pdu.clone();
        }
        if (fc == 16) {
            int start = u16(pdu, 1), qty = u16(pdu, 3);
            if (qty < 1 || qty > 123 || start + qty > n) return new byte[]{(byte) 0x90, 2};
            for (int i = 0; i < qty; i++) reg[start + i + 1] = (short) u16(pdu, 6 + 2 * i);
            return new byte[]{16, pdu[1], pdu[2], pdu[3], pdu[4]};
        }
        if (fc == 7) return new byte[]{7, 0};
        if (fc == 8) return pdu.clone();
        return new byte[]{(byte) (fc | 0x80), 1};
    }

    static synchronized void log(String s) {
        String line = LocalDateTime.now().format(DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss")) + " " + s;
        System.out.println(line);
        try {
            Files.writeString(folder.resolve("sim.log"), line + System.lineSeparator(),
                    StandardOpenOption.CREATE, StandardOpenOption.APPEND);
        } catch (IOException ignored) { }
    }
}