using Shouldly;
using Xunit;

namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests;

public sealed class ModbusRtuFramingTests
{
    private static MemoryStream Canned(params byte[] frame) => new(frame);

    [Fact]
    public void BuildAdu_prefixes_unit_and_appends_crc()
    {
        var adu = ModbusRtuFraming.BuildAdu(0x01, new byte[] { 0x03, 0x00, 0x00, 0x00, 0x01 });
        adu.ShouldBe(new byte[] { 0x01, 0x03, 0x00, 0x00, 0x00, 0x01, 0x84, 0x0A });
    }

    [Fact]
    public async Task ReadResponse_FC03_returns_bare_pdu()
    {
        // addr=01 fc=03 byteCount=02 data=00 0A  + CRC(lo,hi)
        var body = new byte[] { 0x01, 0x03, 0x02, 0x00, 0x0A };
        var frame = ModbusCrc.Append(body);
        await using var s = Canned(frame);
        var pdu = await ModbusRtuFraming.ReadResponsePduAsync(s, 0x01, TestContext.Current.CancellationToken);
        pdu.ShouldBe(new byte[] { 0x03, 0x02, 0x00, 0x0A }); // fc + byteCount + data, no addr/CRC
    }

    [Fact]
    public async Task ReadResponse_exception_frame_throws_ModbusException()
    {
        // addr=01 fc=0x83 exc=0x02 + CRC
        var frame = ModbusCrc.Append(new byte[] { 0x01, 0x83, 0x02 });
        await using var s = Canned(frame);
        var ex = await Should.ThrowAsync<ModbusException>(
            ModbusRtuFraming.ReadResponsePduAsync(s, 0x01, TestContext.Current.CancellationToken));
        ex.FunctionCode.ShouldBe((byte)0x03);
        ex.ExceptionCode.ShouldBe((byte)0x02);
    }

    [Fact]
    public async Task ReadResponse_bad_crc_throws_desync()
    {
        var frame = ModbusCrc.Append(new byte[] { 0x01, 0x03, 0x02, 0x00, 0x0A });
        frame[^1] ^= 0xFF; // corrupt CRC high byte
        await using var s = Canned(frame);
        await Should.ThrowAsync<ModbusTransportDesyncException>(
            ModbusRtuFraming.ReadResponsePduAsync(s, 0x01, TestContext.Current.CancellationToken));
    }

    [Fact]
    public async Task ReadResponse_FC06_write_echo_returns_four_byte_pdu()
    {
        // addr=01 fc=06 addr=00 07 value=00 2A + CRC  -> PDU = 06 00 07 00 2A
        var frame = ModbusCrc.Append(new byte[] { 0x01, 0x06, 0x00, 0x07, 0x00, 0x2A });
        await using var s = Canned(frame);
        var pdu = await ModbusRtuFraming.ReadResponsePduAsync(s, 0x01, TestContext.Current.CancellationToken);
        pdu.ShouldBe(new byte[] { 0x06, 0x00, 0x07, 0x00, 0x2A });
    }
}