using System.Buffers.Binary;
using Shouldly;
using Xunit;
using ZB.MOM.WW.OtOpcUa.Driver.Modbus;

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

[Trait("Category", "Unit")]
public sealed class ModbusDataTypeTests
{
    /// <summary>
    ///     Register-count lookup is per-tag now (strings need StringLength; Int64/Float64 need 4).
    /// </summary>
    [Theory]
    [InlineData(ModbusDataType.BitInRegister, 1)]
    [InlineData(ModbusDataType.Int16, 1)]
    [InlineData(ModbusDataType.UInt16, 1)]
    [InlineData(ModbusDataType.Int32, 2)]
    [InlineData(ModbusDataType.UInt32, 2)]
    [InlineData(ModbusDataType.Float32, 2)]
    [InlineData(ModbusDataType.Int64, 4)]
    [InlineData(ModbusDataType.UInt64, 4)]
    [InlineData(ModbusDataType.Float64, 4)]
    public void RegisterCount_returns_correct_register_count_per_type(ModbusDataType t, int expected)
    {
        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, t);
        ModbusDriver.RegisterCount(tag).ShouldBe((ushort)expected);
    }

    [Theory]
    [InlineData(0, 1)]   // 0 chars → still 1 byte / 1 register (pathological but well-defined: length 0 is 0 bytes)
    [InlineData(1, 1)]
    [InlineData(2, 1)]
    [InlineData(3, 2)]
    [InlineData(10, 5)]
    public void RegisterCount_for_String_rounds_up_to_register_pair(ushort chars, int expectedRegs)
    {
        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.String, StringLength: chars);
        // 0-char is encoded as 0 regs; the test case expects 1 for lengths 1-2, 2 for 3-4, etc.
        if (chars == 0) ModbusDriver.RegisterCount(tag).ShouldBe((ushort)0);
        else ModbusDriver.RegisterCount(tag).ShouldBe((ushort)expectedRegs);
    }

    // --- Int32 / UInt32 / Float32 with byte-order variants ---

    [Fact]
    public void Int32_BigEndian_decodes_ABCD_layout()
    {
        // Value 0x12345678 → bytes [0x12, 0x34, 0x56, 0x78] as PLC wrote them.
        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Int32,
            ByteOrder: ModbusByteOrder.BigEndian);
        var bytes = new byte[] { 0x12, 0x34, 0x56, 0x78 };
        ModbusDriver.DecodeRegister(bytes, tag).ShouldBe(0x12345678);
    }

    [Fact]
    public void Int32_WordSwap_decodes_CDAB_layout()
    {
        // Siemens/AB PLC stored 0x12345678 as register[0] = 0x5678, register[1] = 0x1234.
        // Wire bytes are [0x56, 0x78, 0x12, 0x34]; with ByteOrder=WordSwap we get 0x12345678 back.
        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Int32,
            ByteOrder: ModbusByteOrder.WordSwap);
        var bytes = new byte[] { 0x56, 0x78, 0x12, 0x34 };
        ModbusDriver.DecodeRegister(bytes, tag).ShouldBe(0x12345678);
    }

    [Fact]
    public void Float32_WordSwap_encode_decode_roundtrips()
    {
        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Float32,
            ByteOrder: ModbusByteOrder.WordSwap);
        var wire = ModbusDriver.EncodeRegister(25.5f, tag);
        wire.Length.ShouldBe(4);
        ModbusDriver.DecodeRegister(wire, tag).ShouldBe(25.5f);
    }

    // --- Int64 / UInt64 / Float64 ---

    [Fact]
    public void Int64_BigEndian_roundtrips()
    {
        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Int64);
        var wire = ModbusDriver.EncodeRegister(0x0123456789ABCDEFL, tag);
        wire.Length.ShouldBe(8);
        BinaryPrimitives.ReadInt64BigEndian(wire).ShouldBe(0x0123456789ABCDEFL);
        ModbusDriver.DecodeRegister(wire, tag).ShouldBe(0x0123456789ABCDEFL);
    }

    [Fact]
    public void UInt64_WordSwap_reverses_four_words()
    {
        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.UInt64,
            ByteOrder: ModbusByteOrder.WordSwap);
        var value = 0xAABBCCDDEEFF0011UL;

        var wireBE = new byte[8];
        BinaryPrimitives.WriteUInt64BigEndian(wireBE, value);

        // Word-swap layout: [word3, word2, word1, word0] where each word keeps its bytes big-endian.
        var wireWS = new byte[] { wireBE[6], wireBE[7], wireBE[4], wireBE[5], wireBE[2], wireBE[3], wireBE[0], wireBE[1] };
        ModbusDriver.DecodeRegister(wireWS, tag).ShouldBe(value);

        var roundtrip = ModbusDriver.EncodeRegister(value, tag);
        roundtrip.ShouldBe(wireWS);
    }

    [Fact]
    public void Float64_roundtrips_under_word_swap()
    {
        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.Float64,
            ByteOrder: ModbusByteOrder.WordSwap);
        var wire = ModbusDriver.EncodeRegister(3.14159265358979d, tag);
        wire.Length.ShouldBe(8);
        ((double)ModbusDriver.DecodeRegister(wire, tag)!).ShouldBe(3.14159265358979d, tolerance: 1e-12);
    }

    // --- BitInRegister ---

    [Theory]
    [InlineData(0b0000_0000_0000_0001, 0, true)]
    [InlineData(0b0000_0000_0000_0001, 1, false)]
    [InlineData(0b1000_0000_0000_0000, 15, true)]
    [InlineData(0b0100_0000_0100_0000, 6, true)]
    [InlineData(0b0100_0000_0100_0000, 14, true)]
    [InlineData(0b0100_0000_0100_0000, 7, false)]
    public void BitInRegister_extracts_bit_at_index(ushort raw, byte bitIndex, bool expected)
    {
        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.BitInRegister,
            BitIndex: bitIndex);
        var bytes = new byte[] { (byte)(raw >> 8), (byte)(raw & 0xFF) };
        ModbusDriver.DecodeRegister(bytes, tag).ShouldBe(expected);
    }

    [Fact]
    public void BitInRegister_write_is_not_supported_in_PR24()
    {
        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.BitInRegister,
            BitIndex: 5);
        Should.Throw<InvalidOperationException>(() => ModbusDriver.EncodeRegister(true, tag))
            .Message.ShouldContain("read-modify-write");
    }

    // --- String ---

    [Fact]
    public void String_decodes_ASCII_packed_two_chars_per_register()
    {
        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.String,
            StringLength: 6);
        // "HELLO!" = 0x48 0x45 0x4C 0x4C 0x4F 0x21 across 3 registers.
        var bytes = "HELLO!"u8.ToArray();
        ModbusDriver.DecodeRegister(bytes, tag).ShouldBe("HELLO!");
    }

    [Fact]
    public void String_decode_truncates_at_first_nul()
    {
        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.String,
            StringLength: 10);
        var bytes = new byte[] { 0x48, 0x69, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 };
        ModbusDriver.DecodeRegister(bytes, tag).ShouldBe("Hi");
    }

    [Fact]
    public void String_encode_nul_pads_remaining_bytes()
    {
        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.String,
            StringLength: 8);
        var wire = ModbusDriver.EncodeRegister("Hi", tag);
        wire.Length.ShouldBe(8);
        wire[0].ShouldBe((byte)'H');
        wire[1].ShouldBe((byte)'i');
        for (var i = 2; i < 8; i++) wire[i].ShouldBe((byte)0);
    }

    // --- DL205 low-byte-first strings (AutomationDirect DirectLOGIC quirk) ---

    [Fact]
    public void String_LowByteFirst_decodes_DL205_packed_Hello()
    {
        // HR[1040] = 0x6548 (wire BE bytes [0x65, 0x48]) decodes first char from low byte = 'H',
        // second from high byte = 'e'. HR[1041] = 0x6C6C → 'l','l'. HR[1042] = 0x006F → 'o', nul.
        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.String,
            StringLength: 5, StringByteOrder: ModbusStringByteOrder.LowByteFirst);
        var wire = new byte[] { 0x65, 0x48, 0x6C, 0x6C, 0x00, 0x6F };
        ModbusDriver.DecodeRegister(wire, tag).ShouldBe("Hello");
    }

    [Fact]
    public void String_LowByteFirst_decode_truncates_at_first_nul()
    {
        // Low-byte-first with only 2 real chars in register 0 (lo='H', hi='i') and the rest nul.
        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.String,
            StringLength: 6, StringByteOrder: ModbusStringByteOrder.LowByteFirst);
        var wire = new byte[] { 0x69, 0x48, 0x00, 0x00, 0x00, 0x00 };
        ModbusDriver.DecodeRegister(wire, tag).ShouldBe("Hi");
    }

    [Fact]
    public void String_LowByteFirst_encode_round_trips_with_decode()
    {
        var tag = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.String,
            StringLength: 5, StringByteOrder: ModbusStringByteOrder.LowByteFirst);
        var wire = ModbusDriver.EncodeRegister("Hello", tag);
        // Expect exactly the DL205-documented byte sequence.
        wire.ShouldBe(new byte[] { 0x65, 0x48, 0x6C, 0x6C, 0x00, 0x6F });
        ModbusDriver.DecodeRegister(wire, tag).ShouldBe("Hello");
    }

    [Fact]
    public void String_HighByteFirst_and_LowByteFirst_differ_on_same_wire()
    {
        // Same wire buffer, different byte order → first char switches 'H' vs 'e'.
        var wire = new byte[] { 0x48, 0x65 };
        var hi = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.String,
            StringLength: 2, StringByteOrder: ModbusStringByteOrder.HighByteFirst);
        var lo = new ModbusTagDefinition("T", ModbusRegion.HoldingRegisters, 0, ModbusDataType.String,
            StringLength: 2, StringByteOrder: ModbusStringByteOrder.LowByteFirst);
        ModbusDriver.DecodeRegister(wire, hi).ShouldBe("He");
        ModbusDriver.DecodeRegister(wire, lo).ShouldBe("eH");
    }
}