using Shouldly;
using Xunit;

namespace ZB.MOM.WW.OtOpcUa.Driver.S7.Tests;

/// <summary>
///     Unit tests for the S7 wide-type (8-byte numeric) byte-buffer codec: the pure
///     <see cref="S7Driver.DecodeScalarBlock"/> / <see cref="S7Driver.EncodeScalarBlock"/>
///     helpers and <see cref="S7Driver.ScalarByteWidth"/>. These decode/encode an
///     Int64/UInt64/LReal (Float64) scalar from a contiguous big-endian byte block — the
///     network I/O half (<c>Plc.ReadBytesAsync</c>/<c>WriteBytesAsync</c>) has no in-process
///     fake so only the codec is unit-proven (mirrors <see cref="S7ArrayReadTests"/>).
///     String/DateTime decode is a deferred stub here (T3/T4); this file pins the
///     NotSupportedException contract those land against.
/// </summary>
[Trait("Category", "Unit")]
public sealed class S7ScalarBlockTests
{
    // ── Helpers ──────────────────────────────────────────────────────────────────────────

    // Wide scalars are byte-anchored: DB{n}.DBB{offset}, parser yields S7Size.Byte.
    private static S7TagDefinition Tag(S7DataType dt, int stringLength = 254) =>
        new("WideTag", "DB1.DBB0", dt, StringLength: stringLength);

    private static S7ParsedAddress Addr() =>
        new(S7Area.DataBlock, DbNumber: 1, S7Size.Byte, ByteOffset: 0, BitOffset: 0);

    // S7 is big-endian: most-significant byte first.
    private static byte[] BeUInt64(ulong v)
    {
        var b = new byte[8];
        for (var i = 0; i < 8; i++)
            b[i] = (byte)(v >> (56 - i * 8));
        return b;
    }

    // ── ScalarByteWidth ───────────────────────────────────────────────────────────────────

    /// <summary>Verifies the 8-byte numeric widths and the String/DateTime widths.</summary>
    [Theory]
    [InlineData(S7DataType.Int64, 8)]
    [InlineData(S7DataType.UInt64, 8)]
    [InlineData(S7DataType.Float64, 8)]
    [InlineData(S7DataType.DateTime, 8)]
    public void ScalarByteWidth_fixed_width_types(S7DataType dt, int expected)
        => S7Driver.ScalarByteWidth(Tag(dt)).ShouldBe(expected);

    /// <summary>Verifies String width is StringLength + 2 (S7 STRING header: max-len + actual-len).</summary>
    [Fact]
    public void ScalarByteWidth_String_is_length_plus_two()
        => S7Driver.ScalarByteWidth(Tag(S7DataType.String, stringLength: 10)).ShouldBe(12);

    // ── DecodeScalarBlock — Int64 ─────────────────────────────────────────────────────────

    /// <summary>Verifies an Int64 block decodes from big-endian bytes.</summary>
    [Fact]
    public void DecodeScalarBlock_Int64_reads_big_endian()
    {
        var block = BeUInt64(0x0123456789ABCDEFUL);
        // First byte is the most-significant byte (0x01) — proves big-endian, not little-endian.
        block[0].ShouldBe((byte)0x01);

        var result = S7Driver.DecodeScalarBlock(Tag(S7DataType.Int64), Addr(), block);
        result.ShouldBeOfType<long>().ShouldBe(0x0123456789ABCDEFL);
    }

    /// <summary>Verifies a negative Int64 decodes correctly (two's complement, high bit set).</summary>
    [Fact]
    public void DecodeScalarBlock_Int64_negative()
    {
        var block = BeUInt64(unchecked((ulong)-2L)); // 0xFFFF_FFFF_FFFF_FFFE
        var result = S7Driver.DecodeScalarBlock(Tag(S7DataType.Int64), Addr(), block);
        result.ShouldBeOfType<long>().ShouldBe(-2L);
    }

    // ── DecodeScalarBlock — UInt64 ────────────────────────────────────────────────────────

    /// <summary>Verifies a UInt64 block decodes a value larger than long.MaxValue.</summary>
    [Fact]
    public void DecodeScalarBlock_UInt64_reads_value_above_long_max()
    {
        var block = BeUInt64(ulong.MaxValue); // 0xFFFF_FFFF_FFFF_FFFF
        var result = S7Driver.DecodeScalarBlock(Tag(S7DataType.UInt64), Addr(), block);
        result.ShouldBeOfType<ulong>().ShouldBe(ulong.MaxValue);
    }

    // ── DecodeScalarBlock — Float64 (LReal) ───────────────────────────────────────────────

    /// <summary>Verifies a Float64 (LReal) block decodes from IEEE-754 big-endian.</summary>
    [Fact]
    public void DecodeScalarBlock_Float64_reads_ieee754_big_endian()
    {
        var bits = unchecked((ulong)BitConverter.DoubleToInt64Bits(Math.PI));
        var block = BeUInt64(bits);
        var result = S7Driver.DecodeScalarBlock(Tag(S7DataType.Float64), Addr(), block);
        result.ShouldBeOfType<double>().ShouldBe(Math.PI, tolerance: 1e-12);
    }

    // ── EncodeScalarBlock — big-endian byte production ────────────────────────────────────

    /// <summary>Verifies Int64 encodes to big-endian bytes (MSB first).</summary>
    [Fact]
    public void EncodeScalarBlock_Int64_writes_big_endian()
    {
        var bytes = S7Driver.EncodeScalarBlock(Tag(S7DataType.Int64), 0x0123456789ABCDEFL);
        bytes.Length.ShouldBe(8);
        bytes.ShouldBe(BeUInt64(0x0123456789ABCDEFUL));
        bytes[0].ShouldBe((byte)0x01); // MSB first — little-endian regression guard.
    }

    /// <summary>Verifies UInt64 encodes to big-endian bytes.</summary>
    [Fact]
    public void EncodeScalarBlock_UInt64_writes_big_endian()
    {
        var bytes = S7Driver.EncodeScalarBlock(Tag(S7DataType.UInt64), ulong.MaxValue);
        bytes.ShouldBe(BeUInt64(ulong.MaxValue));
    }

    /// <summary>Verifies Float64 encodes to IEEE-754 big-endian bytes.</summary>
    [Fact]
    public void EncodeScalarBlock_Float64_writes_ieee754_big_endian()
    {
        var bytes = S7Driver.EncodeScalarBlock(Tag(S7DataType.Float64), Math.PI);
        bytes.ShouldBe(BeUInt64(unchecked((ulong)BitConverter.DoubleToInt64Bits(Math.PI))));
    }

    // ── Round-trip identity (encode → decode) ─────────────────────────────────────────────

    /// <summary>Verifies Int64 round-trips through encode→decode for positive, negative and edge values.</summary>
    [Theory]
    [InlineData(0L)]
    [InlineData(1L)]
    [InlineData(-1L)]
    [InlineData(long.MaxValue)]
    [InlineData(long.MinValue)]
    [InlineData(0x0123456789ABCDEFL)]
    public void Int64_round_trips(long value)
    {
        var tag = Tag(S7DataType.Int64);
        var decoded = S7Driver.DecodeScalarBlock(tag, Addr(), S7Driver.EncodeScalarBlock(tag, value));
        decoded.ShouldBeOfType<long>().ShouldBe(value);
    }

    /// <summary>Verifies UInt64 round-trips, including a large value above long.MaxValue.</summary>
    [Theory]
    [InlineData(0UL)]
    [InlineData(70_000UL)]
    [InlineData(ulong.MaxValue)]
    [InlineData(0x8000_0000_0000_0001UL)]
    public void UInt64_round_trips(ulong value)
    {
        var tag = Tag(S7DataType.UInt64);
        var decoded = S7Driver.DecodeScalarBlock(tag, Addr(), S7Driver.EncodeScalarBlock(tag, value));
        decoded.ShouldBeOfType<ulong>().ShouldBe(value);
    }

    /// <summary>Verifies Float64 (LReal) round-trips for representative doubles.</summary>
    [Theory]
    [InlineData(0.0)]
    [InlineData(3.141592653589793)]
    [InlineData(-2.5e-300)]
    [InlineData(1.7976931348623157e308)]
    public void Float64_round_trips(double value)
    {
        var tag = Tag(S7DataType.Float64);
        var decoded = S7Driver.DecodeScalarBlock(tag, Addr(), S7Driver.EncodeScalarBlock(tag, value));
        decoded.ShouldBeOfType<double>().ShouldBe(value);
    }

    // ── Deferred-stub contract: String/DateTime throw NotSupportedException ────────────────

    /// <summary>Verifies String/DateTime decode is a deferred stub (NotSupportedException — T3/T4).</summary>
    /// <param name="dt">The not-yet-implemented wide type.</param>
    [Theory]
    [InlineData(S7DataType.String)]
    [InlineData(S7DataType.DateTime)]
    public void DecodeScalarBlock_String_or_DateTime_throws_NotSupported(S7DataType dt)
    {
        var tag = Tag(dt, stringLength: 10);
        var block = new byte[S7Driver.ScalarByteWidth(tag)];
        Should.Throw<NotSupportedException>(() => S7Driver.DecodeScalarBlock(tag, Addr(), block));
    }

    /// <summary>Verifies String/DateTime encode is a deferred stub (NotSupportedException — T3/T4).</summary>
    /// <param name="dt">The not-yet-implemented wide type.</param>
    [Theory]
    [InlineData(S7DataType.String)]
    [InlineData(S7DataType.DateTime)]
    public void EncodeScalarBlock_String_or_DateTime_throws_NotSupported(S7DataType dt)
        => Should.Throw<NotSupportedException>(() => S7Driver.EncodeScalarBlock(Tag(dt), "x"));
}