using System;
using System.Buffers.Binary;
using AcDream.Core.Net.Messages;
using Xunit;

namespace AcDream.Core.Net.Tests.Messages;

/// <summary>
/// Covers <see cref="UpdateMotion.TryParse"/> — the 0xF74C GameMessage the
/// server sends when an entity's motion state changes (NPC starts walking,
/// creature enters combat, door opens, etc). The parser shares the inner
/// MovementData decoder with CreateObject but reaches it through a
/// different outer layout, so we need standalone coverage.
/// </summary>
public class UpdateMotionTests
{
    [Fact]
    public void RejectsWrongOpcode()
    {
        var body = new byte[32];
        BinaryPrimitives.WriteUInt32LittleEndian(body, 0xDEADBEEFu);
        Assert.Null(UpdateMotion.TryParse(body));
    }

    [Fact]
    public void RejectsTruncated()
    {
        Assert.Null(UpdateMotion.TryParse(new byte[3]));
        Assert.Null(UpdateMotion.TryParse(Array.Empty<byte>()));
    }

    [Fact]
    public void ParsesStanceOnly_WhenForwardCommandFlagUnset()
    {
        // Layout:
        //   u32 opcode = 0xF74C
        //   u32 guid
        //   u16 instanceSeq
        //   u16 movementSeq + u16 serverControlSeq + u8 isAutonomous + 1 pad (= 6 bytes total header, per ACE Align())
        //   u8 movementType = 0 (Invalid)
        //   u8 motionFlags = 0
        //   u16 currentStyle (outer MovementData field) = 0x0042
        //   u32 packed = CurrentStyle flag (0x1) only
        //   u16 inner currentStyle = 0x0005 (overrides outer per InterpretedMotionState semantics)
        var body = new byte[4 + 4 + 2 + 6 + 4 + 4 + 2];
        int p = 0;
        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(p), 0xF74Cu); p += 4;
        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(p), 0x12345678u); p += 4;
        BinaryPrimitives.WriteUInt16LittleEndian(body.AsSpan(p), 0x0001); p += 2;
        // 8-byte header slot — leave zero
        p += 6;
        body[p++] = 0;     // movementType = Invalid
        body[p++] = 0;     // motionFlags
        BinaryPrimitives.WriteUInt16LittleEndian(body.AsSpan(p), 0x0042); p += 2;
        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(p), 0x1u); p += 4;  // flags = CurrentStyle only
        BinaryPrimitives.WriteUInt16LittleEndian(body.AsSpan(p), 0x0005); p += 2;

        var result = UpdateMotion.TryParse(body);
        Assert.NotNull(result);
        Assert.Equal(0x12345678u, result!.Value.Guid);
        Assert.Equal((ushort)0x0005, result.Value.MotionState.Stance);
        Assert.Null(result.Value.MotionState.ForwardCommand);
    }

    [Fact]
    public void ParsesStanceAndForwardCommand()
    {
        // Flags = CurrentStyle (0x1) | ForwardCommand (0x2)
        var body = new byte[4 + 4 + 2 + 6 + 4 + 4 + 2 + 2];
        int p = 0;
        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(p), 0xF74Cu); p += 4;
        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(p), 0xABCDEF01u); p += 4;
        BinaryPrimitives.WriteUInt16LittleEndian(body.AsSpan(p), 0x0010); p += 2;
        p += 6;   // MovementData header slot
        body[p++] = 0;
        body[p++] = 0;
        BinaryPrimitives.WriteUInt16LittleEndian(body.AsSpan(p), 0x0000); p += 2;  // outer style = 0
        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(p), 0x3u); p += 4;    // CurrentStyle + ForwardCommand
        BinaryPrimitives.WriteUInt16LittleEndian(body.AsSpan(p), 0x000D); p += 2;  // stance = 0xD
        BinaryPrimitives.WriteUInt16LittleEndian(body.AsSpan(p), 0x0007); p += 2;  // forward command = 0x7 (Run)

        var result = UpdateMotion.TryParse(body);
        Assert.NotNull(result);
        Assert.Equal(0xABCDEF01u, result!.Value.Guid);
        Assert.Equal((ushort)0x000D, result.Value.MotionState.Stance);
        Assert.Equal((ushort)0x0007, result.Value.MotionState.ForwardCommand);
    }

    [Fact]
    public void ParsesNoFlagsSet_KeepsOuterStance()
    {
        // When the InterpretedMotionState flags are zero, neither the inner
        // currentStyle nor the forward command are present in the payload,
        // so the parser should fall back to the MovementData outer stance
        // field and leave ForwardCommand null.
        var body = new byte[4 + 4 + 2 + 6 + 4 + 4];
        int p = 0;
        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(p), 0xF74Cu); p += 4;
        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(p), 0x55555555u); p += 4;
        BinaryPrimitives.WriteUInt16LittleEndian(body.AsSpan(p), 0); p += 2;
        p += 6;
        body[p++] = 0;
        body[p++] = 0;
        BinaryPrimitives.WriteUInt16LittleEndian(body.AsSpan(p), 0x00AA); p += 2;
        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(p), 0u); p += 4;  // no flags

        var result = UpdateMotion.TryParse(body);
        Assert.NotNull(result);
        Assert.Equal((ushort)0x00AA, result!.Value.MotionState.Stance);
        Assert.Null(result.Value.MotionState.ForwardCommand);
    }

    [Fact]
    public void HandlesNonInvalidMovementType_GracefullyReturnsOuterStance()
    {
        // movementType != 0 means one of the Move* variants we don't parse.
        // The parser must still return a valid Parsed with the outer stance
        // and a null ForwardCommand rather than failing the whole message.
        var body = new byte[4 + 4 + 2 + 6 + 4];
        int p = 0;
        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(p), 0xF74Cu); p += 4;
        BinaryPrimitives.WriteUInt32LittleEndian(body.AsSpan(p), 0x99999999u); p += 4;
        BinaryPrimitives.WriteUInt16LittleEndian(body.AsSpan(p), 0); p += 2;
        p += 6;
        body[p++] = 1;    // movementType = MoveToObject (non-Invalid)
        body[p++] = 0;
        BinaryPrimitives.WriteUInt16LittleEndian(body.AsSpan(p), 0x00CC); p += 2;

        var result = UpdateMotion.TryParse(body);
        Assert.NotNull(result);
        Assert.Equal((ushort)0x00CC, result!.Value.MotionState.Stance);
        Assert.Null(result.Value.MotionState.ForwardCommand);
    }
}