using System.Buffers.Binary;

sealed class PacketMemoryFixture
{
    public Dictionary<nint,byte[]> Blocks { get; }=[];
    public List<(nint Address,int Count)> Reads { get; }=[];
    public bool ShortReads { get; set; }
    public bool Read(nint address,int count,out byte[] bytes)
    {
        Reads.Add((address,count));
        foreach(var (start,block) in Blocks)
        {
            var offset=(long)address-(long)start;
            if(offset<0 || offset>block.Length || count>block.Length-offset) continue;
            bytes=block.AsSpan((int)offset,ShortReads ? Math.Max(0,count-1) : count).ToArray();
            return true;
        }
        bytes=[]; return false;
    }
    public static PacketMemoryFixture Up(byte[] payload,ulong? length=null)
    {
        var memory=new PacketMemoryFixture();
        var header=new byte[16]; BinaryPrimitives.WriteUInt16LittleEndian(header,838);
        BinaryPrimitives.WriteUInt64LittleEndian(header.AsSpan(8),length ?? (ulong)payload.Length+16);
        memory.Blocks[0x1000]=header; memory.Blocks[0x1020]=payload.ToArray();
        return memory;
    }
    public static PacketMemoryFixture Down(byte[] payload,ulong? length=null,uint target=0x12345678)
    {
        var memory=new PacketMemoryFixture();
        var reference=new byte[8]; BinaryPrimitives.WriteUInt64LittleEndian(reference,0x2000);
        memory.Blocks[0x1010]=reference;
        var descriptor=new byte[32]; BinaryPrimitives.WriteUInt32LittleEndian(descriptor,target);
        BinaryPrimitives.WriteUInt64LittleEndian(descriptor.AsSpan(16),length ?? (ulong)payload.Length+16);
        BinaryPrimitives.WriteUInt64LittleEndian(descriptor.AsSpan(24),0x3000);
        memory.Blocks[0x2020]=descriptor;
        var ipc=new byte[16+payload.Length]; BinaryPrimitives.WriteUInt16LittleEndian(ipc.AsSpan(2),354);
        payload.CopyTo(ipc,16); memory.Blocks[0x3000]=ipc;
        return memory;
    }
}