using System.IO.MemoryMappedFiles;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using StitchATon2.Infra.Buffers;

namespace StitchATon2.Infra;

public static class Utils
{
    [UnsafeAccessor(UnsafeAccessorKind.Field, Name = "_buffer")]
    private static extern ref SafeBuffer GetSafeBuffer(this UnmanagedMemoryAccessor view);

    [UnsafeAccessor(UnsafeAccessorKind.Field, Name = "_offset")]
    private static extern ref long GetOffset(this UnmanagedMemoryAccessor view);

    private static unsafe uint AlignedSizeOf<T>() where T : unmanaged
    {
        uint size = (uint)sizeof(T);
        return size is 1 or 2 ? size : (uint)((size + 3) & (~3));
    }

    public static void DangerousReadSpan<T>(this UnmanagedMemoryAccessor view, long position, Span<T> span, int offset, int count)
        where T : unmanaged
    {
        uint sizeOfT = AlignedSizeOf<T>();
        int n = count;
        long spaceLeft = view.Capacity - position;
        if (spaceLeft < 0)
        {
            n = 0;
        }
        else
        {
            ulong spaceNeeded = (ulong)(sizeOfT * count);
            if ((ulong)spaceLeft < spaceNeeded)
            {
                n = (int)(spaceLeft / sizeOfT);
            }
        }

        var byteOffset = (ulong)(view.GetOffset() + position);
        view.GetSafeBuffer().ReadSpan(byteOffset, span.Slice(offset, n));
    }

    public static ArrayOwner<T> Clone<T>(this ArrayOwner<T> arrayOwner, int length) where T : unmanaged
    {
        var newArrayOwner = MemoryAllocator.AllocateArray<T>(length);
        Array.Copy(arrayOwner.Array, 0, newArrayOwner.Array, 0, length);
        return newArrayOwner;
    }

    public static void CopyTo<T>(this ArrayOwner<T> arrayOwner, ArrayOwner<T> target, int length) where T : unmanaged
    {
        Array.Copy(arrayOwner.Array, 0, target.Array, 0, length);
    }
}