StitchATon2/Infra/Utils.cs

using System.IO.MemoryMappedFiles;

namespace StitchATon2.Infra;

internal static class Utils
{
    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);
    }

    internal static void DangerousReadSpan<T>(this MemoryMappedViewAccessor 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.PointerOffset + position);
        view.SafeMemoryMappedViewHandle.ReadSpan(byteOffset, span.Slice(offset, n));
    }

    internal static void DangerousWriteSpan<T>(this MemoryMappedViewAccessor view, long position, Span<T> span, int offset, int count)
        where T : unmanaged
    {
        var byteOffset = (ulong)(view.PointerOffset + position);
        view.SafeMemoryMappedViewHandle.WriteSpan<T>(byteOffset, span.Slice(offset, count));
    }
}