ImageSharp/src/ImageSharp46/Formats/Jpg/Components/MutableSpan.cs

using System.Buffers;
using System.Numerics;
using System.Runtime.CompilerServices;

namespace ImageSharp.Formats
{
    /// <summary>
    /// Like corefxlab Span, but with an AddOffset() method for efficiency.
    /// TODO: When Span will be official, consider replacing this class!
    /// </summary>
    /// <see cref="https://github.com/dotnet/corefxlab/blob/master/src/System.Slices/System/Span.cs"/>
    /// <typeparam name="T"></typeparam>
    internal struct MutableSpan<T>
    {
        public T[] Data;
        public int Offset;

        public int TotalCount => Data.Length - Offset;

        public MutableSpan(int size, int offset = 0)
        {
            Data = new T[size];
            Offset = offset;
        }

        public MutableSpan(T[] data, int offset = 0)
        {
            Data = data;
            Offset = offset;
        }

        public T this[int idx]
        {
            [MethodImpl(MethodImplOptions.AggressiveInlining)] get { return Data[idx + Offset]; }
            [MethodImpl(MethodImplOptions.AggressiveInlining)] set { Data[idx + Offset] = value; }
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public MutableSpan<T> Slice(int offset)
        {
            return new MutableSpan<T>(Data, Offset + offset);
        }

        public static implicit operator MutableSpan<T>(T[] data) => new MutableSpan<T>(data, 0);
        
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public void AddOffset(int offset)
        {
            Offset += offset;
        }
    }

    internal static class MutableSpanExtensions
    {
        public static MutableSpan<T> Slice<T>(this T[] array, int offset) => new MutableSpan<T>(array, offset);

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static void SaveTo(this MutableSpan<float> data, ref Vector4 v)
        {
            v.X = data[0];
            v.Y = data[1];
            v.Z = data[2];
            v.W = data[3];
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static void SaveTo(this MutableSpan<int> data, ref Vector4 v)
        {
            v.X = data[0];
            v.Y = data[1];
            v.Z = data[2];
            v.W = data[3];
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static void LoadFrom(this MutableSpan<float> data, ref Vector4 v)
        {
            data[0] = v.X;
            data[1] = v.Y;
            data[2] = v.Z;
            data[3] = v.W;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static void LoadFrom(this MutableSpan<int> data, ref Vector4 v)
        {
            data[0] = (int)v.X;
            data[1] = (int)v.Y;
            data[2] = (int)v.Z;
            data[3] = (int)v.W;
        }


    }
}