FModel/FModel/Extensions/CollectionExtensions.cs

using System.Collections.Generic;
using System.Linq;
using System.Runtime.CompilerServices;

namespace FModel.Extensions;

public static class CollectionExtensions
{
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static T Next<T>(this IList<T> collection, T value)
    {
        var i = collection.IndexOf(value) + 1;
        return i >= collection.Count ? collection.First() : collection[i];
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static T Next<T>(this IList<T> collection, int index)
    {
        var i = index + 1;
        return i >= collection.Count ? collection.First() : collection[i];
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static T Previous<T>(this IList<T> collection, T value)
    {
        var i = collection.IndexOf(value) - 1;
        return i < 0 ? collection.Last() : collection[i];
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static T Previous<T>(this IList<T> collection, int index)
    {
        var i = index - 1;
        return i < 0 ? collection.Last() : collection[i];
    }
}