ranges.cs

namespace System
{
    public readonly struct Index
    {
        private readonly int _value;

        public int Value => _value < 0 ? ~_value : _value;
        public bool FromEnd => _value < 0;

        public Index(int value, bool fromEnd)
        {
            if (value < 0) throw new ArgumentException("Index must not be negative.", nameof(value));

            _value = fromEnd ? ~value : value;
        }

        public static implicit operator Index(int value)
            => new Index(value, fromEnd: false);
    }

    public readonly struct Range
    {
        public Index Start { get; }
        public Index End { get; }

        private Range(Index start, Index end)
        {
            this.Start = start;
            this.End = end;
        }

        public static Range Create(Index start, Index end) => new Range(start, end);
        public static Range FromStart(Index start) => new Range(start, new Index(0, fromEnd: true));
        public static Range ToEnd(Index end) => new Range(new Index(0, fromEnd: false), end);
        public static Range All() => new Range(new Index(0, fromEnd: false), new Index(0, fromEnd: true));
    }

    static class Extensions
    {
        public static int get_IndexerExtension(this int[] array, Index index) =>
            index.FromEnd ? array[array.Length - index.Value] : array[index.Value];

        public static int get_IndexerExtension(this Span<int> span, Index index) =>
            index.FromEnd ? span[span.Length - index.Value] : span[index.Value];

        public static char get_IndexerExtension(this string s, Index index) =>
            index.FromEnd ? s[s.Length - index.Value] : s[index.Value];

        public static Span<int> get_IndexerExtension(this int[] array, Range range) =>
            array.Slice(range);

        public static Span<int> get_IndexerExtension(this Span<int> span, Range range) =>
            span.Slice(range);

        public static string get_IndexerExtension(this string s, Range range) =>
            s.Substring(range);

        public static Span<T> Slice<T>(this T[] array, Range range)
            => array.AsSpan().Slice(range);

        public static Span<T> Slice<T>(this Span<T> span, Range range)
        {
            var (start, length) = GetStartAndLength(range, span.Length);
            return span.Slice(start, length);
        }

        public static string Substring(this string s, Range range)
        {
            var (start, length) = GetStartAndLength(range, s.Length);
            return s.Substring(start, length);
        }

        private static (int start, int length) GetStartAndLength(Range range, int entityLength)
        {
            var start = range.Start.FromEnd ? entityLength - range.Start.Value : range.Start.Value;
            var end = range.End.FromEnd ? entityLength - range.End.Value : range.End.Value;
            var length = end - start;

            return (start, length);
        }
    }
}