Split for Span

SpanSplitExtensions.cs

using System;
using System.Diagnostics.Contracts;
using System.Runtime.CompilerServices;

public static class SpanSplitExtensions
{
    public ref struct Enumerable1<T> where T : IEquatable<T>
    {
        public Enumerable1(ReadOnlySpan<T> span, T separator)
        {
            Span = span;
            Separator = separator;
        }

        ReadOnlySpan<T> Span { get; }
        T Separator { get; }

        public Enumerator1<T> GetEnumerator() => new Enumerator1<T>(Span, Separator);
    }

    public ref struct Enumerable2<T> where T : IEquatable<T>
    {
        public Enumerable2(ReadOnlySpan<T> span, T separator1, T separator2)
        {
            Span = span;
            Separator1 = separator1;
            Separator2 = separator2;
        }

        ReadOnlySpan<T> Span { get; }
        T Separator1 { get; }
        T Separator2 { get; }

        public Enumerator2<T> GetEnumerator() => new Enumerator2<T>(Span, Separator1, Separator2);
    }

    public ref struct Enumerable3<T> where T : IEquatable<T>
    {
        public Enumerable3(ReadOnlySpan<T> span, T separator1, T separator2, T separator3)
        {
            Span = span;
            Separator1 = separator1;
            Separator2 = separator2;
            Separator3 = separator3;
        }

        ReadOnlySpan<T> Span { get; }
        T Separator1 { get; }
        T Separator2 { get; }
        T Separator3 { get; }

        public Enumerator3<T> GetEnumerator() =>
            new Enumerator3<T>(Span, Separator1, Separator2, Separator3);
    }

    public ref struct EnumerableN<T> where T : IEquatable<T>
    {
        public EnumerableN(ReadOnlySpan<T> span, ReadOnlySpan<T> separators)
        {
            Span = span;
            Separators = separators;
        }

        ReadOnlySpan<T> Span { get; }
        ReadOnlySpan<T> Separators { get; }

        public EnumeratorN<T> GetEnumerator() => new EnumeratorN<T>(Span, Separators);
    }

    public ref struct Enumerator1<T> where T : IEquatable<T>
    {
        public Enumerator1(ReadOnlySpan<T> span, T separator)
        {
            Span = span;
            Separator = separator;
            Current = default;

            if (Span.IsEmpty)
                TrailingEmptyItem = true;
        }

        ReadOnlySpan<T> Span { get; set; }
        T Separator { get; }
        int SeparatorLength => 1;

        ReadOnlySpan<T> TrailingEmptyItemSentinel => Unsafe.As<T[]>(nameof(TrailingEmptyItemSentinel)).AsSpan();

        bool TrailingEmptyItem
        {
            get => Span == TrailingEmptyItemSentinel;
            set => Span = value ? TrailingEmptyItemSentinel : default;
        }

        public bool MoveNext()
        {
            if (TrailingEmptyItem)
            {
                TrailingEmptyItem = false;
                Current = default;
                return true;
            }

            if (Span.IsEmpty)
            {
                Span = Current = default;
                return false;
            }

            int idx = Span.IndexOf(Separator);
            if (idx < 0)
            {
                Current = Span;
                Span = default;
            }
            else
            {
                Current = Span.Slice(0, idx);
                Span = Span.Slice(idx + SeparatorLength);
                if (Span.IsEmpty)
                    TrailingEmptyItem = true;
            }

            return true;
        }

        public ReadOnlySpan<T> Current { get; private set; }
    }

    public ref struct Enumerator2<T> where T : IEquatable<T>
    {
        public Enumerator2(ReadOnlySpan<T> span, T separator1, T separator2)
        {
            Span = span;
            Separator1 = separator1;
            Separator2 = separator2;
            Current = default;

            if (Span.IsEmpty)
                TrailingEmptyItem = true;
        }

        ReadOnlySpan<T> Span { get; set; }
        T Separator1 { get; }
        T Separator2 { get; }
        int SeparatorLength => 1;

        ReadOnlySpan<T> TrailingEmptyItemSentinel => Unsafe.As<T[]>(nameof(TrailingEmptyItemSentinel)).AsSpan();

        bool TrailingEmptyItem
        {
            get => Span == TrailingEmptyItemSentinel;
            set => Span = value ? TrailingEmptyItemSentinel : default;
        }

        public bool MoveNext()
        {
            if (TrailingEmptyItem)
            {
                TrailingEmptyItem = false;
                Current = default;
                return true;
            }

            if (Span.IsEmpty)
            {
                Span = Current = default;
                return false;
            }

            int idx = Span.IndexOfAny(Separator1, Separator2);
            if (idx < 0)
            {
                Current = Span;
                Span = default;
            }
            else
            {
                Current = Span.Slice(0, idx);
                Span = Span.Slice(idx + SeparatorLength);
                if (Span.IsEmpty)
                    TrailingEmptyItem = true;
            }

            return true;
        }

        public ReadOnlySpan<T> Current { get; private set; }
    }

    public ref struct Enumerator3<T> where T : IEquatable<T>
    {
        public Enumerator3(ReadOnlySpan<T> span, T separator1, T separator2, T separator3)
        {
            Span = span;
            Separator1 = separator1;
            Separator2 = separator2;
            Separator3 = separator3;
            Current = default;

            if (Span.IsEmpty)
                TrailingEmptyItem = true;
        }

        ReadOnlySpan<T> Span { get; set; }
        T Separator1 { get; }
        T Separator2 { get; }
        T Separator3 { get; }
        int SeparatorLength => 1;

        ReadOnlySpan<T> TrailingEmptyItemSentinel => Unsafe.As<T[]>(nameof(TrailingEmptyItemSentinel)).AsSpan();

        bool TrailingEmptyItem
        {
            get => Span == TrailingEmptyItemSentinel;
            set => Span = value ? TrailingEmptyItemSentinel : default;
        }

        public bool MoveNext()
        {
            if (TrailingEmptyItem)
            {
                TrailingEmptyItem = false;
                Current = default;
                return true;
            }

            if (Span.IsEmpty)
            {
                Span = Current = default;
                return false;
            }

            int idx = Span.IndexOfAny(Separator1, Separator2, Separator3);
            if (idx < 0)
            {
                Current = Span;
                Span = default;
            }
            else
            {
                Current = Span.Slice(0, idx);
                Span = Span.Slice(idx + SeparatorLength);
                if (Span.IsEmpty)
                    TrailingEmptyItem = true;
            }

            return true;
        }

        public ReadOnlySpan<T> Current { get; private set; }
    }

    public ref struct EnumeratorN<T> where T : IEquatable<T>
    {
        public EnumeratorN(ReadOnlySpan<T> span, ReadOnlySpan<T> separators)
        {
            Span = span;
            Separators = separators;
            Current = default;

            if (Span.IsEmpty)
                TrailingEmptyItem = true;
        }

        ReadOnlySpan<T> Span { get; set; }
        ReadOnlySpan<T> Separators { get; }
        int SeparatorLength => 1;

        ReadOnlySpan<T> TrailingEmptyItemSentinel => Unsafe.As<T[]>(nameof(TrailingEmptyItemSentinel)).AsSpan();

        bool TrailingEmptyItem
        {
            get => Span == TrailingEmptyItemSentinel;
            set => Span = value ? TrailingEmptyItemSentinel : default;
        }

        public bool MoveNext()
        {
            if (TrailingEmptyItem)
            {
                TrailingEmptyItem = false;
                Current = default;
                return true;
            }

            if (Span.IsEmpty)
            {
                Span = Current = default;
                return false;
            }

            int idx = Span.IndexOfAny(Separators);
            if (idx < 0)
            {
                Current = Span;
                Span = default;
            }
            else
            {
                Current = Span.Slice(0, idx);
                Span = Span.Slice(idx + SeparatorLength);
                if (Span.IsEmpty)
                    TrailingEmptyItem = true;
            }

            return true;
        }

        public ReadOnlySpan<T> Current { get; private set; }
    }

    [Pure]
    public static Enumerable1<T> Split<T>(this ReadOnlySpan<T> span, T separator)
        where T : IEquatable<T> => new Enumerable1<T>(span, separator);

    [Pure]
    public static Enumerable2<T> Split<T>(this ReadOnlySpan<T> span, T separator1, T separator2)
        where T : IEquatable<T> => new Enumerable2<T>(span, separator1, separator2);

    [Pure]
    public static Enumerable3<T> Split<T>(this ReadOnlySpan<T> span, T separator1, T separator2, T separator3)
        where T : IEquatable<T> => new Enumerable3<T>(span, separator1, separator2, separator3);

    [Pure]
    public static EnumerableN<T> Split<T>(this ReadOnlySpan<T> span, ReadOnlySpan<T> values)
        where T : IEquatable<T> => new EnumerableN<T>(span, values);
}

SpanSplitExtensionsTests.cs

[TestFixture]
public class SpanSplitExtensionsTests
{
    [TestCase("", 'a')]
    [TestCase("a", 'a')]
    [TestCase("aaaa", 'a')]
    [TestCase("abababab", 'a')]
    [TestCase("babababa", 'a')]
    [TestCase("aaaaab", 'a')]
    [TestCase("baaaaa", 'a')]
    [TestCase("zzzzzazzzzazzzzazzz", 'a')]
    public void Test(string str, char separator)
    {
        List<string> parts = new List<string>();

        foreach (ReadOnlySpan<char> part in str.AsSpan().Split(separator))
            parts.Add(part.ToString());

        CollectionAssert.AreEqual(str.Split(separator), parts);
    }
}

@LordJZ Could you explain to me please, what is the meaning of the property TrailingEmptyItemSentinel.

I think that you can use a classic boolean field - TrailingEmptyItem and ditch the TrailingEmptyItemSentinel completely.

That's absolutely correct. The sole reason for this sentinel is to avoid having an extra boolean field.

That's absolutely correct. The sole reason for this sentinel is to avoid having an extra boolean field.

So it is about size optimization of the Enumerator.

Thank you for your answer @LordJZ.