using System;

namespace gaemstone.Utility;

public static class SpanExtensions
{
	public static T? GetOrNull<T>(this Span<T> span, int index)
		where T : struct => GetOrNull((ReadOnlySpan<T>)span, index);
	public static T? GetOrNull<T>(this ReadOnlySpan<T> span, int index)
		where T : struct => (index >= 0 && index < span.Length) ? span[index] : null;


	public static ReadOnlySpanSplitEnumerator<T> Split<T>(this ReadOnlySpan<T> span, T splitOn)
		where T : IEquatable<T> => new(span, splitOn);

	public ref struct ReadOnlySpanSplitEnumerator<T>
		where T : IEquatable<T>
	{
		private readonly ReadOnlySpan<T> _span;
		private readonly T _splitOn;
		private int _index = -1;
		private int _end = -1;

		public ReadOnlySpanSplitEnumerator(ReadOnlySpan<T> span, T splitOn)
			{ _span = span; _splitOn = splitOn; }

		public ReadOnlySpanSplitEnumerator<T> GetEnumerator() => this;

		public ReadOnlySpan<T> Current
			=> (_end >= 0) && (_end <= _span.Length)
				? _span[_index.._end] : throw new InvalidOperationException();

		public bool MoveNext()
		{
			if (_end == _span.Length) return false;
			_end++; _index = _end;
			while (_end < _span.Length && !_span[_end].Equals(_splitOn)) _end++;
			return true;
		}
	}
}