Two binary search implementations using ReadOnlySpan<T>. One returns the index of the item, the other returns a Boolean. License: MIT

SpanBinarySearch.cs

/// <summary>
/// Searches an entire one-dimensional sorted span for a value using the specified comparer.
/// </summary>
/// <typeparam name="T">The type of the items being searched.</typeparam>
/// <param name="span">The sorted span to search.</param>
/// <param name="item">The item to search for.</param>
/// <param name="comparer">The comparer implementation to use when comparing elements.</param>
/// <returns>True if the item is in the input; false otherwise.</returns>
/// <exception cref="ArgumentNullException">Thrown when <paramref name="comparer"/> is null.</exception>
static bool BinarySearch<T>(ReadOnlySpan<T> span, T item, IComparer<T> comparer)
{
  if (comparer == null) throw new ArgumentNullException(nameof(comparer));

  while (span.Length > 0)
  {
    int i = span.Length / 2;
    int comparison = comparer.Compare(item, span[i]);

    if (comparison < 0) span = span.Slice(0, i);
    else if (comparison > 0) span = span.Slice(i + 1);
    else return true;
  }

  return false;
}

/// <summary>
/// Searches an entire one-dimensional sorted span for a value using the specified comparer.
/// </summary>
/// <typeparam name="T">The type of the items being searched.</typeparam>
/// <param name="span">The sorted span to search.</param>
/// <param name="item">The item to search for.</param>
/// <param name="comparer">The comparer implementation to use when comparing elements.</param>
/// <returns>The index of the item if found; -1 otherwise.</returns>
/// <exception cref="ArgumentNullException">Thrown when <paramref name="comparer"/> is null.</exception>
static int BinarySearch<T>(ReadOnlySpan<T> span, T item, IComparer<T> comparer)
{
  if (comparer == null) { throw new ArgumentNullException(nameof(comparer)); }

  int position = 0;
  while (span.Length > 0)
  {
    int i = span.Length / 2;
    int comparison = comparer.Compare(item, span[i]);

    if (comparison < 0) { span = span.Slice(0, i); }
    else if (comparison > 0)
    {
      span = span.Slice(i + 1);
      position += i + 1;
    }
    else { return position + i; }
  }

  return -1;
}