// Copyright (c) Six Labors.
// Licensed under the Six Labors Split License.

using System;

namespace SixLabors.Fonts.Unicode {
  /// <summary>
  /// An enumerator for retrieving <see cref="CodePoint"/> instances from a <see cref="ReadOnlySpan{Char}"/>.
  /// Methods are pattern-matched by compiler to allow using foreach pattern.
  /// </summary>
  public ref struct SpanCodePointEnumerator
  {
      private ReadOnlySpan<char> source;

      /// <summary>
      /// Initializes a new instance of the <see cref="SpanCodePointEnumerator"/> struct.
      /// </summary>
      /// <param name="source">The buffer to read from.</param>
      public SpanCodePointEnumerator(ReadOnlySpan<char> source)
      {
          this.source = source;
          this.Current = CodePoint.ReplacementChar;
      }

      /// <summary>
      /// Gets the element in the collection at the current position of the enumerator.
      /// </summary>
      public CodePoint Current { get; private set; }

      /// <summary>
      /// Returns an enumerator that iterates through the collection.
      /// </summary>
      /// <returns>An enumerator that iterates through the collection.</returns>
      public readonly SpanCodePointEnumerator GetEnumerator() => this;

      /// <summary>
      /// Advances the enumerator to the next element of the collection.
      /// </summary>
      /// <returns>
      /// <see langword="true"/> if the enumerator was successfully advanced to the next element;
      /// <see langword="false"/> if the enumerator has passed the end of the collection.
      /// </returns>
      public bool MoveNext()
      {
          this.Current = CodePoint.DecodeFromUtf16At(this.source, 0, out int consumed);
          this.source = this.source.Slice(consumed);
          return consumed > 0;
      }
  }
}