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

using System.Runtime.CompilerServices;

namespace SixLabors.Fonts {
  /// <summary>
  /// Provides a mapped view of an underlying slice, selecting arbitrary indices
  /// from the source array.
  /// </summary>
  /// <typeparam name="T">The type of item contained in the underlying array.</typeparam>
  internal readonly struct MappedArraySlice<T>
      where T : struct
  {
      private readonly ArraySlice<T> data;
      private readonly ArraySlice<int> map;

      /// <summary>
      /// Initializes a new instance of the <see cref="MappedArraySlice{T}"/> struct.
      /// </summary>
      /// <param name="data">The data slice.</param>
      /// <param name="map">The map slice.</param>
      public MappedArraySlice(in ArraySlice<T> data, in ArraySlice<int> map)
      {
          Guard.MustBeGreaterThanOrEqualTo(data.Length, map.Length, nameof(map));

          this.data = data;
          this.map = map;
      }

      /// <summary>
      /// Gets the number of items in the map.
      /// </summary>
      public int Length => this.map.Length;

      /// <summary>
      /// Returns a reference to specified element of the slice.
      /// </summary>
      /// <param name="index">The index of the element to return.</param>
      /// <returns>The <typeparamref name="T"/>.</returns>
      /// <exception cref="IndexOutOfRangeException">
      /// Thrown when index less than 0 or index greater than or equal to <see cref="Length"/>.
      /// </exception>
      public readonly ref T this[int index]
      {
          [MethodImpl(MethodImplOptions.AggressiveInlining)]
          get => ref this.data[this.map[index]];
      }
  }
}