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

using System.IO;

namespace SixLabors.Fonts.Tables.General.Colr {
  /// <summary>
  /// Represents the COLR v1 LayerList, which stores an array of offsets to paint tables.
  /// PaintColrLayers references ranges within this list to compose multi-layer color glyphs.
  /// <see href="https://learn.microsoft.com/en-us/typography/opentype/spec/colr#baseglyphlist-layerlist-and-colrglyphs"/>
  /// </summary>
  internal sealed class LayerList
  {
      /// <summary>
      /// Initializes a new instance of the <see cref="LayerList"/> class.
      /// </summary>
      /// <param name="paintOffsets">The array of paint table offsets relative to the beginning of the COLR table.</param>
      public LayerList(uint[] paintOffsets)
          => this.PaintOffsets = paintOffsets;

      /// <summary>
      /// Gets the array of paint table offsets relative to the beginning of the COLR table.
      /// </summary>
      public uint[] PaintOffsets { get; }

      /// <summary>
      /// Gets the number of paint offsets in the list.
      /// </summary>
      public int Count => this.PaintOffsets.Length;

      /// <summary>
      /// Loads a <see cref="LayerList"/> from the given reader at the specified offset.
      /// </summary>
      /// <param name="reader">The binary reader positioned within the COLR table.</param>
      /// <param name="offset">The offset from the beginning of the COLR table to the LayerList.</param>
      /// <returns>The loaded <see cref="LayerList"/>, or <see langword="null"/> if the offset is zero or the list is empty.</returns>
      public static LayerList? Load(BigEndianBinaryReader reader, uint offset)
      {
          if (offset == 0)
          {
              return null;
          }

          reader.Seek(offset, SeekOrigin.Begin);
          uint count = reader.ReadUInt32();

          if (count == 0)
          {
              return null;
          }

          // Offsets are relative to the table start; convert to COLR-relative.
          uint[] offsets = new uint[count];
          for (int i = 0; i < count; i++)
          {
              offsets[i] = offset + reader.ReadOffset32();
          }

          return new LayerList(offsets);
      }
  }
}