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

using System.IO;

namespace SixLabors.Fonts.Tables.General.Colr {
  /// <summary>
  /// Represents the BaseGlyphList table in COLR v1, which maps glyph IDs to their root paint table offsets.
  /// <see href="https://learn.microsoft.com/en-us/typography/opentype/spec/colr#baseglyphlist-layerlist-and-டிcolrglyphs"/>
  /// </summary>
  internal sealed class BaseGlyphList
  {
      /// <summary>
      /// Initializes a new instance of the <see cref="BaseGlyphList"/> class.
      /// </summary>
      /// <param name="records">The array of base glyph paint records.</param>
      public BaseGlyphList(BaseGlyphPaintRecord[] records)
          => this.Records = records;

      /// <summary>
      /// Gets the array of base glyph paint records, sorted by glyph ID.
      /// </summary>
      public BaseGlyphPaintRecord[] Records { get; }

      /// <summary>
      /// Gets the number of base glyph paint records.
      /// </summary>
      public int Count => this.Records.Length;

      /// <summary>
      /// Loads a <see cref="BaseGlyphList"/> 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 BaseGlyphList.</param>
      /// <returns>The loaded <see cref="BaseGlyphList"/>, or <see langword="null"/> if the offset is zero or the list is empty.</returns>
      public static BaseGlyphList? 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.
          BaseGlyphPaintRecord[] records = new BaseGlyphPaintRecord[count];
          for (int i = 0; i < count; i++)
          {
              ushort glyphId = reader.ReadUInt16();
              records[i] = new BaseGlyphPaintRecord(glyphId, offset + reader.ReadOffset32());
          }

          // Spec says records are sorted by glyphId; assume font is correct
          return new BaseGlyphList(records);
      }
  }
}