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

using System;
using System.IO;

namespace SixLabors.Fonts.Tables.AdvancedTypographic {
  /// <summary>
  /// A ChainedClassSequenceRuleSet table contains an array of ChainedClassSequenceRule tables that define
  /// chained context rules for class-based glyph contexts.
  /// <see href="https://learn.microsoft.com/en-us/typography/opentype/spec/chapter2#chained-sequence-context-format-2-class-based-glyph-contexts"/>
  /// </summary>
  internal sealed class ChainedClassSequenceRuleSetTable
  {
      /// <summary>
      /// Initializes a new instance of the <see cref="ChainedClassSequenceRuleSetTable"/> class.
      /// </summary>
      /// <param name="subRules">The array of chained class sequence rule tables.</param>
      private ChainedClassSequenceRuleSetTable(ChainedClassSequenceRuleTable[] subRules) => this.SubRules = subRules;

      /// <summary>
      /// Gets the array of chained class sequence rule tables.
      /// </summary>
      public ChainedClassSequenceRuleTable[] SubRules { get; }

      /// <summary>
      /// Loads the <see cref="ChainedClassSequenceRuleSetTable"/> from the binary reader at the specified offset.
      /// </summary>
      /// <param name="reader">The big endian binary reader.</param>
      /// <param name="offset">Offset from the beginning of the ChainedClassSequenceRuleSet table.</param>
      /// <returns>The <see cref="ChainedClassSequenceRuleSetTable"/>.</returns>
      public static ChainedClassSequenceRuleSetTable Load(BigEndianBinaryReader reader, long offset)
      {
          // ClassSequenceRuleSet
          // +----------+----------------------------------------+---------------------------------------+
          // | Type     | Name                                   | Description                           |
          // +==========+========================================+=======================================+
          // | uint16   | classSeqRuleCount                      | Number of ClassSequenceRule tables    |
          // +----------+----------------------------------------+---------------------------------------+
          // | Offset16 | classSeqRuleOffsets[classSeqRuleCount] | Array of offsets to ClassSequenceRule |
          // |          |                                        | tables, from beginning of             |
          // |          |                                        | ClassSequenceRuleSet table            |
          // +----------+----------------------------------------+---------------------------------------+
          reader.Seek(offset, SeekOrigin.Begin);
          ushort seqRuleCount = reader.ReadUInt16();

          using Buffer<ushort> seqRuleOffsetsBuffer = new(seqRuleCount);
          Span<ushort> seqRuleOffsets = seqRuleOffsetsBuffer.GetSpan();
          reader.ReadUInt16Array(seqRuleOffsets);

          var subRules = new ChainedClassSequenceRuleTable[seqRuleCount];
          for (int i = 0; i < subRules.Length; i++)
          {
              subRules[i] = ChainedClassSequenceRuleTable.Load(reader, offset + seqRuleOffsets[i]);
          }

          return new ChainedClassSequenceRuleSetTable(subRules);
      }
  }
}