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

using System;
using System.IO;

namespace SixLabors.Fonts.Tables.AdvancedTypographic {
  /// <summary>
  /// A ClassSequenceRuleSet table contains an array of ClassSequenceRule tables that define
  /// context rules for class-based glyph contexts in Sequence Context Format 2.
  /// <see href="https://learn.microsoft.com/en-us/typography/opentype/spec/chapter2#sequence-context-format-2-class-based-glyph-contexts"/>
  /// </summary>
  internal sealed class ClassSequenceRuleSetTable
  {
      /// <summary>
      /// Initializes a new instance of the <see cref="ClassSequenceRuleSetTable"/> class.
      /// </summary>
      /// <param name="sequenceRuleTables">The array of class sequence rule tables.</param>
      private ClassSequenceRuleSetTable(ClassSequenceRuleTable[] sequenceRuleTables)
          => this.SequenceRuleTables = sequenceRuleTables;

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

      /// <summary>
      /// Loads the class sequence rule set table.
      /// </summary>
      /// <param name="reader">The big endian binary reader.</param>
      /// <param name="offset">Offset from beginning of the ClassSequenceRuleSet table.</param>
      /// <returns>A class sequence rule set table.</returns>
      public static ClassSequenceRuleSetTable 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 ClassSequenceRuleTable[seqRuleCount];
          for (int i = 0; i < subRules.Length; i++)
          {
              subRules[i] = ClassSequenceRuleTable.Load(reader, offset + seqRuleOffsets[i]);
          }

          return new ClassSequenceRuleSetTable(subRules);
      }
  }
}