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

using System;
using System.IO;

namespace SixLabors.Fonts.Tables.AdvancedTypographic {
  /// <summary>
  /// A ChainedSequenceRuleSet table contains an array of ChainedSequenceRule tables that define
  /// chained context rules for simple glyph contexts (glyph ID based).
  /// <see href="https://learn.microsoft.com/en-us/typography/opentype/spec/chapter2#chained-sequence-context-format-1-simple-glyph-contexts"/>
  /// </summary>
  internal sealed class ChainedSequenceRuleSetTable
  {
      /// <summary>
      /// Initializes a new instance of the <see cref="ChainedSequenceRuleSetTable"/> class.
      /// </summary>
      /// <param name="subRules">The array of chained sequence rule tables.</param>
      private ChainedSequenceRuleSetTable(ChainedSequenceRuleTable[] subRules) => this.SequenceRuleTables = subRules;

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

      /// <summary>
      /// Loads the <see cref="ChainedSequenceRuleSetTable"/> 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 ChainedSequenceRuleSet table.</param>
      /// <returns>The <see cref="ChainedSequenceRuleSetTable"/>.</returns>
      public static ChainedSequenceRuleSetTable Load(BigEndianBinaryReader reader, long offset)
      {
          // ChainedSequenceRuleSet
          // +----------+--------------------------------------------+-----------------------------------------+
          // | Type     | Name                                       | Description                             |
          // +==========+============================================+=========================================+
          // | uint16   | chainedSeqRuleCount                        | Number of ChainedSequenceRule tables    |
          // +----------+--------------------------------------------+-----------------------------------------+
          // | Offset16 | chainedSeqRuleOffsets[chainedSeqRuleCount] | Array of offsets to ChainedSequenceRule |
          // |          |                                            | tables, from beginning of               |
          // |          |                                            | ChainedSequenceRuleSet table            |
          // +----------+--------------------------------------------+-----------------------------------------+
          reader.Seek(offset, SeekOrigin.Begin);
          ushort chainedSeqRuleCount = reader.ReadUInt16();

          using Buffer<ushort> chainedSeqRuleOffsetsBuffer = new(chainedSeqRuleCount);
          Span<ushort> chainedSeqRuleOffsets = chainedSeqRuleOffsetsBuffer.GetSpan();
          reader.ReadUInt16Array(chainedSeqRuleOffsets);

          var chainedSequenceRules = new ChainedSequenceRuleTable[chainedSeqRuleCount];
          for (int i = 0; i < chainedSequenceRules.Length; i++)
          {
              chainedSequenceRules[i] = ChainedSequenceRuleTable.Load(reader, offset + chainedSeqRuleOffsets[i]);
          }

          return new ChainedSequenceRuleSetTable(chainedSequenceRules);
      }
  }
}