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

using System;
using System.IO;

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

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

      /// <summary>
      /// Loads the <see cref="SequenceRuleSetTable"/> 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 SequenceRuleSet table.</param>
      /// <returns>The <see cref="SequenceRuleSetTable"/>.</returns>
      public static SequenceRuleSetTable Load(BigEndianBinaryReader reader, long offset)
      {
          // SequenceRuleSet
          // +----------+------------------------------+----------------------------------------------------------------+
          // | Type     | Name                         | Description                                                    |
          // +==========+==============================+================================================================+
          // | uint16   | seqRuleCount                 | Number of SequenceRule tables                                  |
          // +----------+------------------------------+----------------------------------------------------------------+
          // | Offset16 | seqRuleOffsets[posRuleCount] | Array of offsets to SequenceRule tables, from beginning of the |
          // |          |                              | SequenceRuleSet 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 sequenceRuleTables = new SequenceRuleTable[seqRuleCount];
          for (int i = 0; i < sequenceRuleTables.Length; i++)
          {
              sequenceRuleTables[i] = SequenceRuleTable.Load(reader, offset + seqRuleOffsets[i]);
          }

          return new SequenceRuleSetTable(sequenceRuleTables);
      }
  }
}