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

using System.IO;

namespace SixLabors.Fonts.Tables.AdvancedTypographic {
  /// <summary>
  /// A ClassSequenceRule table describes a context rule using glyph class values.
  /// <see href="https://learn.microsoft.com/en-us/typography/opentype/spec/chapter2#sequence-context-format-2-class-based-glyph-contexts"/>
  /// </summary>
  internal sealed class ClassSequenceRuleTable
  {
      /// <summary>
      /// Initializes a new instance of the <see cref="ClassSequenceRuleTable"/> class.
      /// </summary>
      /// <param name="inputSequence">The array of input sequence classes, beginning with the second glyph position.</param>
      /// <param name="seqLookupRecords">The array of sequence lookup records.</param>
      private ClassSequenceRuleTable(ushort[] inputSequence, SequenceLookupRecord[] seqLookupRecords)
      {
          this.InputSequence = inputSequence;
          this.SequenceLookupRecords = seqLookupRecords;
      }

      /// <summary>
      /// Gets the array of input sequence classes, beginning with the second glyph position.
      /// </summary>
      public ushort[] InputSequence { get; }

      /// <summary>
      /// Gets the array of sequence lookup records specifying actions to be applied.
      /// </summary>
      public SequenceLookupRecord[] SequenceLookupRecords { get; }

      /// <summary>
      /// Loads the <see cref="ClassSequenceRuleTable"/> 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 ClassSequenceRule table.</param>
      /// <returns>The <see cref="ClassSequenceRuleTable"/>.</returns>
      public static ClassSequenceRuleTable Load(BigEndianBinaryReader reader, long offset)
      {
          // ClassSequenceRule
          // +----------------------+----------------------------------+------------------------------------------+
          // | Type                 | Name                             | Description                              |
          // +======================+==================================+==========================================+
          // | uint16               | glyphCount                       | Number of glyphs to be matched           |
          // +----------------------+----------------------------------+------------------------------------------+
          // | uint16               | seqLookupCount                   | Number of SequenceLookupRecords          |
          // +----------------------+----------------------------------+------------------------------------------+
          // | uint16               | inputSequence[glyphCount - 1]    | Sequence of classes to be matched to the |
          // |                      |                                  | input glyph sequence, beginning with the |
          // |                      |                                  | second glyph position                    |
          // +----------------------+----------------------------------+------------------------------------------+
          // | SequenceLookupRecord | seqLookupRecords[seqLookupCount] | Array of SequenceLookupRecords           |
          // +----------------------+----------------------------------+------------------------------------------+
          reader.Seek(offset, SeekOrigin.Begin);
          ushort glyphCount = reader.ReadUInt16();
          ushort seqLookupCount = reader.ReadUInt16();
          ushort[] inputSequence = reader.ReadUInt16Array(glyphCount - 1);
          SequenceLookupRecord[] seqLookupRecords = SequenceLookupRecord.LoadArray(reader, seqLookupCount);

          return new ClassSequenceRuleTable(inputSequence, seqLookupRecords);
      }
  }
}