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

using System.IO;

namespace SixLabors.Fonts.Tables.AdvancedTypographic {
  /// <summary>
  /// A SequenceRule table describes a context rule using glyph IDs 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 SequenceRuleTable
  {
      /// <summary>
      /// Initializes a new instance of the <see cref="SequenceRuleTable"/> class.
      /// </summary>
      /// <param name="inputSequence">The array of input glyph IDs, starting with the second glyph.</param>
      /// <param name="seqLookupRecords">The array of sequence lookup records.</param>
      private SequenceRuleTable(ushort[] inputSequence, SequenceLookupRecord[] seqLookupRecords)
      {
          this.InputSequence = inputSequence;
          this.SequenceLookupRecords = seqLookupRecords;
      }

      /// <summary>
      /// Gets the array of input glyph IDs, starting with the second glyph.
      /// </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="SequenceRuleTable"/> 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 SequenceRule table.</param>
      /// <returns>The <see cref="SequenceRuleTable"/>.</returns>
      public static SequenceRuleTable Load(BigEndianBinaryReader reader, long offset)
      {
          // +----------------------+----------------------------------+---------------------------------------------------------+
          // | Type                 | Name                             | Description                                             |
          // +======================+==================================+=========================================================+
          // | uint16               | glyphCount                       | Number of glyphs in the input glyph sequence            |
          // +----------------------+----------------------------------+---------------------------------------------------------+
          // | uint16               | seqLookupCount                   | Number of SequenceLookupRecords                         |
          // +----------------------+----------------------------------+---------------------------------------------------------+
          // | uint16               | inputSequence[glyphCount - 1]    | Array of input glyph IDs—starting with the second glyph |
          // +----------------------+----------------------------------+---------------------------------------------------------+
          // | SequenceLookupRecord | seqLookupRecords[seqLookupCount] | Array of Sequence lookup records                        |
          // +----------------------+----------------------------------+---------------------------------------------------------+
          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 SequenceRuleTable(inputSequence, seqLookupRecords);
      }
  }
}