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

using System;

namespace SixLabors.Fonts.Tables.AdvancedTypographic.Variations {
  /// <summary>
  /// Implements reading the Font Variations Table `avar`.
  /// <see href="https://docs.microsoft.com/de-de/typography/opentype/spec/avar"/>
  /// </summary>
  internal class AVarTable : Table
  {
      /// <summary>
      /// The table name identifier for the avar table.
      /// </summary>
      internal const string TableName = "avar";

      /// <summary>
      /// Initializes a new instance of the <see cref="AVarTable"/> class.
      /// </summary>
      /// <param name="axisCount">The number of variation axes.</param>
      /// <param name="segmentMaps">The segment maps array, one per axis.</param>
      public AVarTable(uint axisCount, SegmentMapRecord[] segmentMaps)
      {
          this.AxisCount = axisCount;
          this.SegmentMaps = segmentMaps;
      }

      /// <summary>
      /// Gets the number of variation axes for the font.
      /// </summary>
      public uint AxisCount { get; }

      /// <summary>
      /// Gets the segment maps array, one segment map for each axis, in the order of axes specified in the fvar table.
      /// </summary>
      public SegmentMapRecord[] SegmentMaps { get; }

      /// <summary>
      /// Loads the avar table from the specified font reader.
      /// </summary>
      /// <param name="reader">The font reader.</param>
      /// <returns>The <see cref="AVarTable"/>, or <see langword="null"/> if the table is not present.</returns>
      public static AVarTable? Load(FontReader reader)
      {
          if (!reader.TryGetReaderAtTablePosition(TableName, out BigEndianBinaryReader? binaryReader))
          {
              return null;
          }

          using (binaryReader)
          {
              return Load(binaryReader);
          }
      }

      /// <summary>
      /// Loads the avar table from the specified binary reader.
      /// </summary>
      /// <param name="reader">The big-endian binary reader positioned at the start of the avar table.</param>
      /// <returns>The <see cref="AVarTable"/>.</returns>
      public static AVarTable Load(BigEndianBinaryReader reader)
      {
          // VariationsTable `avar`
          // +-----------------+----------------------------------------+-------------------------------------------------------------------------+
          // | Type            | Name                                   | Description                                                             |
          // +=================+========================================+=========================================================================+
          // | uint16          | majorVersion                           | Major version number of the font variations table — set to 1.           |
          // +-----------------+----------------------------------------+-------------------------------------------------------------------------+
          // | uint16          | minorVersion                           | Minor version number of the font variations table — set to 0.           |
          // +-----------------+----------------------------------------+-------------------------------------------------------------------------+
          // | uint16          | (reserved)                             | This field is permanently reserved. Set to zero.                        |
          // +-----------------+----------------------------------------+-------------------------------------------------------------------------+
          // | uint16          | axisCount                              | The number of variation axes in the font                                |
          // |                 |                                        | (the number of records in the axes array).                              |
          // +-----------------+----------------------------------------+-------------------------------------------------------------------------+
          // | SegmentMaps     | axisSegmentMaps[axisCount]             | The segment maps array — one segment map for each axis, in the order of |
          // |                 |                                        | axes specified in the 'fvar' table.                                     |
          // +-----------------+----------------------------------------+-------------------------------------------------------------------------+
          ushort major = reader.ReadUInt16();
          ushort minor = reader.ReadUInt16();
          ushort reserved = reader.ReadUInt16();
          ushort axisCount = reader.ReadUInt16();

          if (major != 1)
          {
              throw new NotSupportedException("Only version 1 of avar table is supported");
          }

          var segmentMaps = new SegmentMapRecord[axisCount];
          for (int i = 0; i < axisCount; i++)
          {
              segmentMaps[i] = SegmentMapRecord.Load(reader);
          }

          return new AVarTable(axisCount, segmentMaps);
      }
  }
}