// 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 `HVAR`.
  /// The HVAR table is used in variable fonts to provide variations for horizontal glyph metrics values.
  /// This can be used to provide variation data for advance widths in the 'hmtx' table.
  /// <see href="https://docs.microsoft.com/en-gb/typography/opentype/spec/hvar"/>
  /// </summary>
  internal class HVarTable : Table
  {
      /// <summary>
      /// The table name identifier for the HVAR table.
      /// </summary>
      internal const string TableName = "HVAR";

      /// <summary>
      /// Initializes a new instance of the <see cref="HVarTable"/> class.
      /// </summary>
      /// <param name="itemVariationStore">The item variation store containing delta data.</param>
      /// <param name="advanceWidthMapping">The optional delta-set index mapping for advance widths.</param>
      /// <param name="lsbMapping">The optional delta-set index mapping for left side bearings.</param>
      /// <param name="rsbMapping">The optional delta-set index mapping for right side bearings.</param>
      public HVarTable(
          ItemVariationStore itemVariationStore,
          DeltaSetIndexMap[]? advanceWidthMapping,
          DeltaSetIndexMap[]? lsbMapping,
          DeltaSetIndexMap[]? rsbMapping)
      {
          this.ItemVariationStore = itemVariationStore;
          this.AdvanceWidthMapping = advanceWidthMapping;
          this.LsbMapping = lsbMapping;
          this.RsbMapping = rsbMapping;
      }

      /// <summary>
      /// Gets the item variation store containing the variation delta data.
      /// </summary>
      public ItemVariationStore ItemVariationStore { get; }

      /// <summary>
      /// Gets the optional delta-set index mapping for advance widths.
      /// </summary>
      public DeltaSetIndexMap[]? AdvanceWidthMapping { get; }

      /// <summary>
      /// Gets the optional delta-set index mapping for left side bearings.
      /// </summary>
      public DeltaSetIndexMap[]? LsbMapping { get; }

      /// <summary>
      /// Gets the optional delta-set index mapping for right side bearings.
      /// </summary>
      public DeltaSetIndexMap[]? RsbMapping { get; }

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

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

      /// <summary>
      /// Loads the HVAR table from the specified binary reader.
      /// </summary>
      /// <param name="reader">The big-endian binary reader positioned at the start of the HVAR table.</param>
      /// <returns>The <see cref="HVarTable"/>.</returns>
      public static HVarTable Load(BigEndianBinaryReader reader)
      {
          // Horizontal metrics variations table
          // +--------------------------+----------------------------------------+-------------------------------------------------------------------------+
          // | 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.           |
          // +--------------------------+----------------------------------------+-------------------------------------------------------------------------+
          // | Offset32                 | itemVariationStoreOffset               | Offset in bytes from the start of this table to the                     |
          // |                          |                                        | item variation store table.                                             |
          // +--------------------------+----------------------------------------+-------------------------------------------------------------------------+
          // | Offset32                 | advanceWidthMappingOffset              | Offset in bytes from the start of this table to the delta-set index     |
          // |                          |                                        | mapping for advance widths (may be NULL).                               |
          // +--------------------------+----------------------------------------+-------------------------------------------------------------------------+
          // | Offset32                 | lsbMappingOffset                       | Offset in bytes from the start of this table to the delta-set index     |
          // |                          |                                        | mapping for left side bearings (may be NULL).                           |
          // +--------------------------+----------------------------------------+-------------------------------------------------------------------------+
          // | Offset32                 | rsbMappingOffset                       | Offset in bytes from the start of this table to the delta-set index     |
          // |                          |                                        | mapping for right side bearings (may be NULL).                          |
          // +--------------------------+----------------------------------------+-------------------------------------------------------------------------+
          ushort major = reader.ReadUInt16();
          ushort minor = reader.ReadUInt16();
          uint itemVariationStoreOffset = reader.ReadOffset32();
          uint advanceWidthMappingOffset = reader.ReadOffset32();
          uint lsbMappingOffset = reader.ReadOffset32();
          uint rsbMappingOffset = reader.ReadOffset32();

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

          ItemVariationStore itemVariationStore = ItemVariationStore.Load(reader, itemVariationStoreOffset);

          DeltaSetIndexMap[]? advanceWidthMapping = DeltaSetIndexMap.Load(reader, advanceWidthMappingOffset);
          DeltaSetIndexMap[]? lsbMapping = DeltaSetIndexMap.Load(reader, lsbMappingOffset);
          DeltaSetIndexMap[]? rsbMapping = DeltaSetIndexMap.Load(reader, rsbMappingOffset);

          return new HVarTable(itemVariationStore, advanceWidthMapping, lsbMapping, rsbMapping);
      }
  }
}