ImageSharp/SixLabors.Fonts/Tables/General/Kern/KerningTable.cs

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

using System.Collections.Generic;
using System.Numerics;

namespace SixLabors.Fonts.Tables.General.Kern {
  /// <summary>
  /// Represents the OpenType 'kern' table, which contains kerning pair adjustments
  /// for positioning glyphs within a font.
  /// <see href="https://learn.microsoft.com/en-us/typography/opentype/spec/kern"/>
  /// </summary>
  internal sealed class KerningTable : Table
  {
      /// <summary>
      /// The table tag name identifying the 'kern' table.
      /// </summary>
      internal const string TableName = "kern";

      /// <summary>
      /// The array of kerning subtables contained in this table.
      /// </summary>
      private readonly KerningSubTable[] kerningSubTable;

      /// <summary>
      /// Initializes a new instance of the <see cref="KerningTable"/> class.
      /// </summary>
      /// <param name="kerningSubTable">The array of kerning subtables.</param>
      public KerningTable(KerningSubTable[] kerningSubTable)
          => this.kerningSubTable = kerningSubTable;

      /// <summary>
      /// Gets the number of kerning subtables.
      /// </summary>
      public int Count => this.kerningSubTable.Length;

      /// <summary>
      /// Loads the <see cref="KerningTable"/> from the specified font reader.
      /// Returns an empty table if the 'kern' table is not present.
      /// </summary>
      /// <param name="fontReader">The font reader to read the table from.</param>
      /// <returns>The loaded <see cref="KerningTable"/>.</returns>
      public static KerningTable Load(FontReader fontReader)
      {
          if (!fontReader.TryGetReaderAtTablePosition(TableName, out BigEndianBinaryReader? binaryReader))
          {
              // this table is optional.
              return new KerningTable([]);
          }

          using (binaryReader)
          {
              // Move to start of table.
              return Load(binaryReader);
          }
      }

      /// <summary>
      /// Loads the <see cref="KerningTable"/> from the specified binary reader.
      /// </summary>
      /// <param name="reader">The binary reader positioned at the start of the kern table data.</param>
      /// <returns>The loaded <see cref="KerningTable"/>.</returns>
      public static KerningTable Load(BigEndianBinaryReader reader)
      {
          // +--------+---------+-------------------------------------------+
          // | Type   | Field   | Description                               |
          // +========+=========+===========================================+
          // | uint16 | version | Table version number(0)                   |
          // +--------+---------+-------------------------------------------+
          // | uint16 | nTables | Number of subtables in the kerning table. |
          // +--------+---------+-------------------------------------------+
          ushort version = reader.ReadUInt16();
          ushort subTableCount = reader.ReadUInt16();

          List<KerningSubTable> tables = new(subTableCount);
          for (int i = 0; i < subTableCount; i++)
          {
              KerningSubTable? t = KerningSubTable.Load(reader); // returns null for unknown/supported table format
              if (t != null)
              {
                  tables.Add(t);
              }
          }

          return new KerningTable([.. tables]);
      }

      /// <summary>
      /// Updates glyph positions by applying kerning adjustments for the specified glyph pair.
      /// </summary>
      /// <param name="fontMetrics">The font metrics used for position calculations.</param>
      /// <param name="collection">The glyph positioning collection to update.</param>
      /// <param name="left">The index of the left glyph in the collection.</param>
      /// <param name="right">The index of the right glyph in the collection.</param>
      public void UpdatePositions(FontMetrics fontMetrics, GlyphPositioningCollection collection, int left, int right)
      {
          if (this.Count == 0 || collection.Count == 0)
          {
              return;
          }

          GlyphShapingData current = collection[left];
          if (current.IsKerned)
          {
              // Already kerned via previous processing.
              return;
          }

          ushort currentId = current.GlyphId;
          ushort nextId = collection[right].GlyphId;

          if (this.TryGetKerningOffset(currentId, nextId, out Vector2 result))
          {
              collection.Advance(fontMetrics, left, currentId, (short)result.X, (short)result.Y);
              current.IsKerned = true;
          }
      }

      /// <summary>
      /// Attempts to get the accumulated kerning offset for the specified pair of glyph indices
      /// by iterating through all kerning subtables.
      /// </summary>
      /// <param name="current">The glyph index of the current (left) glyph.</param>
      /// <param name="next">The glyph index of the next (right) glyph.</param>
      /// <param name="result">When this method returns, contains the accumulated kerning offset vector.</param>
      /// <returns><see langword="true"/> if any kerning was applied; otherwise, <see langword="false"/>.</returns>
      public bool TryGetKerningOffset(ushort current, ushort next, out Vector2 result)
      {
          result = Vector2.Zero;
          if (this.Count == 0 || current == 0 || next == 0)
          {
              return false;
          }

          bool kerned = false;
          foreach (KerningSubTable sub in this.kerningSubTable)
          {
              kerned |= sub.TryApplyOffset(current, next, ref result);
          }

          return kerned;
      }
  }
}