ImageSharp/SixLabors.Fonts/Tables/General/VerticalMetricsTable.cs

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

namespace SixLabors.Fonts.Tables.General {
  /// <summary>
  /// Represents the vertical metrics table, which contains the vertical layout metrics
  /// (advance heights and top side bearings) for each glyph.
  /// <see href="https://learn.microsoft.com/en-us/typography/opentype/spec/vmtx"/>
  /// </summary>
  internal sealed class VerticalMetricsTable : Table
  {
      /// <summary>
      /// The table name identifier.
      /// </summary>
      internal const string TableName = "vmtx";

      /// <summary>
      /// The top side bearings array for all glyphs.
      /// </summary>
      private readonly short[] topSideBearings;

      /// <summary>
      /// The advance heights array for glyphs with full metric records.
      /// </summary>
      private readonly ushort[] advancedHeights;

      /// <summary>
      /// Initializes a new instance of the <see cref="VerticalMetricsTable"/> class.
      /// </summary>
      /// <param name="advancedHeights">The advance heights for each glyph.</param>
      /// <param name="topSideBearings">The top side bearings for each glyph.</param>
      public VerticalMetricsTable(ushort[] advancedHeights, short[] topSideBearings)
      {
          this.advancedHeights = advancedHeights;
          this.topSideBearings = topSideBearings;
      }

      /// <summary>
      /// Gets the advance height for the specified glyph. If the glyph index exceeds the
      /// number of metric records, the first record's advance height is returned.
      /// </summary>
      /// <param name="glyphIndex">The glyph index.</param>
      /// <returns>The advance height in font design units.</returns>
      public ushort GetAdvancedHeight(int glyphIndex)
      {
          if (glyphIndex >= this.advancedHeights.Length)
          {
              return this.advancedHeights[0];
          }

          return this.advancedHeights[glyphIndex];
      }

      /// <summary>
      /// Gets the top side bearing for the specified glyph.
      /// </summary>
      /// <param name="glyphIndex">The glyph index.</param>
      /// <returns>The top side bearing in font design units.</returns>
      internal short GetTopSideBearing(int glyphIndex)
      {
          if (glyphIndex >= this.topSideBearings.Length)
          {
              return this.topSideBearings[0];
          }

          return this.topSideBearings[glyphIndex];
      }

      /// <summary>
      /// Loads the <see cref="VerticalMetricsTable"/> from the specified font reader.
      /// </summary>
      /// <param name="reader">The font reader.</param>
      /// <returns>The <see cref="VerticalMetricsTable"/>, or <see langword="null"/> if the table is not present.</returns>
      public static VerticalMetricsTable? Load(FontReader reader)
      {
          // You should load all dependent tables prior to manipulating the reader
          VerticalHeadTable headTable = reader.GetTable<VerticalHeadTable>();
          MaximumProfileTable profileTable = reader.GetTable<MaximumProfileTable>();

          // Move to start of table
          if (!reader.TryGetReaderAtTablePosition(TableName, out BigEndianBinaryReader? binaryReader))
          {
              return null;
          }

          using (binaryReader)
          {
              return Load(binaryReader, headTable.NumberOfVMetrics, profileTable.GlyphCount);
          }
      }

      /// <summary>
      /// Loads the <see cref="VerticalMetricsTable"/> from the specified binary reader.
      /// </summary>
      /// <param name="reader">The big-endian binary reader.</param>
      /// <param name="metricCount">The number of vertical metric records (from 'vhea').</param>
      /// <param name="glyphCount">The total number of glyphs in the font (from 'maxp').</param>
      /// <returns>The <see cref="VerticalMetricsTable"/>.</returns>
      public static VerticalMetricsTable Load(BigEndianBinaryReader reader, int metricCount, int glyphCount)
      {
          // Type           | Name                                          | Description
          // longVerMetric  | vMetrics[numberOfVMetrics]                    | Paired advance height and top side bearing values for each glyph. Records are indexed by glyph ID.
          // int16          | leftSideBearing[numGlyphs - numberOfVMetrics] | Top side bearings for glyph IDs greater than or equal to numberOfVMetrics.
          int bearingCount = glyphCount - metricCount;
          ushort[] advancedHeights = new ushort[metricCount];
          short[] topSideBearings = new short[glyphCount];

          for (int i = 0; i < metricCount; i++)
          {
              // longVerMetric Record:
              // Type   | Name          | Description
              // -------| ------------- | -----------------------------------------------------------
              // uint16 | advanceHeight | The advance height of the glyph.Signed integer in FUnits.
              // int16  | topSideBearing| The top side bearing of the glyph. Signed integer in FUnits
              advancedHeights[i] = reader.ReadUInt16();
              topSideBearings[i] = reader.ReadInt16();
          }

          for (int i = 0; i < bearingCount; i++)
          {
              topSideBearings[metricCount + i] = reader.ReadInt16();
          }

          return new VerticalMetricsTable(advancedHeights, topSideBearings);
      }
  }
}