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

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

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

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

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

      /// <summary>
      /// Initializes a new instance of the <see cref="HorizontalMetricsTable"/> class.
      /// </summary>
      /// <param name="advancedWidths">The advance widths for each glyph.</param>
      /// <param name="leftSideBearings">The left side bearings for each glyph.</param>
      public HorizontalMetricsTable(ushort[] advancedWidths, short[] leftSideBearings)
      {
          this.advancedWidths = advancedWidths;
          this.leftSideBearings = leftSideBearings;
      }

      /// <summary>
      /// Gets the advance width for the specified glyph. If the glyph index exceeds the
      /// number of metric records, the last record's advance width is returned.
      /// </summary>
      /// <param name="glyphIndex">The glyph index.</param>
      /// <returns>The advance width in font design units.</returns>
      public ushort GetAdvancedWidth(int glyphIndex)
      {
          if (glyphIndex >= this.advancedWidths.Length)
          {
              // Records are indexed by glyph ID. As an optimization, the number of records can
              // be less than the number of glyphs, in which case the advance width value of the
              // last record applies to all remaining glyph IDs.
              return this.advancedWidths[^1];
          }

          return this.advancedWidths[glyphIndex];
      }

      /// <summary>
      /// Gets the left side bearing for the specified glyph.
      /// </summary>
      /// <param name="glyphIndex">The glyph index.</param>
      /// <returns>The left side bearing in font design units.</returns>
      internal short GetLeftSideBearing(int glyphIndex)
      {
          if (glyphIndex >= this.leftSideBearings.Length)
          {
              return this.leftSideBearings[^1];
          }

          return this.leftSideBearings[glyphIndex];
      }

      /// <summary>
      /// Loads the <see cref="HorizontalMetricsTable"/> from the specified font reader.
      /// </summary>
      /// <param name="reader">The font reader.</param>
      /// <returns>The <see cref="HorizontalMetricsTable"/>.</returns>
      public static HorizontalMetricsTable Load(FontReader reader)
      {
          // you should load all dependent tables prior to manipulating the reader
          HorizontalHeadTable headTable = reader.GetTable<HorizontalHeadTable>();
          MaximumProfileTable profileTable = reader.GetTable<MaximumProfileTable>();

          // Move to start of table
          using BigEndianBinaryReader binaryReader = reader.GetReaderAtTablePosition(TableName);
          return Load(binaryReader, headTable.NumberOfHMetrics, profileTable.GlyphCount);
      }

      /// <summary>
      /// Loads the <see cref="HorizontalMetricsTable"/> from the specified binary reader.
      /// </summary>
      /// <param name="reader">The big-endian binary reader.</param>
      /// <param name="metricCount">The number of horizontal metric records (from 'hhea').</param>
      /// <param name="glyphCount">The total number of glyphs in the font (from 'maxp').</param>
      /// <returns>The <see cref="HorizontalMetricsTable"/>.</returns>
      public static HorizontalMetricsTable Load(BigEndianBinaryReader reader, int metricCount, int glyphCount)
      {
          // Type           | Name                                          | Description
          // longHorMetric  | hMetrics[numberOfHMetrics]                    | Paired advance width and left side bearing values for each glyph. Records are indexed by glyph ID.
          // int16          | leftSideBearing[numGlyphs - numberOfHMetrics] | Left side bearings for glyph IDs greater than or equal to numberOfHMetrics.
          int bearingCount = glyphCount - metricCount;
          ushort[] advancedWidth = new ushort[metricCount];
          short[] leftSideBearings = new short[glyphCount];

          for (int i = 0; i < metricCount; i++)
          {
              // longHorMetric Record:
              // Type   | Name         | Description
              // uint16 | advanceWidth | Glyph advance width, in font design units.
              // int16  | lsb          | Glyph left side bearing, in font design units.
              advancedWidth[i] = reader.ReadUInt16();
              leftSideBearings[i] = reader.ReadInt16();
          }

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

          return new HorizontalMetricsTable(advancedWidth, leftSideBearings);
      }
  }
}