ImageSharp/SixLabors.Fonts/Tables/AdvancedTypographic/SkippingGlyphIterator.cs

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

using System;

namespace SixLabors.Fonts.Tables.AdvancedTypographic {
  /// <summary>
  /// An iterator over a glyph shaping collection that respects OpenType lookup flags,
  /// skipping glyphs that should be ignored (marks, base glyphs, ligatures) based on the flags.
  /// </summary>
  internal struct SkippingGlyphIterator
  {
      private readonly FontMetrics fontMetrics;
      private bool ignoreMarks;
      private bool ignoreBaseGlyphs;
      private bool ignoreLigatures;
      private ushort markAttachmentType;
      private bool useMarkFilteringSet;
      private ushort markFilteringSet;

      /// <summary>
      /// Initializes a new instance of the <see cref="SkippingGlyphIterator"/> struct.
      /// </summary>
      /// <param name="fontMetrics">The font metrics for glyph class lookups.</param>
      /// <param name="collection">The glyph shaping collection to iterate over.</param>
      /// <param name="index">The starting index in the collection.</param>
      /// <param name="lookupFlags">The lookup flags that control which glyphs to skip.</param>
      /// <param name="markFilteringSet">The mark filtering set index, used when <see cref="LookupFlags.UseMarkFilteringSet"/> is set.</param>
      public SkippingGlyphIterator(
          FontMetrics fontMetrics,
          IGlyphShapingCollection collection,
          int index,
          LookupFlags lookupFlags,
          ushort markFilteringSet)
      {
          this.fontMetrics = fontMetrics;
          this.Collection = collection;
          this.Index = index;
          this.ignoreMarks = (lookupFlags & LookupFlags.IgnoreMarks) != 0;
          this.ignoreBaseGlyphs = (lookupFlags & LookupFlags.IgnoreBaseGlyphs) != 0;
          this.ignoreLigatures = (lookupFlags & LookupFlags.IgnoreLigatures) != 0;
          this.markAttachmentType = (ushort)((int)(lookupFlags & LookupFlags.MarkAttachmentTypeMask) >> 8);
          this.useMarkFilteringSet = (lookupFlags & LookupFlags.UseMarkFilteringSet) != 0;
          this.markFilteringSet = markFilteringSet;
      }

      /// <summary>
      /// Gets the glyph shaping collection being iterated.
      /// </summary>
      public IGlyphShapingCollection Collection { get; }

      /// <summary>
      /// Gets or sets the current index in the collection.
      /// </summary>
      public int Index { get; set; }

      /// <summary>
      /// Advances to the next non-skipped glyph in the forward direction.
      /// </summary>
      /// <returns>The new index after advancing.</returns>
      public int Next()
      {
          this.Move(1);
          return this.Index;
      }

      /// <summary>
      /// Advances to the next non-skipped glyph in the backward direction.
      /// </summary>
      /// <returns>The new index after moving backward.</returns>
      public int Prev()
      {
          this.Move(-1);
          return this.Index;
      }

      /// <summary>
      /// Moves the iterator by the specified number of non-skipped glyphs. A negative count moves backward.
      /// </summary>
      /// <param name="count">The number of positions to move. Negative values move backward.</param>
      /// <returns>The new index after incrementing.</returns>
      public int Increment(int count = 1)
      {
          int direction = count < 0 ? -1 : 1;
          count = Math.Abs(count);
          while (count-- > 0)
          {
              this.Move(direction);
          }

          return this.Index;
      }

      /// <summary>
      /// Resets the iterator to a new index and lookup flags.
      /// </summary>
      /// <param name="index">The new starting index.</param>
      /// <param name="lookupFlags">The new lookup flags.</param>
      /// <param name="markFilteringSet">The new mark filtering set index.</param>
      public void Reset(int index, LookupFlags lookupFlags, ushort markFilteringSet)
      {
          this.Index = index;
          this.ignoreMarks = (lookupFlags & LookupFlags.IgnoreMarks) != 0;
          this.ignoreBaseGlyphs = (lookupFlags & LookupFlags.IgnoreBaseGlyphs) != 0;
          this.ignoreLigatures = (lookupFlags & LookupFlags.IgnoreLigatures) != 0;
          this.markAttachmentType = (ushort)((int)(lookupFlags & LookupFlags.MarkAttachmentTypeMask) >> 8);
          this.useMarkFilteringSet = (lookupFlags & LookupFlags.UseMarkFilteringSet) != 0;
          this.markFilteringSet = markFilteringSet;
      }

      /// <summary>
      /// Moves the iterator one step in the given direction, skipping glyphs that should be ignored.
      /// </summary>
      /// <param name="direction">The direction to move: 1 for forward, -1 for backward.</param>
      private void Move(int direction)
      {
          this.Index += direction;
          while (this.Index >= 0 && this.Index < this.Collection.Count)
          {
              if (!this.ShouldIgnore(this.Index))
              {
                  break;
              }

              this.Index += direction;
          }
      }

      /// <summary>
      /// Determines whether the glyph at the given index should be ignored based on the current lookup flags.
      /// </summary>
      /// <param name="index">The index of the glyph to check.</param>
      /// <returns><see langword="true"/> if the glyph should be skipped; otherwise, <see langword="false"/>.</returns>
      private readonly bool ShouldIgnore(int index)
      {
          GlyphShapingData data = this.Collection[index];
          GlyphShapingClass shapingClass = AdvancedTypographicUtils.GetGlyphShapingClass(this.fontMetrics, data.GlyphId, data);

          if (this.useMarkFilteringSet && shapingClass.IsMark)
          {
              // Skip marks not in the lookup's MarkFilteringSet.
              // This requires GDEF MarkGlyphSetsDef support.
              if (!AdvancedTypographicUtils.IsInMarkFilteringSet(this.fontMetrics, this.markFilteringSet, data.GlyphId))
              {
                  return true;
              }
          }

          return (this.ignoreMarks && shapingClass.IsMark) ||
              (this.ignoreBaseGlyphs && shapingClass.IsBase) ||
              (this.ignoreLigatures && shapingClass.IsLigature) ||
              (this.markAttachmentType > 0 && shapingClass.IsMark && shapingClass.MarkAttachmentType != this.markAttachmentType);
      }
  }
}