ImageSharp/PolygonClipper/ActiveEdge.cs

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

using System.Runtime.CompilerServices;

namespace SixLabors.PolygonClipper {
  /// <summary>
  /// Represents an edge that is currently active in the sweep-line.
  /// </summary>
  /// <remarks>
  /// The sweep assumes a Y-axis-positive-down coordinate system. "Bottom" and "Top"
  /// refer to the lower and upper scanline endpoints (larger and smaller Y respectively).
  /// </remarks>
  internal sealed class ActiveEdge
  {
#pragma warning disable SA1401 // Hot sweep state uses fields to avoid accessor overhead.
      /// <summary>
      /// The lower endpoint of the edge in scanline order.
      /// </summary>
      public Vertex Bottom;

      /// <summary>
      /// The upper endpoint of the edge in scanline order.
      /// </summary>
      public Vertex Top;

      /// <summary>
      /// The X coordinate where the edge intersects the current scanline.
      /// </summary>
      public double CurrentX;

      /// <summary>
      /// The delta-X per delta-Y for the edge (its scanline slope).
      /// </summary>
      public double Dx;

      /// <summary>
      /// The winding delta contributed by this edge (+1 or -1).
      /// </summary>
      public int WindDelta;

      /// <summary>
      /// The accumulated winding count for this edge.
      /// </summary>
      public int WindCount;

      /// <summary>
      /// The output record this edge is contributing to, if any.
      /// </summary>
      public OutputRecord? OutputRecord;

      /// <summary>
      /// The previous edge in the Active Edge List (AEL).
      /// </summary>
      public ActiveEdge? PrevInAel;

      /// <summary>
      /// The next edge in the Active Edge List (AEL).
      /// </summary>
      public ActiveEdge? NextInAel;

      /// <summary>
      /// The previous edge in the Sorted Edge List (SEL).
      /// </summary>
      public ActiveEdge? PrevInSel;

      /// <summary>
      /// The next edge in the Sorted Edge List (SEL).
      /// </summary>
      public ActiveEdge? NextInSel;

      /// <summary>
      /// The temporary link used when sorting intersections.
      /// </summary>
      public ActiveEdge? Jump;

      /// <summary>
      /// The current top vertex for this edge's bound.
      /// </summary>
      public SweepVertex? VertexTop;

      /// <summary>
      /// The local minima that spawned this edge.
      /// </summary>
      public LocalMinima LocalMin;

      /// <summary>
      /// Indicates whether this edge is the left bound of its pair.
      /// </summary>
      public bool IsLeftBound;

      /// <summary>
      /// The pending join state for this edge.
      /// </summary>
      public JoinWith JoinWith;
#pragma warning restore SA1401

      /// <summary>
      /// Gets a value indicating whether this edge currently contributes to output.
      /// </summary>
      public bool IsHot => this.OutputRecord != null;

      /// <summary>
      /// Gets a value indicating whether the edge is horizontal within tolerance.
      /// </summary>
      public bool IsHorizontal => this.Top.Y == this.Bottom.Y;

      /// <summary>
      /// Gets a value indicating whether a horizontal edge is heading right.
      /// </summary>
      public bool IsHeadingRightHorizontal => double.IsNegativeInfinity(this.Dx);

      /// <summary>
      /// Gets a value indicating whether a horizontal edge is heading left.
      /// </summary>
      public bool IsHeadingLeftHorizontal => double.IsPositiveInfinity(this.Dx);

      /// <summary>
      /// Gets a value indicating whether the current top vertex is a local maxima.
      /// </summary>
      public bool IsMaxima => this.VertexTop != null && this.VertexTop.IsMaxima;

      /// <summary>
      /// Gets a value indicating whether this edge is the front edge of its output record.
      /// </summary>
      public bool IsFront => this.OutputRecord != null && this == this.OutputRecord.FrontEdge;

      /// <summary>
      /// Gets the next input vertex along the bound in the winding direction.
      /// </summary>
      public SweepVertex NextVertex => this.WindDelta > 0 ? this.VertexTop!.Next! : this.VertexTop!.Prev!;

      /// <summary>
      /// Gets the vertex two steps behind the current top, used for turn tests.
      /// </summary>
      public SweepVertex PrevPrevVertex => this.WindDelta > 0 ? this.VertexTop!.Prev!.Prev! : this.VertexTop!.Next!.Next!;

      /// <summary>
      /// Finds the previous hot edge in the AEL, if any.
      /// </summary>
      [MethodImpl(MethodImplOptions.AggressiveInlining)]
      public ActiveEdge? GetPrevHotEdge()
      {
          ActiveEdge? prev = this.PrevInAel;
          while (prev != null && !prev.IsHot)
          {
              prev = prev.PrevInAel;
          }

          return prev;
      }

      /// <summary>
      /// Calculates the X coordinate where this edge intersects the scanline at <paramref name="currentY" />.
      /// </summary>
      // This method sits on the hottest path in large self-intersection workloads.
      // AggressiveOptimization consistently improves codegen here versus tiered defaults.
      [MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
      public static double TopX(ActiveEdge edge, double currentY)
      {
          if (currentY == edge.Top.Y || edge.Top.X == edge.Bottom.X)
          {
              return edge.Top.X;
          }

          if (currentY == edge.Bottom.Y)
          {
              return edge.Bottom.X;
          }

          return edge.Bottom.X + (edge.Dx * (currentY - edge.Bottom.Y));
      }

      /// <summary>
      /// Recomputes <see cref="Dx" /> from the current endpoints.
      /// </summary>
      [MethodImpl(MethodImplOptions.AggressiveInlining)]
      public void UpdateDx() => this.Dx = GetDx(this.Bottom, this.Top);

      /// <summary>
      /// Computes delta-X per delta-Y, returning infinities for horizontal edges.
      /// </summary>
      private static double GetDx(Vertex pt1, Vertex pt2)
      {
          double dy = pt2.Y - pt1.Y;
          if (dy != 0)
          {
              return (pt2.X - pt1.X) / dy;
          }

          return pt2.X > pt1.X ? double.NegativeInfinity : double.PositiveInfinity;
      }
  }
}