ImageSharp/ImageSharp.Drawing/SplitPathExtensions.cs

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

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

namespace SixLabors.ImageSharp.Drawing {
  /// <summary>
  /// Extensions to <see cref="IPath"/> for splitting paths into dash segments
  /// without performing stroke expansion.
  /// </summary>
  public static class SplitPathExtensions
  {
      // Safety limit: if the estimated number of dash segments exceeds this threshold,
      // return the original path unsplit to avoid runaway segmentation from very short
      // patterns applied to very long paths.
      private const int MaxPatternSegments = 10000;

      /// <summary>
      /// Splits the given path into dash segments based on the provided pattern.
      /// Returns a composite path containing only the "on" segments as open sub-paths.
      /// </summary>
      /// <param name="path">The centerline path to split.</param>
      /// <param name="strokeWidth">The stroke width (pattern elements are multiples of this).</param>
      /// <param name="pattern">The dash pattern. Each element is a multiple of <paramref name="strokeWidth"/>.</param>
      /// <returns>A path containing the "on" dash segments.</returns>
      public static IPath GenerateDashes(this IPath path, float strokeWidth, ReadOnlySpan<float> pattern)
          => path.GenerateDashes(strokeWidth, pattern, startOff: false);

      /// <summary>
      /// Splits the given path into dash segments based on the provided pattern.
      /// Returns a composite path containing only the "on" segments as open sub-paths.
      /// </summary>
      /// <param name="path">The centerline path to split.</param>
      /// <param name="strokeWidth">The stroke width (pattern elements are multiples of this).</param>
      /// <param name="pattern">The dash pattern. Each element is a multiple of <paramref name="strokeWidth"/>.</param>
      /// <param name="startOff">Whether the first item in the pattern is off rather than on.</param>
      /// <returns>A path containing the "on" dash segments.</returns>
      public static IPath GenerateDashes(this IPath path, float strokeWidth, ReadOnlySpan<float> pattern, bool startOff)
      {
          if (pattern.Length < 2)
          {
              return path;
          }

          const float eps = 1e-6f;

          // Compute the absolute pattern length in path units to detect degenerate patterns.
          float patternLength = 0f;
          for (int i = 0; i < pattern.Length; i++)
          {
              patternLength += MathF.Abs(pattern[i]) * strokeWidth;
          }

          // Fallback to the original path when the dash pattern is too small to be meaningful.
          if (patternLength <= eps)
          {
              return path;
          }

          IEnumerable<ISimplePath> simplePaths = path.Flatten();
          List<IPath> segments = [];
          List<PointF> buffer = new(64);

          foreach (ISimplePath p in simplePaths)
          {
              bool online = !startOff;
              int patternPos = 0;
              float targetLength = pattern[patternPos] * strokeWidth;

              ReadOnlySpan<PointF> pts = p.Points.Span;
              if (pts.Length < 2)
              {
                  continue;
              }

              // Number of edges to traverse (closed paths wrap; open paths stop one short).
              int edgeCount = p.IsClosed ? pts.Length : pts.Length - 1;

              // Compute total path length to estimate the number of dash segments.
              // This avoids runaway segmentation when a very short pattern is applied
              // to a very long path.
              float totalLength = 0f;
              for (int j = 0; j < edgeCount; j++)
              {
                  int nextIndex = p.IsClosed ? (j + 1) % pts.Length : j + 1;
                  totalLength += Vector2.Distance(pts[j], pts[nextIndex]);
              }

              if (totalLength > eps)
              {
                  float estimatedSegments = (totalLength / patternLength) * pattern.Length;
                  if (estimatedSegments > MaxPatternSegments)
                  {
                      return path;
                  }
              }

              int ei = 0;
              Vector2 current = pts[0];

              while (ei < edgeCount)
              {
                  int nextIndex = p.IsClosed ? (ei + 1) % pts.Length : ei + 1;
                  Vector2 next = pts[nextIndex];
                  float segLen = Vector2.Distance(current, next);

                  // Skip degenerate zero-length segments.
                  if (segLen <= eps)
                  {
                      current = next;
                      ei++;
                      continue;
                  }

                  // Accumulate into the current dash span when the segment is shorter
                  // than the remaining target length.
                  if (segLen + eps < targetLength)
                  {
                      if (online)
                      {
                          buffer.Add(current);
                      }

                      current = next;
                      ei++;
                      targetLength -= segLen;
                      continue;
                  }

                  // Close out a dash span when the segment length matches the target.
                  if (MathF.Abs(segLen - targetLength) <= eps)
                  {
                      if (online)
                      {
                          buffer.Add(current);
                          buffer.Add(next);
                          FlushBuffer(buffer, segments);
                      }

                      buffer.Clear();
                      online = !online;
                      current = next;
                      ei++;
                      patternPos = (patternPos + 1) % pattern.Length;
                      targetLength = pattern[patternPos] * strokeWidth;
                      continue;
                  }

                  // Split inside this segment to end the current dash span.
                  float t = targetLength / segLen;
                  Vector2 split = current + (t * (next - current));

                  if (online)
                  {
                      buffer.Add(current);
                      buffer.Add(split);
                      FlushBuffer(buffer, segments);
                  }

                  buffer.Clear();
                  online = !online;
                  current = split; // continue along the same geometric segment
                  patternPos = (patternPos + 1) % pattern.Length;
                  targetLength = pattern[patternPos] * strokeWidth;
              }

              // Flush the tail of the last dash span, if any.
              if (buffer.Count > 0)
              {
                  if (online)
                  {
                      buffer.Add(current);
                      FlushBuffer(buffer, segments);
                  }

                  buffer.Clear();
              }
          }

          if (segments.Count == 0)
          {
              return path;
          }

          if (segments.Count == 1)
          {
              return segments[0];
          }

          return new ComplexPolygon(segments);
      }

      private static void FlushBuffer(List<PointF> buffer, List<IPath> segments)
      {
          if (buffer.Count >= 2 && buffer[0] != buffer[^1])
          {
              segments.Add(new Path(new LinearLineSegment([.. buffer])));
          }
      }
  }
}