fix(geometry): harden the arc-preserving per-entity offset

GetOffsetPerimeterEntities/GetOffsetPartEntities feed directional-distance
loops (FillLinear, Compactor, RotationSlideStrategy) that handle arcs
natively. Switching them to Clipper line output (plan option B) made
OpenNest.Tests run 48s -> 8m19s, Fill tests ~3x slower, and broke 20
exact-fit tests through tessellation and conservative padding, so they keep
the per-entity offset (option A), hardened:

- Arc, Circle and Line offsets are now side-symmetric. Right on a CCW arc
  shrank instead of growing, Right on a CW circle grew, and Right on a line
  offset to the left and reversed it. Only Left was used on hot paths, so
  this was latent (SimplifierViewer drew both tolerance bands on one side).
- Shape.OffsetEntity closes every gap between consecutive offset pieces:
  convex non-tangent line/arc corners get a round join about the original
  corner, lines across a collapsed fillet are mitered, and any other gap
  (concave arc corner, collapsed entity) is bridged with a line. Before,
  only line-line corners were joined, so a vertex could slip through.
- Zero-area spikes are left in place and documented: they lie inside the
  offset envelope, which is harmless for directional distance.
- OffsetOutward/OffsetInward become internal; PartGeometry is their only
  caller.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

OpenNest.Core/Geometry/Arc.cs

@@ -443,19 +443,23 @@ namespace OpenNest.Geometry
             boundingBox.Width = maxY - minY;
         }
 
+        /// <summary>
+        /// Offsets the arc to the given side of its travel direction. The center lies to
+        /// the left of a CCW arc and to the right of a CW (reversed) one, so the arc grows
+        /// on the other side and shrinks toward its center. Returns null when it shrinks
+        /// to nothing.
+        /// </summary>
         public override Entity OffsetEntity(double distance, OffsetSide side)
         {
-            if (side == OffsetSide.Left && reversed)
+            var grows = (side == OffsetSide.Left) == reversed;
-            {
-                return new Arc(center, radius + distance, startAngle, endAngle, reversed);
-            }
-            else
-            {
-                if (distance >= radius)
-                    return null;
 
-                return new Arc(center, radius - distance, startAngle, endAngle, reversed);
+            if (grows)
-            }
+                return new Arc(center, radius + distance, startAngle, endAngle, reversed);
+
+            if (distance >= radius)
+                return null;
+
+            return new Arc(center, radius - distance, startAngle, endAngle, reversed);
         }
 
         public override Entity OffsetEntity(double distance, Vector pt)

OpenNest.Core/Geometry/Circle.cs

@@ -273,7 +273,10 @@ namespace OpenNest.Geometry
 
         public override Entity OffsetEntity(double distance, OffsetSide side)
         {
-            if (side == OffsetSide.Left && Rotation == RotationType.CCW)
+            // The center lies to the left of a CCW circle and to the right of a CW one.
+            var shrinks = (side == OffsetSide.Left) == (Rotation == RotationType.CCW);
+
+            if (shrinks)
             {
                 return Radius <= distance
                     ? null
@@ -281,7 +284,7 @@ namespace OpenNest.Geometry
             }
             else
             {
-                return new Circle(center, Radius + distance) { Layer = Layer };
+                return new Circle(center, Radius + distance) { Layer = Layer, Rotation = Rotation };
             }
         }
 

OpenNest.Core/Geometry/Line.cs

@@ -398,11 +398,9 @@ namespace OpenNest.Geometry
             var x = System.Math.Cos(angle) * distance;
             var y = System.Math.Sin(angle) * distance;
 
-            var pt = new Vector(x, y);
+            var pt = side == OffsetSide.Left ? new Vector(x, y) : new Vector(-x, -y);
 
-            return side == OffsetSide.Left
+            return new Line(StartPoint + pt, EndPoint + pt);
-                ? new Line(StartPoint + pt, EndPoint + pt)
-                : new Line(EndPoint + pt, StartPoint + pt);
         }
 
         public override Entity OffsetEntity(double distance, Vector pt)

OpenNest.Core/Geometry/Shape.cs

@@ -463,80 +463,60 @@ namespace OpenNest.Geometry
             boundingBox = Entities.Select(geo => geo.BoundingBox).ToList().GetBoundingBox();
         }
 
+        /// <summary>
+        /// Offsets each perimeter entity to the given side and joins the pieces into a
+        /// closed chain: line-line corners get a round join (convex) or a miter (concave),
+        /// other convex corners get a round join, and any remaining gap (a concave corner
+        /// involving an arc, or an entity that collapsed under the offset) is bridged
+        /// with a line. Cutouts are offset the same way.
+        /// <para>
+        /// Where a feature is narrower than twice the distance, the result keeps zero-area
+        /// spikes and inverted loops. They lie inside the true offset envelope, so they are
+        /// harmless to directional-distance queries, which only need a closed boundary
+        /// that never falls inside the envelope. Use <see cref="ClipperBridge"/> when a
+        /// clean region is needed.
+        /// </para>
+        /// </summary>
         public override Entity OffsetEntity(double distance, OffsetSide side)
         {
             var offsetShape = new Shape();
             var definedShape = new ShapeProfile(this);
 
-            Entity firstEntity = null;
+            var pieces = new List<OffsetPiece>();
-            Entity firstOffsetEntity = null;
+            var collapsed = false;
-            Entity lastEntity = null;
-            Entity lastOffsetEntity = null;
 
             foreach (var entity in definedShape.Perimeter.Entities)
             {
                 var offsetEntity = entity.OffsetEntity(distance, side);
 
                 if (offsetEntity == null)
+                {
+                    collapsed = true;
                     continue;
-
-                if (firstEntity == null)
-                {
-                    firstEntity = entity;
-                    firstOffsetEntity = offsetEntity;
                 }
 
-                switch (entity.Type)
+                pieces.Add(new OffsetPiece(entity, offsetEntity, collapsed));
-                {
+                collapsed = false;
-                    case EntityType.Line:
-                    {
-                        var line = (Line)entity;
-                        var offsetLine = (Line)offsetEntity;
-
-                        if (lastOffsetEntity != null && lastOffsetEntity.Type == EntityType.Line)
-                        {
-                            JoinOffsetLines(
-                                (Line)lastEntity,
-                                (Line)lastOffsetEntity,
-                                line,
-                                offsetLine,
-                                distance,
-                                side,
-                                offsetShape
-                            );
-                        }
-
-                        offsetShape.Entities.Add(offsetLine);
-                        break;
-                    }
-
-                    default:
-                        offsetShape.Entities.Add(offsetEntity);
-                        break;
-                }
-
-                lastOffsetEntity = offsetEntity;
-                lastEntity = entity;
             }
 
-            // Close the shape: join last offset entity back to first
+            // Entities that collapsed at the end of the loop sit before the first piece.
-            if (
+            if (collapsed && pieces.Count > 0)
-                lastOffsetEntity != null
+                pieces[0] = pieces[0] with { CollapsedBefore = true };
-                && firstOffsetEntity != null
+
-                && lastOffsetEntity != firstOffsetEntity
+            for (var i = 0; i < pieces.Count; i++)
-                && lastOffsetEntity.Type == EntityType.Line
-                && firstOffsetEntity.Type == EntityType.Line
-            )
             {
-                JoinOffsetLines(
+                offsetShape.Entities.Add(pieces[i].Offset);
-                    (Line)lastEntity,
+
-                    (Line)lastOffsetEntity,
+                if (pieces.Count > 1)
-                    (Line)firstEntity,
+                {
-                    (Line)firstOffsetEntity,
+                    JoinOffsetPieces(
-                    distance,
+                        pieces[i],
-                    side,
+                        pieces[(i + 1) % pieces.Count],
-                    offsetShape
+                        distance,
-                );
+                        side,
+                        offsetShape
+                    );
+                }
             }
 
             foreach (var cutout in definedShape.Cutouts)
@@ -547,6 +527,151 @@ namespace OpenNest.Geometry
             return offsetShape;
         }
 
+        private readonly record struct OffsetPiece(
+            Entity Source,
+            Entity Offset,
+            bool CollapsedBefore
+        );
+
+        private static void JoinOffsetPieces(
+            OffsetPiece last,
+            OffsetPiece next,
+            double distance,
+            OffsetSide side,
+            Shape offsetShape
+        )
+        {
+            // Lines meeting across a collapsed fillet are concave, so a miter trims both at
+            // their intersection. Parallel ones (a round-bottomed slot) fall through to
+            // the bridge below.
+            if (
+                next.CollapsedBefore
+                && last.Offset is Line lastOffsetLine
+                && next.Offset is Line nextOffsetLine
+                && Intersect.IntersectsUnbounded(nextOffsetLine, lastOffsetLine, out var miter)
+            )
+            {
+                lastOffsetLine.EndPoint = miter;
+                nextOffsetLine.StartPoint = miter;
+                return;
+            }
+
+            if (!next.CollapsedBefore && last.Source is Line lastLine && next.Source is Line nextLine)
+            {
+                JoinOffsetLines(
+                    lastLine,
+                    (Line)last.Offset,
+                    nextLine,
+                    (Line)next.Offset,
+                    distance,
+                    side,
+                    offsetShape
+                );
+                return;
+            }
+
+            if (
+                !TryGetEnds(last.Offset, out _, out var gapStart)
+                || !TryGetEnds(next.Offset, out var gapEnd, out _)
+            )
+                return;
+
+            if (gapStart.DistanceTo(gapEnd) <= OpenNest.Math.Tolerance.Epsilon)
+                return;
+
+            if (
+                !next.CollapsedBefore
+                && IsConvexCorner(last.Source, next.Source, side, out var corner)
+            )
+            {
+                offsetShape.Entities.Add(
+                    new Arc(
+                        corner,
+                        distance,
+                        corner.AngleTo(gapStart),
+                        corner.AngleTo(gapEnd),
+                        side == OffsetSide.Left
+                    )
+                );
+                return;
+            }
+
+            // Concave corner or collapsed entity: the neighbors' offsets overlap, so a
+            // straight bridge stays inside the offset envelope and closes the chain.
+            offsetShape.Entities.Add(new Line(gapStart, gapEnd));
+        }
+
+        private static bool IsConvexCorner(
+            Entity last,
+            Entity next,
+            OffsetSide side,
+            out Vector corner
+        )
+        {
+            corner = default;
+
+            if (
+                !TryGetEnds(last, out _, out corner)
+                || !TryGetTangents(last, out _, out var d1)
+                || !TryGetTangents(next, out var d2, out _)
+            )
+                return false;
+
+            var cross = d1.X * d2.Y - d1.Y * d2.X;
+
+            return (side == OffsetSide.Left && cross < -OpenNest.Math.Tolerance.Epsilon)
+                || (side == OffsetSide.Right && cross > OpenNest.Math.Tolerance.Epsilon);
+        }
+
+        private static bool TryGetEnds(Entity entity, out Vector start, out Vector end)
+        {
+            switch (entity)
+            {
+                case Line line:
+                    start = line.StartPoint;
+                    end = line.EndPoint;
+                    return true;
+
+                case Arc arc:
+                    start = arc.StartPoint();
+                    end = arc.EndPoint();
+                    return true;
+
+                default:
+                    start = end = default;
+                    return false;
+            }
+        }
+
+        /// <summary>
+        /// Direction of travel at the start and end of a line or arc.
+        /// </summary>
+        private static bool TryGetTangents(Entity entity, out Vector start, out Vector end)
+        {
+            switch (entity)
+            {
+                case Line line:
+                    start = end = line.EndPoint - line.StartPoint;
+                    return true;
+
+                case Arc arc:
+                    start = ArcTangent(arc, arc.StartAngle);
+                    end = ArcTangent(arc, arc.EndAngle);
+                    return true;
+
+                default:
+                    start = end = default;
+                    return false;
+            }
+        }
+
+        private static Vector ArcTangent(Arc arc, double angle)
+        {
+            var sin = System.Math.Sin(angle);
+            var cos = System.Math.Cos(angle);
+            return arc.IsReversed ? new Vector(sin, -cos) : new Vector(-sin, cos);
+        }
+
         private static void JoinOffsetLines(
             Line lastLine,
             Line lastOffsetLine,
@@ -611,7 +736,7 @@ namespace OpenNest.Geometry
         /// Normalizes to CW winding before offsetting Left (which is outward for CW),
         /// making the method independent of the original contour winding direction.
         /// </summary>
-        public Shape OffsetOutward(double distance)
+        internal Shape OffsetOutward(double distance)
         {
             var poly = ToPolygon();
 
@@ -660,7 +785,7 @@ namespace OpenNest.Geometry
         /// Normalizes to CCW winding before offsetting Left (which is inward for CCW),
         /// making the method independent of the original contour winding direction.
         /// </summary>
-        public Shape OffsetInward(double distance)
+        internal Shape OffsetInward(double distance)
         {
             var poly = ToPolygon();
 

OpenNest.Tests/Geometry/ShapeOffsetTests.cs

@@ -0,0 +1,155 @@
+using System.Collections.Generic;
+using System.Linq;
+using OpenNest.Geometry;
+using OpenNest.Math;
+
+namespace OpenNest.Tests.Geometry;
+
+public class ShapeOffsetTests
+{
+    [Theory]
+    [InlineData(false, OffsetSide.Left, 4)] // CCW: center on the left, shrinks.
+    [InlineData(false, OffsetSide.Right, 6)]
+    [InlineData(true, OffsetSide.Left, 6)] // CW: center on the right, grows.
+    [InlineData(true, OffsetSide.Right, 4)]
+    public void ArcOffset_GrowsAwayFromCenter(bool reversed, OffsetSide side, double radius)
+    {
+        var arc = new Arc(0, 0, 5, 0, Angle.HalfPI, reversed);
+
+        var offset = (Arc)arc.OffsetEntity(1, side);
+
+        Assert.Equal(radius, offset.Radius, 9);
+        Assert.Equal(reversed, offset.IsReversed);
+    }
+
+    [Theory]
+    [InlineData(RotationType.CCW, OffsetSide.Left, 4)]
+    [InlineData(RotationType.CCW, OffsetSide.Right, 6)]
+    [InlineData(RotationType.CW, OffsetSide.Left, 6)]
+    [InlineData(RotationType.CW, OffsetSide.Right, 4)]
+    public void CircleOffset_GrowsAwayFromCenter(RotationType rotation, OffsetSide side, double radius)
+    {
+        var circle = new Circle(0, 0, 5) { Rotation = rotation };
+
+        var offset = (Circle)circle.OffsetEntity(1, side);
+
+        Assert.Equal(radius, offset.Radius, 9);
+        Assert.Equal(rotation, offset.Rotation);
+    }
+
+    [Theory]
+    [InlineData(OffsetSide.Left, 1)]
+    [InlineData(OffsetSide.Right, -1)]
+    public void LineOffset_MovesToSideAndKeepsDirection(OffsetSide side, double y)
+    {
+        var line = new Line(0, 0, 10, 0);
+
+        var offset = (Line)line.OffsetEntity(1, side);
+
+        Assert.True(offset.StartPoint.DistanceTo(new Vector(0, y)) < 1e-9);
+        Assert.True(offset.EndPoint.DistanceTo(new Vector(10, y)) < 1e-9);
+    }
+
+    [Fact]
+    public void OffsetOutward_NonTangentLineArcCorners_GetRoundJoins()
+    {
+        // D shape: right half of an r=5 circle closed by the Y axis. Both corners are
+        // convex and not tangent, so the offset needs a round join at each.
+        var shape = new Shape();
+        shape.Entities.Add(new Arc(0, 0, 5, -Angle.HalfPI, Angle.HalfPI));
+        shape.Entities.Add(new Line(0, 5, 0, -5));
+
+        var offset = shape.OffsetOutward(1);
+
+        AssertClosedChain(offset.Entities);
+        Assert.Equal(2, offset.Entities.OfType<Arc>().Count(a => a.Radius.IsEqualTo(1)));
+        Assert.All(Samples(offset.Entities), p => Assert.True(DistanceTo(shape, p) > 1 - 1e-6));
+    }
+
+    [Fact]
+    public void OffsetOutward_CollapsedFillet_ClosesTheChain()
+    {
+        // 10x4 part with a 0.2-wide slot down from the top, with a round (r=0.1) bottom.
+        // Offsetting outward by 0.25 collapses the slot's end arc.
+        var shape = new Shape();
+        shape.Entities.Add(new Line(0, 0, 10, 0));
+        shape.Entities.Add(new Line(10, 0, 10, 4));
+        shape.Entities.Add(new Line(10, 4, 5.1, 4));
+        shape.Entities.Add(new Line(5.1, 4, 5.1, 2));
+        shape.Entities.Add(new Arc(5, 2, 0.1, 0, System.Math.PI, reversed: true));
+        shape.Entities.Add(new Line(4.9, 2, 4.9, 4));
+        shape.Entities.Add(new Line(4.9, 4, 0, 4));
+        shape.Entities.Add(new Line(0, 4, 0, 0));
+
+        var offset = shape.OffsetOutward(0.25);
+
+        AssertClosedChain(offset.Entities);
+        Assert.All(
+            Samples(offset.Entities),
+            p => Assert.True(DistanceTo(shape, p) > 0.25 - 1e-6 || IsInsideSlot(p))
+        );
+    }
+
+    // The collapsed slot leaves a line bridging its walls' offsets, which lies inside
+    // the offset envelope (closer than the spacing) by design.
+    private static bool IsInsideSlot(Vector p) => p.X > 4.8 && p.X < 5.2 && p.Y > 1.7;
+
+    private static void AssertClosedChain(List<Entity> entities)
+    {
+        for (var i = 0; i < entities.Count; i++)
+        {
+            var end = End(entities[i]);
+            var start = Start(entities[(i + 1) % entities.Count]);
+
+            Assert.True(
+                end.DistanceTo(start) < 1e-6,
+                $"Gap of {end.DistanceTo(start)} after entity {i} ({entities[i].Type})."
+            );
+        }
+    }
+
+    private static IEnumerable<Vector> Samples(List<Entity> entities)
+    {
+        foreach (var entity in entities)
+        {
+            for (var t = 0.0; t <= 1.0; t += 0.1)
+            {
+                yield return entity switch
+                {
+                    Line l => l.StartPoint + (l.EndPoint - l.StartPoint) * t,
+                    Arc a => ArcPoint(a, t),
+                    _ => Start(entity),
+                };
+            }
+        }
+    }
+
+    private static Vector ArcPoint(Arc arc, double t)
+    {
+        var sweep = arc.SweepAngle();
+        var angle = arc.StartAngle + (arc.IsReversed ? -sweep : sweep) * t;
+        return new Vector(
+            arc.Center.X + arc.Radius * System.Math.Cos(angle),
+            arc.Center.Y + arc.Radius * System.Math.Sin(angle)
+        );
+    }
+
+    private static double DistanceTo(Shape shape, Vector p) =>
+        shape.Entities.Min(e => e.ClosestPointTo(p).DistanceTo(p));
+
+    private static Vector Start(Entity e) =>
+        e switch
+        {
+            Line l => l.StartPoint,
+            Arc a => a.StartPoint(),
+            _ => default,
+        };
+
+    private static Vector End(Entity e) =>
+        e switch
+        {
+            Line l => l.EndPoint,
+            Arc a => a.EndPoint(),
+            _ => default,
+        };
+}