feat: add FillLinear class with geometry-aware copy distance

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

OpenNest.Engine/FillLinear.cs

@@ -0,0 +1,66 @@
+using System.Collections.Generic;
+using OpenNest.Geometry;
+using OpenNest.Math;
+
+namespace OpenNest
+{
+    public class FillLinear
+    {
+        public FillLinear(Box workArea, double partSpacing)
+        {
+            WorkArea = workArea;
+            PartSpacing = partSpacing;
+        }
+
+        public Box WorkArea { get; }
+
+        public double PartSpacing { get; }
+
+        /// <summary>
+        /// Finds the geometry-aware copy distance between two identical parts along an axis.
+        /// Places part B at bounding box offset from part A, then pushes B back toward A
+        /// using directional distance to find the tightest non-overlapping position.
+        /// </summary>
+        private double FindCopyDistance(Part partA, NestDirection direction)
+        {
+            var bbox = partA.BoundingBox;
+            double bboxDim;
+            PushDirection pushDir;
+            Vector copyOffset;
+
+            if (direction == NestDirection.Horizontal)
+            {
+                bboxDim = bbox.Width;
+                pushDir = PushDirection.Left;
+                copyOffset = new Vector(bboxDim, 0);
+            }
+            else
+            {
+                bboxDim = bbox.Height;
+                pushDir = PushDirection.Down;
+                copyOffset = new Vector(0, bboxDim);
+            }
+
+            // Create part B offset by bounding box dimension (guaranteed no overlap).
+            var partB = (Part)partA.Clone();
+            partB.Offset(copyOffset);
+
+            // Get geometry lines for push calculation.
+            var opposite = Helper.OppositeDirection(pushDir);
+
+            var movingLines = PartSpacing > 0
+                ? Helper.GetOffsetPartLines(partB, PartSpacing, pushDir)
+                : Helper.GetPartLines(partB, pushDir);
+
+            var stationaryLines = Helper.GetPartLines(partA, opposite);
+
+            // Find how far B can slide toward A.
+            var slideDistance = Helper.DirectionalDistance(movingLines, stationaryLines, pushDir);
+
+            if (slideDistance >= double.MaxValue || slideDistance < 0)
+                return bboxDim;
+
+            return bboxDim - slideDistance;
+        }
+    }
+}

OpenNest.Engine/OpenNest.Engine.csproj

@@ -42,6 +42,7 @@
     <Compile Include="CirclePacking\FillEndOdd.cs" />
     <Compile Include="CirclePacking\FillEngine.cs" />
     <Compile Include="CirclePacking\Item.cs" />
+    <Compile Include="FillLinear.cs" />
     <Compile Include="NestDirection.cs" />
     <Compile Include="NestEngine.cs" />
     <Compile Include="NestItem.cs" />