feat(engine): add Compactor for post-fill gravity compaction

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Engine/Compactor.cs

@@ -0,0 +1,93 @@
+using System.Collections.Generic;
+using System.Linq;
+using OpenNest.Geometry;
+
+namespace OpenNest
+{
+    /// <summary>
+    /// Pushes a group of parts left and down to close gaps after placement.
+    /// Uses the same directional-distance logic as PlateView.PushSelected
+    /// but operates on Part objects directly.
+    /// </summary>
+    public static class Compactor
+    {
+        private const double ChordTolerance = 0.001;
+
+        /// <summary>
+        /// Compacts movingParts toward the bottom-left of the plate work area.
+        /// Everything already on the plate (excluding movingParts) is treated
+        /// as stationary obstacles.
+        /// </summary>
+        public static void Compact(List<Part> movingParts, Plate plate)
+        {
+            if (movingParts == null || movingParts.Count == 0)
+                return;
+
+            Push(movingParts, plate, PushDirection.Left);
+            Push(movingParts, plate, PushDirection.Down);
+        }
+
+        private static void Push(List<Part> movingParts, Plate plate, PushDirection direction)
+        {
+            var stationaryParts = plate.Parts
+                .Where(p => !movingParts.Contains(p))
+                .ToList();
+
+            var stationaryBoxes = new Box[stationaryParts.Count];
+
+            for (var i = 0; i < stationaryParts.Count; i++)
+                stationaryBoxes[i] = stationaryParts[i].BoundingBox;
+
+            var stationaryLines = new List<Line>[stationaryParts.Count];
+            var opposite = Helper.OppositeDirection(direction);
+            var halfSpacing = plate.PartSpacing / 2;
+            var isHorizontal = Helper.IsHorizontalDirection(direction);
+            var workArea = plate.WorkArea();
+
+            foreach (var moving in movingParts)
+            {
+                var distance = double.MaxValue;
+                var movingBox = moving.BoundingBox;
+
+                // Plate edge distance.
+                var edgeDist = Helper.EdgeDistance(movingBox, workArea, direction);
+                if (edgeDist > 0 && edgeDist < distance)
+                    distance = edgeDist;
+
+                List<Line> movingLines = null;
+
+                for (var i = 0; i < stationaryBoxes.Length; i++)
+                {
+                    var gap = Helper.DirectionalGap(movingBox, stationaryBoxes[i], direction);
+                    if (gap < 0 || gap >= distance)
+                        continue;
+
+                    var perpOverlap = isHorizontal
+                        ? movingBox.IsHorizontalTo(stationaryBoxes[i], out _)
+                        : movingBox.IsVerticalTo(stationaryBoxes[i], out _);
+
+                    if (!perpOverlap)
+                        continue;
+
+                    movingLines ??= halfSpacing > 0
+                        ? Helper.GetOffsetPartLines(moving, halfSpacing, direction, ChordTolerance)
+                        : Helper.GetPartLines(moving, direction, ChordTolerance);
+
+                    stationaryLines[i] ??= halfSpacing > 0
+                        ? Helper.GetOffsetPartLines(stationaryParts[i], halfSpacing, opposite, ChordTolerance)
+                        : Helper.GetPartLines(stationaryParts[i], opposite, ChordTolerance);
+
+                    var d = Helper.DirectionalDistance(movingLines, stationaryLines[i], direction);
+                    if (d < distance)
+                        distance = d;
+                }
+
+                if (distance < double.MaxValue && distance > 0)
+                {
+                    var offset = Helper.DirectionToOffset(direction, distance);
+                    moving.Offset(offset);
+                }
+            }
+        }
+    }
+}