From c552372f81d0d84d23b71e79577b99f099fc651f Mon Sep 17 00:00:00 2001
From: AJ Isaacs <ajisaacs27@gmail.com>
Date: Thu, 19 Mar 2026 21:10:20 -0400
Subject: [PATCH] fix(core): copy-on-write for shared Program in tiled parts

CloneAtOffset shares the Program instance for tiling performance,
but rotating a part on the plate mutated the shared Program, causing
all parts from the same tile template to rotate together.

Added ownsProgram flag with EnsureOwnedProgram() that clones the
Program before first mutation, preserving tiling performance while
making user rotations independent.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
---
 OpenNest.Core/Part.cs | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/OpenNest.Core/Part.cs b/OpenNest.Core/Part.cs
index e0c3b0d..b47793c 100644
--- a/OpenNest.Core/Part.cs
+++ b/OpenNest.Core/Part.cs
@@ -20,6 +20,7 @@ namespace OpenNest
     public class Part : IPart, IBoundable
     {
         private Vector location;
+        private bool ownsProgram;
 
         public readonly Drawing BaseDrawing;
 
@@ -32,6 +33,7 @@ namespace OpenNest
         {
             BaseDrawing = baseDrawing;
             Program = baseDrawing.Program.Clone() as Program;
+            ownsProgram = true;
             this.location = location;
             UpdateBounds();
         }
@@ -67,6 +69,7 @@ namespace OpenNest
         /// <param name="angle">Angle of rotation in radians.</param>
         public void Rotate(double angle)
         {
+            EnsureOwnedProgram();
             Program.Rotate(angle);
             location = Location.Rotate(angle);
             UpdateBounds();
@@ -79,6 +82,7 @@ namespace OpenNest
         /// <param name="origin">The origin to rotate the part around.</param>
         public void Rotate(double angle, Vector origin)
         {
+            EnsureOwnedProgram();
             Program.Rotate(angle);
             location = Location.Rotate(angle, origin);
             UpdateBounds();
@@ -222,6 +226,15 @@ namespace OpenNest
             return part;
         }
 
+        private void EnsureOwnedProgram()
+        {
+            if (!ownsProgram)
+            {
+                Program = Program.Clone() as Program;
+                ownsProgram = true;
+            }
+        }
+
         private Part(Drawing baseDrawing, Program program, Vector location, Box boundingBox)
         {
             BaseDrawing = baseDrawing;