feat: serialize variable definitions and \$references in NestWriter

Emit variable definitions before G-code in program text entries and use
\$varName syntax for coordinate fields that have VariableRefs, so programs
round-trip through NestWriter → NestReader without losing variable information.

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

OpenNest.IO/NestWriter.cs

@@ -305,6 +305,15 @@ namespace OpenNest.IO
             var writer = new StreamWriter(stream);
             writer.AutoFlush = true;
 
+            // Emit variable definitions before G-code
+            foreach (var v in program.Variables.Values)
+            {
+                var line = $"{v.Name} = {v.Expression}";
+                if (v.Inline) line += " inline";
+                if (v.Global) line += " global";
+                writer.WriteLine(line);
+            }
+
             writer.WriteLine(program.Mode == Mode.Absolute ? "G90" : "G91");
 
             for (var i = 0; i < drawing.Program.Length; ++i)
@@ -316,6 +325,13 @@ namespace OpenNest.IO
             stream.Position = 0;
         }
 
+        private string FormatCoord(double value, string axis, Dictionary<string, string> variableRefs)
+        {
+            if (variableRefs != null && variableRefs.TryGetValue(axis, out var varName))
+                return $"${varName}";
+            return System.Math.Round(value, OutputPrecision).ToString(CoordinateFormat);
+        }
+
         private string GetCodeString(ICode code)
         {
             switch (code.Type)
@@ -324,16 +340,16 @@ namespace OpenNest.IO
                     {
                         var sb = new StringBuilder();
                         var arcMove = (ArcMove)code;
+                        var refs = arcMove.VariableRefs;
 
-                        var x = System.Math.Round(arcMove.EndPoint.X, OutputPrecision).ToString(CoordinateFormat);
+                        var x = FormatCoord(arcMove.EndPoint.X, "X", refs);
-                        var y = System.Math.Round(arcMove.EndPoint.Y, OutputPrecision).ToString(CoordinateFormat);
+                        var y = FormatCoord(arcMove.EndPoint.Y, "Y", refs);
-                        var i = System.Math.Round(arcMove.CenterPoint.X, OutputPrecision).ToString(CoordinateFormat);
+                        var i = FormatCoord(arcMove.CenterPoint.X, "I", refs);
-                        var j = System.Math.Round(arcMove.CenterPoint.Y, OutputPrecision).ToString(CoordinateFormat);
+                        var j = FormatCoord(arcMove.CenterPoint.Y, "J", refs);
 
-                        if (arcMove.Rotation == RotationType.CW)
+                        sb.Append(arcMove.Rotation == RotationType.CW
-                            sb.Append(string.Format("G02X{0}Y{1}I{2}J{3}", x, y, i, j));
+                            ? $"G02X{x}Y{y}I{i}J{j}"
-                        else
+                            : $"G03X{x}Y{y}I{i}J{j}");
-                            sb.Append(string.Format("G03X{0}Y{1}I{2}J{3}", x, y, i, j));
 
                         if (arcMove.Layer != LayerType.Cut)
                             sb.Append(GetLayerString(arcMove.Layer));
@@ -354,10 +370,9 @@ namespace OpenNest.IO
                     {
                         var sb = new StringBuilder();
                         var linearMove = (LinearMove)code;
+                        var refs = linearMove.VariableRefs;
 
-                        sb.Append(string.Format("G01X{0}Y{1}",
+                        sb.Append($"G01X{FormatCoord(linearMove.EndPoint.X, "X", refs)}Y{FormatCoord(linearMove.EndPoint.Y, "Y", refs)}");
-                            System.Math.Round(linearMove.EndPoint.X, OutputPrecision).ToString(CoordinateFormat),
-                            System.Math.Round(linearMove.EndPoint.Y, OutputPrecision).ToString(CoordinateFormat)));
 
                         if (linearMove.Layer != LayerType.Cut)
                             sb.Append(GetLayerString(linearMove.Layer));
@@ -371,15 +386,16 @@ namespace OpenNest.IO
                 case CodeType.RapidMove:
                     {
                         var rapidMove = (RapidMove)code;
+                        var refs = rapidMove.VariableRefs;
 
-                        return string.Format("G00X{0}Y{1}",
+                        return $"G00X{FormatCoord(rapidMove.EndPoint.X, "X", refs)}Y{FormatCoord(rapidMove.EndPoint.Y, "Y", refs)}";
-                            System.Math.Round(rapidMove.EndPoint.X, OutputPrecision).ToString(CoordinateFormat),
-                            System.Math.Round(rapidMove.EndPoint.Y, OutputPrecision).ToString(CoordinateFormat));
                     }
 
                 case CodeType.SetFeedrate:
                     {
                         var setFeedrate = (Feedrate)code;
+                        if (setFeedrate.VariableRef != null)
+                            return $"F${setFeedrate.VariableRef}";
                         return "F" + setFeedrate.Value;
                     }
 

OpenNest.Tests/IO/NestWriterVariableTests.cs

@@ -0,0 +1,91 @@
+using System.IO;
+using System.Linq;
+using System.Text;
+using OpenNest.CNC;
+using OpenNest.Geometry;
+using OpenNest.IO;
+
+namespace OpenNest.Tests.IO;
+
+public class NestWriterVariableTests
+{
+    [Fact]
+    public void RoundTrip_VariableDefinitions_Preserved()
+    {
+        var nest = CreateNestWithVariableProgram(
+            "width = 48.0 global\ndiameter = 0.3\nG90\nG01X$widthY$diameter");
+
+        var loaded = RoundTrip(nest);
+        var pgm = loaded.Drawings.First().Program;
+
+        Assert.Equal(2, pgm.Variables.Count);
+        Assert.Equal(48.0, pgm.Variables["width"].Value);
+        Assert.True(pgm.Variables["width"].Global);
+        Assert.Equal(0.3, pgm.Variables["diameter"].Value);
+        Assert.False(pgm.Variables["diameter"].Global);
+    }
+
+    [Fact]
+    public void RoundTrip_VariableRefs_Preserved()
+    {
+        var nest = CreateNestWithVariableProgram(
+            "width = 48.0\nG90\nG01X$widthY0");
+
+        var loaded = RoundTrip(nest);
+        var pgm = loaded.Drawings.First().Program;
+
+        var linear = (LinearMove)pgm.Codes[0];
+        Assert.Equal(48.0, linear.EndPoint.X);
+        Assert.NotNull(linear.VariableRefs);
+        Assert.Equal("width", linear.VariableRefs["X"]);
+    }
+
+    [Fact]
+    public void RoundTrip_InlineFlag_Preserved()
+    {
+        var nest = CreateNestWithVariableProgram(
+            "kerf = 0.06 inline\nG90\nG01X1Y0");
+
+        var loaded = RoundTrip(nest);
+        var pgm = loaded.Drawings.First().Program;
+
+        Assert.True(pgm.Variables["kerf"].Inline);
+    }
+
+    [Fact]
+    public void RoundTrip_NoVariables_WorksAsNormal()
+    {
+        var nest = CreateNestWithVariableProgram("G90\nG01X1Y2");
+
+        var loaded = RoundTrip(nest);
+        var pgm = loaded.Drawings.First().Program;
+
+        Assert.Empty(pgm.Variables);
+        var linear = (LinearMove)pgm.Codes[0];
+        Assert.Equal(1.0, linear.EndPoint.X);
+    }
+
+    private static Nest CreateNestWithVariableProgram(string gcode)
+    {
+        var stream = new MemoryStream(Encoding.UTF8.GetBytes(gcode));
+        var reader = new ProgramReader(stream);
+        var program = reader.Read();
+        reader.Close();
+
+        var drawing = new Drawing("TestPart", program);
+        var nest = new Nest { Name = "Test" };
+        nest.Drawings.Add(drawing);
+        var plate = new Plate(new Size(100, 100));
+        plate.Parts.Add(new Part(drawing, new Vector(0, 0)));
+        nest.Plates.Add(plate);
+        return nest;
+    }
+
+    private static Nest RoundTrip(Nest nest)
+    {
+        var ms = new MemoryStream();
+        new NestWriter(nest).Write(ms);
+        ms.Position = 0;
+        return new NestReader(ms).Read();
+    }
+}