refactor(engine): thread explicit placement strategy through multi-plate orchestrators

OpenNest.Engine/Jobs/Placement/PlateFillService.cs

@@ -33,8 +33,7 @@ public static class PlateFillService
         CancellationToken token
     )
     {
-        var filler = CreateFiller(strategy, plate);
-        return filler.Fill(item, workArea, progress, token);
+        return RequireFiller(strategy, plate).Fill(item, workArea, progress, token);
     }
 
     public static List<Part> FillGroup(
@@ -46,8 +45,7 @@ public static class PlateFillService
         CancellationToken token
     )
     {
-        var filler = CreateFiller(strategy, plate);
-        return filler.Fill(groupParts, workArea, progress, token);
+        return RequireFiller(strategy, plate).Fill(groupParts, workArea, progress, token);
     }
 
     public static List<Part> PackArea(
@@ -59,33 +57,71 @@ public static class PlateFillService
         CancellationToken token
     )
     {
-        var filler = CreateFiller(strategy, plate);
-        return filler.PackArea(box, items, progress, token);
+        return RequireFiller(strategy, plate).PackArea(box, items, progress, token);
     }
 
-    private static PlateFillerBase CreateFiller(string strategy, Plate plate)
+    public static List<Part> Nest(
+        string strategy,
+        Plate plate,
+        List<NestItem> items,
+        IProgress<NestProgress> progress,
+        CancellationToken token
+    )
     {
-        ArgumentNullException.ThrowIfNull(strategy);
-        ArgumentNullException.ThrowIfNull(plate);
+        return RequireFiller(strategy, plate).Nest(items, progress, token);
+    }
+
+    /// <summary>
+    /// Resolves a caller-supplied strategy name: null or empty means "Default"; otherwise the name
+    /// must match a built-in strategy, matched case-insensitively like the legacy registry's
+    /// ActiveEngineName so tolerant interactive callers keep working. Returns the canonical name;
+    /// unknown names throw <see cref="NotSupportedException"/>.
+    /// </summary>
+    internal static string ResolveStrategy(string strategy, bool allowEmpty = true)
+    {
+        if (string.IsNullOrWhiteSpace(strategy))
+        {
+            if (allowEmpty)
+                return "Default";
+            throw new NotSupportedException(
+                $"Unknown placement strategy: '{strategy}'. Known strategies: {string.Join(", ", BuiltInStrategies)}."
+            );
+        }
 
-        // OrdinalIgnoreCase mirrors the legacy registry's ActiveEngineName matching so the
-        // interactive callers keep their tolerant name handling while moving off global state.
         foreach (var candidate in BuiltInStrategies)
         {
             if (candidate.Equals(strategy, StringComparison.OrdinalIgnoreCase))
-            {
-                return candidate switch
-                {
-                    "Default" => new DefaultPlateFiller(plate),
-                    "Strip" => new StripPlateFiller(plate),
-                    "Vertical Remnant" => new RemnantPlateFiller(plate, RemnantFillPolicy.Vertical),
-                    _ => new RemnantPlateFiller(plate, RemnantFillPolicy.Horizontal),
-                };
-            }
+                return candidate;
         }
 
         throw new NotSupportedException(
             $"Unknown placement strategy: {strategy}. Known strategies: {string.Join(", ", BuiltInStrategies)}."
         );
     }
+
+    /// <summary>
+    /// Builds the filler for an optional strategy (null/empty = Default). Internal so the
+    /// engine-side multi-plate orchestrators share one resolution/rejection contract.
+    /// </summary>
+    internal static PlateFillerBase CreateFiller(string strategy, Plate plate)
+    {
+        ArgumentNullException.ThrowIfNull(plate);
+        return ResolveStrategy(strategy) switch
+        {
+            "Default" => new DefaultPlateFiller(plate),
+            "Strip" => new StripPlateFiller(plate),
+            "Vertical Remnant" => new RemnantPlateFiller(plate, RemnantFillPolicy.Vertical),
+            _ => new RemnantPlateFiller(plate, RemnantFillPolicy.Horizontal),
+        };
+    }
+
+    /// <summary>Public operations require an explicit strategy name (null is an argument error).</summary>
+    private static PlateFillerBase RequireFiller(string strategy, Plate plate)
+    {
+        ArgumentNullException.ThrowIfNull(strategy);
+        // An explicit empty string is an unknown strategy, not the orchestrator's
+        // null-means-Default defaulting; only the orchestrator boundary may default.
+        ResolveStrategy(strategy, allowEmpty: false);
+        return CreateFiller(strategy, plate);
+    }
 }

OpenNest.Engine/MultiPlateNester.cs

@@ -3,6 +3,8 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Threading;
 using OpenNest.Engine.Fill;
+using OpenNest.Engine.Jobs.Placement;
+using OpenNest.Engine.Jobs.Placement.Fillers;
 using OpenNest.Geometry;
 using OpenNest.Math;
 
@@ -26,6 +28,7 @@ namespace OpenNest.Engine
         private readonly IProgress<NestProgress> _progress;
         private readonly CancellationToken _token;
         private readonly MultiPlateNestOptions _options;
+        private readonly string _strategy;
 
         private bool HasPlateOptions => _plateOptions != null && _plateOptions.Count > 0;
 
@@ -45,8 +48,12 @@ namespace OpenNest.Engine
             _platePool = InitializePlatePool(existingPlates);
             _progress = progress;
             _token = token;
+            _strategy = PlateFillService.ResolveStrategy(options.Strategy);
         }
 
+        private PlateFillerBase CreateFiller(Plate plate) =>
+            PlateFillService.CreateFiller(_strategy, plate);
+
         // --- Static Utility Methods ---
 
         public static bool FitsBounds(Box container, Box part)
@@ -226,7 +233,7 @@ namespace OpenNest.Engine
 
         private int FillAndPlace(PlateResult pr, Box zone, NestItem item)
         {
-            var engine = NestEngineRegistry.Create(pr.Plate);
+            var engine = CreateFiller(pr.Plate);
             var clonedItem = CloneItem(item);
             var parts = engine.Fill(clonedItem, zone, _progress, _token);
 
@@ -381,7 +388,7 @@ namespace OpenNest.Engine
                     if (remnants.Count == 0)
                         break;
 
-                    var engine = NestEngineRegistry.Create(pr.Plate);
+                    var engine = CreateFiller(pr.Plate);
 
                     foreach (var remnant in remnants)
                     {
@@ -430,7 +437,7 @@ namespace OpenNest.Engine
                     if (remnants.Count == 0)
                         break;
 
-                    var engine = NestEngineRegistry.Create(plate);
+                    var engine = CreateFiller(plate);
 
                     foreach (var remnant in remnants)
                     {
@@ -643,7 +650,7 @@ namespace OpenNest.Engine
                                 upgradeOption,
                                 remnants =>
                                 {
-                                    var engine = NestEngineRegistry.Create(target.Plate);
+                                    var engine = CreateFiller(target.Plate);
                                     var tempItems = donorParts
                                         .GroupBy(p => p.BaseDrawing)
                                         .Select(g => new NestItem

OpenNest.Engine/MultiPlateResult.cs

@@ -10,6 +10,14 @@ namespace OpenNest.Engine
         public PartSortOrder SortOrder { get; set; } = PartSortOrder.BoundingBoxArea;
         public double MinRemnantSize { get; set; } = 12.0;
         public bool AllowPlateCreation { get; set; } = true;
+
+        /// <summary>
+        /// Explicit placement strategy for every single-plate fill/pack this run performs
+        /// ("Default", "Strip", "Vertical Remnant", "Horizontal Remnant"). Null or empty means
+        /// "Default"; unknown names are rejected up front. This replaces reading the
+        /// process-global engine registry.
+        /// </summary>
+        public string Strategy { get; set; }
     }
 
     public class MultiPlateResult

OpenNest.Engine/PlateOptimizer.cs

@@ -5,6 +5,7 @@ using System.Linq;
 using System.Threading;
 using OpenNest.Engine;
 using OpenNest.Engine.BestFit;
+using OpenNest.Engine.Jobs.Placement;
 using OpenNest.Geometry;
 using OpenNest.Math;
 
@@ -18,7 +19,8 @@ namespace OpenNest.Engine
             double salvageRate,
             Plate templatePlate,
             IProgress<NestProgress> progress = null,
-            CancellationToken token = default
+            CancellationToken token = default,
+            string strategy = null
         )
         {
             if (
@@ -29,6 +31,10 @@ namespace OpenNest.Engine
             )
                 return null;
 
+            // Explicit strategy at the top-level boundary (null/empty = Default, unknown = throw);
+            // the size search never consults the process-global engine registry.
+            var resolvedStrategy = PlateFillService.ResolveStrategy(strategy);
+
             // Find the minimum dimension needed to fit the largest part,
             // skipping items that are too large for every plate option.
             var minPartWidth = 0.0;
@@ -94,7 +100,8 @@ namespace OpenNest.Engine
                     salvageRate,
                     templatePlate,
                     progress,
-                    token
+                    token,
+                    resolvedStrategy
                 );
                 if (result == null)
                     continue;
@@ -149,7 +156,8 @@ namespace OpenNest.Engine
             double salvageRate,
             Plate templatePlate,
             IProgress<NestProgress> progress,
-            CancellationToken token
+            CancellationToken token,
+            string strategy
         )
         {
             // Create a temporary plate with candidate size + settings from template.
@@ -178,8 +186,7 @@ namespace OpenNest.Engine
                 })
                 .ToList();
 
-            var engine = NestEngineRegistry.Create(tempPlate);
-            var parts = engine.Nest(clonedItems, progress, token);
+            var parts = PlateFillService.Nest(strategy, tempPlate, clonedItems, progress, token);
 
             if (parts == null || parts.Count == 0)
                 return null;