feat(engine): select from mixed plate inventory

OpenNest.Engine/Jobs/NestJobCandidateComparer.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace OpenNest;
+
+/// <summary>Ranks independent plate trials: priority fulfillment, sheet area, placement envelope, then input order.</summary>
+public sealed class NestJobCandidateComparer
+{
+    private readonly IReadOnlyList<NestJobPart> parts;
+
+    public NestJobCandidateComparer(IReadOnlyList<NestJobPart> parts)
+    {
+        this.parts = parts ?? throw new ArgumentNullException(nameof(parts));
+    }
+
+    /// <summary>Returns positive when the left trial is preferred.</summary>
+    public int Compare(PlateCandidate left, NestPlateStock leftStock, int leftIndex,
+        PlateCandidate right, NestPlateStock rightStock, int rightIndex)
+    {
+        var priorities = parts.Select(part => part.Priority).Distinct().OrderBy(priority => priority);
+        foreach (var priority in priorities)
+        {
+            var leftCount = Count(left, priority);
+            var rightCount = Count(right, priority);
+            if (leftCount != rightCount) return leftCount.CompareTo(rightCount);
+        }
+
+        var area = Area(rightStock).CompareTo(Area(leftStock));
+        if (area != 0) return area;
+
+        var envelope = Envelope(right).CompareTo(Envelope(left));
+        if (envelope != 0) return envelope;
+
+        return rightIndex.CompareTo(leftIndex);
+    }
+
+    private int Count(PlateCandidate candidate, int priority) => candidate.Placements.Count(placement =>
+        parts.First(part => part.Id == placement.PartId).Priority == priority);
+
+    private static double Area(NestPlateStock stock) => stock.Size.Width * stock.Size.Length;
+
+    private static double Envelope(PlateCandidate candidate)
+    {
+        if (candidate.Placements.Count == 0) return 0;
+        var xs = candidate.Placements.Select(placement => placement.X);
+        var ys = candidate.Placements.Select(placement => placement.Y);
+        return (xs.Max() - xs.Min()) * (ys.Max() - ys.Min());
+    }
+}