Various optimizations and cleanups. "in" removed from GpuUInt128 to avoid defensive copies from IL.

PerfectNumbers.Core/PartialFactorResult.cs

@@ -0,0 +1,76 @@
+using System.Buffers;
+
+namespace PerfectNumbers.Core
+{
+    public sealed class PartialFactorResult
+    {
+        [ThreadStatic]
+        private static PartialFactorResult? s_poolHead;
+
+        public FactorEntry[]? Factors;
+        private PartialFactorResult? _next;
+        public ulong Cofactor;
+        public bool FullyFactored;
+        public int Count;
+
+        private PartialFactorResult()
+        {
+        }
+
+		public static PartialFactorResult Rent(in FactorEntry[]? factors, ulong cofactor, bool fullyFactored, int count)
+		{
+			PartialFactorResult? instance = s_poolHead;
+			if (instance is null)
+			{
+				instance = new();
+			}
+			else
+			{
+				s_poolHead = instance._next;
+			}
+
+			instance._next = null;
+			instance.Factors = factors;
+			instance.Cofactor = cofactor;
+			instance.FullyFactored = fullyFactored;
+			instance.Count = count;
+			return instance;
+		}
+
+		public static readonly PartialFactorResult Empty = new() { Cofactor = 1UL, FullyFactored = true };
+
+        public PartialFactorResult WithAdditionalPrime(ulong prime)
+        {
+            FactorEntry[]? source = Factors;
+            if (source is null || Count == 0)
+            {
+                FactorEntry[] local = ArrayPool<FactorEntry>.Shared.Rent(1);
+                local[0] = new FactorEntry(prime, 1);
+                return Rent(local, 1UL, true, 1);
+            }
+
+            FactorEntry[] extended = ArrayPool<FactorEntry>.Shared.Rent(Count + 1);
+            Array.Copy(source, 0, extended, 0, Count);
+            extended[Count] = new FactorEntry(prime, 1);
+            Span<FactorEntry> span = extended.AsSpan(0, Count + 1);
+            span.Sort(static (a, b) => a.Value.CompareTo(b.Value));
+            return Rent(extended, 1UL, true, Count + 1);
+        }
+
+        public void Dispose()
+		{
+			if (this != Empty)
+			{
+				FactorEntry[]? factors = Factors;
+				if (factors is not null)
+				{
+					Factors = null;
+					ArrayPool<FactorEntry>.Shared.Return(factors, clearArray: false);
+				}
+
+				_next = s_poolHead;
+				s_poolHead = this;
+			}
+        }
+    }
+}