[Except] Implement Visitor to improve performance.

Author: reegeek

Documents/BenchmarksResults/Except.md

@@ -14,10 +14,10 @@ Intel Core i7-7700 CPU 3.60GHz (Kaby Lake), 1 CPU, 8 logical and 4 physical core
 
 
 ```
-|                            Method |     Mean |   Error |  StdDev | Ratio |   Gen 0 |   Gen 1 |   Gen 2 | Allocated |
-|---------------------------------- |---------:|--------:|--------:|------:|--------:|--------:|--------:|----------:|
-|                              Linq | 405.5 μs | 4.10 μs | 3.64 μs |  1.00 | 90.8203 | 90.8203 | 90.8203 |  524848 B |
-|                        StructLinq | 169.1 μs | 1.25 μs | 1.05 μs |  0.42 |       - |       - |       - |      64 B |
-|               StructLinqZeroAlloc | 169.5 μs | 1.44 μs | 1.28 μs |  0.42 |       - |       - |       - |         - |
-|    StructLinqZeroAllocAndComparer | 148.6 μs | 0.81 μs | 0.76 μs |  0.37 |       - |       - |       - |         - |
-| StructLinqZeroAllocAndComparerSum | 129.5 μs | 0.62 μs | 0.52 μs |  0.32 |       - |       - |       - |         - |
+|                            Method |     Mean |   Error |   StdDev | Ratio |   Gen 0 |   Gen 1 |   Gen 2 | Allocated |
+|---------------------------------- |---------:|--------:|---------:|------:|--------:|--------:|--------:|----------:|
+|                              Linq | 501.7 μs | 9.40 μs | 16.47 μs |  1.00 | 90.8203 | 90.8203 | 90.8203 |  524848 B |
+|                        StructLinq | 176.3 μs | 0.64 μs |  0.57 μs |  0.36 |       - |       - |       - |      64 B |
+|               StructLinqZeroAlloc | 177.2 μs | 1.65 μs |  1.54 μs |  0.36 |       - |       - |       - |         - |
+|    StructLinqZeroAllocAndComparer | 147.7 μs | 1.51 μs |  1.34 μs |  0.30 |       - |       - |       - |         - |
+| StructLinqZeroAllocAndComparerSum | 113.7 μs | 1.06 μs |  0.94 μs |  0.23 |       - |       - |       - |         - |

src/StructLinq.Tests/ExceptTests.cs

@@ -28,5 +28,16 @@ public void SameAsSystem()
             var value = array1.ToStructEnumerable().Except(array2.ToStructEnumerable()).ToArray();
             Assert.Equal(expected, value);
         }
+
+        [Fact]
+        public void SameAsSystemEnumerable()
+        {
+            var array1 = new int[] { 1, 1, 2, 3, 4, 4, 5 };
+            var array2 = new int[] { 4, 5, 6, 6, 7, 8, 9 };
+
+            var expected = array1.Except(array2);
+            var value = array1.ToStructEnumerable().Except(array2.ToStructEnumerable()).ToEnumerable();
+            Assert.Equal(expected, value);
+        }
     }
 }

src/StructLinq/Except/ExceptEnumerable.cs

@@ -44,13 +44,43 @@ public ExceptEnumerator<T, TEnumerator1, TEnumerator2, TComparer> GetEnumerator(
         public VisitStatus Visit<TVisitor>(ref TVisitor visitor)
             where TVisitor : IVisitor<T>
         {
-            foreach (var input in this)
+            var exceptVisitor = new ExceptVisitor<TVisitor>(capacity, bucketPool, slotPool, comparer, ref visitor);
+            enumerable2.Visit(ref exceptVisitor);
+            exceptVisitor.Add = false;
+            var visitStatus = enumerable1.Visit(ref exceptVisitor);
+            visitor = exceptVisitor.Visitor;
+            exceptVisitor.Dispose();
+            return visitStatus;
+        }
+
+        private struct ExceptVisitor<TVisitor> : IVisitor<T>
+            where TVisitor : IVisitor<T>
+        {
+            public TVisitor Visitor;
+            public bool Add;
+            private PooledSet<T, TComparer> set;
+
+            public ExceptVisitor(int capacity, ArrayPool<int> bucketPool, ArrayPool<Slot<T>> slotPool, TComparer comparer, ref TVisitor visitor)
             {
-                if (!visitor.Visit(input))
-                    return VisitStatus.VisitorFinished;
+                this.Visitor = visitor;
+                set = new PooledSet<T, TComparer>(capacity, bucketPool, slotPool, comparer);
+                Add = true;
             }
 
-            return VisitStatus.EnumeratorFinished;
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            public bool Visit(T input)
+            {
+                if (!Add)
+                    return !set.AddIfNotPresent(input) || Visitor.Visit(input);
+                set.AddIfNotPresent(input);
+                return true;
+            }
+
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            public void Dispose()
+            {
+                set.Dispose();
+            }
         }
     }
 }