[Distinct] use visitor to improve performance.

Author: reegeek

Documents/BenchmarksResults/Distinct.md

@@ -7,16 +7,16 @@
 ``` ini
 
 BenchmarkDotNet=v0.12.1, OS=Windows 10.0.19042
-Intel Core i7-8750H CPU 2.20GHz (Coffee Lake), 1 CPU, 12 logical and 6 physical cores
-.NET Core SDK=5.0.101
-  [Host]     : .NET Core 5.0.1 (CoreCLR 5.0.120.57516, CoreFX 5.0.120.57516), X64 RyuJIT
-  DefaultJob : .NET Core 5.0.1 (CoreCLR 5.0.120.57516, CoreFX 5.0.120.57516), X64 RyuJIT
+Intel Core i7-7700 CPU 3.60GHz (Kaby Lake), 1 CPU, 8 logical and 4 physical cores
+.NET Core SDK=5.0.202
+  [Host]     : .NET Core 5.0.5 (CoreCLR 5.0.521.16609, CoreFX 5.0.521.16609), X64 RyuJIT
+  DefaultJob : .NET Core 5.0.5 (CoreCLR 5.0.521.16609, CoreFX 5.0.521.16609), X64 RyuJIT
 
 
 ```
 |                 Method |     Mean |   Error |  StdDev | Ratio |   Gen 0 |   Gen 1 |   Gen 2 | Allocated |
 |----------------------- |---------:|--------:|--------:|------:|--------:|--------:|--------:|----------:|
-|                   Linq | 485.9 μs | 2.88 μs | 2.70 μs |  1.00 | 90.8203 | 90.8203 | 90.8203 |  524784 B |
-|             StructLinq | 176.9 μs | 3.32 μs | 3.41 μs |  0.36 |       - |       - |       - |      32 B |
-|    StructLinqZeroAlloc | 173.8 μs | 0.75 μs | 0.70 μs |  0.36 |       - |       - |       - |         - |
-| StructLinqZeroAllocSum | 160.3 μs | 0.57 μs | 0.53 μs |  0.33 |       - |       - |       - |         - |
+|                   Linq | 544.3 μs | 7.98 μs | 7.08 μs |  1.00 | 90.8203 | 90.8203 | 90.8203 |  524784 B |
+|             StructLinq | 215.7 μs | 2.56 μs | 2.14 μs |  0.40 |       - |       - |       - |      32 B |
+|    StructLinqZeroAlloc | 220.7 μs | 0.77 μs | 0.64 μs |  0.41 |       - |       - |       - |         - |
+| StructLinqZeroAllocSum | 152.6 μs | 0.53 μs | 0.44 μs |  0.28 |       - |       - |       - |         - |

src/StructLinq.Tests/DistinctTests.cs

@@ -30,5 +30,21 @@ public void ShouldEquals()
                 .ToEnumerable();
             Assert.Equal(expected, value);
         }
+
+        [Fact]
+        public void ToArrayShouldEquals()
+        {
+            var expected = Enumerable.Range(0, 100)
+                                     .Select(x => x % 10)
+                                     .Distinct()
+                                     .ToArray();
+
+            var value = Enumerable.Range(0, 100)
+                                  .Select(x => x % 10)
+                                  .ToStructEnumerable()
+                                  .Distinct(x => x)
+                                  .ToArray();
+            Assert.Equal(expected, value);
+        }
     }
 }

src/StructLinq/Distinct/DistinctEnumerable.cs

@@ -38,13 +38,36 @@ public DistinctEnumerator<T, TEnumerator, TComparer> GetEnumerator()
         public VisitStatus Visit<TVisitor>(ref TVisitor visitor)
             where TVisitor : IVisitor<T>
         {
-            foreach (var input in this)
+            var distinctVisitor = new DistinctVisitor<TVisitor>(capacity, bucketPool, slotPool, comparer, ref visitor);
+            var visitStatus = enumerable.Visit(ref distinctVisitor);
+            visitor = distinctVisitor.visitor;
+            distinctVisitor.Dispose();
+            return visitStatus;
+        }
+
+        private struct DistinctVisitor<TVisitor> : IVisitor<T>
+            where TVisitor : IVisitor<T>
+        {
+            public TVisitor visitor;
+            private PooledSet<T, TComparer> set;
+
+            public DistinctVisitor(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);
             }
 
-            return VisitStatus.EnumeratorFinished;
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            public bool Visit(T input)
+            {
+                return !set.AddIfNotPresent(input) || visitor.Visit(input);
+            }
+
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            public void Dispose()
+            {
+                set.Dispose();
+            }
         }
     }
 }