Index: stable/HeuristicLab.Encodings.PermutationEncoding/3.3/HeuristicLab.Encodings.PermutationEncoding-3.3.csproj
===================================================================
--- stable/HeuristicLab.Encodings.PermutationEncoding/3.3/HeuristicLab.Encodings.PermutationEncoding-3.3.csproj	(revision 15131)
+++ stable/HeuristicLab.Encodings.PermutationEncoding/3.3/HeuristicLab.Encodings.PermutationEncoding-3.3.csproj	(revision 15217)
@@ -203,4 +203,5 @@
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="ShakingOperators\PermutationShakingOperator.cs" />
+    <Compile Include="SimilarityCalculators\HammingSimilarityCalculator.cs" />
   </ItemGroup>
   <ItemGroup>
Index: stable/HeuristicLab.Encodings.PermutationEncoding/3.3/PermutationEqualityComparer.cs
===================================================================
--- stable/HeuristicLab.Encodings.PermutationEncoding/3.3/PermutationEqualityComparer.cs	(revision 15131)
+++ stable/HeuristicLab.Encodings.PermutationEncoding/3.3/PermutationEqualityComparer.cs	(revision 15217)
@@ -30,6 +30,8 @@
   public class PermutationEqualityComparer : EqualityComparer<Permutation> {
     public override bool Equals(Permutation x, Permutation y) {
+      if (ReferenceEquals(x, y)) return true;
+      if (x == null || y == null) return false;
+      if (x.Length != y.Length) return false;
       if (x.PermutationType != y.PermutationType) return false;
-      if (x.Length != y.Length) return false;
       switch (x.PermutationType) {
         case PermutationTypes.Absolute:
@@ -67,5 +69,5 @@
 
     public override int GetHashCode(Permutation obj) {
-      if (obj == null) return 0;
+      if (obj == null) throw new ArgumentNullException("obj", "PermutationEqualityComparer: Cannot compute hash value of null.");
       return GenerateHashString(obj).GetHashCode();
     }
Index: stable/HeuristicLab.Encodings.PermutationEncoding/3.3/SimilarityCalculators/HammingSimilarityCalculator.cs
===================================================================
--- stable/HeuristicLab.Encodings.PermutationEncoding/3.3/SimilarityCalculators/HammingSimilarityCalculator.cs	(revision 15217)
+++ stable/HeuristicLab.Encodings.PermutationEncoding/3.3/SimilarityCalculators/HammingSimilarityCalculator.cs	(revision 15217)
@@ -0,0 +1,115 @@
+﻿#region License Information
+/* HeuristicLab
+ * Copyright (C) 2002-2017 Heuristic and Evolutionary Algorithms Laboratory (HEAL)
+ *
+ * This file is part of HeuristicLab.
+ *
+ * HeuristicLab is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * HeuristicLab is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with HeuristicLab. If not, see <http://www.gnu.org/licenses/>.
+ */
+#endregion
+
+using System;
+using HeuristicLab.Common;
+using HeuristicLab.Core;
+using HeuristicLab.Optimization.Operators;
+using HeuristicLab.Persistence.Default.CompositeSerializers.Storable;
+
+namespace HeuristicLab.Encodings.PermutationEncoding {
+  [Item("Hamming Similarity Calculator for Permutation", "An operator that performs similarity calculation between two permutation-encoded solutions.")]
+  [StorableClass]
+  public sealed class HammingSimilarityCalculator : SingleObjectiveSolutionSimilarityCalculator {
+    protected override bool IsCommutative { get { return true; } }
+
+    [StorableConstructor]
+    private HammingSimilarityCalculator(bool deserializing) : base(deserializing) { }
+    private HammingSimilarityCalculator(HammingSimilarityCalculator original, Cloner cloner) : base(original, cloner) { }
+    public HammingSimilarityCalculator() : base() { }
+
+    public override IDeepCloneable Clone(Cloner cloner) {
+      return new HammingSimilarityCalculator(this, cloner);
+    }
+
+    public static double CalculateSimilarity(Permutation left, Permutation right) {
+      if (left == null || right == null)
+        throw new ArgumentException("Cannot calculate similarity because one or both of the provided solutions is null.");
+      if (left.PermutationType != right.PermutationType)
+        throw new ArgumentException("Cannot calculate similarity because the provided solutions have different permutation types.");
+      if (left.Length != right.Length)
+        throw new ArgumentException("Cannot calculate similarity because the provided solutions have different lengths.");
+      if (left.Length == 0)
+        throw new ArgumentException("Cannot calculate similarity because solutions are of length 0.");
+      if (ReferenceEquals(left, right)) return 1.0;
+
+      switch (left.PermutationType) {
+        case PermutationTypes.Absolute:
+          return CalculateAbsolute(left, right);
+        case PermutationTypes.RelativeDirected:
+          return CalculateRelativeDirected(left, right);
+        case PermutationTypes.RelativeUndirected:
+          return CalculateRelativeUndirected(left, right);
+        default:
+          throw new InvalidOperationException("unknown permutation type");
+      }
+    }
+
+    private static double CalculateAbsolute(Permutation left, Permutation right) {
+      double similarity = 0.0;
+      for (int i = 0; i < left.Length; i++)
+        if (left[i] == right[i]) similarity++;
+
+      return similarity / left.Length;
+    }
+
+    private static double CalculateRelativeDirected(Permutation left, Permutation right) {
+      int[] edgesR = CalculateEdgesVector(right);
+      int[] edgesL = CalculateEdgesVector(left);
+
+      double similarity = 0.0;
+      for (int i = 0; i < left.Length; i++) {
+        if (edgesL[i] == edgesR[i]) similarity++;
+      }
+
+      return similarity / left.Length;
+    }
+
+    private static double CalculateRelativeUndirected(Permutation left, Permutation right) {
+      int[] edgesR = CalculateEdgesVector(right);
+      int[] edgesL = CalculateEdgesVector(left);
+
+      double similarity = 0.0;
+      for (int i = 0; i < left.Length; i++) {
+        if ((edgesL[i] == edgesR[i]) || (edgesL[edgesR[i]] == i))
+          similarity++;
+      }
+
+      return similarity / left.Length;
+    }
+
+    private static int[] CalculateEdgesVector(Permutation permutation) {
+      // transform path representation into adjacency representation
+      int[] edgesVector = new int[permutation.Length];
+      for (int i = 0; i < permutation.Length - 1; i++)
+        edgesVector[permutation[i]] = permutation[i + 1];
+      edgesVector[permutation[permutation.Length - 1]] = permutation[0];
+      return edgesVector;
+    }
+
+    public override double CalculateSolutionSimilarity(IScope leftSolution, IScope rightSolution) {
+      var sol1 = leftSolution.Variables[SolutionVariableName].Value as Permutation;
+      var sol2 = rightSolution.Variables[SolutionVariableName].Value as Permutation;
+
+      return CalculateSimilarity(sol1, sol2);
+    }
+  }
+}
