Add cohesion metric

lib/mosaik/metrics/cohesion.rb

@@ -0,0 +1,60 @@
+# frozen_string_literal: true
+
+module MOSAIK
+  module Metrics
+    ##
+    # Cohesion (S. Chidamber and C. Kemerer, 1994)
+    #
+    class Cohesion < Metric
+      def evaluate
+        # Total cohesion
+        cohesion = 0.0
+
+        # Iterate over each cluster
+        graph.clusters.each_value do |cluster|
+          # Find all vertices in the cluster
+          vertices_in_cluster = cluster.vertices
+          vertices_in_cluster_id = vertices_in_cluster.map(&:id)
+
+          # Calculate the cardinality of the cluster
+          cardinality_c = vertices_in_cluster.sum do |v|
+            warn "No `methods` attribute found for #{v.id}, did you extract structural coupling information?" unless v.attributes.key?(:methods)
+
+            v.attributes.fetch(:methods, 0.0)
+          end
+
+          # Skip if the vertex has less than 2 methods (denominator would be 0)
+          if cardinality_c < 2
+            warn "Cluster #{cluster.id} has less than 2 methods, skipping cohesion calculation"
+
+            # Store cohesion value in the cluster
+            cluster.attributes[:cohesion] = 0.0
+
+            next
+          end
+
+          # Calculate sum of edges between vertices in the cluster
+          sum = vertices_in_cluster
+            .flat_map { |v| v.edges.filter_map { |i, e| e if i.in?(vertices_in_cluster_id) } }
+            .uniq
+            .count
+
+          # Calculate cohesion value for the cluster
+          cohesion_c = sum.to_f / (cardinality_c * (cardinality_c - 1) / 2)
+
+          # Store cohesion value in the cluster
+          cluster.attributes[:cohesion] = cohesion_c
+
+          # Calculate cohesion contribution from this cluster
+          cohesion += cohesion_c
+        end
+
+        # Store cohesion value in the graph
+        graph.attributes[:cohesion] = cohesion
+
+        # Return the total cohesion
+        cohesion
+      end
+    end
+  end
+end

spec/mosaik/metrics/cohesion_spec.rb

@@ -0,0 +1,56 @@
+# frozen_string_literal: true
+
+RSpec.describe MOSAIK::Metrics::Cohesion do
+  subject(:metric) { build(:cohesion_metric, graph:) }
+
+  # Cohesion value calculated by hand:
+  #
+  # Directed:
+  #   Cluster A: |A| = 11: (2 + 1 + 0) / (11 * 10 / 2) = 0.05454545454545454
+  #   Cluster B: |B| = 8:  (1 + 0) / (8 * 7 / 2) = 0.03571428571428571
+  #   Cluster C: |C| = 1: 0
+  #
+  #   Cohesion = 0.05454545454545454 + 0.03571428571428571 + 0 = 0.09025974025974026
+  #
+  # Undirected:
+  #   Cluster A: |A| = 11: (2 + 1 + 0) / (11 * 10 / 2) = 0.05454545454545454
+  #   Cluster B: |B| = 8:  (1 + 0) / (8 * 7 / 2) = 0.03571428571428571
+  #   Cluster C: |C| = 1: 0
+  #
+  #   Cohesion = 0.05454545454545454 + 0.03571428571428571 + 0 = 0.09025974025974026
+  #
+
+  context "when the graph is directed" do
+    include_context "with a simple directed graph"
+
+    it "sets the cohesion values for each cluster, and for the graph" do
+      metric.evaluate
+
+      expect(graph.find_cluster("A").attributes[:cohesion]).to eq 0.05454545454545454
+      expect(graph.find_cluster("B").attributes[:cohesion]).to eq 0.03571428571428571
+      expect(graph.find_cluster("C").attributes[:cohesion]).to eq 0.0
+      expect(graph.attributes[:cohesion]).to eq 0.09025974025974026
+    end
+
+    it "returns the total cohesion" do
+      expect(metric.evaluate).to eq 0.09025974025974026
+    end
+  end
+
+  context "when the graph is undirected" do
+    include_context "with a simple undirected graph"
+
+    it "sets the cohesion values for each cluster, and for the graph" do
+      metric.evaluate
+
+      expect(graph.find_cluster("A").attributes[:cohesion]).to eq 0.05454545454545454
+      expect(graph.find_cluster("B").attributes[:cohesion]).to eq 0.03571428571428571
+      expect(graph.find_cluster("C").attributes[:cohesion]).to eq 0.0
+      expect(graph.attributes[:cohesion]).to eq 0.09025974025974026
+    end
+
+    it "returns the total cohesion" do
+      expect(metric.evaluate).to eq 0.09025974025974026
+    end
+  end
+end

spec/support/contexts/directed_graph.rb

@@ -4,12 +4,12 @@
   let(:graph) do
     graph = build(:graph, directed: true)
 
-    a = graph.add_vertex("A")
-    b = graph.add_vertex("B")
-    c = graph.add_vertex("C")
-    d = graph.add_vertex("D")
-    e = graph.add_vertex("E")
-    f = graph.add_vertex("F")
+    a = graph.add_vertex("A", methods: 3)
+    b = graph.add_vertex("B", methods: 2)
+    c = graph.add_vertex("C", methods: 3)
+    d = graph.add_vertex("D", methods: 6)
+    e = graph.add_vertex("E", methods: 5)
+    f = graph.add_vertex("F", methods: 1)
 
     graph.add_edge("A", "B", weight: 3.0)
     graph.add_edge("A", "C", weight: 1.0)

spec/support/contexts/undirected_graph.rb

@@ -4,12 +4,12 @@
   let(:graph) do
     graph = build(:graph, directed: false)
 
-    a = graph.add_vertex("A")
-    b = graph.add_vertex("B")
-    c = graph.add_vertex("C")
-    d = graph.add_vertex("D")
-    e = graph.add_vertex("E")
-    f = graph.add_vertex("F")
+    a = graph.add_vertex("A", methods: 3)
+    b = graph.add_vertex("B", methods: 2)
+    c = graph.add_vertex("C", methods: 3)
+    d = graph.add_vertex("D", methods: 6)
+    e = graph.add_vertex("E", methods: 5)
+    f = graph.add_vertex("F", methods: 1)
 
     graph.add_edge("A", "B", weight: 3.0)
     graph.add_edge("A", "C", weight: 1.0)

spec/support/factories/metrics.rb

@@ -10,5 +10,7 @@
 
   factory :coupling_metric, parent: :metric, class: "MOSAIK::Metrics::Coupling"
 
+  factory :cohesion_metric, parent: :metric, class: "MOSAIK::Metrics::Cohesion"
+
   factory :modularity_metric, parent: :metric, class: "MOSAIK::Metrics::Modularity"
 end