Extract domain knowledge responsability to CSP

lib/winston/backtrack.rb

@@ -34,7 +34,7 @@ def select_unassigned_variable(assignments)
     end
 
     def domain_values(var)
-      csp.variables[var.name].domain.values
+      csp.domain_for(var.name)
     end
   end
 end

lib/winston/csp.rb

@@ -29,6 +29,13 @@ def validate(changed_var, assignments)
       true
     end
 
+    def domain_for(variable_name)
+      variable = variables[variable_name]
+      return [] if variable.nil? || variable.domain.nil?
+
+      variable.domain.values
+    end
+
     private
 
     def var_assignments

spec/winston/csp_spec.rb

@@ -60,6 +60,25 @@
     end
   end
 
+  describe "#domain_for" do
+    before do
+      subject.add_variable :a, value: 1
+      subject.add_variable :b, domain: [1, 2]
+    end
+
+    it "returns domain values for a given variable" do
+      expect(subject.domain_for(:b)).to eq([1, 2])
+    end
+
+    it "returns an empty list when a variable doesn't exist in the problem" do
+      expect(subject.domain_for(:c)).to be_empty
+    end
+
+    it "return an empty list when a variable doesn't have a domain" do
+      expect(subject.domain_for(:a)).to be_empty
+    end
+  end
+
   describe "#solve" do
     let(:solver) { double("Solver") }
     before do