Fix NaN coverage percentage when no resources are collected

lib/chefspec/coverage.rb

@@ -161,7 +161,7 @@ def report!
       report = {}.tap do |h|
         h[:total]     = @collection.size
         h[:touched]   = @collection.count { |_, resource| resource.touched? }
-        h[:coverage]  = ((h[:touched] / h[:total].to_f) * 100).round(2)
+        h[:coverage]  = coverage_percentage(h[:touched], h[:total])
       end
 
       report[:untouched_resources] = @collection.collect do |_, resource|
@@ -179,6 +179,18 @@ def report!
 
     private
 
+    #
+    # The percentage of covered resources, guarding against a division by zero
+    # when no resources were collected (which would otherwise produce NaN).
+    #
+    # @return [Float]
+    #
+    def coverage_percentage(touched, total)
+      return 0.0 if total == 0
+
+      ((touched / total.to_f) * 100).round(2)
+    end
+
     def find(resource)
       @collection[resource.to_s]
     end

spec/unit/coverage_spec.rb

@@ -0,0 +1,21 @@
+require "spec_helper"
+
+describe ChefSpec::Coverage do
+  subject(:coverage) { described_class.instance }
+
+  describe "#coverage_percentage" do
+    it "returns 0.0 when there are no resources instead of NaN" do
+      result = coverage.send(:coverage_percentage, 0, 0)
+      expect(result).to eq(0.0)
+      expect(result).not_to be_nan
+    end
+
+    it "calculates the touched percentage" do
+      expect(coverage.send(:coverage_percentage, 3, 4)).to eq(75.0)
+    end
+
+    it "rounds to two decimal places" do
+      expect(coverage.send(:coverage_percentage, 1, 3)).to eq(33.33)
+    end
+  end
+end