Fix coverage

lib/mutant/parallel/connection.rb

@@ -21,11 +21,11 @@ class Reader
         attr_reader :log
 
         def error
-          @errors.first
+          Util.max_one(@errors)
         end
 
         def result
-          @results.first
+          Util.max_one(@results)
         end
 
         def initialize(*)
@@ -96,7 +96,7 @@ def advance_result
         end
 
         def length
-          @lengths.first
+          Util.max_one(@lengths)
         end
 
         def advance_log

lib/mutant/util.rb

@@ -19,5 +19,23 @@ def self.one(array)
         fail SizeError, "expected size to be exactly 1 but size was #{array.size}"
       end
     end
+
+    # Return only element in array if it contains max one member
+    #
+    # @param array [Array]
+    #
+    # @return [Object] first entry
+    # @return [nil] if empty
+    #
+    # rubocop:disable Lint/EmptyInPattern
+    def self.max_one(array)
+      case array
+      in []
+      in [value]
+        value
+      else
+        fail SizeError, "expected size to be max 1 but size was #{array.size}"
+      end
+    end
   end # Util
 end # Mutant

spec/unit/mutant/util_spec.rb

@@ -53,3 +53,55 @@ def apply
     end
   end
 end
+
+RSpec.describe Mutant::Util, '.max_one' do
+  let(:item) { instance_double(Object) }
+
+  def apply
+    described_class.max_one(array)
+  end
+
+  context 'when array has exactly one element' do
+    context 'and that element is nil' do
+      let(:array) { [nil] }
+
+      it 'returns nil' do
+        expect(apply).to be(nil)
+      end
+    end
+
+    context 'and that element is false' do
+      let(:array) { [false] }
+
+      it 'returns false' do
+        expect(apply).to be(false)
+      end
+    end
+
+    context 'and that element is a regular object' do
+      let(:array) { [item] }
+
+      it 'returns first element' do
+        expect(apply).to be(item)
+      end
+    end
+  end
+
+  context 'when array is empty' do
+    let(:array) { [] }
+
+    it 'returns nil' do
+      expect(apply).to be(nil)
+    end
+  end
+
+  context 'when array has more than one element' do
+    let(:array) { [1, 2] }
+
+    it 'raises expected error' do
+      expect { apply }
+        .to raise_error(described_class::SizeError)
+        .with_message('expected size to be max 1 but size was 2')
+    end
+  end
+end