Fix bug in Mutant::Util.one

lib/mutant/util.rb

@@ -12,9 +12,12 @@ module Util
     #
     # @return [Object] first entry
     def self.one(array)
-      return array.first if array.one?
-
-      fail SizeError, "expected size to be exactly 1 but size was #{array.size}"
+      case array
+      in [value]
+        value
+      else
+        fail SizeError, "expected size to be exactly 1 but size was #{array.size}"
+      end
     end
   end # Util
 end # Mutant

spec/unit/mutant/util_spec.rb

@@ -0,0 +1,55 @@
+# frozen_string_literal: true
+
+RSpec.describe Mutant::Util, '.one' do
+  let(:item) { instance_double(Object) }
+
+  def apply
+    described_class.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 'raises expected error' do
+      expect { apply }
+        .to raise_error(described_class::SizeError)
+        .with_message('expected size to be exactly 1 but size was 0')
+    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 exactly 1 but size was 2')
+    end
+  end
+end