Add Mutant::Timer

lib/mutant.rb

@@ -177,6 +177,7 @@ require 'mutant/expression/method'
 require 'mutant/expression/methods'
 require 'mutant/expression/namespace'
 require 'mutant/test'
+require 'mutant/timer'
 require 'mutant/integration'
 require 'mutant/selector'
 require 'mutant/selector/expression'

lib/mutant/timer.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+module Mutant
+  module Timer
+    # Monotonic elapsed time of block execution
+    #
+    # @return [Float]
+    def self.elapsed
+      start = now
+      yield
+      now - start
+    end
+
+    # The now monotonic time
+    #
+    # @return [Float]
+    def self.now
+      Process.clock_gettime(Process::CLOCK_MONOTONIC)
+    end
+  end # Timer
+end # Mutant

spec/unit/mutant/clock_monotonic_spec.rb

@@ -0,0 +1,52 @@
+# frozen_string_literal: true
+
+RSpec.describe Mutant::Timer do
+  let(:events) { [] }
+
+  let(:times) { [1.0, 2.0] }
+
+  before do
+    allow(Process).to receive(:clock_gettime) do |argument|
+      expect(argument).to be(Process::CLOCK_MONOTONIC)
+
+      events << :clock_gettime
+
+      times.fetch(events.count(:clock_gettime).pred)
+    end
+  end
+
+  describe '.elapsed' do
+    def apply
+      described_class.elapsed { events << :yield }
+    end
+
+    it 'executes events in expected sequence' do
+      expect { apply }
+        .to change(events, :to_a)
+        .from([])
+        .to(%i[clock_gettime yield clock_gettime])
+    end
+
+    it 'returns elapsed time' do
+      expect(apply).to be(1.0)
+    end
+  end
+
+  describe '.now' do
+    def apply
+      described_class.now
+    end
+
+    it 'returns current monotonic time' do
+      expect(apply).to be(1.0)
+      expect(apply).to be(2.0)
+    end
+
+    it 'calls expected system API' do
+      expect { apply }
+        .to change(events, :to_a)
+        .from([])
+        .to(%i[clock_gettime])
+    end
+  end
+end