Basic setup for an RSpec based benchmark suite

This benchmark suite uses benchmark-ips
(https://github.com/evanphx/benchmark-ips) behind the scenes. Specs can
be turned into benchmark specs by setting "benchmark" to "true" in the
top-level describe block like so:

    describe SomeClass, benchmark: true do

    end

Writing benchmarks can be done using custom RSpec matchers, for example:

    describe MaruTheCat, benchmark: true do
      describe '#jump_in_box' do
        it 'should run 1000 iterations per second' do
          maru = described_class.new

          expect { maru.jump_in_box }.to iterate_per_second(1000)
        end
      end
    end

By default the "iterate_per_second" expectation requires a standard
deviation under 30% (this is just an arbitrary default for now). You can
change this by chaining "with_maximum_stddev" on the expectation:

    expect { maru.jump_in_box }.to iterate_per_second(1000)
      .with_maximum_stddev(10)

This will change the expectation to require a maximum deviation of 10%.

Alternatively you can use the it block style to write specs:

    describe MaruTheCat, benchmark: true do
      describe '#jump_in_box' do
        subject { -> { described_class.new } }

        it { is_expected.to iterate_per_second(1000) }
      end
    end

Because "iterate_per_second" operates on a block, opposed to a static
value, the "subject" method must return a Proc. This looks a bit goofy
but I have been unable to find a nice way around this.

1 files changed, 42 insertions, 0 deletions

diff --git a/spec/support/matchers/benchmark_matchers.rb b/spec/support/matchers/benchmark_matchers.rb
new file mode 100644
index 00000000000..45a1d49345f
--- /dev/null
+++ b/spec/support/matchers/benchmark_matchers.rb
@@ -0,0 +1,42 @@
+module BenchmarkMatchers
+  extend RSpec::Matchers::DSL
+
+  matcher :iterate_per_second do |min_iterations|
+    supports_block_expectations
+
+    match do |block|
+      @max_stddev ||= 30
+
+      @entry = benchmark(&block)
+
+      expect(@entry.ips).to be >= min_iterations
+      expect(@entry.stddev_percentage).to be <= @max_stddev
+    end
+
+    chain :with_maximum_stddev do |value|
+      @max_stddev = value
+    end
+
+    description do
+      "run at least #{min_iterations} iterations per second"
+    end
+
+    failure_message do
+      ips    = @entry.ips.round(2)
+      stddev = @entry.stddev_percentage.round(2)
+
+      "expected at least #{min_iterations} iterations per second " \
+        "with a maximum stddev of #{@max_stddev}%, instead of " \
+        "#{ips} iterations per second with a stddev of #{stddev}%"
+    end
+  end
+
+  # Benchmarks the given block and returns a Benchmark::IPS::Report::Entry.
+  def benchmark(&block)
+    report = Benchmark.ips(quiet: true) do |bench|
+      bench.report(&block)
+    end
+
+    report.entries[0]
+  end
+end