add support for derivative metrics

lib/metriks.rb

@@ -1,4 +1,3 @@
-
 module Metriks
   VERSION = '0.9.9.1'
 
@@ -22,6 +21,10 @@ def self.meter(name)
     Metriks::Registry.default.meter(name)
   end
 
+  def self.derive(name)
+    Metriks::Registry.default.derive(name)
+  end
+
   def self.histogram(name)
     Metriks::Registry.default.histogram(name)
   end

lib/metriks/derive.rb

@@ -0,0 +1,13 @@
+require 'atomic'
+
+require 'metriks/meter'
+
+module Metriks
+  class Derive < Metriks::Meter
+    def mark(val = 1)
+      @last ||= Atomic.new(val)
+      last = @last.swap(val)
+      super(last > val ? val : val - last)
+    end
+  end
+end

lib/metriks/registry.rb

@@ -2,6 +2,7 @@
 require 'metriks/timer'
 require 'metriks/utilization_timer'
 require 'metriks/meter'
+require 'metriks/derive'
 
 module Metriks
   # Public: A collection of metrics
@@ -86,6 +87,21 @@ def meter(name)
       add_or_get(name, Metriks::Meter)
     end
 
+    # Public: Fetch or create a new derivative metric. Derivatives are a meter
+    # that tracks throughput by calculating the derivative to the previous
+    # value.
+    #
+    # name - The String name of the metric to define or fetch
+    #
+    # Examples
+    #
+    #   registry.derive('network.bytes')
+    #
+    # Returns the Metricks::Derive identified by the name.
+    def derive(name)
+      add_or_get(name, Metriks::Derive)
+    end
+
     # Public: Fetch or create a new timer metric. Timers provide the means to
     # time the execution of a method including statistics on the number of
     # invocations, average length of time, throughput.
@@ -188,4 +204,4 @@ def add_or_get(name, klass, &create_metric)
       end
     end
   end
-end
+end

test/derive_test.rb

@@ -0,0 +1,32 @@
+require 'test_helper'
+
+require 'metriks/derive'
+
+class DeriveTest < Test::Unit::TestCase
+  include ThreadHelper
+
+  def setup
+    @meter = Metriks::Derive.new
+  end
+
+  def teardown
+    @meter.stop
+  end
+
+  def test_meter
+    @meter.mark(100)
+    @meter.mark(150)
+
+    assert_equal 50, @meter.count
+  end
+
+  def test_one_minute_rate
+    @meter.mark(1000)
+    @meter.mark(2000)
+
+    # Pretend it's been 5 seconds
+    @meter.tick
+
+    assert_equal 200, @meter.one_minute_rate
+  end
+end