[ryan] added derivative stat

.rvmrc

@@ -1 +1 @@
-rvm ree@am
+rvm 1.9.3-p0@am

lib/active_metric/stat.rb

@@ -30,6 +30,14 @@ def calculate(measurement)
     def complete
     end
 
+    def sample_duration_in_seconds
+      calculable.duration_in_seconds
+    end
+
+    def summary_duration_in_seconds
+      subject.summary.duration_in_seconds
+    end
+
     def self.class_for(stat)
       eval(stat.to_s.classify)
     end
@@ -50,7 +58,6 @@ def self.create_custom_stat(name_of_stat, value_type, default, axis, calculate_b
       klass.send(:field, :value, :type => value_type, :default => default)
       ActiveMetric.const_set(class_name,klass)
       return klass
-
     end
 
     def subject

lib/active_metric/statistics/defaults.rb

@@ -27,6 +27,22 @@ def complete
     end
   end
 
+  class Derivative < Stat
+    field :first
+    field :last
+
+    def calculate(measurement)
+      self.last  = (measurement.send(self.property))
+      self.first ||= self.last
+    end
+
+    def complete
+      self.value = (self.last - self.first).to_f / sample_duration_in_seconds
+      super
+    end
+
+  end
+
   class Eightieth < Stat
     def calculate(measurement)
     end
@@ -44,4 +60,5 @@ def complete
       super
     end
   end
+
 end

test/stat_test.rb

@@ -59,6 +59,13 @@ class StatTest < ActiveSupport::TestCase
       assert_equal 4.5, stat.value
     end
 
+    test "can calculate derivative" do
+      stat = Derivative.new(:value, :calculable => @sample)
+      stat.expects(:sample_duration_in_seconds).returns(10)
+      test_stat(stat, 10.times)
+      assert_equal 0.9, stat.value
+    end
+
     #this test is here for the user, not for automated tests
     #test "random distributions are good too" do
     #  stat = StandardDeviation.new(:value, :calculable => @sample)
@@ -79,7 +86,7 @@ class StatTest < ActiveSupport::TestCase
 
     def test_stat(stat, values)
       values.each  do |value|
-        stat.calculate TestMeasurement.new(:value => value)
+        stat.calculate TestMeasurement.new(:value => value, :timestamp => value)
       end
       stat.complete
     end