From cbde37818da31051fa02131875054ef60d32774d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Timothe=CC=81e=20Peignier?= <timothee.peignier@tryphon.org>
Date: Tue, 23 Oct 2012 03:23:44 +0200
Subject: [PATCH] add derive instrument

---
 docs/instruments.rst              |  7 +++++++
 metrology/__init__.py             |  4 ++++
 metrology/instruments/__init__.py |  1 +
 metrology/instruments/derive.py   | 28 ++++++++++++++++++++++++++++
 metrology/registry.py             |  5 ++++-
 tests/instruments/test_derive.py  | 28 ++++++++++++++++++++++++++++
 6 files changed, 72 insertions(+), 1 deletion(-)
 create mode 100644 metrology/instruments/derive.py
 create mode 100644 tests/instruments/test_derive.py

diff --git a/docs/instruments.rst b/docs/instruments.rst
index da0e1b1..02ef55b 100644
--- a/docs/instruments.rst
+++ b/docs/instruments.rst
@@ -17,6 +17,13 @@ Counters
 .. automodule:: metrology.instruments.counter
     :members:
 
+Derive
+======
+
+.. automodule:: metrology.instruments.derive
+  :members:
+
+
 Meters
 ======
 
diff --git a/metrology/__init__.py b/metrology/__init__.py
index 636dabb..624b85a 100644
--- a/metrology/__init__.py
+++ b/metrology/__init__.py
@@ -10,6 +10,10 @@ def get(cls, name):
     def counter(cls, name):
         return registry.counter(name)
 
+    @classmethod
+    def derive(cls, name):
+        return registry.derive(name)
+
     @classmethod
     def meter(cls, name):
         return registry.meter(name)
diff --git a/metrology/instruments/__init__.py b/metrology/instruments/__init__.py
index 40d220c..15d5893 100644
--- a/metrology/instruments/__init__.py
+++ b/metrology/instruments/__init__.py
@@ -1,5 +1,6 @@
 # -*- flake8: noqa -*-
 from metrology.instruments.counter import Counter
+from metrology.instruments.derive import Derive
 from metrology.instruments.gauge import Gauge
 from metrology.instruments.histogram import Histogram, HistogramUniform, HistogramExponentiallyDecaying
 from metrology.instruments.meter import Meter
diff --git a/metrology/instruments/derive.py b/metrology/instruments/derive.py
new file mode 100644
index 0000000..94b56d0
--- /dev/null
+++ b/metrology/instruments/derive.py
@@ -0,0 +1,28 @@
+from atomic import Atomic
+
+from metrology.instruments.meter import Meter
+from metrology.stats import EWMA
+
+
+class Derive(Meter):
+    """
+    A derive is like a meter but accepts an absolute counter as input.
+
+      derive = Metrology.derive('network.io')
+      derive.mark()
+      derive.count
+
+    """
+    def __init__(self, average_class=EWMA):
+        self.last = Atomic(0)
+        super(Derive, self).__init__(average_class)
+
+    def mark(self, value=1):
+        """Record an event with the derive.
+
+        :param value: counter value to record
+        """
+        last = self.last.get_and_set(value)
+        if last <= value:
+            value = value - last
+        super(Derive, self).mark(value)
diff --git a/metrology/registry.py b/metrology/registry.py
index fbc0c8d..14a4684 100644
--- a/metrology/registry.py
+++ b/metrology/registry.py
@@ -3,7 +3,7 @@
 from threading import RLock
 
 from metrology.exceptions import RegistryException
-from metrology.instruments import Counter, Profiler, Meter, Timer, UtilizationTimer, HistogramUniform
+from metrology.instruments import Counter, Derive, Profiler, Meter, Timer, UtilizationTimer, HistogramUniform
 
 
 class Registry(object):
@@ -41,6 +41,9 @@ def histogram(self, name, klass=None):
             klass = HistogramUniform
         return self.add_or_get(name, klass)
 
+    def derive(self, name):
+        return self.add_or_get(name, Derive)
+
     def profiler(self, name):
         return self.add_or_get(name, Profiler)
 
diff --git a/tests/instruments/test_derive.py b/tests/instruments/test_derive.py
new file mode 100644
index 0000000..d823d70
--- /dev/null
+++ b/tests/instruments/test_derive.py
@@ -0,0 +1,28 @@
+from unittest import TestCase
+
+from metrology.instruments.derive import Derive
+
+
+class DeriveTest(TestCase):
+    def setUp(self):
+        self.derive = Derive()
+
+    def test_derive(self):
+        self.derive.mark()
+        self.assertEqual(1, self.derive.count)
+
+    def test_blank_derive(self):
+        self.assertEqual(0, self.derive.count)
+        self.assertEqual(0.0, self.derive.mean_rate)
+
+    def test_derive_value(self):
+        self.derive.mark(3)
+        self.assertEqual(3, self.derive.count)
+
+    def test_one_minute_rate(self):
+        self.derive.mark(1000)
+        self.derive.tick()
+        self.assertEqual(200, self.derive.one_minute_rate)
+
+    def tearDown(self):
+        self.derive.stop()