From 8e76a5c3ef674ecf0392d06cdc0385b3b552daf1 Mon Sep 17 00:00:00 2001
From: James Turk <james.p.turk@gmail.com>
Date: Mon, 23 Mar 2015 23:45:53 -0400
Subject: [PATCH] some basic math operators

---
 bia/test_units.py | 20 ++++++++++++++++++++
 bia/units.py      | 16 ++++++++++++++--
 2 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/bia/test_units.py b/bia/test_units.py
index 321bcbe..dcdb9d0 100644
--- a/bia/test_units.py
+++ b/bia/test_units.py
@@ -16,3 +16,23 @@ def test_basic_conversions():
     assert g.unit == 'g'
 
     assert g.as_unit('lb').scalar == a.as_unit('lb').scalar
+
+
+def test_basic_cmp():
+    assert Mass(1, 'kg') < Mass(2, 'kg')
+    assert Mass(1, 'kg') <= Mass(2, 'kg')
+    assert Mass(2, 'kg') <= Mass(2, 'kg')
+    assert Mass(2, 'kg') == Mass(2, 'kg')
+    assert Mass(2, 'kg') >= Mass(2, 'kg')
+    assert Mass(2, 'kg') > Mass(1, 'kg')
+    assert Mass(2, 'kg') >= Mass(1, 'kg')
+
+
+def test_conversion_cmp():
+    assert Mass(1, 'kg') < Mass(100, 'lb')
+    assert Mass(10000000, 'g') > Mass(100, 'lb')
+
+
+def test_add_sub():
+    assert Mass(1, 'kg') + Mass(2, 'kg') == Mass(3, 'kg')
+    assert Mass(2, 'kg') - Mass(1, 'kg') == Mass(1, 'kg')
diff --git a/bia/units.py b/bia/units.py
index 77f395a..17f8328 100644
--- a/bia/units.py
+++ b/bia/units.py
@@ -38,8 +38,20 @@ class Unit(object):
     def __repr__(self):
         return 'U({}, {!r})'.format(self.scalar, self.unit)
 
-    def __cmp__(self):
-        pass
+    def __cmp__(self, other):
+        if self.unit != other.unit:
+            other = other.as_unit(self.unit)
+        return self.scalar - other.scalar
+
+    def __add__(self, other):
+        if self.unit != other.unit:
+            other = other.as_unit(self.unit)
+        return self.__class__(self.scalar.__add__(other.scalar), self.unit)
+
+    def __sub__(self, other):
+        if self.unit != other.unit:
+            other = other.as_unit(self.unit)
+        return self.__class__(self.scalar.__sub__(other.scalar), self.unit)
 
 
 class Mass(Unit):