Optimizations to unit conversion

wrappers/python/simtk/unit/basedimension.py

@@ -42,6 +42,7 @@ class BaseDimension(object):
         if not self.name in BaseDimension._index_by_name.keys():
             BaseDimension._index_by_name[name] = BaseDimension._next_unused_index
             BaseDimension._next_unused_index += 1
+        self._index = BaseDimension._index_by_name[name]
         
     def __cmp__(self, other):
         """
@@ -51,13 +52,13 @@ class BaseDimension(object):
         
         Returns 0 if self == other, -1 if self < other, and 1 if self > other.
         """
-        return cmp(BaseDimension._index_by_name[self.name], BaseDimension._index_by_name[other.name])
+        return cmp(self._index, other._index)
         
     def __hash__(self):
         """
         Needed for using BaseDimensions as hash keys.
         """
-        return hash(BaseDimension._index_by_name[self.name])
+        return self._index
 
     def __repr__(self):
         return 'BaseDimension("%s")' % self.name

wrappers/python/simtk/unit/unit.py

@@ -89,7 +89,6 @@ class Unit(object):
         """
         Yields (BaseDimension, exponent) tuples comprising this unit.
         """
-        result = {}
         # There might be two units with the same dimension? No.
         for dimension in sorted(self._all_base_units.iterkeys()):
             exponent = 0
@@ -105,7 +104,6 @@ class Unit(object):
         
         There might be multiple BaseUnits with the same dimension.
         """
-        result = {}
         for dimension in sorted(self._all_base_units.iterkeys()):
             for base_unit in sorted(self._all_base_units[dimension].iterkeys()):
                 exponent = self._all_base_units[dimension][base_unit]
@@ -526,6 +524,7 @@ class ScaledUnit(object):
 class UnitSystem(object):
     def __init__(self, units):
         self.units = units
+        self._unit_conversion_cache = {}
         # Create a set of base units to be used for dimension conversion
         base_units = {}
         for unit in self.units:
@@ -577,6 +576,9 @@ class UnitSystem(object):
     def express_unit(self, old_unit):
         """
         """
+        unit_name = old_unit.get_name()
+        if unit_name in self._unit_conversion_cache:
+            return self._unit_conversion_cache[unit_name]
         # First express unit in terms of base dimensions found in this unit system
         # (plus other dimensions not found)
         m = len(self.dimensions)
@@ -606,6 +608,7 @@ class UnitSystem(object):
                 found_dims[dim] = base_unit
                 exponent = other_dims[dim]
                 new_unit *= Unit({base_unit: exponent})
+        self._unit_conversion_cache[unit_name] = new_unit
         return new_unit
 
 def is_unit(x):