unit_operators.py

#!/bin/env python
"""
Module simtk.unit.unit_operators

Physical quantities with units, intended to produce similar functionality
to Boost.Units package in C++ (but with a runtime cost).
Uses similar API as Scientific.Physics.PhysicalQuantities
but different internals to satisfy our local requirements.
In particular, there is no underlying set of 'canonical' base
units, whereas in Scientific.Physics.PhysicalQuantities all
units are secretly in terms of SI units.  Also, it is easier
to add new fundamental dimensions to simtk.dimensions.  You
might want to make new dimensions for, say, "currency" or 
"information".

Two possible enhancements that have not been implemented are
  1) Include uncertainties with propagation of errors
  2) Incorporate offsets for celsius <-> kelvin conversion



This is part of the OpenMM molecular simulation toolkit originating from
Simbios, the NIH National Center for Physics-Based Simulation of
Biological Structures at Stanford, funded under the NIH Roadmap for
Medical Research, grant U54 GM072970. See https://simtk.org.

Portions copyright (c) 2012 Stanford University and the Authors.
Authors: Christopher M. Bruns
Contributors: Peter Eastman

Permission is hereby granted, free of charge, to any person obtaining a 
copy of this software and associated documentation files (the "Software"),
to deal in the Software without restriction, including without limitation
the rights to use, copy, modify, merge, publish, distribute, sublicense,
and/or sell copies of the Software, and to permit persons to whom the
Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
THE AUTHORS, CONTRIBUTORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
USE OR OTHER DEALINGS IN THE SOFTWARE.
"""

__author__ = "Christopher M. Bruns"
__version__ = "0.5"

from unit import Unit, is_unit
from quantity import Quantity, is_quantity

# Attach methods of Unit class that return a Quantity to Unit class.
# I put them here to avoid circular dependence in imports.
# i.e. Quantity depends on Unit, but not vice versa

def _unit_class_rdiv(self, other):
    """
    Divide another object type by a Unit.
    
    Returns a new Quantity with a value of other and units
    of the inverse of self.
    """
    if is_unit(other):
        raise NotImplementedError('programmer is surprised __rtruediv__ was called instead of __truediv__')
    else:
        # print "R scalar / unit"
        unit = pow(self, -1.0)
        value = other
        return Quantity(value, unit).reduce_unit(self)

Unit.__rtruediv__ = _unit_class_rdiv
Unit.__rdiv__ = _unit_class_rdiv


def _unit_class_mul(self, other):
    """Multiply a Unit by an object.
    
    If other is another Unit, returns a new composite Unit.  
    Exponents of similar dimensions are added.  If self and 
    other share similar BaseDimension, but
    with different BaseUnits, the resulting BaseUnit for that
    BaseDimension will be that used in self.
    
    If other is a not another Unit, this method returns a 
    new Quantity...  UNLESS other is a Quantity and the resulting
    unit is dimensionless, in which case the underlying value type
    of the Quantity is returned.
    """
    if is_unit(other):
        if self in Unit._multiplication_cache:
            if other in Unit._multiplication_cache[self]:
                return Unit._multiplication_cache[self][other]
        else:
            Unit._multiplication_cache[self] = {}
        # print "unit * unit"
        result1 = {} # dictionary of dimensionTuple: (BaseOrScaledUnit, exponent)
        for unit, exponent in self.iter_base_or_scaled_units():
            d = unit.get_dimension_tuple()
            if d not in result1:
                result1[d] = {}
            assert unit not in result1[d]
            result1[d][unit] = exponent
        for unit, exponent in other.iter_base_or_scaled_units():
            d = unit.get_dimension_tuple()
            if d not in result1:
                result1[d] = {}
            if unit not in result1[d]:
                result1[d][unit] = 0
            result1[d][unit] += exponent
        result2 = {} # stripped of zero exponents
        for d in result1:
            for unit in result1[d]:
                exponent = result1[d][unit]
                if exponent != 0:
                    assert unit not in result2
                    result2[unit] = exponent
        new_unit = Unit(result2)
        Unit._multiplication_cache[self][other] = new_unit
        return new_unit
    elif is_quantity(other):
        # print "unit * quantity"
        value = other._value
        unit = self * other.unit
        return Quantity(value, unit).reduce_unit(self)
    else:
        # print "scalar * unit"
        value = other
        unit = self
        # Is reduce_unit needed here?  I hope not, there is a performance issue...
        # return Quantity(other, self).reduce_unit(self)
        return Quantity(other, self)
        
Unit.__mul__ = _unit_class_mul
Unit.__rmul__ = Unit.__mul__
Unit._multiplication_cache = {}


# run module directly for testing
if __name__=='__main__':
    # Test the examples in the docstrings
    import doctest, sys
    doctest.testmod(sys.modules[__name__])