"""
Copyright 2013 Steven Diamond

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
"""

# An object oriented model of a circuit.
import cvxpy as cp
import abc

class Node(object):
    """ A node connecting devices. """
    def __init__(self):
        self.voltage = cp.Variable()
        self.current_flows = []

    # The current entering a node equals the current leaving the node.
    def constraints(self):
        return [sum(f for f in self.current_flows) == 0]

class Ground(Node):
    """ A node at 0 volts. """
    def constraints(self):
        return [self.voltage == 0] + super(Ground, self).constraints()

class Device(object):
    __metaclass__ = abc.ABCMeta
    """ A device on a circuit. """
    def __init__(self, pos_node, neg_node):
        self.pos_node = pos_node
        self.pos_node.current_flows.append(-self.current())
        self.neg_node = neg_node
        self.neg_node.current_flows.append(self.current())

    # The voltage drop on the device.
    @abc.abstractmethod
    def voltage(self):
        return NotImplemented

    # The current through the device.
    @abc.abstractmethod
    def current(self):
        return NotImplemented

    # Every path between two nodes has the same voltage drop.
    def constraints(self):
        return [self.pos_node.voltage - self.voltage() == self.neg_node.voltage]

class Resistor(Device):
    """ A resistor with V = R*I. """
    def __init__(self, pos_node, neg_node, resistance):
        self._current = cp.Variable()
        self.resistance = resistance
        super(Resistor, self).__init__(pos_node, neg_node)

    def voltage(self):
        return -self.resistance*self.current()

    def current(self):
        return self._current

class VoltageSource(Device):
    """ A constant source of voltage. """
    def __init__(self, pos_node, neg_node, voltage):
        self._current = cp.Variable()
        self._voltage = voltage
        super(VoltageSource, self).__init__(pos_node, neg_node)

    def voltage(self):
        return self._voltage

    def current(self):
        return self._current

class CurrentSource(Device):
    """ A constant source of current. """
    def __init__(self, pos_node, neg_node, current):
        self._current = current
        self._voltage = cp.Variable()
        super(CurrentSource, self).__init__(pos_node, neg_node)

    def voltage(self):
        return self._voltage

    def current(self):
        return self._current

# # Create a simple circuit and find the current and voltage.
nodes = [Ground(),Node(),Node()]
# A 5 V battery
devices = [VoltageSource(nodes[0], nodes[2], 10)]
# A series of pairs of parallel resistors.
# 1/4 Ohm resistor and a 1 Ohm resistor in parallel.
devices.append( Resistor(nodes[0], nodes[1], 0.25) )
devices.append( Resistor(nodes[0], nodes[1], 1) )
# 4 Ohm resistor and a 1 Ohm resistor in parallel.
devices.append( Resistor(nodes[1], nodes[2], 4) )
devices.append( Resistor(nodes[1], nodes[2], 1) )

# Create the problem.
constraints = []
for obj in nodes + devices:
    constraints += obj.constraints()
cp.Problem(cp.Minimize(0), constraints).solve()
for node in nodes:
    print(node.voltage.value)
