# Source code for pants.ant

```
"""
.. module:: ant
:platform: Linux, Unix, Windows
:synopsis: Provides functionality for finding each solution step as well
as representing a complete solution.
.. moduleauthor:: Robert Grant <rhgrant10@gmail.com>
"""
import sys
import random
import bisect
import itertools
import functools
from .world import World
@functools.total_ordering
[docs]class Ant:
"""
A single independent finder of solutions to a :class:`World`.
Each :class:`Ant` finds a solution to a world one move at a time. They
also represent the solution they find, and are capable of reporting which
nodes and edges they visited, in what order they were visited, and the
total length of the solution.
Two properties govern the decisions each :class:`Ant` makes while finding
a solution: *alpha* and *beta*. *alpha* controls the importance placed on
pheromone while *beta* controls the importance placed on distance. In
general, *beta* should be greater than *alpha* for best results.
:class:`Ant`\s also have a *uid* property that can be used to identify a
particular instance.
Using the :func:`initialize` method, each :class:`Ant` *must be
initialized* to a particular :class:`World`, and optionally may be given an
initial node from which to start finding a solution. If a starting node is
not given, one is chosen at random. Thus a few examples of instantiation
and initialization might look like:
.. code-block:: python
ant = Ant()
ant.initialize(world)
.. code-block:: python
ant = Ant().initialize(world)
.. code-block:: python
ant = Ant(alpha=0.5, beta=2.25)
ant.initialize(world, start=world.nodes[0])
.. note::
The examples above assume the world has already been created!
Once an :class:`Ant` has found a solution (or at any time), the solution
may be obtained and inspected by accessing its ``tour`` property, which
returns the nodes visited in order, or its ``path`` property, which
returns the edges visited in order. Also, the total distance of the
solution can be accessed through its ``distance`` property. :class:`Ant`\s
are even sortable by their distance:
.. code-block:: python
ants = [Ant() for ...]
# ... have each ant in the list solve a world
ants = sorted(ants)
for i in range(1, len(ants)):
assert ants[i - 1].distance < ants[i].distance
:class:`Ant`\s may be cloned, which will return a shallow copy while not
preserving the *uid* property. If this behavior is not desired, simply use
the :func:`copy.copy` or :func:`copy.deepcopy` methods as necessary.
The remaining methods mainly govern the mechanics of making each move.
:func:`can_move` determines whether all possible moves have been made,
:func:`remaining_moves` returns the moves not yet made, :func:`choose_move`
returns a single move from a list of moves, :func:`make_move` actually
performs the move, and :func:`weigh` returns the weight of a given move.
The :func:`move` method governs the move-making process by gathering the
remaining moves, choosing one of them, making the chosen move, and
returning the move that was made.
"""
uid = 0
def __init__(self, alpha=1, beta=3):
"""Create a new Ant for the given world.
:param float alpha: the relative importance of pheromone (default=1)
:param float beta: the relative importance of distance (default=3)
"""
self.uid = self.__class__.uid
self.__class__.uid += 1
self.world = None
self.alpha = alpha
self.beta = beta
self.start = None
self.distance = 0
self.visited = []
self.unvisited = []
self.traveled = []
[docs] def initialize(self, world, start=None):
"""Reset everything so that a new solution can be found.
:param World world: the world to solve
:param Node start: the starting node (default is chosen randomly)
:return: `self`
:rtype: :class:`Ant`
"""
self.world = world
if start is None:
self.start = random.randrange(len(self.world.nodes))
else:
self.start = start
self.distance = 0
self.visited = [self.start]
self.unvisited = [n for n in self.world.nodes if n != self.start]
self.traveled = []
return self
[docs] def clone(self):
"""Return a shallow copy with a new UID.
If an exact copy (including the uid) is desired, use the
:func:`copy.copy` method.
:return: a clone
:rtype: :class:`Ant`
"""
ant = Ant(self.alpha, self.beta)
ant.world = self.world
ant.start = self.start
ant.visited = self.visited[:]
ant.unvisited = self.unvisited[:]
ant.traveled = self.traveled[:]
ant.distance = self.distance
return ant
@property
def node(self):
"""Most recently visited node."""
try:
return self.visited[-1]
except IndexError:
return None
@property
def tour(self):
"""Nodes visited by the :class:`Ant` in order."""
return [self.world.data(i) for i in self.visited]
@property
def path(self):
"""Edges traveled by the :class:`Ant` in order."""
return [edge for edge in self.traveled]
def __eq__(self, other):
"""Return ``True`` if the distance is equal to the other distance.
:param Ant other: right-hand argument
:rtype: bool
"""
return self.distance == other.distance
def __lt__(self, other):
"""Return ``True`` if the distance is less than the other distance.
:param Ant other: right-hand argument
:rtype: bool
"""
return self.distance < other.distance
[docs] def can_move(self):
"""Return ``True`` if there are moves that have not yet been made.
:rtype: bool
"""
# This is only true after we have made the move back to the starting
# node.
return len(self.traveled) != len(self.visited)
[docs] def move(self):
"""Choose, make, and return a move from the remaining moves.
:return: the :class:`Edge` taken to make the move chosen
:rtype: :class:`Edge`
"""
remaining = self.remaining_moves()
choice = self.choose_move(remaining)
return self.make_move(choice)
[docs] def remaining_moves(self):
"""Return the moves that remain to be made.
:rtype: list
"""
return self.unvisited
[docs] def choose_move(self, choices):
"""Choose a move from all possible moves.
:param list choices: a list of all possible moves
:return: the chosen element from *choices*
:rtype: node
"""
if len(choices) == 0:
return None
if len(choices) == 1:
return choices[0]
# Find the weight of the edges that take us to each of the choices.
weights = []
for move in choices:
edge = self.world.edges[self.node, move]
weights.append(self.weigh(edge))
# Choose one of them using a weighted probability.
total = sum(weights)
cumdist = list(itertools.accumulate(weights)) + [total]
return choices[bisect.bisect(cumdist, random.random() * total)]
[docs] def make_move(self, dest):
"""Move to the *dest* node and return the edge traveled.
When *dest* is ``None``, an attempt to take the final move back to the
starting node is made. If that is not possible (because it has
previously been done), then ``None`` is returned.
:param node dest: the destination node for the move
:return: the edge taken to get to *dest*
:rtype: :class:`Edge`
"""
# Since self.node simply refers to self.visited[-1], which will be
# changed before we return to calling code, store a reference now.
ori = self.node
# When dest is None, all nodes have been visited but we may not
# have returned to the node on which we started. If we have, then
# just do nothing and return None. Otherwise, set the dest to the
# node on which we started and don't try to move it from unvisited
# to visited because it was the first one to be moved.
if dest is None:
if self.can_move() is False:
return None
dest = self.start # last move is back to the start
else:
self.visited.append(dest)
self.unvisited.remove(dest)
edge = self.world.edges[ori, dest]
self.traveled.append(edge)
self.distance += edge.length
return edge
[docs] def weigh(self, edge):
"""Calculate the weight of the given *edge*.
The weight of an edge is simply a representation of its perceived value
in finding a shorter solution. Larger weights increase the odds of the
edge being taken, whereas smaller weights decrease those odds.
:param Edge edge: the edge to weigh
:return: the weight of *edge*
:rtype: float
"""
pre = 1 / (edge.length or 1)
post = edge.pheromone
return post ** self.alpha * pre ** self.beta
```