Assignment 1: Search CSC 384H


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In this assignment you will use some supplied search routines to solve a simplified version of the Amazon
warehouse delivery problem.
What is supplied. You will be supplied with python code implementing various search routines. The file contains a set of classes for doing search:
1. StateSpace is an abstract base class for implementing search spaces. This base class defines a
fixed interface to a state space that is used by the SearchEngine class to perform search in that
state space.
For a specific problem one defines a concrete sub-class that inherits from StateSpace. This concrete sub-class inherits some of the “utility” routines already implemented in the base class, and
overrides some other core routines to provide functionality specific to the problem being represented.
The file contains an example of a concrete StateSpace for solving the waterjugs problem. This WaterJugs class inherits some functions (methods)1
from the abstract class
StateSpace and overrides the following methods:
(a) The class initializer method “ init ”. This method initializes the specific state data structures
for a WaterJugs object, and calls the base class init method to initialize the general data shared
by all state space objects.
(b) The successors method. Note that each WaterJugs object represents a state in the waterjugs state space. So successors is implemented as a “self” successor function. That is,
the state uses this method to generate its own successors. The successors method returns a
list of WaterJugs objects—each of which is reachable from “self” by a single action. Note
that every WaterJugs object contains a reference to its parent, the action used to obtain it (a
string is used to represent the action), the cost of getting to this state (the g-value), along with
the WaterJugs specific data structures.
(c) The hashable state method. The search engine uses a python dictionary to implement
cycle checking. This means that it must use a state to index into that dictionary. Often is it
necessary to use lists and other mutable python objects in the representation of a state, and
these mutable data structures cannot be used to index into a dictionary. So to allow the search
engine to operate correctly it is the responsibility of the implementor of a state space to write
a hashable state function. This function returns a data item such that (a) this data can be
used to index into a dictionary (e.g., it might convert a list representation into an immutable
tuple) and (b) two states generate the same hashable state if and only if they represent the
same state. For example, in water jugs we might have two different WaterJugs objects with
different parents and different g-values, but they might represent the same state (i.e., the same
configuration of the jugs). In this case, both objects should return the same data item when their
hashable state method is called.
Note that a sub-class of StateSpace can use additional methods to make implementing the
successor state method or other methods more modular. These extra methods will not be called
directly by the search routines.
2. SearchEngine is a class that implements a number of search routines. By creating an object of
this class initialized with various parameters we can set that object’s search strategy and invoke that
object’s search algorithm on a problem. The SearchEngine class uses two other auxiliary classes:
(a) sNode is a class used by the search routines to implement nodes in the search space. Remember
that nodes in the search space represent paths in the State Space. In addition, the nodes also store
other information useful for the search routines and implement comparison functions used to sort
OPEN in the search routines.
(b) Open is a class used by the search routines to implement the OPEN set. Dependent on the search
strategy different types of data structures are used in the class to store the OPEN set. sNode
and Open are internal classes used by SearchEngine.
1Note that a function contained in a class is typically called a method.
Become familiar with this code. You will need to understand its general operation when debugging
your implementation. Note that the SearchEngine method trace on can be useful in debugging your
implementation. In addition the search engine object can be configured in various ways so that it performs
various types of search.
2 Warehouse Domain
In this question you are to solve a simplified warehouse delivery problem. The problem is modeled after the
much more complex system used by Amazon in their warehouses (see
The warehouse has various products, a set of packing stations, and a set of robots. The products and
packing stations are located at various fixed locations around the warehouse. The robots can move around
the warehouse.
The typical problem for this domain is to operate the robots so as to complete a set of orders. Each
order is for a particular product to be moved to a particular packing station. A robot must be assigned to this
job and it will move to the product’s location, pick up the product, and then move to the packing station to
deliver the product. After the delivery the robot will remain at the packing station until it is assigned another
The problem is solved when all orders have been assigned to robots and the robots have completed all
jobs they have been assigned. The quality of a solution is measured as the time at which the final delivery is
completed. That is, a better solution delivers the orders to the packing stations more quickly.
The problem is simplified in a number of ways from the real application, some of these simplifications
include: the robot can only carry one product at a time; each order consists of only one product to be
delivered; we do not worry about the robots occupying the same location or otherwise crashing into each
other; we don’t worry about the actions involved in picking up the product and dropping it off at the packing
station—we just ask the robot to “pick-up at location (x1, y1) and deliver at location (x2, y2)” without
worrying about the intermediate steps.
2.0.1 Locations and Robot Moves
Locations are specified using a simple x-y coordinate system in which x and y are both integers greater than
or equal to zero.
Robots can move between any two location. If the robot is currently at (x1, y1) it can move to location
(x2, y2). It can only move horizontally or vertically and it takes 1 unit of time to move +1 or -1 in either
direction. For example, to move from (1, 2) to (4, 5) the robot will need abs(1 − 4) + abs(2 − 5) = 6 units
of time. In particular it needs to move +3 in the x direction and then +3 in the y direction.
2.0.2 Concurrency
When we have multiple robots we have to manage them executing delivery jobs concurrently. To achieve
this the states of the state space will include (a) a current time and (b) some pending events that are going
to occur in the future (i.e., in the future of this particular state—each state can have its own pending future
events). This will allow us have a state space in which multiple concurrent activities can be taking place in
any state. These activities all have some end time in the future.
In this domain the future events will all be completions of deliveries by different robots. So we can have
a state where a bunch of different robots are all concurrently working on different deliveries. Each delivery
will complete as some different time in the future.
To manage such states the state space will employ two different types of actions.
1. Actions that start up activities and update the state to include information about the future time when
these activities will be completed. These actions generate a new state but that new state will have the
same time as the current state. That is, these actions do not move time forward and cost zero (since
we are trying to find a solution that completes at the earliest time).
2. A single action that moves the time forward. This action operates on a state by moving time forward
to the completion of the earliest future event. In this domain, these are that events involving robots
finishing deliveries. This action can only move time forward to the time of the earliest delivery among
the state’s concurrent active deliveries. These actions cost an amount equal to the amount the move
time forward.
2.1 Example
Say we have the following products with x-y locations (locations are represented as pairs (x, y)).
[[’prod1’, (0,5)], [’prod2’, (1,5)], [’prod3’, (2,5)]]
The following packing stations with x-y locations
[[’pack1’, (0,0)], [’pack2’, (1,0)]]]
And the following list of delivery orders that have to be completed
[[’order2’, ’prod3’, ’pack1’], [’order3’, ’prod2’, ’pack2’],
[’order4’, ’prod3’, ’pack1’]]
That is we want to deliver two items of prod3 to packing station pack1. In our simplified domain this can
only be accomplished with two separate orders. We also want to deliver prod2 to packing station pack2.
Finally, in our example we have two robots. To specify the robots and to capture the fact that a robot
might be currently working on a delivery we need to provide information about the status of each robot.
[[’r1’, ’idle’, (0,0)], [’r2’, ’on_delivery’, (1,0), 8]]
Here robot r1 is currently idle. That is, it is not working on any delivery. It is located at coordinates
(0, 0). Robot r2 on the other hand is currently on_delivery, we do not know its current location (as it
is moving) but we know that after its delivery is over it will be located at coordinates (1, 0). Furthermore,
we know that it will complete its delivery at time 8.
Finally, the current time in the state is 0.
2.1.1 Actions
In this example state there are 4 possible actions that could be executed. We can pick any idle robot and
get them to work on any order. In this case we have one idle robot and 3 possible orders, so these generate
3 actions. Finally we can move time forward to the completion of the earliest pending delivery, this is the
fourth action.
1. Assign r1 to order2. To deliver this order r1 will have to move from its current location (0, 0)
to pick up product prod3 which is located at (2, 5) then it will have to move from (2, 5) to packing
station pack1 located at (0, 0). This means that the delivery will be finished in 14 time units from
the current state time. The current time is 0 so the delivery will be finished at time 14, and will leave
r1 at location (0, 0).
The new state generated by this action will have an updated list of delivery orders where order2 has
been removed.
[[’order3’, ’prod2’, ’pack2’], [’order4’, ’prod3’, ’pack1’]]
The new state will also have an updated status for robot r1, so the robot status information will
[[’r1’, ’on_delivery, (0,0), 14], [’r2’, ’on_delivery’, (1,0), 8]]
Note that this action does not affect r2.
The action does not change anything else in the state. In particular, the current time remains as 0.
2. We can assign r1 to order3. This action updates the list of orders and the status of r1.
3. We can assign r1 to order4.
4. We can move time forward to the time of completion of the earliest delivery. In this example, there is
only one delivery in progress and it will complete at time 8. So this action does not change the list or
orders, but it will change the robot status information to:
[[’r1’, ’idle’, (0,0)], [’r2’, ’idle’, (1,0)]]
That is, r2 now becomes idle and is located at the final location of the delivery it was on. Furthermore,
the time of this new state has been moved forward to 8. Since we started at time 0, the cost of this
action is 8 (the other three actions which are all delivery actions cost 0).
Some important notes:
1. It is important to be able to wait for a delivery to complete even when there is an idle robot and an
unfulfilled order. In some cases the shortest completion time might be achieved by waiting for an
active robot to finish and then using that robot, rather than using a far away idle robot.
2. In the new state after doing the first action of assigning r1 to order1, we have the robot status
[[’r1’, ’on_delivery’, (0,0), 14], [’r2’, ’on_delivery’, (1,0), 8]]
In this new state there are no idle robots, so there will be only one possible action that could be
executed: the action of updating time. Furthermore, we can only update time to move forward to the
smallest of 8 and 14 as 8 is the time of the earliest delivery. If we wanted to wait until time 14, we
have to that by executing two move time forward actions: move time to 8, then move time to 14. Note
the action that moves the time from 0 to 8 will cost 8, while the action that moves the time from 8 to
14 will cost 14-8=6.
3. A “move time forward” action is performed to move the time forward to the smallest of the completion
times of the future deliveries. However, it could be the case that multiple deliveries are scheduled to
complete at that time. In this case the robot status of all completed deliveries must be updated. For
example, suppose that the state has the following robot status
[[’r1’, ’on_delivery’, (0,0), 14], [’r2’, ’on_delivery’, (1,0), 8]]
[’r3’, ’on_delivery’, (0,5), 8]]
A move time forward action will move the time to 8 (the smallest of the future delivery times), and
will update the status of both robots r2 and r3. The robot status in the new state after this action will
[[’r1’, ’on_delivery’, (0,0), 14], [’r2’, ’idle’, (1,0)]]
[’r3’, ’idle’, (0,5)]]
2.2 To Do
1. Implement a state representation for this problem and a successor state function. You will need to
keep track of at least the following items in your state objects:
(a) The time in that state.
(b) The list of unfulfilled orders in that state.
(c) The status of each robot in that state. When the robot is idle its location needs to be known.
When the robot is on an delivery the state needs to track the delivery’s completion time and the
location of the robot after that the delivery is finished.
2. Implement the successor state function for class warehouse. In particular you must implement two
types of actions:
(a) Actions named
deliver(, ,