Algorithms and Data Structures  ASSIGNMENT 2

Algorithms and Data Structures (ECE 345) 
ASSIGNMENT 2
1. Assignments must be submitted by 5:00PM EST on the due date through the Quercus submission system as a single
PDF file.
2. Assignments must be completed individually except the programming exercise for which you can work in group
of up to two students. Report for the programming exercise must be submitted separately at the same time. Only
one report is needed for each group.
3. All pages must be numbered and no more than a single answer for any question.
4. Use LATEX or Microsoft Word for your assignment writeup. You can find a LATEX and a Word template for writing
your assignment on Quercus.
5. Any problem encountered with submission must be reported to the head TA as soon as possible.
6. Unless otherwise stated, you need to justify the correctness and complexity of algorithms you designed for the
problems.
EXERCISE 1 Another me?!, 10 points
Every UofT student has his/her unique UTORid. A UTORid is an eight-character (lowercase) alphanumeric string. We
call two UTORids similar if one can be converted to the other by replacing one single character (but not adding or
deleting). e.g., abcd0123 is similar to abcde123 or abcd0124, but not similar to abcd1023.You are given a set of n
UTORids, and your task is to find every UTORid in the set which has a similar UTORid in the set. Describe a worst-case
O(n) algorithm to complete your task. Justify your answer.
EXERCISE 2 Linear Time Sorting, 10 points
You are given an unsorted array A of b
√
nc integers ranging from −n to n. The absolute value of each element is a square
number. In other words, each of them is either of the form i
2 or −i
2
, where i is an integer. Devise an algorithm to sort A in
O(
√
n) time using only O(
√
n) memory. Partial credit will be awarded if your algorithm requires more time or memory
than what is specified above.
EXERCISE 3 AVL Trees, 10 points
Your friend Quincy and you established a very unreliable communication protocol: He consistently sends messages to
you in the form of (k, v), where k is the timestamp when he sends you the message, and v is the content of the message
represented by an integer value. We call two messages (k1, v1) and (k2, v2) you received out of order if k1 < k2 but
v1 v2. Assume all messages have distinct k values and distinct v values, describe an O(logn) algorithm to determine
whether a new message is received out of order, where n is the number of messages you have already received.
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EXERCISE 4 Hashing, 10 points
Given a list of n requests r1,··· ,rn, each request ri contains the ID of the server to which the request was sent. Your task
now is to find the first request in the list which is sent to a unique server, or report that no such request exists. Design an
algorithm that runs in expected O(n) time. Justify your answer.
EXERCISE 5 Hashing in Depth, 20 points
Suppose we have a hash table with m slots, and we have n values to hash into the hash table. Suppose m n and the
hashing function we use satisfies the simple uniform hashing assumption.
(a) Give a lower bound for the probability that a specific slot is empty after hashing n values. Your answer should be
given in the form of c0 +c1
n
m
, where c0 and c1 are constants. (Hint: You may want to use Binomial expansion).
(b) Give a lower bound for the probability that a specific slot stores only one value after hashing n values. Your answer
should be given in the form of f1(n)
m +
f2(n)
m2
, where f1 and f2 are polynomial functions of n.
(c) Give an upper bound for the probability that a specific slot stores at least two value after hashing n values. (Hint:
Use part (a) and (b)).
EXERCISE 6 Programming Exercise, 30 points
(In)secure Password Storage/Checking: In this programming assignment, you will experiment with simple and (in)secure
password storage/checking, and you will measure some performance statistics.
1. Outline:
• Assume that you will be given a file containing a list of existing passwords. The name of the file will be
passwords.txt, and the format will be one password per line without comma separators. Assume the file
will reside in the same path as your executables. A sample file will be provided to you on Blackboard. Your
program will need to parse the file and hash the passwords into a hash table. You may assume that your
program will be tested on files with 100−10000 passwords.
• Each password in passwords.txt will be alphanumeric without any complex characters such as !, ?,
#, &, etc. It will only consist of a mix of lower and/or upper case English alphabet letters with digits ∈ {0,...,9}. Each password will be 6 to 12 characters long. For example, ece345LoVe is a valid
password, whereas HATEece345HATE is invalid due to length violation (and probably other reasons...!), and
Pa33word!!?? is invalid due to containing complex characters.
• You are free to decide how to implement your hashing. Use only methods described in lecture. You are not
allowed to use cryptographic libraries such as OpenSSL. Security is not our concern in this exercise.
• Your executable should be named as checkpass, and it’s behavior should be as follows:
– It should be callable from command line as checkpass passwords.txt password, where passwords.txt
is a file containing passwords and password is user provided. For example: checkpass passwords.txt
23CdB9a13
For C/C++ please include a Makefile to build your sources into the executable. For any other compiled
language, please follow the same format as for C/C++.
For Java please create a Makefile that compiles your sources and a checkpass.sh script that runs your
code for the given input, with the correct classpath.
For Python please name your script checkpass.py, have #!/usr/bin/env pythonX (where X is the version of python you are using) as the first line of your file. Additionally please do chmod +x checkpass.py
so that you can run your file as ./checkpass.py passwords.txt password. For any other scripting
language (supported by the UG machines), please follow the same format as for python, except name
your file as per that language (ie for perl use checkpass.pl)
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– Your program then should print in standard output VALID if and only if the password is valid according
to the constraints described above AND the password does not already exist in passwords.txt AND
the reverse of the password does not exist in passwords.txt. Think how to make the check for reverse
passwords efficient. In any other case your program will print INVALID.
– If the password entered is valid then your program should hash it and store the password (not its hash)
into passwords.txt, and then it should terminate. If the password is not valid then the program should
simply terminate after outputting INVALID.
2. Deliverables:
• Your full source code. Any code that does not build or does not run will receive a mark of 0. All code will be
marked on the UG machines. This should be handed in electronically.
• A written report with readable graphs (with proper lables, grid ticks, and legends as needed). The report must
address the following points:
– Implementation details regarding your hashing approach. What is the size of your hash table and which
hash function are you using? Are you applying chaining or open addressing? If you are using open
addressing state what probing sequence you are implementing.
– A brief justification of your implementation decisions above.
– For a fixed number of entries n = 1000, vary the size of your table to get various load factors. For each
load factor, run your code on five different password.txt files which you will create. Each file should
include 1000 password entries. Plot the average number of collisions vs. load factor. You will need
several values of load factor to see a trend on your plot. Briefly explain what you observe in the plot and
why.
NOTE: To set up this experiment, you can easily create another version of your program where there is
no password input from the command line. You only need to parse the passwords.txt files that you
will create and insert the entries in your hashtable. However, DO NOT submit this version of the code.
• A version of submission.toml file that contains your group information and details about your source files.
You can reuse the one from previous assignments. This should be handed in electronically.
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