Assignment Description
In this lab, you will be exploring a different tree application (Huffman
Trees), which allow for efficient
lossless compression of files. There are a lot of files in this lab, but you
will only be modifying huffman_tree.cpp
.
Lab Insight
Huffman encoding is a fundamental compression algorithms for data. Compressing data is a very powerful tool that can represent a given set of information in less space, thus allowing the data to be transferred more efficiently. Different types of compression can be seen within images formats like JPG(lossy) or PNG(lossless). It can also be seen in ZIP files for compressing multiple files. The concept of encoding data can be seen in future courses CS 438, Communication Networks, dealing with transferring large amounts of data, and CS 461, Computer Security, which deals with encoding data for a layer of privacy.
Checking Out the Code
You can find the code for all our released assignments at the following public git repo.
You can either clone this or pull a zip file of the repo to your local computer. All the files for this MP are in the lab_huffman
directory. You should copy that directory into the directory you setup for your docker container.
Preparing Your Code
This semester for MPs we are using CMake rather than just make. This allows for us to use libraries such as Catch2 that can be installed in your system rather than providing them with each assignment. This change does mean that for each assignment you need to use CMake to build your own custom makefiles. To do this you need to run the following in the base directory of the assignment. Which in this assignment is the lab_huffman
directory.
mkdir build
cd build
This first makes a new directory in your assignment directory called build
. This is where you will actually build the assignment and then moves to that directory. This is not included in the provided code since we are following industry standard practices and you would normally exclude the build directory from any source control system.
Now you need to actually run CMake as follows.
cmake ..
This runs CMake to initialize the current directory which is the build
directory you just made as the location to build the assignment. The one argument to CMake here is ..
which referes to the parent of the current directory which in this case is top of the assignment. This directory has the files CMake needs to setup your assignment to be build.
At this point you can in the build directory run make as described to build the various programs for the MP.
Video Intro
The following is meant to help you understand the task for this lab. It is strongly recommended that you watch the video to understand the motivation for why we’re talking about Huffman Encoding as well as how the algorithm works.
There is a video introduction for this lab! If you are interested in seeing a step-by-step execution of the Huffman Tree algorithms, please watch it:
The Huffman Encoding
In 1951, while taking an Information Theory class as a student at MIT, David A. Huffman and his classmates were given a choice by the professor Robert M. Fano: they can either take the final exam, or if they want to opt out of it they need to find the most efficient binary code. Huffman took the road less traveled and the rest they say is history.
Put simply, Huffman encoding takes in a text input and generates a binary code (a string of 0’s and 1’s) that represents that text. Let’s look at an example: Input message: “feed me more food”
Building the Huffman tree
Input: “feed me more food”
Step 1: Calculate frequency of every character in the text, and order by increasing frequency. Store in a queue.
r : 1 | d : 2 | f : 2 | m : 2 | o : 3 | 'SPACE' : 3 | e : 4
Step 2: Build the tree from the bottom up. Start by taking the two least frequent characters and merging them (create a parent node for them). Store the merged characters in a new queue:
SINGLE: f : 2 | m : 2 | o : 3 | 'SPACE' : 3 | e : 4
MERGED: rd : 3
Step 3: Repeat Step 2 this time also considering the elements in the new queue. ‘f’ and ‘m’ this time are the two elements with the least frequency, so we merge them:
SINGLE: o : 3 | 'SPACE' : 3 | e : 4
MERGED: rd : 3 | fm : 4
Step 4: Repeat Step 3 until there are no more elements in the SINGLE queue, and only one element in the MERGED queue:
SINGLE: e : 4
MERGED: rd : 3 | fm : 4 | o+SPACE : 6
SINGLE:
MERGED: fm : 4 | o+SPACE : 6 | rde: 7
SINGLE:
MERGED: rde: 7 | fmo+SPACE: 10
SINGLE:
MERGED: rdefmo+SPACE: 17
From Text to Binary
Now that we built our Huffman tree, its time to see how to encode our original message “feed me more food” into binary code.
Step 1: Label the branches of the Huffman tree with a ‘0’ or ‘1’. BE CONSISTENT: in this example we chose to label all left branches with ‘0’ and all right branches with ‘1’. In the case of our visualizations, some of the nodes may be swapped for convenience of placement. However, any path with label ‘1’ is intended to be on the right side, and any path with label ‘0’ is intended to be on the left side.
Step 2: Taking one character at a time from our message, traverse the Huffman tree to find the leaf node for that character. The binary code for the character is the string of 0’s and 1’s in the path from the root to the leaf node for that character. For example: ‘f’ has the binary code: 100
So our message “feed me more food” becomes 10001010011111111010111111110111000001111111100110110001
Efficiency of Huffman Encoding Notice that in our Huffman tree, the more frequent a character is, the closer it is to the root, and as a result the shorter its binary code is. Can you see how this will result in compressing the encoded text?
From Binary Code to Text
We can also decode strings of 0’s and 1’s into text using our Huffman tree. What word does the code 00001001
translate to?
What About the Rest of the Alphabet?
Notice that in our example above, the Huffman tree that we built does not have all the alphabet’s letters; so while we can encode our message and some other words like “door” or “deer”, it won’t help us if we need to send a message containing a letter that’s not in the tree. For our Huffman encoding to be applicable in the real world we need to build a huffman tree that contains all the letters of the alphabet; which means instead of using “feed me more food” to build our tree, we should use a text document that contains all letters of the alphabet to build our Huffman tree. As a fun example, here is the Huffman tree that results when we use the text of the Declaration of Independence to build it.
Here is the Doxygen generated list of files and their uses.
Static Keyword
The static keyword means that the variable or function is shared by all
instances of the class. This means that if a static function is used that
inside the function, no references to the functions member variables may be
used (no access to this
pointer). Static functions can be beneficial when
it is inconvient to make a new instance of a class, but it would be nice to
use the member function. For example, if you’re inside a member function
and want to call a static function of that class you can do
myStaticHelper(args)
the same way you’d call another member function.
Implement buildTree()
and removeSmallest()
Your first task will be to implement the buildTree()
function on a
HuffmanTree
. This function builds a HuffmanTree
based on a collection of
sorted Frequency
objects. Please see the Doxygen for
buildTree()
for details on the algorithm. You also will
probably want to consult the list of constructors for
TreeNode
s.
You should implement removeSmallest()
first as it will help you in writing
buildTree()
!
Tie Breaking To facilitate grading, make sure that when building internal nodes, the left child has the smallest frequency.
In removeSmallest()
, break ties by taking the front of the singleQueue
!
Implement decode()
Your next task will be using an existing HuffmanTree
to decode a given binary
file. You should start at the root and traverse the tree using the description
given in the Doxygen. Here is the Doxygen for decode()
.
You will probably find the Doxygen for BinaryFileReader
useful
here.
We’re using a standard stringstream
here to build up our output. To append
characters to it, use the following syntax:
ss << myChar;
Implement writeTree()
and readTree()
Finally, you will write a function used for writing HuffmanTrees
to files in
an efficient way, and a function to read this efficiently stored file-based
representation of a HuffmanTree
.
Here is the Doxygen for writeTree()
and the Doxygen for
readTree()
.
You will probably find the Doxygen for BinaryFileWriter
useful
here.
Testing Your Code!
We’ve provided you with a collection of data files to help you explore Huffman encoding. Run the following command to download and extract the files. They will be in a newly-created data
directory.
wget https://courses.engr.illinois.edu/cs225/fa2022/assets/assignments/labs/huffman/lab_huffman_data.tar && tar -xf lab_huffman_data.tar && rm lab_huffman_data.tar
If you are working on your own, non-linux machine, you can enter the path in a browser to download the tar file and manually extract it in a local directory.
When you run make
, two programs should be generated: encoder
and decoder
, with the following usages:
$ ./encoder
Usage:
./encoder input output treefile
input: file to be encoded
output: encoded output
treefile: compressed huffman tree for decoding
$ ./decoder
Usage:
./decoder input treefile output
input: file to be decoded
treefile: compressed huffman tree to use for decoding
output: decompressed file
Use your encoder
to encode a file in the data directory, and then use your
compressed file an the huffman tree it built to decode
it again using the
decoder. If diff
-ing the files produces no output, your HuffmanTree
should
be working!
When testing, try using small files at first such as data/small.txt
. Open it
up and look inside. Imagine what the tree should look like, and see what’s
happening when you run your code.
Now try running your code:
$ ./encoder data/small.txt output.dat treefile.tree
______________ 28 _____________
______________/ \______________
______ 11 _____ ______ 17 _____
______/ \______ ______/ \______
s:5 __ 6 __ __ 8 __ __ 9 __
__/ \__ __/ \__ __/ \__
y:3 3 l:4 i:4 4 :5
/ \ / \
h:1 t:2 2 2
/ \ / \
r:1 o:1 a:1 \n:1
Saving HuffmanTree to file...
Differing Output It is possible to get different output than this tree and still pass catch. To be clear you should not spend time agonizing over the specific details of the tree but if the tree correctly puts the smaller frequencies on the longer paths. Use the provided test cases on catch to see if your code is passing.
You can also test under catch as usual by running:
make test && ./test
Submitting Your Work
The following files are used to grade this assignment:
huffman_tree.cpp
huffman_tree.h
All other files including any testing files you have added will not be used for grading.