# Huffman Coding by Akas

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HUFFMAN CODING 2010

1 FEASIBILITY STUDY

In computer science and information theory, Huffman coding is an entropy encoding algorithm used for lossless data compression. The term refers to the use of a variable-length code table for encoding a source symbol (such as a character in a file) where the variable-length code table has been derived in a particular way based on the estimated probability of occurrence for each possible value of the source symbol. It was developed by David A. Huffman at MIT, and published in the 1952 paper "A Method for the Construction of Minimum-Redundancy Codes".

Huffman coding uses a specific method for choosing the representation for each symbol, resulting in a prefix code (sometimes called "prefix-free codes") (that is, the bit string representing some particular symbol is never a prefix of the bit string representing any other symbol) that expresses the most common characters using shorter strings of bits than are used for less common source symbols. Huffman was able to design the most efficient compression method of this type: no other mapping of individual source symbols to unique strings of bits will produce a smaller average output size when the actual symbol frequencies agree with those used to create the code

1.1 History:

In 1951, David A. Huffman and his MIT information theory classmates were given the choice of a term paper or a final exam. The professor, Robert M. Fano, assigned a term paper on the problem of finding the most efficient binary code. Huffman, unable to prove any codes were the most efficient, was about to give up and start studying for the final when he hit upon the idea of using a frequency-sorted binary tree and quickly proved this method the most efficient. In doing so, the student outdid his professor, who had worked with information theory inventor Claude Shannon to develop a similar code.

1.2 The algorithm:1. Scan text to be compressed and tally occurrence of all characters.

2. Sort or prioritize characters based on number of occurrences in text.

3. Build Huffman code tree based on prioritized list.

4. Perform a traversal of tree to determine all code words.

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5. Scan text again and create new file using the Huffman code.

The above construction algorithm uses a priority queue where the node with lowest probability is given highest priority. This priority queue is used to build the Huffman tree which assigns more frequent symbols less number of bits and symbols that occur less frequently would take up more number of bits. In other words assign shorter code words to more common bits and long code words to less common bits and that would be ok because they are less frequent so that the data compression is achieved.

1.3 Problem definition:

“To design the system that will allow the user to enter the total number of characters with their frequencies at the terminal and then display the Huffman codes on the terminal in an interactive manner. “

The main aim of the feasibility study activity is to determine whether it would be financially, and technically feasible to develop the product. After thoroughly analyzing the problem definition and Huffman coding algorithm from various standard books on information theory and internet various strategies for solving the problem were analyzed and finally the algorithm based on priority queue (singly linked list) was chosen.

1.3.1 Technical feasibility

We need C++ compiler software and a computer for our Huffman algorithm which is available. The personal involved with the project should be well versed with the basic concepts of C, C++ and data structures.

1.3.2 Economical Feasibility:

Economic feasibility is dependent on the complexity of the problem and number of personnels involved with the project. Huffman coding is a moderate problem and thus will need atleast 4 group members. Our team comprises of 4

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engineering students who are familiar with data structures and thus is economically feasible

1.3.3 Operational Feasibility:

This project is developed for information theory professionals and the proposed system provides the fast and efficient operations. So it will be acceptable to a large extent. So the proposed system is operationally feasible.

1.3.4 Time feasibility

Our Huffman project should be completed by the 12th of April 2010. This time is feasible as per scheduling conducted by the project manager

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2 REQUIREMENT ANALYSIS AND SPECIFICATIONThe aim of the requirements analysis and specification phase is to understand the exact requirements of the customer and to document them properly. This phase consists of two distinct activities:

2.1 Requirement gathering:

The goal of the requirement gathering activity is to collect all relevant information regarding the product to be developed from the customer with a view to clearly understand the customer requirements .To thoroughly understand the problem, first a study of Graph theory was conducted using standard text books (refer Bibliography) to understand the Huffman tree. The the study of Huffman coding was taken up from various relevant sources like internet and books on data compression. Finally, the method used by Huffman coding to compress the data was understood to understand the motive behind the project.

2.2 Requirement analysis:

The goal of requirement analysis activity is to weed out the incompleteness and inconsistencies in the above gathered requirements. The data collected from various sources including a group of users usually contain several contradictions, ambiguities, incompleteness, inconsistencies, etc since each user typically has only a partial and incomplete view of the system. In case of the Huffman coding algorithm the main requirement is the frequency distribution table which needs to be checked for contradictions, ambiguities, and incompleteness.

For example, the user should not enter the same symbol twice resulting in inconsistency. This would lead to errors as each symbol has only one Huffman code. if the user enters only one symbol this leads to incompleteness which is removed by not using of algorithm and we can conventionally assign 1 or 0 to the symbol. Similarly, the number of symbols cant be greater than 94.

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ASCII

Hex

Symbol

0123456789

101112131415

0123456789ABCDEF

NULSOHSTXETXEOTENQACKBELBS

TABLFVTFFCRSOSI

ASCII

Hex

Symbol

16171819202122232425262728293031

101112131415161718191A1B1C1D1E1F

DLEDC1DC2DC3DC4NAKSYNETBCANEMSUBESCFSGSRSUS

ASCII

Hex

Symbol

32333435363738394041424344454647

202122232425262728292A2B2C2D2E2F

(space)!"#$%&'()*+,-./

ASCII

Hex

Symbol

48495051525354555657585960616263

303132333435363738393A3B3C3D3E3F

0123456789:;<=>?

ASCII

Hex

Symbol

6465666768697071727374

404142434445464748494A

@ABCDEFGHIJ

ASCII

Hex

Symbol

8081828384858687888990

505152535455565758595A

PQRSTUVWXYZ

ASCII

Hex

Symbol

96979899

100101102103104105106

606162636465666768696A

`abcdefghij

ASCII

Hex

Symbol

112113114115116117118119120121122

707172737475767778797A

pqrstuvwxyz

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7576777879

4B4C4D4E4F

KLMNO

9192939495

5B5C5D5E5F

[\]^_

107108109110111

6B6C6D6E6F

kl

mno

123124125126127

7B7C7D7E7F

{|}~�

As can be seen from the above table, the number of valid symbols that can be entered can’t be greater than 94, because the standard ascii uses 32 among 128 for control purposes.

This incompleteness needs to removed by prompting the proper error message. The Huffman coding has important feature of being “prefix free” i.e., none of the code words are prefix of any other codeword’s, so that ambiguity will not occur among the Huffman code words because of the prefix property and thus leads to correct decoding of the message .

2.3 SRS (software requirements specification):

The customer requirements identified during the requirements gathering and analysis activity are organized into a SRS document. The important components of this document are:

Purpose :

a) Data compression: -The main purpose of the software is generating Huffman codes used for data compression.

b) Overview:- In computer science and information theory, Huffman coding is an algorithm used for lossless data compression. The term refers to the use of a variable-length code table for encoding a source symbol (such as a character in a file) where the variable- length code table has been derived in a particular way based on the estimated probability of occurrence for each possible value of the source symbol. It was developed by David A. Huffman at MIT, and published in the 1952 paper "A Method for the Construction of Minimum-Redundancy Codes”. Huffman coding uses a specific method for choosing the representation for each symbol, resulting in

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a prefix code that expresses the most common characters using shorter strings of bits than are used for less common source symbols.

c) End users:-Information theory is a branch of applied mathematics and electrical engineering involving quantification of information. It is the study of ways in which information can be represented or encoded. The Huffman Coding Software is meant for information theory professionals.

Functional requirements :

Functional requirements describe the functions to be supported by the system. Each function can be characterized by the input data, the processing required on the input data and the output data to be produced. The main functions to be supported are:-

1. The user provides symbol and its corresponding frequency as input. Frequency has to be a positive integer.

2. The symbol and its corresponding frequency are inserted into the node of priority queue.

3. Once we have inserted all the symbols and their corresponding frequencies into the priority queue , we build the Huffman tree for the symbols

4. Once the complete tree is created we determine the Huffman code of each symbol by traversing the tree.

Non functional requirements :

Hardware requirements: 16 MB RAM

Software requirements: Windows, UNIX or Macintosh OS and C++ compiler.

Ways for executing on Unix or Mac

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1. download gcc ;in unix type “sudo apt-get install build-essential” at the terminal

2. Make minor changes to run on posix plateform i.e unix or mac eg change clrscr with clear which is the corresponding command in linux for same purpose.

3. The GNU Compiler Collection (usually shortened to GCC) is a compiler system produced by the GNU Project supporting various programming languages

4. GCC has been adopted as the standard compiler by most other modern Unix-like computer operating systems, including GNU/Linux, the BSD family and Mac OS X.

5. command for running on unix after the installation of gcc components “g++ -Wall -W -Werror huff.cpp -o huff”

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Goals of implementation :

In computer science data compression or source coding is the process of encoding information using fewer bits (or other information-bearing units) than an unenclosed representation would use, through use of specific encoding schemes.

As with any communication, compressed data communication only works when both the sender and receiver of the information understand the encoding scheme. For example, this text makes sense only if the receiver understands that it is intended to be interpreted as characters representing the English language. Similarly, compressed data can only be understood if the decoding method is known by the receiver.

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Compression is useful because it helps reduce the consumption of expensive resources, such as hard disk space or transmission bandwidth. On the downside, compressed data must be decompressed to be used, and this extra processing may be detrimental to some applications. For instance, a compression scheme for video may require expensive hardware for the video to be decompressed fast enough to be viewed as it's being decompressed (the option of decompressing the video in full before watching it may be inconvenient, and requires storage space for the decompressed video). The design of data compression schemes therefore involves trade-offs among various factors, including the degree of compression, the amount of distortion introduced (if using a lossy compression scheme), and the computational resources required to compress and uncompress the data.

The goal of the Huffman coding implementation is the data compression-process of reducing the amount of data required to represent a given quantity of information as illustrated in figure below:

A source generates 4 different symbols {a1,a2,a3,a4} with probability {0.4;0.35;0.2;0.05}. A binary tree is generated from left to right taking the two less probable symbols, putting them together to form another equivalent symbol having a probability that equals the sum of the two symbols. The process is repeated until there is just one symbol. The tree can then be read backwards, from right to left, assigning different bits to different branches. The final Huffman code is:

The standard way to represent a signal made of 4 symbols is by using 2 bits/symbol. If this Huffman code is used to represent the signal, then the average length is lowered to 1.85 bits/symbol. The technique works by creating a binary tree of nodes. Thus, on an average ,this achieves a saving of 0.15 bits/symbol and thus achieving data compression.

In run-length encoding, large runs of consecutive identical data values are replaced by a simple code with the data value and length of the run. This is an example of lossless data compression. It is often used to optimize disk space on office computers, or better use the connection bandwidth in a

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Symbol

Code

a1 0

a2 10

a3 110

a4 111

HUFFMAN CODING 2010

computer network. For symbolic data such as spreadsheets, text, executable programs, etc., losslessness is essential because changing even a single bit cannot be tolerated (except in some limited cases).

For visual and audio data, some loss of quality can be tolerated without losing the essential nature of the data. By taking advantage of the limitations of the human sensory system, a great deal of space can be saved while producing an output which is nearly indistinguishable from the original. These lossy data compression methods typically offer a three-way tradeoff between compression speed, compressed data size and quality loss.

Lossy image compression is used in digital cameras, to increase storage capacities with minimal degradation of picture quality. Similarly, DVDs use the lossy MPEG-2 Video codec for video compression.

In lossy audio compression, methods of psychoacoustics are used to remove non-audible (or less audible) components of the signal. Compression of human speech is often performed with even more specialized techniques, so that "speech compression" or "voice coding" is sometimes distinguished as a separate discipline from "audio compression". Different audio and speech compression standards are listed under audio codecs. Voice compression is used in Internet telephony for example, while audio compression is used for CD ripping and is decoded by audio players.

Huffman coding today is often used as a "back-end" to some other compression method. DEFLATE (PKZIP's algorithm) and multimedia codecs such as JPEG and MP3 have a front-end model and quantization followed by Huffman coding.

Example implementations: DEFLATE (a combination of LZ77 and Huffman coding) – used by ZIP,

gzip and PNG files

Unix pack utility (the .z file format) used Huffman coding

bzip2 (a combination of the Burrows-Wheeler transform and Huffman coding)

JPEG (image compression using a discrete cosine transform, then quantization, then Huffman coding)

MPEG (audio and video compression standards family in wide use, using DCT and motion-compensated prediction for video)

o MP3 (a part of the MPEG-1 standard for sound and music compression, using subbanding and MDCT, perceptual modeling, quantization, and Huffman coding)

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o AAC (part of the MPEG-2 and MPEG-4 audio coding specifications, using MDCT, perceptual modeling, quantization, and Huffman coding)

DESIGN PROCEES FOR HUFFMAN CODING

During the software design phase, the designer transforms the SRS document document into the design document. The design document produced at the end of design phase should be implemented using a programming language in the coding phase.

The items that are taken into consideration in the design phase are the different modules which constitute it. Control relationships and interfaces among different modules are identified. Suitable data structures for the data to be stored need to be properly designed and documented.

One of the basic steps in the design process involves graphical representation of our main problem. We use DFD’S for graphical representation.

Context level DFD

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The Huffman software takes the frequency distribution table as input and computes the corresponding Huffman codes for each symbol.

Level 1 DFD

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Huffman

Coding

Frequency distribution table

Huffman codes

input

HUFFMAN CODING 2010

In Level 1 DFD we provide character and its corresponding frequency as input, if the input is valid then the input is inserted into the priority queue. Once we have inserted all the symbols and their corresponding frequencies , we deque two nodes of lowest frequencies from the priority queue and form the composite node of type tree which is then reinserted back in the priority queue if it is not empty. Once the complete tree is created we determine the Huffman code of each symbol by traversing the tree .

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Insert into Priority Queue 0.2

Insert into tree node

0.3

Generate codes

0.4

Take input

From user 0.1 valid

Priority queue node

Composite node of tree

symbol

code

HUFFMAN CODING 2010

Level 2 DFD (0.1)

input

valid

valid

valid

In this DFD , we take the number of symbols as input , the user can enter only integers from 1 to 94. If the user enters any other value e.g characters then the error message is to be displayed and the user must enter the valid input. After getting the valid number of characters the user must enter the symbol for which the Huffman code is required. Here the user can enter the symbols. After the valid input has been taken, we take the frequency of the character. In this case we have to accept only the integer values; if the user provides the input other than integers then error message is to be displayed.

Level 2 DFD (0.2)

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Take no. of symbols

0.1.1

Enter frequency

0.1.3

Enter the symbol

0.1.2

HUFFMAN CODING 2010

In this DFD, the proper location of the valid symbol or composite node is determined & placed at the same location. In this case if the front of the priority queue is null then the node is inserted at the beginning of the priority queue else the node is placed at specific position depending on its frequency.

LEVEL 2 DFD (0.4)

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Calculate location in Pque 0.2.1

Insert node in Pque 0.2.2

valid

position

Priority queue node

Composite node of tree

HUFFMAN CODING 2010

symbol

path found

code

Here the input is the symbol for which we have to generate the Huffman code. While traversing from root to leaf (symbol) we assign zero as we move to left child and 1 as we move to right.

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Calculate path from root to leaf

0.4.1

Traverse path assigning 0 to left and 1 to right child

0.4.2

HUFFMAN CODING 2010

Data Dictionary

Name DescriptionComposite node Tree or priority queue nodeTree Symbol,its frequency and pointers to left and right childernPriority queue Symbol,its frequency and pointer to next node in the queueFrequency distribution table Symbols & their frequencyHuffman code Bit pattern used to represent a symbolRoot Root node of huffman treeLeaf Symbols entered(having no children)Path Unique sequence of edges from root to a symbol

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3.2 Structured design

3.2.1 Data design:

The main operations that the data structure must support are as follows:

- It must represent a binary Huffman tree with nonnegative frequencies.- It must store a list of internal priority queue nodes in non decreasing

order.

Huffman algorithm consists of two main data structures:-

1. Priority queue:

Priority queue holds the symbol, frequency and the pointer to the next node in the queue. T his datastructure is used to create a node in the increasing order priority queue. The main functions that manipulate this priority queue like finding position for a node in the queue based on frequency, dequeing the first node of the priority queue for composite node creation.

2. Binary tree:

Binary tree is a tree that holds symbol, frequency of the character, and pointers to right and left children. This datastructure is used to create a node of Huffman tree. Finally, the Huffman tree is traversed to generate the Huffman code.

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3.2.2 Work break down structure (wbs):

The main module calls input module which in turn calls enque module which inserts the nodes in the priority queue at proper position based on frequency by calling the find position module. Main module then calls the insert module. Insert module calls the deque module which returns the lowest priority node. This information is used to create a composite node of type tree .The summation of the frequencies and concatenation of the symbols of two nodes dequeued are then used as parameters to create a composite node of type ‘priority queue’ into the queue if it is empty . The main module may call the display module to display the symbols in the in-order manner for debugging purposes. At last the encode module is used to generate the Huffman codes for each symbol.

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MAIN

INPUT INSERT DISPLAY ENCODE

ENQUE DEQUE

FP

WELCOME

HUFFMAN CODING 2010

3.2.3 Procedural design

3.2.3.1 Flowcharts:

no

yes

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All symbols entered

Insert symbol in priority queue in increasing order

Traversal Huffman tree to generate code

Build Huffman tree

start

Enter frequency and symbol

stop

HUFFMAN CODING 2010

Flowchart for main

yes

no

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start

Create a priority queue node

Find position

Position =Null

Insert at beginning

Insert at specific position

Stop

HUFFMAN CODING 2010

Flowchat for enque

Flow chart for deque

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start

Pointer =front

Front=front->next

Return pointer

stop

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yes

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start

Root!=Null

Root= left child

Display root

Root= right child

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Flowchart for display

no

yes

yes

no

yes

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start

Front != Null

Dequeue two nodes of lowest frequency

Node has children

Create composite node of tree

Adjust nodes of tree

Assign left & right child

Priority

queue empty

Insert composite node into priority queue

stop

HUFFMAN CODING 2010

no

Flowchart for Insertion

no

yes

yes

no

Flow chart for Encode

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stop

start

Symbol found in left child

Print 0

Node= left child

Print 1

Node= right child

stopSymbol != node

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3.2.3.2 Algorithms

Algorithm for main :-

STEP1: begin

Step2: take frequency & symbol as input.

STEP3: repeat step 2 until there is no input

Step4: insert symbols in priority queue in increasing order.

Step5: build Huffman tree.

Step6: traverse the tree to generate the Huffman code.

Step7: stop.

Algorithm for encode :-

STEP1: begin

Step2: while symbol != Node

if symbol found in left subtree

Print 0

Node = left child

Goto 2

Else

Print 1

Node = right child

Goto 2

Step3: stop.

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Algorithm for display:-

STEP1: begin

Step2: If root!= NULL

Root=left child

Display (root)

Display the symbol.

Root=right child

Display(root)

Step3: stop.

Algorithm for enque:-

Step1: Begin

Step2: Create node of priority queue.

STEP3: Find the position of the symbol in priority queue.

STEP4: Insert node at specific position.

STEP5: STOP

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Algorithm for deque:-

STEP1: Begin.

STEP2: return the node having the lowest frequency from priority queue.

Step 3: update front of queue.

Step4: stop.

Algorithm for insertion:-

STEP1: begin

Step2: Dequeue two nodes of lowest frequency.

STEP3: create composite nodes of tree.

Step4: adjust the tiny trees

Step5: If front!= Null

Step6: insert composite node into priority queue

Step7: stop.

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2 CODING/* HUFFMAN CODING (Mini Project)*/

/*

Implemented By :

Asif Iqbal,Aquib Rahid Pandit,Kaleem Dar & Sahil Sholla

*/

/*header files:*/

#include<iostream.h>

#include<string.h>

#include<math.h>

#include<stdlib.h>

#include<conio.h>

#include<ctype.h>

#include<graphics.h>

/*Global declarations*/

int n;

char b[94][2];

/* Structure Specifications */

/* Binary Tree Node Specification */

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/* 'tree' is a Binary Tree, that holds symbol,its

frequency and pointers to left and right children */

struct tree

{

char a[94];

int s;

struct tree *left,*right;

}*root=NULL,*tt[47]={NULL},*temp,*temp2,*t2,*ri,*le;

/* Priority Queue Node Specification */

/* 'pqu'is a priority queue node that holds symbol,its

frequency and pointer to next node in the queue*/

struct pqu

{

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int info;

char a[94];

struct pqu *ptr;

}*front=NULL,*t,*par,*t1,*p1,*p2;

Code has been hidden for security reasons

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//main program

void main()

{

int i;

welcome();

input();

insert();

//disp(root);

clrscr();

cout<<"\n\nThe corresponding codes are...\n\n";

for(i=0;i<n;i++)

{

cout<<b[i] << "==>";

encode(b[i]);

cout<<"\t";

getch();

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}

}

3 TESTING

Our Huffman coding software consists of different modules. An error in any one of these modules will result in system error and if these errors are not debugged it will result in defective system. This may result in rejection by the customer at the customer testing phase, and thus will result in the project failure. Thus testing is an important activity. We need not to carry out unit testing as our program is simple with fewer modules. However we do carry out phased testing of our code to verify that the code is working properly. Various modules which perform the function of inputting the frequency table, enquing the symbols into priority queue at their specific position as per their frequency; this task is performed by find position (fp) module; and then dequeinq the symbols from priority queues and inserting it into the binary tree.

At this phase we include a debugging module named display to display the symbols of complete binary tree by inorder traversal. If the result of display module is not what is expected then the errors in the code were identified and corrected. We have taken some conventions as specified in SRS document. They are:

1. We assign 0 to the left child and 1 to the right child in the tree.

2. The position of a new composite node with same frequency as of those symbol present in the priority queue, we place the one which enters the priority queue next at high priority.

3. We assign symbol with higher frequency as left child & lower frequency as right child in the tree.

4. We assign 1 for any string or character in the case of only one character.

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By taking all the above conventions into consideration we conducted integration testing using phased approach because of the fewer number of modules.

Test Case (0)

Input:

Total number of symbols = 6.

Frequency distribution table as:-

{ a=3,b=3,c=4,d=4,e=5,f=5 }

Expected output:

ffeefebadcbbaabadcddcc

Result:

Verification successful

Elaboration of above case

Iteration (1)

Priority queue

Processing:

for i=o, z=0

p1=address of (a) p2=address of (b)

a1=a b1=b

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a=3 b=3 c=4 d=4 e=5 f=5front

HUFFMAN CODING 2010

Iteration (2)

Priority queue

Processing:

for (i=0, z=1)

p1=address of (c) p2=address of (d)

a1=c b1=d

Iteration (3)

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b=3 a=3

Null

Null

Null

Null

ba=6root

temp2

temp

tt[0]

front d=4 e=5 f=5 ab=6c=4

d=4 c=4

Null

Null

Null

Null

dc=6root

temp2tt[1]

temp

HUFFMAN CODING 2010

Priority queue

Processing:

for (i=0, z=2)

p1=address of (e) p2=address of (f)

a1=e b1=f

Iteration (4)

Priority queue

Processing:

for (i=0, z=3)

p1=address of (ab) p2=address of (cd)

a1=ab b1=cd

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e=5 f=5 ba=6 dc=8front

f=5 e=5

Null

Null

Null

Null

fe=10root

temp2tt[2]

temp

ba=6 dc=8 fe=10front

HUFFMAN CODING 2010

Iteration (5)

Priority queue

Processing:

for (i=0, z=4)

p1=address of (fe) p2=address of (badc)

a1=fe b1=badc

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ba=66666

dc=85

badcroot

temp2tt[2]

temp

b=3 a=3 d=4 c=4

null

null

null

null

null

null

null

null

fe=10 badc=14front

HUFFMAN CODING 2010

The inorder traversal of the above tree yields “ffeefebadcbbaabadcddcc”

Which was the expected output, thus the routines involved in creation of priority queue and Huffman tree are verified to be correct. This completes the phased integration testing .

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ba=66666

dc=85

badc=14

b=3 a=3 d=4 c=4

fe=10

f=5 e=5

febadc =24

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Once the phased testing was complete, all the other modules where integrated to form the complete Huffman software. Then system testing was conducted according to the following plan:

We first designed various test cases wit which the software was tested with various test cases used:

CASE(1)

Input:

no. of symbols = a

Expected = error message

Result: Success

For the test case(1) we incorporated certain lines of code which checked whether the input; for number of symbols; is a number or not. This validation was performed by using isdigit( )

function which is included in ctype library.

CASE (2)

Input:

no of symbols = 0

Expected = error message

Result: success

We simply checked whether the input; for no. of characters; is 0, and display a message to enter non-zero number and then continue the program to take new input

CASE (3).

Input:

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no. of symbols = -1

Expected = error message

Result: success

tn[0] contains negative sign and thus isdigit( ) will be false and message to enter valid positive integers will again be displayed.

CASE(4).

Input:

no. of symbols = 1

Expected=error message

Result: success

We don’t need to execute Huffman algorithm because it won’t be necessary nor will it result in any data compression. Thus we simply exit the program.

CASE (5).input

enter symbol = asdd

Expected = error message

Result: success

Strlen is used to first calculate the length of string entered and if it is more than 1 message is displayed to enter one symbol only

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CASE (6).

Input:

no.of symbols = 98

Expected = error message

Result: success

We check the condition if n>94 i.e. if the no. of symbols is specified more than 94 we display the message to the user to enter numbers less than 94.

CASE (7).

Input

enter symbol = aa

Expected = error message

Result: success

To eliminate redundancy or ambiguity we include a loop which runs for the number of symbols, continuously comparing the newly input symbol with the previous input symbols which are stored in b[ ], if the match is found then message is displayed notifying the user that the newly input symbol is already entered and current loop is exited.

After all the above validation we performed regression testing of all the previous cases and verify the result. Now that all the cases behaved as expected & thus system testing is complete.

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Future scope:

The huffman coding the we have considered is simple binary Huffman coding but many variations of Huffman coding exist,

1. n-ary Huffman coding:

The n-ary Huffman algorithm uses the {0, 1, ... , n − 1} alphabet to encode message and build an n-ary tree. This approach was considered by Huffman in his original paper. The same algorithm applies as for binary (n equals 2) codes, except that the n least probable symbols are taken together, instead of just the 2 least probable. Note that for n greater than 2, not all sets of source words can properly form an n-ary tree for Huffman coding. In this case, additional 0-probability place holders must be added. If the number of source words is congruent to 1 modulo n-1, then the set of source words will form a proper Huffman tree.

2.Adaptive Huffman coding:

A variation called adaptive Huffman coding calculates the probabilities dynamically based on recent actual frequencies in the source string. This is somewhat related to the LZ family of algorithms.

3.Huffman template algorithm:

Most often, the weights used in implementations of Huffman coding represent numeric probabilities, but the algorithm given above does not require this; it requires only a way to order weights and to add them. The Huffman template algorithm enables one to use any kind of weights (costs, frequencies etc)

4. Length-limited Huffman coding:

Length-limited Huffman coding is a variant where the goal is still to achieve a minimum weighted path length, but there is an additional restriction that the length of each codeword must be less than a given constant. The package-merge algorithm solves this problem with a simple greedy approach very similar to that used by Huffman's algorithm. Its time complexity is O(nL), where L is the maximum length of a codeword. No algorithm is known to solve this problem in linear or linearithmic time, unlike the presorted and unsorted conventional Huffman problems, respectively.

5.Huffman coding with unequal letter costs:

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In the standard Huffman coding problem, it is assumed that each symbol in the set that the code words are constructed from has an equal cost to transmit: a code word whose length is N digits will always have a cost of N, no matter how many of those digits are 0s, how many are 1s, etc. When working under this assumption, minimizing the total cost of the message and minimizing the total number of digits are the same thing.Huffman coding with unequal letter costs is the generalization in which this assumption is no longer assumed true: the letters of the encoding alphabet may have non-uniform lengths, due to characteristics of the transmission medium. An example is the encoding alphabet of Morse code, where a 'dash' takes longer to send than a 'dot', and therefore the cost of a dash in transmission time is higher. The goal is still to minimize the weighted average codeword length, but it is no longer sufficient just to minimize the number of symbols used by the message. No algorithm is known to solve this in the same manner or with the same efficiency as conventional Huffman coding.

Moreover we can extend the range of the huffman coding software to incorporate unicode whicb will require an interfacing module that will interpret a perticular key in different languages based on the option selected. In that case the maximum number of symbols will be of the order of 232 .

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