In this project, there are four files to do coding.
One of these files are Node.java file. This file contain the node properties, such as left child of the node or possibility of node's key.
FileOperations.java file is also an utility file to doing file operations, such as read original file or write an object to specified file that you want.
Encode.java file is a leading file of this project. Because this is file which coding original file (This file is file.txt file. You can replace your file with it).
You can execute this file on your command line with the following commands:
Compile command:
javac Encode.java
Execute command:
java Encode
Well, but how does it work? In other word how does Shannon-Fano Coding Algorithm work?
How does Shannon-Fano Algorithm work?
Shannon-Fano Coding Algorithm is a data compression algorithm. It uses entropy coding technique. That means for each letter can be represent with 0 and 1 as ASCII. But there is an import difference between ASCII and Shannon-Fano Algorithm. This difference is that ASCII table represents each symbol with a number according to its Alphabetical order. Shannon-Fano Algorithm represents according to frequency of this letter, in the whole file. To do this, Shannon-Fano Algorithm uses a tree.
An example:
We have a word that contain abcde letters and we want to coding this word. Frequency of this letters are;
a -> 0.35,
b -> 0.17, c -> 0.17, d -> 0.16, e -> 0.15.At this point we should generate a balanced tree according as this freqeuncies and we have to sign left side of tree with zeros, right side of three with ones and also each subtree's sides.
Finally we get codes for each letters.
a represents by 00,
b represents by 01, c represents by 10, d represents by 110, e represents by 111.If we had a word like aaabbccde, it was going to represent 00000001011010110111 or we had a word like bbccaaaed, it was going to represent 01011010000000111110.
End of all you will get a two file. First file is encoded file and second file is codes file.
Encoded file contains your compressed file and codes file contains each letter's properties (node class) for using decoding operation.
Decode.java file do this decoding operation (It uses encode.txt and codes.bin files). To decode this encoded file, we are going to use codes in codes.bin file.
Recall that:
a -> 00,
b -> 01, c -> 10, d -> 110, e -> 111.Our encoded file was 00000001011010110111 for aaabbccde. Let's try turn this code to the original form.
Steps : 1- We begin first code.
2- We check code list to find letter which represent with our first code.
2.1- If there is no any letter that represent with our first code we take first two code and we turn step two. If still there is no, we take our first three code e.t.c.
2.2- If there is, we write our corresponding letter to the decode.txt file.
3- We remove codes that we have encoded and we continue through rest of codes.
For example:
We look at first code. First code is 0, but 0 not in our code list. Therefore, we look at first two code and this is 00. 00 equals a, thus our first letter a and we remove 00 from encoded file. Then again we look at first code and again first code not in the code list. Therefore, we look at first two code and first two code represent a. According as our first three letter aaa and we continue like that through encoded file.
End of the decoding, we get aaabbccde and our operation had finished.
To execute Decode.java file, write following commands on your command line:
Compile command:
javac Decode.java
Execute command:
java Decode
You can also create documentation with the following commands:
javadoc **which file that you want to generate documentation**