This document summarizes a simple dictionary compression algorithm that operates in two passes. In the first pass, it analyzes the data file and creates a dictionary of unique bytes and their frequencies. In the second pass, it replaces each byte in the file with an index value from the dictionary, writing these values to the compressed file along with their bit lengths. Compression is achieved because the dictionary is sorted by frequency, allowing each byte to be represented by 4 to 11 bits rather than 8 bits. While compression is slow, decompression is not.

1. DATA
COMPRESSION
Simple Dictionary
Compression
Manish T I

2. • It is a two pass algorithm in which first pass
analyze the data in the source file and second
pass will compress the data to a file.
First Pass:-
• In the source file distinct bytes are identified.
• Check the number of times it occurs in the
source file.
• A new list is sorted in descending order of the
frequencies, in such a manner in which higher
count of byte (alphabets) appear at the top of
the list which is known as the dictionary.

3. Second Pass:-
• The source file is read again byte by byte
• Each byte is located in the dictionary by a direct
search and its index is noted.
• Index value is written on the compressed file,
preceded by its length.
• The index value consist of 256 values and range
spans from 0 to 255.
• The index is written on the compressed file,
preceded by a 3-bit code denoting the index’s
length.

4. • Index Table
Binary
Value
Value Bit
000 0 1
001 1 2
010 2 3
011 3 4
100 4 5
101 5 6
110 6 7
111 7 8

5. Input File sample data : - TTVVVEGTVEN
Dictionary File : -

6. Compressed File (4 – 11 bits)
T
V
V
V
E
G
T
V
E
N
0 0 1 1 0
0 0 0 1
0 0 0 1
0 0 0 1
0 0 1 1 1
0 1 0 1 0 0
0 0 1 1 0
0 0 0 1
0 0 1 1 1
0 1 0 1 0 1
No: of bits
used
5
4
4
4
5
6
5
4
5
6

7. • Compression is achieved because the
dictionary is sorted by the frequency of the
bytes. Each byte is replaced by a quantity of
between 4 and 11 bits.
• Dictionary is not sorted by byte values.
• Disadvantage :- Slow compression not in the
case of decompression.

8. Reference:-
Data Compression : The Complete Reference,
David Salomon, Springer Science & Business
Media, 2004
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