Huffman Compression
A minimal huffman compression program using C++ language.
Theory
Huffman compression is based on huffman coding technique. The huffman coding creates an optimal binary tree that is constructed based on frequency of an item/character in a file.
Let's take an example of a file or string containing data like "AAABBCAAADDFFAAAADCCCDAADDDAAACGAAACACA"
character frequency
A 20
B 2
C 7
D 7
F 2
G 1
The above following data will generate a binary tree starting from the characters having the lowest frequency, and construct till we use all the characters as follows:
Pass 1:
(A, 20) (C, 7) (D, 7) (B, 2) (F, 2) (G, 1) (A, 20) (C, 7) (D, 7) (**, 3) (B, 2)
=> / \
/ \
(F, 2) (G, 1)
Pass 2:
(A, 20) (C, 7) (D, 7) (**, 3) (B, 2) (A, 20) (C, 7) (D, 7) (**, 5)
/ \ / \
/ \ / \
(F, 2) (G, 1) => (**, 3) (B, 2)
/ \
/ \
(F, 2) (G, 1)
Pass 3:
(A, 20) (C, 7) (D, 7) (**, 5) (A, 20) (C, 7) (**, 12)
/ \ / \
/ \ / \
(**, 3) (B, 2) => (D, 7) (**, 5)
/ \ / \
/ \ / \
(F, 2) (G, 1) (**, 3) (B, 2)
/ \
/ \
(F, 2) (G, 1)
Pass 4:
(A, 20) (**, 12) (C, 7) (A, 20) (**, 19)
/ \ / \
/ \ / \
(D, 7) (**, 5) (**, 12) (C, 7)
/ \ / \
/ \ => / \
(**, 3) (B, 2) (D, 7) (**, 5)
/ \ / \
/ \ / \
(F, 2) (G, 1) (**, 3) (B, 2)
/ \
/ \
(F, 2) (G, 1)
Pass 5 (Final), with huffman code:
Left Branch denoting 0 and right 1
(A, 20) (**, 19) (**, 39)
/ \ / \
/ \ (0)/ \(1)
(**, 12) (C, 7) [0] <= (A, 20) (**, 19)
/ \ / \
/ \ (0)/ \(1)
(D, 7) (**, 5) => (**, 12) (C, 7) => [11]
/ \ / \
/ \ (0)/ \(1)
(**, 3) (B, 2) [100] <= (D, 7) (**, 5)
/ \ / \
/ \ / \(1)
(F, 2) (G, 1) (**, 3) (B, 2) => [1011]
/ \
(0)/ \(1)
[10100] <= (F, 2) (G, 1) => [10101]
Final Huffman Codes:
character frequency Huffman Codes Actual Binary
A 20 0 01000001
B 2 1011 01000010
C 7 11 01000011
D 7 100 01000100
F 2 10100 01000110
G 1 10101 01000111
__________________________________________________________________
39 75 312
Hence the resultant data is stored as binary written as '000101110110001001001010010100000010011111110000100100100000111010100011011000000', which has 75 bits plus 5 digits appended to round off the remaining bits while storing in the file.
Executing program
For huff.cpp
g++ huff.cpp
./a.out [filename_to_be_compressed]
For huff-sep.cpp
g++ huff-sep.cpp
[a.exe | ./a.out] -c|-dc [filename_to_be_compressed]
(The order must be same: first: option to compress/decompress and then second: filename)
The file to be compressed will generate a file with extension '.abiz', which is the compressed file and the output file which will be the uncompressed version of '.abiz' file.
Note that compressing files other than ASCII based text files (e.g., audio (.mp3), video (.mp4), pdfs, document (.doc/.docx), etc.) can have little or no effect on the resulting size.