Question

I am using huffman algorithm to develop a file compressor and right now I am facing a problem which is:

By using the algorithm to the word:
stackoverflow, i get the following result:

a,c,e,f,k,l,r,s,t,v,w = 1 time repeated
o = 2 times repeated

a,c,e,f,k,l,r,s,t,v,w = 7.69231%
and
o = 15.3846%

So I start inserting then into a Binary Tree, which will get me the results:

o=00
a=010
e=0110
c=0111
t=1000
s=1001
w=1010
v=1011
k=1100
f=1101
r=1110
l=1111

which means the path for the character in the tree, considering 0 to be left and 1 to right.

then the word “stackoverflow” will be:
100110000100111010011111000010110110111011011111001010

and well, I want to put that whole value into a binary file to be in bits, which will result in 47bits, which would happen to be 6bytes, but instead I can only make it 47bytes because the minimun to put into a file with fwrite or fprintf is 1byte, by using sizeof(something).

Than my question is: how can I print in my file only a single bit?

Answer 1

Just write the “header” to the file: the number of bits and then “pack” the bits into bytes padding the last one. Here’s a sample.

#include <stdio.h>

FILE* f;

/* how many bits in current byte */
int bit_counter;
/* current byte */
unsigned char cur_byte;

/* write 1 or 0 bit */
void write_bit(unsigned char bit)
{
    if(++bit_counter == 8)
    {
        fwrite(&cur_byte,1,1,f);
        bit_counter = 0;
        cur_byte = 0;
    }

    cur_byte <<= 1;
    cur_byte |= bit;
}

int main()
{
    f = fopen("test.bits", "w");

    cur_byte = 0;
    bit_counter = 0;

    /* write the number of bits here to decode the bitstream later (47 in your case) */
    /* int num = 47; */           
    /* fwrite(num, 1, 4, f); */

    write_bit(1);
    write_bit(0);
    write_bit(0);
    /* etc...  - do this in a loop for each encoded character */
    /* 100110000100111010011111000010110110111011011111001010 */

    if(bit_counter > 0)
    {
         // pad the last byte with zeroes
         cur_byte <<= 8 - bit_counter;
         fwrite(&cur_byte, 1, 1, f);
    }

    fclose(f);

    return 0;
}

To do the full Huffman encoder you’ll have to write the bit codes at the beginning, of course.