Huffman's encoding and decoding - huffman-code

Question

I have to build a compressor based on the Huffman algorithm.
So far, I managed to create the tree with the frequencies of each character and generate a representation with a smaller number of bits for each character.
Is something like this, for the phrase "good this sugarplum":
'o' 000, '' 001, 't' 0100, 'r' 0101, 'p' 0110, 'm' 0111, 'l' 1000, 'i' 1001, 'h' 1010, 'd' 1011, 'a'1100, 'u' 1101, 'g' 1110, 's' 1111
The problem I'm having now is finding a way to save the tree in the archive, so I can rebuild it and then decompress the file.
Any suggestions?
I did some research but found it difficult to understand, so if you can explain in detail, I would appreciate it.
The code I used to read the frequencies from file is:
int main (int argc, char *argv[])
{
int i;
TipoSentinela *sentinela;
TipoLista *no = NULL;
Arv *arvore, *arvore2, *arvore3;
int *repete = (int *) calloc (256, sizeof(int));
if(argc == 2)
{
in = load_base(argv[1]);
le_dados_arquivo (repete); //read the frequencies from the file
sentinela = cria_lista (); //create a marker for the tree node list
for (i = 0; i < 256; i++)
{
if(repete[i] > 0 && i != 0)
{
arvore = arv_cria (Cria_info (i, repete[i])); //create a tree node with the character i and the frequence of it in the file
no = inicia_lista (arvore, no, sentinela); //create the list of tree nodes
}
}
Ordena (sentinela); //sort the tree nodes list by the frequencies
for(Seta_primeiro(sentinela); Tamanho_lista(sentinela) != 1; Move_marcador(sentinela))
{
Seta_primeiro(sentinela); //put the marker in the first element of the list
no = Retorna_marcador(sentinela);
arvore2 = Retorna_arvore (no); //return the tree represented by the list marker
Move_marcador(sentinela); //put the marker to the next element
arvore3 = Retorna_arvore (Retorna_marcador (sentinela)); //return the tree represented by the list marker
arvore = Cria_pai (arvore2, arvore3); //create a tree node that will contain the both arvore2 and arvore3
Insere_arvoreFinal (sentinela, arvore); //insert the node at the end of the list
Remove_arvore (sentinela); //remove the node arvore2 from the list
Remove_arvore (sentinela); //remove the node arvore3 from the lsit
Ordena (sentinela); //sort the list again
}
out = load_out(argv[1]); //open the output file
Codificacao (arvore); //generate the code from each node of the tree
rewind(in);
char c;
while(!feof(in))
{
c = fgetc(in);
if(c != EOF)
arvore2 = Procura_info (arvore, c); //search the character c in the tree
if(arvore2 != NULL)
imprimebit(Retorna_codigo(arvore2), out); //write the code in the file
}
fclose(in);
fclose(out);
free(repete);
arvore = arv_libera (arvore);
Libera_Lista(sentinela);
}
return 0;
}
//bit_counter and cur_byte are global variables
void write_bit (unsigned char bit, FILE *f)
{
static k = 0;
if(k != 0)
{
if(++bit_counter == 8)
{
fwrite(&cur_byte,1,1,f);
bit_counter = 0;
cur_byte = 0;
}
}
k = 1;
cur_byte <<= 1;
cur_byte |= ('0' != bit);
}
//aux is the code of a character in the tree
void imprimebit(char *aux, FILE *f)
{
int i, j;
if(aux == NULL)
return;
for(i = 0; i < strlen(aux); i++)
{
write_bit(aux[i], f); //write the bits of the code in the file
}
}
With this, I can write the code of all characters in the output file, but I can't see a way to store the tree too.

Answer 1

You don't need to send the tree. Just send the lengths. Then establish a consistent algorithm to convert the lengths to codes on both ends. The consistency is called a "canonical" Huffman code. You sort the codes by length, and within each length, sort by the symbol. Then assign codes starting at 0. So you would end up with (_ means space):
_ 000
o 001
a 0100
d 0101
g 0110
h 0111
i 1000
l 1001
m 1010
p 1011
r 1100
s 1101
t 1110
u 1111

Answer 2

I did found a way to store the code of each character.
For example:
I write the tree, starting by the root and going down to the left, then right.
So, if my tree was something like
0
/ \
0 1
/ \ / \
'a' 'b' 'c' 'd'
The header of my file would be someting like this:
001[8 bits from 'a']1[8 bits from b]01[8 bits from c]1[8 bits from d]
With this, I would be able to rebuild my tree.
My problem now is in read bit-by-bit of the header of file to know in wich direction I have to create a new node.