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Sunday, February 10, 2008

Huffman Coding

/* Huffman Coding in C . This program reads a text file named on the command line, then compresses it using Huffman coding. The file is read twice, once to determine the frequencies of the characters, and again to do the actual compression. */

#include < stdio.h>
#include < stdlib.h>
#include < string.h>
#include < time.h>

/* there are 256 possible characters */

#define NUM_CHARS 256

/* tree node, heap node */

typedef struct _treenode treenode;
struct _treenode {
int freq; /* frequency; is the priority for heap */
unsigned char ch; /* character, if any */
treenode *left, /* left child of Huffman tree (not heap!) */
*right; /* right child of Huffman tree */

/* this is a priority queue implemented as a binary heap */
typedef struct _pq {
int heap_size;
treenode *A[NUM_CHARS];
} PQ;

/* create an empty queue */

void create_pq (PQ *p) {
p->heap_size = 0;

/* this heap node's parent */

int parent (int i) {
return (i-1) / 2;

/* this heap node's left kid */

int left (int i) {
return i * 2 + 1;

/* this heap node's right kid */

int right (int i) {
return i * 2 + 2;

/* makes the subheap with root i into a heap , assuming left(i) and
* right(i) are heaps
void heapify (PQ *p, int i) {
int l, r, smallest;
treenode *t;

l = left (i);
r = right (i);

/* find the smallest of parent, left, and right */

if (l < p->heap_size && p->A[l]->freq < p->A[i]->freq)
smallest = l;
smallest = i;
if (r < p->heap_size && p->A[r]->freq < p->A[smallest]->freq)
smallest = r;

/* swap the parent with the smallest, if needed. */

if (smallest != i) {
t = p->A[i];
p->A[i] = p->A[smallest];
p->A[smallest] = t;
heapify (p, smallest);

/* insert an element into the priority queue. r->freq is the priority */
void insert_pq (PQ *p, treenode *r) {
int i;

i = p->heap_size - 1;

/* we would like to place r at the end of the array,
* but this might violate the heap property. we'll start
* at the end and work our way up
while ((i > 0) && (p->A[parent(i)]->freq > r->freq)) {
p->A[i] = p->A[parent(i)];
i = parent (i);
p->A[i] = r;

/* remove the element at head of the queue (i.e., with minimum frequency) */
treenode *extract_min_pq (PQ *p) {
treenode *r;

if (p->heap_size == 0) {
printf ("heap underflow!\n");
exit (1);

/* get return value out of the root */

r = p->A[0];

/* take the last and stick it in the root (just like heapsort) */

p->A[0] = p->A[p->heap_size-1];

/* one less thing in queue */


/* left and right are a heap, make the root a heap */

heapify (p, 0);
return r;

/* read the file, computing the frequencies for each character
* and placing them in v[]
unsigned int get_frequencies (FILE *f, unsigned int v[]) {
int r, n;

/* n will count characters */

for (n=0;;n++) {

/* fgetc() gets an unsigned char, converts to int */

r = fgetc (f);

/* no more? get out of loop */

if (feof (f)) break;

/* one more of this character */

return n;

/* make the huffman tree from frequencies in freq[] (Huffman's Algorithm) */

treenode *build_huffman (unsigned int freqs[]) {
int i, n;
treenode *x, *y, *z;
PQ p;

/* make an empty queue */

create_pq (&p);

/* for each character, make a heap/tree node with its value
* and frequency
for (i=0; i< NUM_CHARS; i++) {
x = malloc (sizeof (treenode));

/* its a leaf of the Huffman tree */

x->left = NULL;
x->right = NULL;
x->freq = freqs[i];
x->ch = (char) i;

/* put this node into the heap */

insert_pq (&p, x);

/* at this point, the heap is a "forest" of singleton trees */

n = p.heap_size-1; /* heap_size isn't loop invariant! */

/* if we insert two things and remove one each time,
* at the end of heap_size-1 iterations, there will be
* one tree left in the heap
for (i=0; i< n; i++) {

/* make a new node z from the two least frequent
* nodes x and y
z = malloc (sizeof (treenode));
x = extract_min_pq (&p);
y = extract_min_pq (&p);
z->left = x;
z->right = y;

/* z's frequency is the sum of x and y */

z->freq = x->freq + y->freq;

/* put this back in the queue */

insert_pq (&p, z);

/* return the only thing left in the queue, the whole Huffman tree */

return extract_min_pq (&p);

/* traverse the Huffman tree, building up the codes in codes[] */

void traverse (treenode *r, /* root of this (sub)tree */
int level, /* current level in Huffman tree */
char code_so_far[], /* code string up to this point in tree */
char *codes[]) {/* array of codes */

/* if we're at a leaf node, */

if ((r->left == NULL) && (r->right == NULL)) {

/* put in a null terminator */

code_so_far[level] = 0;

/* make a copy of the code and put it in the array */

codes[r->ch] = strdup (code_so_far);
} else {

/* not at a leaf node. go left with bit 0 */

code_so_far[level] = '0';
traverse (r->left, level+1, code_so_far, codes);

/* go right with bit 1 */

code_so_far[level] = '1';
traverse (r->right, level+1, code_so_far, codes);

/* global variables, a necessary evil */

int nbits, current_byte, nbytes;

/* output a single bit to an open file */

void bitout (FILE *f, char b) {

/* shift current byte left one */

current_byte < < = 1;

/* put a one on the end of this byte if b is '1' */

if (b == '1') current_byte |= 1;

/* one more bit */


/* enough bits? write out the byte */

if (nbits == 8) {
fputc (current_byte, f);
nbits = 0;
current_byte = 0;

/* using the codes in codes[], encode the file in infile, writing
* the result on outfile
void encode_file (FILE *infile, FILE *outfile, char *codes[]) {
unsigned char ch;
char *s;

/* initialize globals for bitout() */

current_byte = 0;
nbits = 0;
nbytes = 0;

/* continue until end of file */

for (;;) {

/* get a char */

ch = fgetc (infile);
if (feof (infile)) break;

/* put the corresponding bitstring on outfile */

for (s=codes[ch]; *s; s++) bitout (outfile, *s);

/* finish off the last byte */

while (nbits) bitout (outfile, '0');

/* main program */

int main (int argc, char *argv[]) {
FILE *f, *g;
treenode *r; /* root of Huffman tree */
unsigned int n, /* number of bytes in file */
freqs[NUM_CHARS]; /* frequency of each char */
char *codes[NUM_CHARS], /* array of codes, 1 per char */
code[NUM_CHARS], /* a place to hold one code */
fname[100]; /* what to call output file */

/* hassle user */

if (argc != 2) {
fprintf (stderr, "Usage: %s < filename>\n", argv[0]);
exit (1);

/* set all frequencies to zero */

memset (freqs, 0, sizeof (freqs));

/* open command line argument file */

f = fopen (argv[1], "r");
if (!f) {
perror (argv[1]);
exit (1);

/* compute frequencies from this file */

n = get_frequencies (f, freqs);
fclose (f);

/* make the huffman tree */

r = build_huffman (freqs);

/* traverse the tree, filling codes[] with the codes */

traverse (r, 0, code, codes);

/* name the output file something.huf */

sprintf (fname, "%s.huf", argv[1]);
g = fopen (fname, "w");
if (!g) {
perror (fname);
exit (1);

/* write frequencies to file so they can be reproduced */

fwrite (freqs, NUM_CHARS, sizeof (int), g);

/* write number of characters to file as binary int */

fwrite (&n, 1, sizeof (int), g);

/* open input file again */

f = fopen (argv[1], "r");
if (!f) {
perror (argv[1]);
exit (1);

/* encode f to g with codes[] */

encode_file (f, g, codes);
fclose (f);
fclose (g);
/* brag */
printf ("%s is %0.2f%% of %s\n",
fname, (float) nbytes / (float) n, argv[1]);
exit (0);


Fatima79 said...

I tried to run this code, but it doesn't seems to be working.
where is this line "FILE *f, *g;" referring to?

Below are the error message.

C:\Users\Fatmata\Desktop\Data Structure HW\paA_6710_huffmanCoding.cpp(92) : error C2601: 'get_frequencies' : local function definitions are illegal
C:\Users\Fatmata\Desktop\Data Structure HW\paA_6710_huffmanCoding.cpp(108) : error C2601: 'build_huffman' : local function definitions are illegal
C:\Users\Fatmata\Desktop\Data Structure HW\paA_6710_huffmanCoding.cpp(170) : error C2601: 'traverse' : local function definitions are illegal
C:\Users\Fatmata\Desktop\Data Structure HW\paA_6710_huffmanCoding.cpp(198) : error C2601: 'bitout' : local function definitions are illegal
C:\Users\Fatmata\Desktop\Data Structure HW\paA_6710_huffmanCoding.cpp(215) : error C2601: 'encode_file' : local function definitions are illegal
C:\Users\Fatmata\Desktop\Data Structure HW\paA_6710_huffmanCoding.cpp(239) : error C2601: 'main' : local function definitions are illegal
C:\Users\Fatmata\Desktop\Data Structure HW\paA_6710_huffmanCoding.cpp(293) : fatal error C1004: unexpected end of file found
Error executing cl.exe.

Could you please respond asap?

Thanks in advance

Anonymous said...

can you put up the .h files also this doesnt seem to be working ty!

Anonymous said...

doesnt work

pow said...

With gcc compiler you need to remove space characters after "<" in some lines to make it works.

Anonymous said...

Great code!

The only problem is that I still don't know how to decompress ><

Hugo said...

me neither...

Anonymous said...

first of save the file using .c and not .cpp
and for removing the erros remove space to include header files:

Anonymous said...

i got this error msg,
BFD 2.15.91 20040904 internal error, aborting at ../../src/bfd/cache.c line 495 in bfd_cache_lookup_worker

rui said...

Hello. good job. some of your posts helps me a lot.
I wanted to ask you if you could see a code that I made the Huffman algorithm. it is a mistake. but I can not figure out. can you help me?
My email: rp.braga2009 @

I hope you can help me.

Shruti said...

Hi..ur work is great:) luving it!!

Anonymous said...


Anonymous said...

Compiler: Default compiler
Compiler: Default compiler
Executing gcc.exe...
gcc.exe "C:\Documents and Settings\Awais Yousaf\My Documents\C Coding\Huffman.c" -o "C:\Documents and Settings\Aw\My Documents\C Coding\Huffman.exe" -I"C:\Dev-Cpp\include" -L"C:\Dev-Cpp\lib"
/mingw/lib/crt2.o(.text+0x37):crt1.c: undefined reference to `__cpu_features_init'
collect2: ld returned 1 exit status

Execution terminated

How to remove this linker error?

Anonymous said...

Awesome job!! love the 'traverse' (thing is, thats wer i was stuck)

Anonymous said...

brilliant job :* thanks

Anonymous said...

please help me.have some kind of a linking problem.have to submit this project by mail id is

Soumya said...

I tried it but its showing 26 ERRORS !! So could you please rectify this ??

Soumya said...

i tried this and it showed 26 ERRORS !! could you please rectify this and post the verified one...

Turcanu Stefan said...

Thank you very much!!! It works in VS2010:D

Turcanu Stefan said...

Thank you very much! It works well on VS2010 :D

Anonymous said...

How do you decode this to give you rht original file?

Carlos Cortez said...

fwrite() uses parameters in wrong order

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