Task: Huffman coding
C: #include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define BYTES 256

struct huffcode {
  int nbits;
  int code;
};
typedef struct huffcode huffcode_t;

struct huffheap {
  int *h;
  int n, s, cs;
  long *f;
};
typedef struct huffheap heap_t;


static heap_t *_heap_create(int s, long *f)
{
  heap_t *h;
  h = malloc(sizeof(heap_t));
  h->h = malloc(sizeof(int)*s);
  h->s = h->cs = s;
  h->n = 0;
  h->f = f;
  return h;
}

static void _heap_destroy(heap_t *heap)
{
  free(heap->h);
  free(heap);
}

#define swap_(I,J) do { int t_; t_ = a[(I)];	\
      a[(I)] = a[(J)]; a[(J)] = t_; } while(0)
static void _heap_sort(heap_t *heap)
{
  int i=1, j=2; 
  int *a = heap->h;

  while(i < heap->n) { 
    if ( heap->f[a[i-1]] >= heap->f[a[i]] ) {
      i = j; j++;
    } else {
      swap_(i-1, i);
      i--;
      i = (i==0) ? j++ : i;
    }
  }
}
#undef swap_

static void _heap_add(heap_t *heap, int c)
{
  if ( (heap->n + 1) > heap->s ) {
    heap->h = realloc(heap->h, heap->s + heap->cs);
    heap->s += heap->cs;
  }
  heap->h[heap->n] = c;
  heap->n++;
  _heap_sort(heap);
}

static int _heap_remove(heap_t *heap)
{
  if ( heap->n > 0 ) {
    heap->n--;
    return heap->h[heap->n];
  }
  return -1;
}


huffcode_t **create_huffman_codes(long *freqs)
{
  huffcode_t **codes;
  heap_t *heap;
  long efreqs[BYTES*2];
  int preds[BYTES*2];
  int i, extf=BYTES;
  int r1, r2;

  memcpy(efreqs, freqs, sizeof(long)*BYTES);
  memset(&efreqs[BYTES], 0, sizeof(long)*BYTES);

  heap = _heap_create(BYTES*2, efreqs);
  if ( heap == NULL ) return NULL;

  for(i=0; i < BYTES; i++) if ( efreqs[i] > 0 ) _heap_add(heap, i);

  while( heap->n > 1 )
  {
    r1 = _heap_remove(heap);
    r2 = _heap_remove(heap);
    efreqs[extf] = efreqs[r1] + efreqs[r2];
    _heap_add(heap, extf);
    preds[r1] = extf;
    preds[r2] = -extf;
    extf++;
  }
  r1 = _heap_remove(heap);
  preds[r1] = r1;
  _heap_destroy(heap);

  codes = malloc(sizeof(huffcode_t *)*BYTES);

  int bc, bn, ix;
  for(i=0; i < BYTES; i++) {
    bc=0; bn=0;
    if ( efreqs[i] == 0 ) { codes[i] = NULL; continue; }
    ix = i;
    while( abs(preds[ix]) != ix ) {
      bc |= ((preds[ix] >= 0) ? 1 : 0 ) << bn;
      ix = abs(preds[ix]);
      bn++;
    }
    codes[i] = malloc(sizeof(huffcode_t));
    codes[i]->nbits = bn;
    codes[i]->code = bc;
  }
  return codes;
}

void free_huffman_codes(huffcode_t **c)
{
  int i;

  for(i=0; i < BYTES; i++) free(c[i]);
  free(c);
}

#define MAXBITSPERCODE 100

void inttobits(int c, int n, char *s)
{
  s[n] = 0;
  while(n > 0) {
    s[n-1] = (c%2) + '0';
    c >>= 1; n--;
  }
}

const char *test = "this is an example for huffman encoding";

int main()
{
  huffcode_t **r;
  int i;
  char strbit[MAXBITSPERCODE];
  const char *p;
  long freqs[BYTES];

  memset(freqs, 0, sizeof freqs);

  p = test;
  while(*p != '\0') freqs[*p++]++;

  r = create_huffman_codes(freqs);

  for(i=0; i < BYTES; i++) {
    if ( r[i] != NULL ) {
      inttobits(r[i]->code, r[i]->nbits, strbit);
      printf("%c (%d) %s\n", i, r[i]->code, strbit);
    }
  }

  free_huffman_codes(r);

  return 0;
}

Java: import java.util.*;

abstract class HuffmanTree implements Comparable<HuffmanTree> {
    public final int frequency; 
    public HuffmanTree(int freq) { frequency = freq; }

    
    public int compareTo(HuffmanTree tree) {
        return frequency - tree.frequency;
    }
}

class HuffmanLeaf extends HuffmanTree {
    public final char value; 
   
    public HuffmanLeaf(int freq, char val) {
        super(freq);
        value = val;
    }
}

class HuffmanNode extends HuffmanTree {
    public final HuffmanTree left, right; 
   
    public HuffmanNode(HuffmanTree l, HuffmanTree r) {
        super(l.frequency + r.frequency);
        left = l;
        right = r;
    }
}

public class HuffmanCode {
    
    public static HuffmanTree buildTree(int[] charFreqs) {
        PriorityQueue<HuffmanTree> trees = new PriorityQueue<HuffmanTree>();
        
        
        for (int i = 0; i < charFreqs.length; i++)
            if (charFreqs[i] > 0)
                trees.offer(new HuffmanLeaf(charFreqs[i], (char)i));

        assert trees.size() > 0;
        
        while (trees.size() > 1) {
            
            HuffmanTree a = trees.poll();
            HuffmanTree b = trees.poll();

            
            trees.offer(new HuffmanNode(a, b));
        }
        return trees.poll();
    }

    public static void printCodes(HuffmanTree tree, StringBuffer prefix) {
        assert tree != null;
        if (tree instanceof HuffmanLeaf) {
            HuffmanLeaf leaf = (HuffmanLeaf)tree;

            
            System.out.println(leaf.value + "\t" + leaf.frequency + "\t" + prefix);

        } else if (tree instanceof HuffmanNode) {
            HuffmanNode node = (HuffmanNode)tree;

            
            prefix.append('0');
            printCodes(node.left, prefix);
            prefix.deleteCharAt(prefix.length()-1);

            
            prefix.append('1');
            printCodes(node.right, prefix);
            prefix.deleteCharAt(prefix.length()-1);
        }
    }

    public static void main(String[] args) {
        String test = "this is an example for huffman encoding";

        
        
        int[] charFreqs = new int[256];
        
        for (char c : test.toCharArray())
            charFreqs[c]++;

        
        HuffmanTree tree = buildTree(charFreqs);

        
        System.out.println("SYMBOL\tWEIGHT\tHUFFMAN CODE");
        printCodes(tree, new StringBuffer());
    }
}