#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>

#define INT_SIZE 2

void decodeH(unsigned short in[], unsigned short out[],int N){

  unsigned int 
    j=0,    /* The C index of the next code word                   */
    i,      /* Index into the block of code words                  */
    c[N+1], /* The table of control words                          */  
    p=0,    /* The C index of first free control word, j=2^p-1     */
    q,      /* Index into the table of computed control words      */
    bit,    /* Bit position in a code word of a possible error     */
    marks=0 /* Bit positions to indicate presense of errors        */
    ;

  for (i=0; p<=N; i++)                 /* Find control words 0..p  */ 
    if (i == j) { c[p] = in[i]; p++; j += j+1; } else 
      for (bit=1,q=0;q<p; bit<<=1,q++) 
	if (bit & (i+1)) c[q]^=in[i];

  /* Check for presense of errors and find their bit positions     */
  for (q=0; q<N; q++) marks |= c[q];

  if (marks) { /* Correct errors in bit positions of marks         */
    for (bit=1; marks; marks>>=1,bit<<=1) 
      if (marks & 1) { /* On error: find its index position        */
	i=0;
	for (q=N; q; q--) if (c[q-1] & bit) i += i+1; else i += i;
	in[i-1] ^= bit;  /* Correct the bit in the received word   */
      }
  }

  /* Move data from in to out                                      */
  for (j=p=i=0; p<=N; i++) 
    if (i==j) {j += j+1; p++;} else out[i-p] = in[i];
}


void readAndDecode(int codeblock, int datablock, int N){

  
  short
    buffer1[datablock],       // for decoded data to be written
    buffer2[datablock],       // a lookahead block is needed
    *prev,*last, *tmp,        // pointers to buffers
    inbuffer[codeblock],      // encoded data to be read
    wanted,                   // count of bytes to be read
    count;                    // count of bytes actually read

  char 
    *inptr;                   // pointer to next free in inbuffer

  count = 0; last = 0; prev = 0;
  do {

    // read a block of data from stdin and decode it

    inptr = (char*) inbuffer;
    wanted = codeblock*INT_SIZE;
    while (wanted && 0 < (count = read(STDIN_FILENO, inptr, wanted))) {
      wanted -= count; inptr += count;
    }

    if (count == 0) {
      // eof has occurred
      // write(STDOUT_FILENO,"|2|",3);
      if (last[datablock-1] >= datablock*INT_SIZE) {
	// the unwritten data are all in the prev-block
	write(STDOUT_FILENO,prev,2*datablock*INT_SIZE-last[datablock-1]);
	break;
      }
      // write(STDOUT_FILENO,"|3|",3);
      if (prev) write(STDOUT_FILENO,prev,datablock*INT_SIZE);
      // write(STDOUT_FILENO,"|4|",3);
      if (last) write(STDOUT_FILENO,last,datablock*INT_SIZE-last[datablock-1]);
      break;
    }
    
    // a new block of encoded data has been read
    // find a buffer for the decoded data

    // write the prev-block to stdout, if present


    if (prev) { 
      // write(STDOUT_FILENO,"|1|",3);
      write(STDOUT_FILENO,prev,datablock*INT_SIZE); 
      tmp = prev; prev = last; last = tmp; 
    } else if (last) { prev = last; last = buffer2; }
    else last = buffer1;

    if (count > 0) {
      decodeH(inbuffer,last,N);
    } else {
      perror("Decoder"); exit(1);
    }
  } while (1);
}


int main(int argc, char *argv[]){

  int
    N,                // The number of auxiliary bit in a Hamming code.
    codeblock,        // The length of an encoded block
    datablock;        // The length of a block of data

  N = 4;

  codeblock = (1<<N) - 1; datablock = codeblock - N;

  readAndDecode(codeblock,datablock,N);

  return 0;
}