/********************************************************************
 *
 * CSE 271 Programming Assignment #2
 *
 * Brian D. Davison
 * Last revised 9 Feb 2005
 *
 * A simple character filter to compress a text sequence.
 * 
 ********************************************************************/

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

const int STDIN = 0;

// outputchar() //////////////////////////////////////////////////
//
// Output a single character, duplicating escape if needed
// Errors cause program to exit.
// Assumes never called with ~ more than twice in a row
//
void outputchar(char c) {
  int result;

  if (c == '~') // need to send escape
    result = printf("~~");
  else
    result = printf("%c", c);

  if (result < 0) { // failed to write
    perror("Write failure in outputchar");
    exit(-1);
  }
}

// outputseq() //////////////////////////////////////////////////
//
// Outputs the compressed version of num copies of c
// Output errors cause program to exit.
// Assumes that next output character will be different from c
//
void outputseq(char c, int num) {
  int result;
  
  if (num==1)
    outputchar(c);
  else if (num==2) {
    outputchar(c);
    outputchar(c);
  } else  // print compressed output
    while (num) {
      // only allow compression of 9 at a time
      int t = num > 9 ? 9 : num;
      result = printf("~%d%c", t, c);
      
      if (result < 0) { // failed to write
	perror("Write failure in outputseq");
	exit(-1);
      }   
      
      num -= t;
    }
  fflush(stdout);
}


// main() ////////////////////////////////////////////////////////////
//
// main() reads from standard input until there is no more to read.
// Keeps track of how many times a character has been read.  When
// the next character is different, it outputs the compressed sequence
// for the previous character.
//
int main(void) {
  char c, oldc = 0;
  int count = 0;
  int result;

  // I'm using read here, but others, such as getchar() should work
  while ((result = read(STDIN, &c, 1)) == 1) {
    if (c == oldc)
      count++;
    else {
      if (oldc) {
	outputseq(oldc, count);
      }
      count = 1;
      oldc = c;
    }
  }
  if (result < -1) {
     perror("read failure");
     exit(-1);
  }

  if (count) // if input was non-empty
    outputseq(oldc, count);

  return 0;
}