/*
 * revdec.c - Reverse decimal numbers.
 *            Computes the reverse of any valid input integer in place.
 *
 * Naveed Hasan <naveed@cs.columbia.edu>
 * 2002.12.27
 * v1.1
 */


/* Returns the integral portion of the logarithm base 10 of the input number. */
int log_10( int num ){
  int result = -1;

  while( num > 0 ){
    num /= 10;
    result++;
  }

  return result;
}


/* Returns the integral value of 10 raised to the power of the input number. */
int pow_10( int num ){
  int result = 1;

  while( num > 0 ){
    num--;
    result *= 10;
  }

  return result;
}


/* Returns the reverse of the input integer */
int rev_10( int num ){

  int sign, width, lopos, hipos, lonum, hinum, diff, lodif, hidif;

  sign = num >= 0 ? 1 : -1;   /* What is the sign of the number? */
  num *= sign;                /* Will be reversing the absolute value... */
  width = log_10(num) + 1;    /* What is the width of the number in digits? */

  /* Swap the low & high halves of the decimal number. */
  for( lopos = 1; lopos < width/2 + 1; lopos++ ){
    hipos = width - lopos + 1;

    lonum = num % pow_10(lopos) / pow_10(lopos - 1);        /* Digit in low & */
    hinum = num % pow_10(hipos) / pow_10(hipos - 1);        /* high position. */

    diff = lonum >= hinum ? lonum - hinum : hinum - lonum;  /* Difference. */
    lodif = lonum >= hinum ? -diff : diff;
    hidif = lonum >= hinum ? diff : -diff;

    /* Actual swaps: simply addition of the appropriate values. */
    num += (lodif * pow_10(lopos - 1) + hidif * pow_10(hipos - 1));
  }

  num *= sign;                /* Put the sign back on the reversed number! */

  return num;
}


/* Main function. Rejects invalid input, which is defined as input which when
 * reversed may underflow or overflow an integer. */
int main( int argc, char** argv ){

  int N = -1000000000;

  if( argc == 2 ) N = atoi(argv[1]);

  if( N < -999999999 || N > 999999999 ){
    printf("Usage: %s number (%d, %d)\n", argv[0], -999999999, 999999999);
    return -1;
  }

  printf("%d\n", N);
  printf("%d\n", rev_10(N));

  return 0;
}