/**** Start of file calc.c ****/
#include "defines.h"

/* The interactive calculator simulates a hand-held
 * calculator. The calculator evaluates expressions
 * until the user signals halt by EOF. An expression
 * consists of a left operand, an operator, and a right
 * operand. The left operand is initialized to zero
 * and is automatically displayed as part of the 
 * prompt for user input. The user inputs the right
 * operand, an expression that consists of constants
 * and operators. To halt, the user enters the stop
 * character ! as the first operator in the right operand.
 */

/* Left-hand operands of expression
 * to be calculated. Left-hand side is the target
 * as well as one of the sources.
 */
int lhs = 0;

/* input/output buffer -- + 2 for newline and null */
char buffer[ MaxExp + 2 ]; 

/* legal operators and stop flag */
static char *ops = "+-*/%!";

/* function declarations */
void 
  prompt( void ), input( void ), output( void ), update_lhs( void );
int bad_op( char );

main()
{
  while ( Forever ) {
    prompt();
    input();
    output();
  }
}

int bad_op( char c )
{
  return NULL == strchr( ops, c );
}
	
/* Parse the right operand, an expression that
 * consists of binary operators and integer
 * constants. In case of error, do not update
 * left operand. Otherwise, set left operand
 * to current value of calculation.
 */
void update_lhs( void )
{
  char *next = buffer, *last;
  char op[ 10 ], numStr[ 20 ];
  int num;
  int temp = lhs; /* copy lhs in case of error */

  /* Loop until right operand parsed. */
  last = next + strlen( buffer );
  while ( next < last ) {
    /* read next operator and integer constant */
    if ( sscanf( next, "%s %s", op, numStr ) < 2 ||
         strlen( op ) != 1                       ||
         bad_op( op[ 0 ] ) )
      return;

    /* Exit? */
    if ( Stop == op[ 0 ] ) {
      printf( " **** End of Calculation ****\n\n" );
      exit( EXIT_SUCCESS );
    }

    /* convert numeric string to integer */
    num = atoi( numStr );

    switch ( op[ 0 ] ) {
    case '+': temp += num; break;
    case '-': temp -= num; break;
    case '*': temp *= num; break;
    case '/': temp /= num; break;
    case '%': temp %= num; break;
    }

    /* reset next -- + 2 for blanks that separate
     * operators and integer constants.
     */
    next += strlen( op ) + strlen( numStr ) + 2;
  }
  lhs = temp; /* new current value */
}
		    /**** End of file calc.c ****/