#include <stdio.h>
#include <string.h>
#include <math.h>


typedef enum {ADD, SUBTRACT, MULTIPLY, DIVIDE, POWER} operation_t;

typedef struct
{
  double operand1;
  operation_t operation;
  double operand2;
} expressionStruct;

expressionStruct parseText(char* inputText);
void performAndPrintOperation(expressionStruct *operation);


/*
 * This is the main method for the program.  It essentially causes the program to loop until the program is to exit.
 * Parameters:
 *  argc(int) Not used
 *  argv(char*) Not used
 * Return value:
 *  int always returns 0.
 */
int main(int argc, char* argv[])
{
  char inputText[100];

  memset(&inputText[0], '\0', sizeof(inputText));

  do {
    printf("Enter an expression (quit to quit): ");

    // Read in a new string
    fgets(&inputText[0], sizeof(inputText), stdin);

    // Process it accordingly if it is not quit.  Quit is the text that will cause this program to exit.
    if (strcmp(&inputText[0], "quit\n")!=0)
    {
      // Parse the text that was input.
      expressionStruct op = parseText(&inputText[0]);

      // Perform the operation and print the results.
      performAndPrintOperation(&op);
    }
  } while (strcmp(&inputText[0], "quit\n")!=0);
}

/*
 * This method will parse the input string and return a structure representing the expression supplied.
 * Parameters:
 *  inputText (char*) This is the input string that was entered by the user.  It is a null terminated string.  If NULL, the return for the structure will be somewhat undefined.
 * Return value:
 *  expressionStruct -> This is the structure which represents the pared expression that was entered.
 */
expressionStruct parseText(char* inputText)
{

  expressionStruct retVal;
  char operation[2];

  if (inputText != NULL)
    {
      sscanf(inputText, "%lf %s %lf", &retVal.operand1, &operation[0], &retVal.operand2);

      switch (operation[0])
	{
	case '+':
	  retVal.operation = ADD;
	  break;
	case '-':
	  retVal.operation = SUBTRACT;
	  break;
	case '*':
	  retVal.operation = MULTIPLY;
	  break;
	case '/':
	  retVal.operation = DIVIDE;
	  break;
	case '^':
	  retVal.operation = POWER;
	  break;
	default:
	  break;
	}
    }
  return retVal;
}

/*
 * This method will perform the given operation and print the results to the console.
 * Parameters:
 *  expression (expressionStruct*) This is a pointer to the structure that is going to be processed.  If NULL, an error is to be printed.
 * Return value:
 *  None
 */
void performAndPrintOperation(expressionStruct *expression)
{
  double result = 0.0;
  char operand = '+';

  if (expression== NULL)
    {
      printf("An error occurred.  The expression was NULL.");
    }
  else
    {
      switch (expression->operation)
	{
	case ADD:
	  result = expression->operand1 + expression->operand2;
	  operand = '+';
	  break;
	case SUBTRACT:
	  result = expression->operand1 - expression->operand2;
	  operand = '-';
	  break;
	case MULTIPLY:
	  result = expression->operand1* expression->operand2;
	  operand = '*';
	  break;
	case DIVIDE:
	  result = expression->operand1 / expression->operand2;
	  operand = '/';
	  break;
	case POWER:
	  operand = '^';
	  result = pow(expression->operand1,expression->operand2);
	  break;
	default:
	  result = 0.0;
	}
    printf("%lg %c %lg = %lg\n", expression -> operand1, operand, expression -> operand2, result);
    }
}