/**
 * rpn.c - a simple Reverse Polish Notation calculator that reads its
 *   input from standard input.
 */

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <limits.h>
#include <ctype.h>

// +-----------+------------------------------------------------------
// | Constants |
// +-----------+

// The capacity of the stack
#define STACK_CAPACITY 16


// +------+-----------------------------------------------------------
// | Main |
// +------+

int 
main (int argc, char *argv[])
{
  int stack[STACK_CAPACITY] = { 0 }; 
                        // The stack of numbers for RPN evaluation
  int top = 0;          // The top of the stack.  That is, the first
                        //   empty space on the stack.  Also the number
                        //   of items on the stack.
  int ch;               // A character we're reading
  int i;                // Everyone's favorite counter variable
  int n;                // The next value read.
  char *negate = "negate";

  while ((ch = getchar()) != EOF)
    {
      switch (ch)
        {
        case 'p':       // Print
          if (top == 0)
            printf ("NO VALUE\n");
          else
            printf ("%d\n", stack[top-1]);
          break;

        case '+':
        case '-':
        case '*':
        case '/':
          top--;
          if (top <= 0)
            {
              printf ("ERROR: Stack underflow\n");
              return EXIT_FAILURE;
            }
          switch (ch)
            {
            case '+':
              stack[top-1] = stack[top-1] + stack[top];
              break;
            case '-':
              stack[top-1] = stack[top-1] - stack[top];
              break;
            case '*':
              stack[top-1] = stack[top-1] * stack[top];
              break;
            case '/':
              stack[top-1] = stack[top-1] / stack[top];
              break;
            } // inner switch
          break;

        case 'n':         // Negate
          for (i = 1; i < 6; i++)
            {
              if ((ch = getchar ()) != negate[i])
                printf ("Invalid prefix to negate.\n");
            } // for
          if (top <= 0)
            {
              printf ("ERROR: Stack underflow\n");
              return EXIT_FAILURE;
            }
          stack[top-1] = -stack[top-1];
          break;

        case '0':
        case '1':
        case '2':
        case '3':
        case '4':
        case '5':
        case '6':
        case '7':
        case '8':
        case '9':
          n = ch - '0';
          while (isdigit (ch = getchar()))
            n = 10 * n + ch - '0';
          if (top >= STACK_CAPACITY)
            {
              printf ("ERROR!  Stack overflow\n");
              return EXIT_FAILURE;
            }
          stack[top] = n;
          top++;
          break;

        default:
          if ((ch == '\n') ||  (ch == ' ') || (ch == '\n'))
            ; // Skip whitespace
          else
            printf ("Invalid input: '%c'\n", ch);
          break;
        } // switch
    } // while

  return EXIT_SUCCESS;
} // main