#include<stdlib.h>
#include<stdio.h>
#include<ctype.h>
#include<math.h>

#define MAXOP 100
#define NUMBER  0
#define TRUE 1
#define FALSE 0

/* This programme is a basic calculator.

Extra cases have been added to:
1. Show the top item of the stack without permanently popping it.
2. Swap the top two items on the stack.
3. Duplicate the top item on the stack.
4. Clear the stack.

I have used functions for each of the new cases rather than have the
code inline in order to limit the physical size of the switch block.

In anticipation of the following exercise the following characters have
been used for the operations (in the same order as above): ? ~ # !
rather than use alphabetic characters.

It is actually rather difficult to be original in this exercise.

This is exercise 4-4 from Kernighan & Ritchie, page 79.

 */

int Getop(char s[]);
void push(double val);
double pop(void);
void showTop(void);
void duplicate(void);
void swapItems(void);
void clearStack();

int main(void)
{
    int type;
    double op2;
    char s[MAXOP];
    int flag = TRUE;

    while((type = Getop(s)) != EOF)
    {
        switch(type)
        {
            case NUMBER:
                                push(atof(s));
                break;
            case '+':
                push(pop() + pop());
                break;
            case '*':
                push(pop() * pop());
                break;
            case '-':
                op2 = pop();
                push(pop()- op2);
                break;
            case '/':
                op2 = pop();
                if(op2)
                    push(pop() / op2);
                else
                    printf("\nError: division by zero!");
                break;
            case '%':
                op2 = pop();
                if(op2)
                    push(fmod(pop(), op2));
                else
                    printf("\nError: division by zero!");
                break;
            case '?':
                showTop();
                break;
            case '#':
                duplicate();
                break;
            case '~':
                swapItems();
                break;
            case '!':
                clearStack();
            case '\n':
                printf("\n\t%.8g\n", pop());
                break;
            default:
                printf("\nError: unknown command %s.\n", s);
                break;
        }
    }
    return EXIT_SUCCESS;
}

#define MAXVAL 100

int sp = 0;          /* Next free stack position. */
double val[MAXVAL];  /* value stack. */

/* push: push f onto stack. */
void push(double f)
{
    if(sp < MAXVAL)
        val[sp++] = f;
    else
         printf("\nError: stack full can't push %g\n", f);
}

/*pop: pop and return top value from stack.*/
double pop(void)
{
    if(sp > 0)
        return val[--sp];
    else
    {
        printf("\nError: stack empty\n");
        return 0.0;
    }
}

void showTop(void)
{
    if(sp > 0)
        printf("Top of stack contains: %8g\n", val[sp-1]);
    else
        printf("The stack is empty!\n");
}


void duplicate(void)
{
    double temp = pop();

    push(temp);
    push(temp);
}

void swapItems(void)
{
    double item1 = pop();
    double item2 = pop();

    push(item1);
    push(item2);
}

/* pop only returns a value if sp is greater than zero. So setting the
stack pointer to zero will cause pop to return its error */

void clearStack(void)
{
    sp = 0;
}

int getch(void);
void unGetch(int);

/* Getop: get next operator or numeric operand. */
int Getop(char s[])
{
    int i = 0;
    int c;
    int next;

    /* Skip whitespace */
    while((s[0] = c = getch()) == ' ' || c == '\t')
        ;
    s[1] = '\0';

    /* Not a number but may contain a unary minus. */
    if(!isdigit(c) && c != '.' && c != '-')
        return c;               

    if(c == '-')
    {
        next = getch();
        if(!isdigit(next) && next != '.')
        {
           return c;
        }
        c = next;
    }
    else
        c = getch();
 
    while(isdigit(s[++i] = c))
            c = getch();
    if(c == '.')                        /* Collect fraction part. */
        while(isdigit(s[++i] = c = getch()))
                        ;
    s[i] = '\0';
    if(c != EOF)
        unGetch(c);
    return NUMBER;
}

#define BUFSIZE 100

char buf[BUFSIZE];
int bufp = 0;

/* Getch: get a ( possibly pushed back) character. */
int getch(void)
{
     return (bufp > 0) ? buf[--bufp]: getchar();
}

/* unGetch: push character back on input. */
void unGetch(int c)
{
    if(bufp >= BUFSIZE)
        printf("\nUnGetch: too many characters\n");
    else
        buf[bufp++] = c;
}