#define DEBUG

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

#include "stack.h"

#define NUMBER 0		
#define STACKSIZE 4		// The maximum number of items can be stored to the stack.
#define BUFSIZE 8

static char s[BUFSIZE];	// Temporary buffer to store data return by getToken()

double operandStack[STACKSIZE];		// An array to store the data in the stack.
int stackPointer;					// The index of the top of the stack.

//////////////////////////////////////////////////////////////////////////
// Implementation of getToken()
//////////////////////////////////////////////////////////////////////////
static char buf[4];
static int bufp;

// Clear the read buffer.
static void clearBuffer()
{
	bufp = 0;
}

// Get a character from the buffer.
static int getch()
{
	return (bufp > 0) ? *(buf + --bufp) : getchar();
}

// Push back a character to the buffer.
static void ungetch(int c)
{
	*(buf + bufp++) = c;
}

// Get a token from the input. If the input is a number, the return
// value of this function is NUMBER, and 's' contains the numeric string.
// If the input is not a number, the return value is the ASCII value
// of the character.
static int getToken(char* s)
{
	int i, c;
	while((*s = c = getch()) == ' ' || c == '\t');
	*(s + 1) = 0;
	
	if(!isdigit(c) && c != '.')
		return c;	// not a number
		
	i = 0;
	if(isdigit(c))	// Collect integer part
	{
		while(isdigit(*(s + ++i) = c = getch()))
		{
			// Check if the next character will cause buffer overflow.
			// If so, stop reading.
			// Ideally, the code should also skip the rest of the line.
			if(i >= BUFSIZE - 2)
			{
				*(s + BUFSIZE - 1) = '\0';
				printf("ERROR: Buffer Overflow.\n");
				return NUMBER;
			}
		
		}
	}
		
	if(c == '.')	// Collect fraction part
	{
		while(isdigit(*(s + ++i) = c = getch()))
		{
			// Check if the next character will cause buffer overflow.
			// If so, stop reading.
			if(i >= BUFSIZE - 2)
			{
				*(s + BUFSIZE - 1) = '\0';
				printf("ERROR: Buffer Overflow.\n");
				return NUMBER;
			}
		}
	}
		
	*(s + i) = 0;
	if(c != EOF)
		ungetch(c);
		
	return NUMBER;
}
///////////////////////////////////////////////////////////////////////////

int main()
{
	int type;
	double op1, op2;

	// Initialize the stack.
	stack_clear(operandStack, &stackPointer, STACKSIZE);
	
	printf("Enter an expression in postfix order. E.g. 1 234 5 * 6 / + 7 - 89 -\n");
	
	// continuously read characters from the keyboard until CTRL+D is pressed.
	while((type = getToken(s)) != EOF)
	{
		if(type == NUMBER)	// The input is a number -> Push onto the stack.
		{
			printf("Read a number \'%s\'\n", s);
			stack_push(operandStack, &stackPointer, STACKSIZE, atof(s));
		}
		else if(type == '\n')	// The input is a new line -> Reset
		{
			printf("Result = %.8g\n", stack_pop(operandStack, &stackPointer));
			stack_clear(operandStack, &stackPointer, STACKSIZE);
		}
		else if(type == '+' || type == '-' || type == '*' || type == '/')	// Operator
		{
				op2 = stack_pop(operandStack, &stackPointer);
				op1 = stack_pop(operandStack, &stackPointer);
				if(type == '+')
					stack_push(operandStack, &stackPointer, STACKSIZE, op1 + op2);
				else if(type == '-')
					stack_push(operandStack, &stackPointer, STACKSIZE, op1 - op2);
				else if(type == '*')
					stack_push(operandStack, &stackPointer, STACKSIZE, op1 * op2);
				else if(type == '/')
					stack_push(operandStack, &stackPointer, STACKSIZE, op1 / op2);
		}
		else	// Invalid expression
		{
			printf("\tInvalid Expression \'%s\'\n", s);
			clearBuffer();
			stack_clear(operandStack, &stackPointer, STACKSIZE);
		}
		
		// Print out the stack only if DEBUG is defined.
		#ifdef DEBUG
		stack_print(operandStack, &stackPointer);
		#endif
	}
	return 0;
}