/*
 * this file contains the functions that do the arithmetic/math calculations
 *
 * to add a function do the following:
 * 1. decide on a *single* character to invoke it
 * 2. write the function and put a prototype under the ones already there
 * 3. add to the table the character to invoke it and the function to do it
 *    (note this ltter is done by giving the function pointer)
 *
 * all functions operate using the stack, so no parameters needed
 */
#include <stdio.h>
#include "stack.h"

/*
 * protptypes
 */
void add(void);
void subtract(void);
void multiply(void);

/*
 * the switching table
 */
struct tbl {
	char op;		/* single char operator */
	void (*func)();		/* address of function to be invoked */
} fnarray[] = {
	{ '+',	add },
	{ '-',	subtract },
	{ '*',	multiply },
	{ '\0',	NULL },
};

/*
 * the switching function
 * it takes the one-character operator and calls the associated function
 */
void fnswitch(char oper)
{
	struct tbl *e;	/* pointer to table entries */

	/*
	 * loop through the table until you either find the
	 * function or you reach the end
	 */
	for(e = fnarray; e->op != '\0'; e++){
		if (e->op == oper){
			/* got it! run the associated function */
			(e->func)();
			return;
		}
	}

	/*
	 * if you get here there was no corresponding function
	 * so complain
	 */
	fprintf(stderr, "%c: no such operation\n", oper);
}

/*
 * pop the top two numbers off the stack, add them, and
 * push the result onto the stack
 */
void add(void)
{
	int v1, v2;	/* values that are popped */

	/* pop the top 2 elements */
	if (pop(&v1) == 0)
		return;
	if (pop(&v2) == 0)
		return;
	/* add them and push it back onto the stack */
	push(v2 + v1);
}

/*
 * pop the top two numbers off the stack, subtract them, and
 * push the result onto the stack
 */
void subtract(void)
{
	int v1, v2;	/* values that are popped */

	/* pop the top 2 elements */
	if (pop(&v1) == 0)
		return;
	if (pop(&v2) == 0)
		return;
	/* subtract them and push it back onto the stack */
	push(v2 - v1);
}

void multiply(void)
{
	int v1, v2;	/* values that are popped */

	/* pop the top 2 elements */
	if (pop(&v1) == 0)
		return;
	if (pop(&v2) == 0)
		return;
	/* multiply them and push it back onto the stack */
	push(v2 * v1);
}