/*
	Implementation of an algorithm to multiply two 16-bit signed integers using the 
	same method as ``by hand''. The sign of the result is first determined and the
	multiplication is then performed on the absolute values of the operands.

	Author: Frédéric Goualard <Frederic.Goualard@univ-nantes.fr>
	Last modified: 2022-11-02/09:07:26/+0100
 */
#include <stdio.h>
#include <stdint.h>

int16_t multiply(int16_t a, int16_t b)
{
	uint16_t ua = (uint16_t)a;
	uint16_t ub = (uint16_t)b;
	
	uint16_t sign = 0;
	if (ua & (1<<15)) { // a<0 ?
		sign = 1;
		ua = ~ua + 1;
	}
	if (ub & (1<<15)) { // b<0 ?
		sign = sign ^ 1;
		ub = ~ub + 1;
	}

	uint16_t res = 0;
	uint16_t mask = 1;
	while (mask != (1<<15)) {
		if ((ub & mask) != 0) {
			res = res + ua;
		}
		ua = ua + ua;
		mask = mask + mask;
	}
	if (sign) {
		return (int16_t)(~res + 1);
	} else {
		return (int16_t)res;
	}
}

int main(void)
{
	{
		int16_t a = -56;
		int16_t b = 13;
		printf("%d x %d = %d\n",a,b,multiply(a,b));
	}
	{
		int16_t a = -61;
		int16_t b = -13;
		printf("%d x %d = %d\n",a,b,multiply(a,b));
	}
}