mul.c

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

static unsigned short lookup_table[512];

unsigned short mul(unsigned char a, unsigned char b)
{
	unsigned short x = a + b, y = (a > b) ? (a - b) : (b - a);
	return lookup_table[x] - lookup_table[y];
}

void initialize()
{
	int i;
	for (i = 0; i < 512; ++i) {
		// value ends with binary 00 (even) or 01 (odd)
		// so we can cut it off.
		lookup_table[i] = (i * i) >> 2;
	}
}

int test()
{
	int exit_code = EXIT_SUCCESS;

	int a, b;
	for (a = 0; a < 256; ++a) {
		for (b = 0; b < 256; ++b) {
			unsigned short expected = a * b, got = mul(a, b);
			if (expected != got) {
				warnx("mul(%d, %d) produces incorrect value. Expected: %d, got %d.", a, b, expected, got);
				exit_code = EXIT_FAILURE;
			}
		}
	}

	return exit_code;
}

int main()
{
	initialize();

	int exit_code = test();

	exit(exit_code);
}