fizzbuzz.cpp

#include <cstdio>

static int abs(int n)
{
	const auto mask = n << 31;
	return (n ^ mask) - mask;
}

static int odd_digit_sum_base4(int n)
{
	// Shift and add half the digits onto the other half with each pass
	n = (n & 0x03030303) + ((n & 0x30303030) >> 4);
	n = (n & 0x00FF00FF) + ((n & 0xFF00FF00) >> 8);
	n = (n & 0x0000FFFF) + ((n & 0xFFFF0000) >> 16);
	return n;
}

static void fizzbuzz(int n)
{
	// Divisibility by 3 for 0-48
	constexpr auto div3_mask = 0b1001001001001001001001001001001001001001001001001;
	// Divisibility by 5 for 0-24
	constexpr auto div5_mask = 0b0000100001000010000100001;

	const auto odd_sum  = odd_digit_sum_base4(n);
	const auto even_sum = odd_digit_sum_base4(n >> 2);

	// Check digit sum divisibility for divisibility by base - 1
	const auto div3 = (div3_mask >> (odd_sum + even_sum)) & 1;
	// Check alternating digit sum divisibility for divisibility by base + 1
	const auto div5 = (div5_mask >> abs(odd_sum - even_sum)) & 1;

	printf((const char*[]) {
		"%d\n",
		"fizz\n",
		"buzz\n",
		"fizzbuzz\n"
	}[div3 | (div5 << 1)], n);
}

int main()
{
	for (auto i = 1; i <= 100; i++)
		fizzbuzz(i);
}