## Tuesday, December 13, 2016

### Fermat Numbers and Bit Mask

There is a little known application of Fermat numbers for generating bit masks, that I discovered back in 2010 while implementing "counting the number of one bits" using C++ template meta-programming. In the algorithm, bit masks such as 0x55555555, 0x33333333, 0x0f0f0f0f, etc. are used to mask out bits in specific locations. These bit masks can be generated by dividing the maximum integer 0xffffffff (or simply ~0u) against Fermat numbers 3, 5, 17, 257, 65537, etc.

 Hex Binary Expression 0x5555... 0101010101010101... ~0u / 3 0x3333... 0011001100110011... ~0u / 5 0x0f0f... 0000111100001111... ~0u / 17 0x00ff... 0000000011111111... ~0u / 257 and so on...

It's not a coincidence that integers with all one bits set can be divided cleanly by a Fermat number, and this works with 8-bit, 16-bit, 32-bit, 64-bit, and beyond. To understand why, a $$k$$-bit all-one integer is $$2^k - 1$$. In particular, $$k = 2^n$$ in our case. We can factor $$2^{2^n} - 1$$ using the formula $$a^2 - b^2 = (a + b)(a - b)$$ we learned in grade school, here using the special case $$b = 1$$, so $$a^2 - 1 = (a + 1)(a - 1)$$.
\begin{align}
2^{2^n} - 1 & = (2^{2^{n - 1}} + 1)(2^{2^{n - 1}} - 1) \\
& = (2^{2^{n - 1}} + 1)(2^{2^{n - 2}} + 1)(2^{2^{n - 2}} - 1) \\
& = (2^{2^{n - 1}} + 1)(2^{2^{n - 2}} + 1)(2^{2^{n - 3}} + 1)(2^{2^{n - 3}} - 1) \\
& = \dots
\end{align}
Conveniently, the last term $$2^{2^0} - 1$$ is just $$1$$, so we can write the product in terms of $$F_i = 2^{2^i} + 1$$ for the $$i$$-th Fermat number,
$2^{2^n} - 1 = \prod_{i=0\dots n-1}{2^{2^i} + 1} = \prod_{i=0\dots n-1}{F_i}$
For example,
 $$k$$ $$2^k - 1$$ Expression 2 3 $$3$$ 4 15 $$3 \times 5$$ 8 255 $$3 \times 5 \times 17$$ 16 65535 $$3 \times 5 \times 17 \times 257$$ 32 4294967295 $$3 \times 5 \times 17 \times 257 \times 65537$$ and so on...

So how are Fermat numbers related to bit mask construction? It turns out that if you write 3, 5, 17, 257, 65537 in binary, you get 11, 101, 1 0001, 1 0000 0001, and 1 0000 0000 0000 0000 0000 0000 0000 0001 respectively. If you multiply 101 (5) and 1 0001 (17), you get alternating patterns of 0101 0101 (85). If you multiply this by 11 (3), you get an 8-bit all-one integer 1111 1111 (255). Conversely, you can take a pattern "out of circulation" by dividing the all-one integer by a Fermat number which is one of its factors.

#### Appendix: Counting the number of one bits using C++ template meta-programming

x = ((x >> 1) & 0x55555555) + (x & 0x55555555);
x = ((x >> 2) & 0x33333333) + (x & 0x33333333);
x = ((x >> 4) & 0x0f0f0f0f) + (x & 0x0f0f0f0f);
// etc.

which can be represented by the following for-loop:
for (int n = 1; n < sizeof(Tp) * 8; n *= 2)
Tp mask = ~0 / ((1 << n) + 1));
}

In the template meta-programming version, the template __unroll_shift_mask_add essentially unrolls into the for-loop above, but in a manner that is agnostic to the actual integer type.
template<unsigned int n, typename Tp>
static Tp compute(Tp x) {
Tp mask = ~Tp(0) / ((Tp(1) << n) + 1);