calculates mean via pop count approximation

mean_bit_set.cpp

struct ConstexprBitset64 {
    uint64_t block = 0;

    constexpr void set(size_t idx) {
        block |= uint64_t{1} << idx;
    }

    constexpr void clear(size_t idx) {
        block &= ~(uint64_t{1} << idx);
    }

    constexpr bool test(size_t idx) const {
        return (block >> idx) & 1u;
    }

    // mean approximate functions

    constexpr size_t getMeanApprox64() const {
        // Uses 64 bit approx
        return __builtin_popcountll(block) >> 1;
    }

    constexpr size_t count() const {
    	return __builtin_popcountll(block);
    }
};

struct BitSet256 {
	ConstexprBitset64 bins[4];

	constexpr size_t count() const {
        return bins[0].count() +
               bins[1].count() +
               bins[2].count() +
               bins[3].count();
	}

    constexpr size_t mean() const {
        return bins[0].getMeanApprox64() +
               bins[1].getMeanApprox64() +
               bins[2].getMeanApprox64() +
               bins[3].getMeanApprox64();
    }

	constexpr void set(size_t idx) {
        const size_t block = idx >> 6;
        const size_t offset = idx & 63;
        bins[block].set(offset);
	}

	constexpr long blockBasedDeviation64(size_t point) const {
		const size_t block = point >> 6;
		const long myCount = bins[block].count();
		return (myCount - (this->count())) + myCount;
	}
};