perfect nums

nevertested.cc

#include <iostream>
#include <cstdint>

template<typename T>
class MerssenePerfect {
    // A000043
    static_assert(std::is_unsigned<T>::value,
        "MerssenePerfect should use an unsigned type");
    const static int_fast8_t primes[18] = { 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61 };
    ptrdiff_t offset;
    ptrdiff_t max;
    T make_value(int_fast8_t prime) {
        // (2^p - 1) * (2 ^ p)
        return ((1 << prime) - 1) << prime;
    }
    public:
    bool operator==(MerssenePerfect& b) { return this.offset == b.offset; }
    bool operator!=(MerssenePerfect& b) { return this.offset != b.offset; }
    operator*() { return make_value(primes[offset]); }
    MerssenePerfect& operator++() { ++offset; return *this; }
    MerssenePerfect  operator++(int) { MerssenePerfect<T> clone(*this); ++offset; return clone; }
    MerssenePerfect begin() { return MerssenePerfect( *this, 0 ); }
    MerssenePerfect end()   { return MerssenePerfect( *this, max ); }
    MerssenePerfect()(ptrdiff_t _o = 0): offset(_o) {
        ptrdiff_t oo;
        for (oo = 0; o < 18; ++o)
            if (primes[oo] * 2 - 1 > CHAR_BIT * sizeof(T))
                break;
        max = oo;
    }
}

// divisors:
// 1 .. 2^p, (2^p) * (1 .. 2^p)
// and sort