Partition does not cross bytes

Question

I am trying to divide by uint128_t consisting of 2 uint64_t s. Oddly enough, the function only works for uint64_t with a lower value and upper value = 0. I don’t understand why.

Here is the division and shift code

 class uint128_t{ private: uint64_t UPPER, LOWER; public: // lots of stuff uint128_t operator<<(int shift){ uint128_t out; if (shift >= 128) out = uint128_t(0, 0); else if ((128 > shift) && (shift >= 64)) out = uint128_t(LOWER << (64 - shift), 0); else if (shift < 64) out = uint128_t((UPPER << shift) + (LOWER >> (64 - shift)), LOWER << shift); return out; } uint128_t operator<<=(int shift){ *this = *this << shift; return *this; } uint128_t operator/(uint128_t rhs){ // copy of numerator = copyn uint128_t copyn(*this), quotient = 0;// constructor: uint128_t(T), uint128_t(S, T), uint128_t(uint128_t), etc while (copyn >= rhs){ // copy of denomiator = copyd // temp is the current quotient bit being worked with uint128_t copyd(rhs), temp(1); // shift the divosr to the highest bit while (copyn > (copyd << 1)){ copyd <<= 1; temp <<= 1; } copyn -= copyd; quotient += temp; } return quotient; } // more stuff }; 

Please ignore my blatant disregard for memory management.

Answer 1

out = uint128_t(LOWER << (64 - shift), 0); false - shift - 64 should be shift - 64 instead.

As a style note, ALL_CAPITALS is usually reserved for constants only. Variables and members should use mostly lowercase letters.

Answer 2

try the following:

 // some bit operations stuff const unsigned char de_brujin_bit_map_64 [] = { 0,1,2,7,3,13,8,19,4,25,14,28,9,34,20,40,5,17,26,38,15,46,29,48,10,31,35,54,21,50,41,57, 63,6,12,18,24,27,33,39,16,37,45,47,30,53,49,56,62,11,23,32,36,44,52,55,61,22,43,51,60,42,59,58 }; inline uint8_t trailing_zero_count(uint64_t x) { return x?de_brujin_bit_map_64[(lower_bit(x)*0x0218A392CD3D5DBFL) >> 58]:64; } inline uint8_t leading_zero_count(uint64_t x) { return x?(63-de_brujin_bit_map_64[(upper_bit(x)*0x0218A392CD3D5DBFL) >> 58]):64; } inline uint64_t lower_bit(uint64_t x) { return x & -(int64_t&)x; } inline uint64_t upper_bit(uint64_t x) { if(!x) return 0; x |= x >> 1; x |= x >> 2; x |= x >> 4; x |= x >> 8; x |= x >> 16; x |= x >> 32; return (x >> 1) + 1; } inline uint128_t upper_bit(const uint128_t x) { if(x.upper()) return uint128_t(upper_bit(x.upper()), 0); else return uint128_t(0, upper_bit(x.lower())); } inline uint128_t lower_bit(const uint128_t x) { if(x.lower()) return uint128_t(0, lower_bit(x.lower())); else return uint128_t(lower_bit(x.upper()), 0); } inline uint8_t trailing_zero_count(const uint128_t& x) { return x.lower()?trailing_zero_count(x.lower()):(64+trailing_zero_count(x.upper())); } inline uint8_t leading_zero_count(const uint128_t& x) { return x.upper()?leading_zero_count(x.upper()):(64+leading_zero_count(x.lower())); } // division operator uint128_t uint128_t::operator/(const uint128_t& rhs) const { if(rhs == 0) return uint128_t(0); // !!!! zero division if(rhs == rhs) return uint128_t(1); if(rhs > *this) return uint128_t(0); if(rhs == 1) return *this; if(!upper_ && !rhs.upper_) return uint128_t(0, lower_/rhs.lower_); if(lower_bit(rhs) == rhs) return *this >> trailing_zero_count(rhs); uint128_t result; uint128_t bit_mask = upper_bit(); uint128_t denom = 1; do { bit_mask >>= 1; denom <<= 1; if(*this & bit_mask) denom |= 1; result <<= 1; if(denom >= rhs) { denom -= rhs; result |= 1; } } while (bit_mask.lower_ != 1); return result; } 

Answer 3

anyway, this version is a bit faster :)

provide 4000 iterations vs 127:

 uint128_t divident = uint128_t(0xffffffffffffffffULL, 0xffffffffffffffffULL); uint128_t divisor = 10; { uint32_t iter_count = 0; uint128_t copyn(divident), quotient = 0; while (copyn >= divisor) { ++iter_count; uint128_t copyd(divisor), temp(1); while ((copyn >> 1) > copyd) { ++iter_count; copyd <<= 1; temp <<= 1; } copyn -= copyd; quotient += temp; } std::cout << "iterations: " << std::dec << iter_count << std::endl; } { uint32_t iter_count = 0; uint128_t bit_pos = dtl::bits::upper_bit(divident); uint128_t tmp = 1, quotient = 0; do { ++iter_count; bit_pos >>= 1; tmp <<= 1; if(divident & bit_pos) tmp |= 1; quotient <<= 1; if(tmp >= divisor) { tmp -= divisor; quotient |= 1; } } while (bit_pos != 1); std::cout << "iterations: " << std::dec << iter_count << std::endl; }