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<random>: Prevent full-width shifts in independent_bits_engine #5740

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Merged
StephanTLavavej merged 3 commits into from
Sep 25, 2025
Merged

<random>: Prevent full-width shifts in independent_bits_engine #5740

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30fb4af
Prevent full-width shifts in `independent_bits_engine`
MattStephanson Sep 24, 2025
cae489e
Fix typo
MattStephanson Sep 25, 2025
32bf2ea
Add newline.
StephanTLavavej Sep 25, 2025
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32 changes: 18 additions & 14 deletions stl/inc/random
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Original file line number Diff line number Diff line change
Expand Up @@ -1900,29 +1900,27 @@ public:
}

_NODISCARD result_type operator()() {
size_t _Idx = 0;
result_type _Res = 0;
result_type _Mask = ((result_type{1} << (_Wx0 - 1)) << 1) - 1;
_Eres _Val;

do { // get a small enough value
_Val = _Eng() - (_Engine::min) ();
} while (_Val > _Yx0);
result_type _Res = static_cast<result_type>(_Val) & _Mask;

size_t _Idx = 1;
for (; _Idx < _Nx0; ++_Idx) { // pack _Wx0-bit values
for (;;) { // get a small enough value
do {
_Val = _Eng() - (_Engine::min) ();
if (_Val <= _Yx0) {
break;
}
}
} while (_Val > _Yx0);
_Res = _Res << _Wx0 | (static_cast<result_type>(_Val) & _Mask);
}

_Mask = _Mask << 1 | 1;
for (; _Idx < _Nx; ++_Idx) { // pack _Wx0+1-bit values
for (;;) { // get a small enough value
do {
_Val = _Eng() - (_Engine::min) ();
if (_Val <= _Yx1) {
break;
}
}
} while (_Val > _Yx1);
_Res = _Res << (_Wx0 + 1) | (static_cast<result_type>(_Val) & _Mask);
}
return _Res;
Expand Down Expand Up @@ -1974,8 +1972,14 @@ private:
_Nx = (_Wx + _Mx - 1) / _Mx + _Nfix; // trial _Nx
_Wx0 = _Wx / _Nx;
_Nx0 = _Nx - _Wx % _Nx;
_Yx0 = (_Rx >> _Wx0) << _Wx0;
_Yx1 = (((_Rx >> _Wx0) >> 1) << _Wx0) << 1;
if (_Wx0 < sizeof(_Eres) * CHAR_BIT) {
_Yx0 = (_Rx >> _Wx0) << _Wx0;
_Yx1 = (((_Rx >> _Wx0) >> 1) << _Wx0) << 1;
} else {
_Yx0 = 0;
_Yx1 = 0;
}

if (_Nfix == 1 || _Rx - _Yx0 <= _Yx0 / _Nx) {
break; // also works if _Rx == 0 (_Mx == all bits)
}
Expand Down