random.tcc: binomial_distribution: mitigate undocumented behaviour #79
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In param_type::_M_initialize(): Change integer literal to corresponding floating-point literal in multiplication by _M_t to avoid unsigned overflow and consequent chain of unpredicted behaviour
Pignotto
commented
Jan 21, 2023
| @@ -1500,7 +1500,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION | |||
| // sqrt(pi / 2) | |||
| const double __spi_2 = 1.2533141373155002512078826424055226L; | |||
| _M_s1 = std::sqrt(__np * __1p) * (1 + _M_d1 / (4 * __np)); | |||
| _M_s2 = std::sqrt(__np * __1p) * (1 + _M_d2 / (4 * _M_t * __1p)); | |||
| _M_s2 = std::sqrt(__np * __1p) * (1 + _M_d2 / (4. * _M_t * __1p)); | |||
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In particular, for values of _M_t that overflow to 0 by the 4* multiplication, the operator()(_UniformRandomNumberGenerator& __urng, const param_type& __param) hangs in an infinite loop as noted and described on https://stackoverflow.com/q/75179395/15114624
Pignotto
changed the title
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In
param_type::_M_initialize(): Change integer literal to corresponding floating-point literal in multiplication by_M_tto avoid unsigned overflow and consequent chain of unpredicted behaviour