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/* boost random/detail/large_arithmetic.hpp header file
*
* Copyright Steven Watanabe 2011
* Distributed under the Boost Software License, Version 1.0. (See
* accompanying file LICENSE_1_0.txt or copy at
* http://www.boost.org/LICENSE_1_0.txt)
*
* See http://www.boost.org for most recent version including documentation.
*
* $Id$
*/
#ifndef BOOST_RANDOM_DETAIL_LARGE_ARITHMETIC_HPP
#define BOOST_RANDOM_DETAIL_LARGE_ARITHMETIC_HPP
#include <boost/cstdint.hpp>
#include <boost/integer.hpp>
#include <boost/limits.hpp>
#include <boost/random/detail/integer_log2.hpp>
#include <boost/random/detail/disable_warnings.hpp>
namespace boost {
namespace random {
namespace detail {
struct div_t {
boost::uintmax_t quotient;
boost::uintmax_t remainder;
};
inline div_t muldivmod(boost::uintmax_t a, boost::uintmax_t b, boost::uintmax_t m)
{
const int bits =
::std::numeric_limits< ::boost::uintmax_t>::digits / 2;
const ::boost::uintmax_t mask = (::boost::uintmax_t(1) << bits) - 1;
typedef ::boost::uint_t<bits>::fast digit_t;
int shift = std::numeric_limits< ::boost::uintmax_t>::digits - 1
- detail::integer_log2(m);
a <<= shift;
m <<= shift;
digit_t product[4] = { 0, 0, 0, 0 };
digit_t a_[2] = { digit_t(a & mask), digit_t((a >> bits) & mask) };
digit_t b_[2] = { digit_t(b & mask), digit_t((b >> bits) & mask) };
digit_t m_[2] = { digit_t(m & mask), digit_t((m >> bits) & mask) };
// multiply a * b
for(int i = 0; i < 2; ++i) {
digit_t carry = 0;
for(int j = 0; j < 2; ++j) {
::boost::uint64_t temp = ::boost::uintmax_t(a_[i]) * b_[j] +
carry + product[i + j];
product[i + j] = digit_t(temp & mask);
carry = digit_t(temp >> bits);
}
if(carry != 0) {
product[i + 2] += carry;
}
}
digit_t quotient[2];
if(m == 0) {
div_t result = {
((::boost::uintmax_t(product[3]) << bits) | product[2]),
((::boost::uintmax_t(product[1]) << bits) | product[0]) >> shift,
};
return result;
}
// divide product / m
for(int i = 3; i >= 2; --i) {
::boost::uintmax_t temp =
::boost::uintmax_t(product[i]) << bits | product[i - 1];
digit_t q = digit_t((product[i] == m_[1]) ? mask : temp / m_[1]);
::boost::uintmax_t rem =
((temp - ::boost::uintmax_t(q) * m_[1]) << bits) + product[i - 2];
::boost::uintmax_t diff = m_[0] * ::boost::uintmax_t(q);
int error = 0;
if(diff > rem) {
if(diff - rem > m) {
error = 2;
} else {
error = 1;
}
}
q -= error;
rem = rem + error * m - diff;
quotient[i - 2] = q;
product[i] = 0;
product[i-1] = static_cast<digit_t>((rem >> bits) & mask);
product[i-2] = static_cast<digit_t>(rem & mask);
}
div_t result = {
((::boost::uintmax_t(quotient[1]) << bits) | quotient[0]),
((::boost::uintmax_t(product[1]) << bits) | product[0]) >> shift,
};
return result;
}
inline boost::uintmax_t muldiv(boost::uintmax_t a, boost::uintmax_t b, boost::uintmax_t m)
{ return detail::muldivmod(a, b, m).quotient; }
inline boost::uintmax_t mulmod(boost::uintmax_t a, boost::uintmax_t b, boost::uintmax_t m)
{ return detail::muldivmod(a, b, m).remainder; }
} // namespace detail
} // namespace random
} // namespace boost
#include <boost/random/detail/enable_warnings.hpp>
#endif // BOOST_RANDOM_DETAIL_LARGE_ARITHMETIC_HPP
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