80 lines
2.8 KiB
C++
80 lines
2.8 KiB
C++
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///////////////////////////////////////////////////////////////
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// Copyright 2012 John Maddock. Distributed under the Boost
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// Software License, Version 1.0. (See accompanying file
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// LICENSE_1_0.txt or copy at https://www.boost.org/LICENSE_1_0.txt
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#ifndef BOOST_MP_MAX_DIGITS10_HPP
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#define BOOST_MP_MAX_DIGITS10_HPP
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namespace boost {
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namespace multiprecision {
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namespace detail {
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template <unsigned digits>
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struct calc_max_digits10
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{
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static constexpr unsigned max_digits_10(unsigned d)
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{
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//
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// We need ceil(log10(2) * d) + 1 decimal places to
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// guarantee round tripping, see: https://www.exploringbinary.com/number-of-digits-required-for-round-trip-conversions/
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// and references therein. Since log10(2) is irrational, then d*log10(2) will
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// never be exactly an integer so we can replace by trunc(log10(2) * d) + 2
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// and avoid the call to ceil:
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//
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return static_cast<unsigned>(0.301029995663981195213738894724493026768189881462108541310 * d) + 2;
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}
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static constexpr unsigned value = max_digits_10(digits);
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};
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template <std::size_t digits>
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struct calc_max_digits10_s
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{
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static constexpr std::size_t max_digits_10(std::size_t d)
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{
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//
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// We need ceil(log10(2) * d) + 1 decimal places to
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// guarantee round tripping, see: https://www.exploringbinary.com/number-of-digits-required-for-round-trip-conversions/
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// and references therein. Since log10(2) is irrational, then d*log10(2) will
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// never be exactly an integer so we can replace by trunc(log10(2) * d) + 2
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// and avoid the call to ceil:
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//
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return static_cast<std::size_t>(static_cast<std::size_t>(0.301029995663981195213738894724493026768189881462108541310 * static_cast<double>(d)) + 2u);
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}
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static constexpr std::size_t value = max_digits_10(digits);
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};
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template <unsigned digits>
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struct calc_digits10
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{
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static constexpr unsigned digits_10(unsigned d)
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{
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//
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// We need floor(log10(2) * (d-1)), see:
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// https://www.exploringbinary.com/number-of-digits-required-for-round-trip-conversions/
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// and references therein.
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//
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return static_cast<unsigned>(0.301029995663981195213738894724493026768189881462108541310 * static_cast<double>(d - 1u));
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}
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static constexpr unsigned value = digits_10(digits);
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};
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template <std::size_t digits>
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struct calc_digits10_s
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{
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static constexpr std::size_t digits_10(std::size_t d)
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{
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//
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// We need floor(log10(2) * (d-1)), see:
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// https://www.exploringbinary.com/number-of-digits-required-for-round-trip-conversions/
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// and references therein.
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//
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return static_cast<std::size_t>(0.301029995663981195213738894724493026768189881462108541310 * static_cast<double>(d - 1u));
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}
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static constexpr std::size_t value = digits_10(digits);
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};
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}}} // namespace boost::multiprecision::detail
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#endif
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