// -----------------------------------------------------------
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// integer_log2.hpp
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//
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// Gives the integer part of the logarithm, in base 2, of a
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// given number. Behavior is undefined if the argument is <= 0.
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//
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// Copyright (c) 2003-2004, 2008 Gennaro Prota
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//
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// Distributed under the Boost Software License, Version 1.0.
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// (See accompanying file LICENSE_1_0.txt or copy at
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// http://www.boost.org/LICENSE_1_0.txt)
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//
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// -----------------------------------------------------------
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#ifndef BOOST_INTEGER_INTEGER_LOG2_HPP
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#define BOOST_INTEGER_INTEGER_LOG2_HPP
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#include <boost/limits.hpp>
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#include <boost/config.hpp>
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#include <boost/assert.hpp>
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#if defined(BOOST_BORLANDC)
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#include <climits>
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#endif
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namespace boost {
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namespace detail {
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template <typename T>
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int integer_log2_impl(T x, int n) {
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int result = 0;
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while (x != 1) {
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const T t = static_cast<T>(x >> n);
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if (t) {
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result += n;
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x = t;
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}
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n /= 2;
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}
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return result;
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}
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// helper to find the maximum power of two
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// less than p (more involved than necessary,
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// to avoid PTS)
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//
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template <int p, int n>
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struct max_pow2_less {
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enum { c = 2*n < p };
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BOOST_STATIC_CONSTANT(int, value =
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c ? (max_pow2_less< c*p, 2*c*n>::value) : n);
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};
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template <>
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struct max_pow2_less<0, 0> {
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BOOST_STATIC_CONSTANT(int, value = 0);
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};
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// this template is here just for Borland :(
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// we could simply rely on numeric_limits but sometimes
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// Borland tries to use numeric_limits<const T>, because
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// of its usual const-related problems in argument deduction
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// - gps
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template <typename T>
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struct width {
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#ifdef BOOST_BORLANDC
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BOOST_STATIC_CONSTANT(int, value = sizeof(T) * CHAR_BIT);
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#else
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BOOST_STATIC_CONSTANT(int, value = (std::numeric_limits<T>::digits));
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#endif
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};
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} // detail
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// ---------
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// integer_log2
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// ---------------
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//
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template <typename T>
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int integer_log2(T x) {
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BOOST_ASSERT(x > 0);
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const int n = detail::max_pow2_less<
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detail::width<T> :: value, 4
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> :: value;
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return detail::integer_log2_impl(x, n);
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}
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}
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#endif // include guard
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