float_int_conversion.hxx

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#ifndef INCLUDED_O3TL_FLOAT_INT_CONVERSION_HXX
#define INCLUDED_O3TL_FLOAT_INT_CONVERSION_HXX
 
#include <sal/config.h>
 
#include <cmath>
#include <limits>
#include <type_traits>
 
namespace o3tl
{
// Return true iff `value` of floating-point type `F` converts to a value of integral type `I` no
// smaller than `min`:
template <typename F, typename I>
constexpr std::enable_if_t<std::is_floating_point_v<F> && std::is_integral_v<I>, bool>
convertsToAtLeast(F value, I min)
{
    // If `F(min)`, `F(min) - F(1)` are too large in magnitude for `F`'s precision, then they either
    // fall into the same bucket, in which case we should return false if `value` represents that
    // bucket, or they are on the boundary of two adjacent buckets, in which case we should return
    // true if `value`represents the higher bucket containing `F(min)`:
    return value > F(min) - F(1);
}
 
// Return true iff `value` of floating-point type `F` converts to a value of integral type `I` no
// larger than `max`:
template <typename F, typename I>
constexpr std::enable_if_t<std::is_floating_point_v<F> && std::is_integral_v<I>, bool>
convertsToAtMost(F value, I max)
{
    // If `F(max)`, `F(max) + F(1)` are too large in magnitude for `F`'s precision, then they either
    // fall into the same bucket, in which case we should return false if `value` represents that
    // bucket, or they are on the boundary of two adjacent buckets, in which case we should return
    // true if `value`represents the lower bucket containing `F(max)`:
    return value < F(max) + F(1);
}
 
// Casts a floating-point to an integer, avoiding overflow. Used like:
//     sal_Int64 n = o3tl::saturating_cast<sal_Int64>(f);
template <typename I, typename F>
constexpr std::enable_if_t<std::is_floating_point_v<F> && std::is_integral_v<I>, I>
saturating_cast(F f)
{
    if constexpr (std::is_signed_v<I>)
        if (!convertsToAtLeast(f, std::numeric_limits<I>::min()))
            return std::numeric_limits<I>::min();
    if (!convertsToAtMost(f, std::numeric_limits<I>::max()))
        return std::numeric_limits<I>::max();
    return f;
}
 
// Return `value` of floating-point type `F` rounded to the nearest integer away from zero (which
// can be useful in calls to convertsToAtLeast/Most(roundAway(x), n), to reject x that are
// smaller/larger than n because they have a fractional part):
template <typename F> std::enable_if_t<std::is_floating_point_v<F>, F> roundAway(F value)
{
    return value >= 0 ? std::ceil(value) : std::floor(value);
}
}
 
#endif
 
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