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This is a simplification of the fix proposed in #2152, with the critical function split out so that it can be tested directly, without having to go through the ULP matcher. Closes #2152
89 lines
3.2 KiB
C++
89 lines
3.2 KiB
C++
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// Copyright Catch2 Authors
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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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// https://www.boost.org/LICENSE_1_0.txt)
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// SPDX-License-Identifier: BSL-1.0
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#ifndef CATCH_FLOATING_POINT_HELPERS_HPP_INCLUDED
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#define CATCH_FLOATING_POINT_HELPERS_HPP_INCLUDED
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#include <catch2/internal/catch_polyfills.hpp>
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#include <cmath>
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#include <cstdint>
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#include <utility>
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#include <limits>
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namespace Catch {
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namespace Detail {
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uint32_t convertToBits(float f);
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uint64_t convertToBits(double d);
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} // end namespace Detail
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/**
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* Calculates the ULP distance between two floating point numbers
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*
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* The ULP distance of two floating point numbers is the count of
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* valid floating point numbers representable between them.
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*
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* There are some exceptions between how this function counts the
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* distance, and the interpretation of the standard as implemented.
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* by e.g. `nextafter`. For this function it always holds that:
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* * `(x == y) => ulpDistance(x, y) == 0` (so `ulpDistance(-0, 0) == 0`)
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* * `ulpDistance(maxFinite, INF) == 1`
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* * `ulpDistance(x, -x) == 2 * ulpDistance(x, 0)`
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*
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* \pre `!isnan( lhs )`
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* \pre `!isnan( rhs )`
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* \pre floating point numbers are represented in IEEE-754 format
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*/
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template <typename FP>
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uint64_t ulpDistance( FP lhs, FP rhs ) {
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assert( std::numeric_limits<FP>::is_iec559 &&
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"ulpDistance assumes IEEE-754 format for floating point types" );
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assert( !Catch::isnan( lhs ) &&
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"Distance between NaN and number is not meaningful" );
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assert( !Catch::isnan( rhs ) &&
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"Distance between NaN and number is not meaningful" );
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// We want X == Y to imply 0 ULP distance even if X and Y aren't
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// bit-equal (-0 and 0), or X - Y != 0 (same sign infinities).
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if ( lhs == rhs ) { return 0; }
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// We need a properly typed positive zero for type inference.
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static constexpr FP positive_zero{};
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// We want to ensure that +/- 0 is always represented as positive zero
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if ( lhs == positive_zero ) { lhs = positive_zero; }
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if ( rhs == positive_zero ) { rhs = positive_zero; }
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// If arguments have different signs, we can handle them by summing
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// how far are they from 0 each.
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if ( std::signbit( lhs ) != std::signbit( rhs ) ) {
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return ulpDistance( std::abs( lhs ), positive_zero ) +
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ulpDistance( std::abs( rhs ), positive_zero );
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}
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// When both lhs and rhs are of the same sign, we can just
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// read the numbers bitwise as integers, and then subtract them
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// (assuming IEEE).
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uint64_t lc = Detail::convertToBits( lhs );
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uint64_t rc = Detail::convertToBits( rhs );
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// The ulp distance between two numbers is symmetric, so to avoid
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// dealing with overflows we want the bigger converted number on the lhs
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if ( lc < rc ) {
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std::swap( lc, rc );
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}
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return lc - rc;
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}
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} // end namespace Catch
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#endif // CATCH_FLOATING_POINT_HELPERS_HPP_INCLUDED
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