floating_point.h

// -*- c++ -*-
//
// Copyright 2005, Google Inc.
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//
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//
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// Authors: Zhanyong Wan, Sean Mcafee
//
// Taken from The Google C++ Testing Framework (Google Test).
// Modified for this discussion by Fred Richards.
//
 
#ifndef FLOATINGPOINT_H
#define FLOATINGPOINT_H
 
#include <cstddef>
#if defined _MSC_VER  // may need to check your VC++ version
  typedef unsigned __int32 uint32_t;
  typedef unsigned __int64 uint64_t;
#else
  #include <stdint.h>
#endif
#include <limits>
 
 
namespace
{
    template <size_t bytes>
    struct TypeWithSize
    {
        typedef void UInt;
    };
 
    template <>
    struct TypeWithSize<4>
    {
        typedef uint32_t UInt;
    };
 
    template <>
    struct TypeWithSize<8>
    {
        typedef uint64_t UInt;
    };
}
 
 
template <typename RawType>
class FloatingPoint
{
public:
    typedef typename TypeWithSize<sizeof(RawType)>::UInt Bits;
 
    static const size_t kBitCount = 8*sizeof(RawType);
    static const size_t kFracBitCount = std::numeric_limits<RawType>::digits - 1;
    static const size_t kExpBitCount = kBitCount - 1 - kFracBitCount;
 
    static const Bits kSignBitMask = static_cast<Bits>(1) << (kBitCount - 1);
    static const Bits kFracBitMask = ~static_cast<Bits>(0) >> (kExpBitCount + 1);
    static const Bits kExpBitMask = ~(kSignBitMask | kFracBitMask);
 
    static const size_t kMaxUlps = 4;
 
 
    explicit FloatingPoint(const RawType& x) : value_(x) {}
 
    //
    // Now checking for NaN to match == behavior.
    //
    bool AlmostEquals(const FloatingPoint& rhs) const {
        if (is_nan() || rhs.is_nan()) return false;
        return ULP_diff(bits_, rhs.bits_) <= kMaxUlps;
    }
 
private:
    bool is_nan() const {
        return ((kExpBitMask & bits_) == kExpBitMask) &&
	        ((kFracBitMask & bits_) != 0);
    }
 
    Bits SignAndMagnitudeToBiased(const Bits& sam) const {
        if (kSignBitMask & sam) {
            return ~sam + 1;  // two's complement
        } else {
            return kSignBitMask | sam;  // * 2
        }
    }
 
    Bits ULP_diff(const Bits& sam1, const Bits& sam2) const
    {
        const Bits biased1 = SignAndMagnitudeToBiased(sam1);
        const Bits biased2 = SignAndMagnitudeToBiased(sam2);
 
        return (biased1 >= biased2) ? (biased1 - biased2) : (biased2 - biased1);
    }
 
    union {
        RawType value_;
        Bits bits_;
    };
};
 
#endif // FLOATINGPOINT_H