SnapValue.h

/* Distributed under the Apache License, Version 2.0.
   See accompanying NOTICE file for details.*/
 
//----------------------------------------------------------------------------
// This class acts like an ordinary double precision floating point value, except
// that it "snaps" to the nearest integer whenever it is close enough to an
// integer that we can presume that the difference is due to rounding error. 
// The purpose of having this is that we want to be able to represent fractional
// exponents of a unit symbol in a CompoundUnitElement, but we want to make sure
// that when doing conversions to other units, we don't spuriously find a mismatch
// in the corresponding UnitDimension objects of the two CompoundUnit objects, simply
// because, for example, one has a meters exponent of 1.0 and the other has a
// meters exponent of 0.9999998 due to some roundabout computation in which we
// took the square root of an acre. 
//----------------------------------------------------------------------------
#pragma once
#include <float.h>
 
// The use of FLT_EPSILON instead of
// DBL_EPSILON is just in case our values are the result of some
// funky compiler intrinsic that does stuff in single precision. This should
// be fine because the only thing we're trying to do, really, is represent
// rational numbers of the form [m / n] where n is small, e.g. 1/2, 1/3, etc.
// the scale factor (16) is to account for the fact that the rounding error
// will be larger for values with a larger integer portion (due to the finite
// number of mantissa bits). Ideally, we should scale-up FLT_EPSILON by the 
// size of the value being snapped, and compare that to the error, but then we
// have to do that extra multiplication even though most values will be within 
// small range anyway, and we'd have to consider when the original value is close 
// to zero, the multiplication by FLT_EPSILON might underflow. I don't think it
// will, but I don't trust my intuition on IEEE 754.
#define SNAP_TOLERANCE (FLT_EPSILON * 16.0)
 
class CSnapValue
{
private:
  // A negating constructor for use by unary-minus only. Add in a dummy arg
  // so that the compiler can resolve to the correct constructor. This
  // skips the snapping because we're negating the value from an existing
  // CSnapValue
  CSnapValue(const CSnapValue &src, bool /*dummyflag*/)
    :m_dVal(-src.m_dVal)
  {
    // nothing
  }
 
public:
  // Make it look, smell, and act like a double
 
  // Constructors
  CSnapValue()
  {
    m_dVal = 0.0;
  }
 
  CSnapValue(const CSnapValue &src)
    :m_dVal(src.m_dVal)
  {
    // nothing
  }
 
  CSnapValue(double val)
    :m_dVal(val)
  {
    Snap();
  }
 
  CSnapValue(float val)
    :m_dVal(val)
  {
    Snap();
  }
 
  CSnapValue(int val)
    :m_dVal(val)
  {
    // No need to snap if we're initializing with an int
  }
 
  operator double() const
  {
    return m_dVal;
  }
 
  // Assignment and arithmetic operators
  // See Myers, "More Effective C++", Addison-Wesley
  CSnapValue & operator=(const CSnapValue &rhs)
  {
    m_dVal = rhs.m_dVal;
    return *this;
  }
 
  CSnapValue & operator*=(const CSnapValue &rhs)
  {
    m_dVal *= rhs.m_dVal;
    Snap();
    return *this;
  }
 
  const CSnapValue operator*(const CSnapValue &rhs) const
  {
    return CSnapValue(*this) *= rhs;
  }
 
  CSnapValue & operator/=(const CSnapValue &rhs)
  {
    m_dVal /= rhs.m_dVal;
    Snap();
    return *this;
  }
 
  const CSnapValue operator/(const CSnapValue &rhs) const
  {
    return CSnapValue(*this) /= rhs;
  }
 
  CSnapValue & operator+=(const CSnapValue &rhs)
  {
    m_dVal += rhs.m_dVal;
    Snap();
    return *this;
  }
 
  const CSnapValue operator+(const CSnapValue &rhs) const
  {
    return CSnapValue(*this) += rhs;
  }
 
  const CSnapValue operator+() const
  {
    return CSnapValue(*this);
  }
 
  CSnapValue & operator-=(const CSnapValue &rhs)
  {
    m_dVal -= rhs.m_dVal;
    Snap();
    return *this;
  }
 
  const CSnapValue operator-(const CSnapValue &rhs) const
  {
    return CSnapValue(*this) -= rhs;
  }
 
  const CSnapValue operator-() const
  {
    // Invoke the special negating constructor!!!
    return CSnapValue(*this,true);
  }
 
  // Increment and decrement
  // prefix
  CSnapValue & operator++()
  {
    *this += CSnapValue(1.0);
    return *this;
  }
 
  CSnapValue & operator--()
  {
    *this -= CSnapValue(1.0);
    return *this;
  }
 
  // postfix
  const CSnapValue operator++(int)
  {
    CSnapValue oldValue = *this;
    ++(*this);
    return oldValue;
  }
 
  const CSnapValue operator--(int)
  {
    CSnapValue oldValue = *this;
    --(*this);
    return oldValue;
  }
 
  const double &GetValue() const
  {
    return m_dVal;
  };
 
 
  // Relational operators
 
  // http://support.microsoft.com/kb/168958 says we need to define operators < and ==
  // for this if we want to export the vector of these contained in UnitDimension. 
  bool operator< (const CSnapValue &rhs) const
  {
    return (m_dVal < rhs.m_dVal);
  }
 
  bool operator== (const CSnapValue &rhs) const
  {
    return  (m_dVal == rhs.m_dVal);
  }
  
  bool operator!=(const CSnapValue &rhs) const
  {
    return (m_dVal != rhs.m_dVal);
  }
 
  bool operator>(const CSnapValue &rhs) const
  {
    return (m_dVal > rhs.m_dVal);
  }
 
  bool operator<=(const CSnapValue &rhs) const
  {
    return (m_dVal <= rhs.m_dVal);
  }
 
  bool operator>=(const CSnapValue &rhs) const
  {
    return (m_dVal >= rhs.m_dVal);
  }
 
  // Provide versions of relational operators that take other types to avoid 
  // method resolution ambiguity 
  bool operator< (const double &rhs) const
  {
    return (m_dVal < rhs);
  }
 
  bool operator== (const double &rhs) const
  {
    return  (m_dVal == rhs);
  }
 
  bool operator!=(const double &rhs) const
  {
    return (m_dVal != rhs);
  }
 
  bool operator>(const double &rhs) const
  {
    return (m_dVal > rhs);
  }
 
  bool operator<=(const double &rhs) const
  {
    return (m_dVal <= rhs);
  }
 
  bool operator>=(const double &rhs) const
  {
    return (m_dVal >= rhs);
  }
 
  bool operator< (const int &rhs) const
  {
    return (m_dVal < rhs);
  }
 
  bool operator== (const int &rhs) const
  {
    return  (m_dVal == rhs);
  }
 
  bool operator!=(const int &rhs) const
  {
    return (m_dVal != rhs);
  }
 
  bool operator>(const int &rhs) const
  {
    return (m_dVal > rhs);
  }
 
  bool operator<=(const int &rhs) const
  {
    return (m_dVal <= rhs);
  }
 
  bool operator>=(const int &rhs) const
  {
    return (m_dVal >= rhs);
  }
 
protected:
  void Snap()
  {
 
    // Need to be careful here, because the int and frac portions
    // that modf returns are THE SAME SIGN, and we need to snap
    // when the fractional value is close to 0 or to +/- 1. 
    double intval;
    double bump = 1.0;
    double frac = modf(m_dVal,&intval);
    if (frac < 0)
    {
      frac = -frac;
      bump = -1.0;
    }
 
    // frac is positive now. We need to snap differently
    // depending on whether it's close to zero or close to 1
    if (frac < 0.5)
    {
      // See if it's close to zero
      if (frac < SNAP_TOLERANCE)
      {
        m_dVal = intval;
      }
    }
    else
    {
      // See if it's close to one
      if ((1.0 - frac) < SNAP_TOLERANCE)
      {
        m_dVal = intval + bump;
      };
    }
 
  }
 
private:
  double m_dVal;
};
 
inline double pow(double x, const CSnapValue &y)
{
  return pow(x,y.GetValue());
}