gum::GammaLog2 Class Reference

The class for computing Log2 (Gamma(x)) More...

#include <agrum/tools/core/math/gammaLog2.h>

Collaboration diagram for gum::GammaLog2:

Public Member Functions
Constructors / Destructors
	GammaLog2 (bool requires_precision=false)
	Default constructor. More...
	GammaLog2 (const GammaLog2 &from)
	Copy constructor. More...
	GammaLog2 (GammaLog2 &&from)
	Move constructor. More...
	~GammaLog2 ()
	Class destructor. More...
Operators
double	operator() (double x) const
	Returns log2 ( gamma (x) ) for x > 0. More...
void	setPrecision (bool p)
	Sets whether we need more precision for small values. More...
double	gammaLog2 (double x) const
	Returns log2 ( gamma (x) ) for x >= 0. More...

Detailed Description

The class for computing Log2 (Gamma(x))

Definition at line 47 of file gammaLog2.h.

Constructor & Destructor Documentation

GammaLog2() [1/3]

gum::GammaLog2::GammaLog2

(

bool

requires_precision = false

)

Default constructor.

Parameters

requires_precision

Set if precision is required or not.

Definition at line 38 of file gammaLog2.cpp.

References gum::Set< Key, Alloc >::emplace().

                                              :
      requires_precision__{requires_precision} {
    GUM_CONSTRUCTOR(GammaLog2);
  }

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GammaLog2() [2/3]

gum::GammaLog2::GammaLog2

(

const GammaLog2 &

from

)

Copy constructor.

Parameters

from	The gum::GammaLog2 to copy.

Definition at line 44 of file gammaLog2.cpp.

References gum::Set< Key, Alloc >::emplace().

                                            :
      requires_precision__{from.requires_precision__} {
    GUM_CONS_CPY(GammaLog2);
  }

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GammaLog2() [3/3]

gum::GammaLog2::GammaLog2

(

GammaLog2 &&

from

)

Move constructor.

Parameters

from	The gum::GammaLog2 to move.

Definition at line 50 of file gammaLog2.cpp.

References gum::Set< Key, Alloc >::emplace().

                                       :
      requires_precision__{from.requires_precision__} {
    GUM_CONS_MOV(GammaLog2);
  }

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~GammaLog2()

gum::GammaLog2::~GammaLog2

(

)

Class destructor.

Definition at line 56 of file gammaLog2.cpp.

References gum::Set< Key, Alloc >::emplace().

{ GUM_DESTRUCTOR(GammaLog2); }

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Member Function Documentation

gammaLog2()

ALWAYS_INLINE double gum::GammaLog2::gammaLog2

(

double

x

)

const

Returns log2 ( gamma (x) ) for x >= 0.

Parameters

x	A positive double.

Returns

Returns log2 ( gamma (x) ) for x >= 0.

Exceptions

OutOfBounds

Raised if x <= 0.

Definition at line 31 of file gammaLog2_inl.h.

References gum::Set< Key, Alloc >::emplace().

                                                          {
    if (x <= 0)
      GUM_ERROR(OutOfBounds,
                "log2(gamma()) should be called with a positive argument");

    // if x is small, use precomputed values
    if (x < 50) {
      if (x >= 0.01) {
        if (requires_precision__) {
          const Idx index = int(x * 100);
          return small_values__[index]
               + (small_values__[index + 1] - small_values__[index])
                    * double(x * 100 - index);
        } else {
          const Idx index = int(x * 100 + 0.5);
          return small_values__[index];
        }
      } else {
        // for very small values of x, Gamma(x) is approximately equal to
        // 1/x. Hence gammaLog2(x) is approximately equal to log2(1/x)
        return std::log2(1.0 / x);
      }
    }

    // returns the approximation by the stirling formula
    return (log_sqrt_2pi__ + (x - 0.5f) * log(x) - x + log(1.0 + 1.0 / (12 * x)))
         * inv_log2__;
  }

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operator()()

ALWAYS_INLINE double gum::GammaLog2::operator()

(

double

x

)

const

Returns log2 ( gamma (x) ) for x > 0.

Exceptions

OutOfBounds

Raised if raised if x <= 0.

Definition at line 60 of file gammaLog2_inl.h.

References gum::Set< Key, Alloc >::emplace().

                                                           {
    return gammaLog2(x);
  }

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setPrecision()

INLINE void gum::GammaLog2::setPrecision

(

bool

p

)

Sets whether we need more precision for small values.

Parameters

p	If true, precision is enable.

Definition at line 64 of file gammaLog2_inl.h.

References gum::Set< Key, Alloc >::emplace().

{ requires_precision__ = prec; }

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Member Data Documentation

inv_log2__

constexpr double gum::GammaLog2::inv_log2__ {M_LOG2E}

staticprivate

The value of 1 / std::log(2).

Definition at line 116 of file gammaLog2.h.

log_sqrt_2pi__

constexpr double gum::GammaLog2::log_sqrt_2pi__ {GUM_LOG_SQRT_2PI}

staticprivate

The value of std::log ( std::sqrt(2pi) ).

Definition at line 119 of file gammaLog2.h.

requires_precision__

bool gum::GammaLog2::requires_precision__ {false}

private

Indicates whether we need more precision for small values.

Definition at line 113 of file gammaLog2.h.

small_values__

const std::vector< double > gum::GammaLog2::small_values__

staticprivate

The 5000 values from 0 to 50 by step of 1/100.

Definition at line 122 of file gammaLog2.h.

---

The documentation for this class was generated from the following files:

• agrum/tools/core/math/gammaLog2.h
• agrum/tools/core/math/gammaLog2.cpp
• agrum/tools/core/math/gammaLog2_inl.h