gammaLog2_inl.h

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 *       - Pierre-Henri WUILLEMIN(_at_LIP6)                                 *
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#pragma once

 
namespace gum {
 
  ALWAYS_INLINE double GammaLog2::gammaLog2(double x) const {
    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_;
  }
 
  ALWAYS_INLINE double GammaLog2::operator()(double x) const { return gammaLog2(x); }
 
  INLINE void GammaLog2::setPrecision(bool prec) { _requires_precision_ = prec; }
 
} /* namespace gum */