lentz_b

eve::lentz_b = functor<lentz_b_t>

inlineconstexpr

Defined in header

#include <eve/module/math.hpp>

Callable Signatures

namespace eve
{
  template< typename Gen, eve::floating_value T> auto lentz_b(Gen g, const T& tol, size_t & max_terms) noexcept;
}

Parameters

• g : generator function.
• tol : tolerance value. If negative the effective tolerance will be abs(tol)*eveeps(as(< u_t>) where u_t is the underlying floating type associated to the return type of the invocable g.
• max_terms : no more than max_terms calls to the generator will be made.

The generator type should be an invocable which supports the following operations:

• The call to g() returns a floating value or a pair (kumi::tuple) of such. Each time this operator is called then the next pair of a and b values has to be returned, or, if result_type is not a pair type, then the next b value has to be returned and all the a values are assumed to be equal to one.
• In all the continued fraction evaluation functions the effective tol parameter is the relative precision desired in the result, The evaluation of the fraction will continue until the last term evaluated leaves the relative error in the result less than tolerance or the max_terms iteration is reached.

Return value

The value of the continued fraction is returned. \(\displaystyle b_0+\frac{a_1}{b_1+\frac{a_2}{b_2+\frac{a_3}{b_3+\cdots\vphantom{\frac{1}{1}} }}}\)

Note that the the first a value (a0) generated is not used here.

Note

the implementation is largely inspired by the boost/math/fraction one, with less requirements on the invocable. Peculiarly lambda functions can be used.

Example

// revision 1
#include <eve/module/math.hpp>
#include <iostream>
 
template <class T>
struct const_fraction
{
  auto operator()(){ return T{1.0, 2.0, 3.0, 4.0}; }
};
 
template <typename T>
struct tan_fraction
{
  T a, b;
  tan_fraction(T v) : a(-v * v), b(-1) {}
  auto operator()()
  {
    b += T(2);
    return kumi::tuple{a, b};
  }
};
 
template <class T>
T mytan(T a)
{
  tan_fraction<T> fract(a);
  return a/eve::lentz_b(fract, eve::eps(eve::as<eve::underlying_type_t<T>>()), 100);
}
 
int main()
{
  using w_t = eve::wide<double, eve::fixed<4>>;
  const_fraction<w_t> func;
  eve::wide zz{1.0, 2.0, 3.0, 4.0};
  std::cout << "ref constant fracs " << (zz+eve::sqrt(eve::sqr(zz)+4))/2 << std::endl;
  auto gr = eve::lentz_b(func,eve::eps(eve::as<double>()), 100);
  std::cout << "    constant fracs " << gr << std::endl;
  w_t z{eve::pio_4(eve::as<double>()), eve::pio_3(eve::as<double>()), 0.0, 1.0};
  std::cout << "frac tan(" << z << ") is: " << mytan(z) << std::endl;
  std::cout << "ref  tan(" << z << ") is: " << eve::tan(z) << std::endl;
}