double_factorial

eve::double_factorial = functor<double_factorial_t>

inlineconstexpr

THe double factorial is defined as \(\displaystyle (2n)!! = \prod_{i=1}^n (2i)\) and \(\displaystyle (2n+1)!! = \prod_{i=0}^n (2i+1)\)

Header file

#include <eve/module/special.hpp>

Callable Signatures

namespace eve
{
   // Regular overload
   template <unsigned_value T> constexpr as_wide_as_t<double,T> double_factorial(T x) noexcept; // 1
 
   // Lanes masking
   constexpr auto double_factorial[conditional_expr auto c](unsigned_value auto x)    noexcept; // 2
   constexpr auto double_factorial[logical_value auto m](unsigned_value auto x)       noexcept; // 2
}

Parameters

• n: unsigned argument.
• c: Conditional expression masking the operation.
• m: Logical value masking the operation.

Return value

1. The value of the double factorial of n is returned.
2. The operation is performed conditionnaly.

External references

• Wolfram MathWorld: Double Factorial
• Wikipedia: Double factorial

Example

// revision 1
#include <eve/module/special.hpp>
#include <iostream>
 
int main()
{
  eve::wide wu = {1u, 2u, 3u, 4u, 5u, 6u, 181u, 182u};
 
  std::cout << "<- wu = " << wu << "\n";
 
  std::cout << "-> double_factorial(wu)                = " << eve::double_factorial(wu) << "\n";
  std::cout << "-> double_factorial[ignore_last(2)](wu)= " << eve::double_factorial[eve::ignore_last(2)](wu) << "\n";
  std::cout << "-> double_factorial[wu != 182u](wu)    = " << eve::double_factorial[wu != 182u](wu) << "\n";
}