TheAlgorithms-C-Plus-Plus/math/inv_sqrt.cpp

/**
 * @file
 * @brief Calculate the inverse inverse root.
 * @details
 * Two implementation to calculate inverse inverse root,
 * from Quake III Arena (C++ version) and with standard lib (cmath)
 */

#include <cmath>        /// for `std::sqrt`
#include <iostream>     /// for IO operations
#include <type_traits>  /// for `static_assert`

/**
 * @brief This is the function that calculates the fast inverse square root.
 * The following code is the fast inverse square root implementation from
 * Quake III Arena (Adapted for C++) More information can be found at
 * [Wikipedia](https://en.wikipedia.org/wiki/Fast_inverse_square_root)
 * @tparam T floating type
 * @tparam iterations inverse square root, the greater the number of
 * iterations, the more exact the result will be (1 or 2).
 * @param x value to calculate
 * @return T return inverse square root
 */
template <typename T, char iterations = 2>
inline T Fast_InvSqrt(T x) {
    using Tint = typename std::conditional<sizeof(T) == 8, std::int64_t,
                                           std::int32_t>::type;
    T y = x;
    T x2 = y * 0.5;
    Tint i = *(Tint *)&y;
    i = (sizeof(T) == 8 ? 0x5fe6eb50c7b537a9 : 0x5f3759df) - (i >> 1);
    y = *(T *)&i;

    y = *reinterpret_cast<T *>(&i);

    y = y * (1.5 - (x2 * y * y));
    if (iterations == 2) {
        y = y * (1.5 - (x2 * y * y));
    }
    return y;
}

/**
 * @brief This is the function that calculates the fast inverse square root.
 * The following code is the fast inverse square root with standard lib (cmath)
 * More info:
 * https://www.linkedin.com/pulse/fast-inverse-square-root-still-armin-kassemi-langroodi
 * @tparam T floating type
 * @param x value to calculate
 * @return T return inverse square root
 */
template <typename T>
T Standard_InvSqrt(T number) {
    T squareRoot = sqrt(number);
    return 1.0f / squareRoot;
}

/**
 * @brief Main function
 * @returns 0 on exit
 */
int main() {
    std::cout << "The Fast inverse square root of 36 is: "
              << Fast_InvSqrt<float, 1>(36.0f) << std::endl;
    std::cout << "The Fast inverse square root of 36 is: "
              << Fast_InvSqrt<double, 2>(36.0f) << " (2 iterations)"
              << std::endl;
    std::cout << "The Standard inverse square root of 36 is: "
              << Standard_InvSqrt<float>(36.0f) << std::endl;
}