Kelvin to Celsius

```/**
* @file
* @brief Conversion from [Kelvin to
* Celsius](https://byjus.com/chemistry/kelvin-to-celsius/) degrees.
* @details
* The algorithm consists on converting a Kelvin degree value to a Celsius
* value.
* The formula to convert a Kelvin to a Celsius value is:
* @f[ C = K - 273.15 @f] where:
* - C is the Celsius temperature
* - K is the Kelvin temperature
*
* Check out [Kelvin](https://en.wikipedia.org/wiki/Kelvin) and
* [Celsius](https://en.wikipedia.org/wiki/Celsius) on Wikipedia for more
* information about their story, how do they work, when and why they should be
* used, etc..
* @author [David Leal](https://github.com/Panquesito7)
*/

#include <cassert>   /// for assert
#include <cmath>     /// for std::abs
#include <iostream>  /// for IO operations

/**
* @namespace
* @brief Other algorithms
*/
namespace others {
/**
* @brief Compare two floating point numbers with a certain tolerance.
* This is needed as with some values, the result (e.g.: -196.15) might be a bit
* lower (in this case, -196.499999...).
* @param a the first number to compare
* @param b the second number to compare
* @param tolerance the tolerance to use when comparing the numbers
* @returns true if the numbers ARE equal within the given tolerance
* @returns false if the numbers are NOT equal within the given tolerance
* otherwise
*/
bool are_almost_equal(double a, double b, double absolute_tolerance = 0.0001) {
return std::abs(a - b) < absolute_tolerance;
}

/**
* @brief Conversion from Kelvin to Celsius algorithm.
* @param number the Celsius number that will be used to convert
* @returns the Kelvin number converted to Celsius
*/
double kelvin_to_celsius(double temperature_in_k) {
const double absolute_zero_in_c = -273.15;
if (temperature_in_k < absolute_zero_in_c) {
throw std::invalid_argument("input temperature below absolute zero");
}
return temperature_in_k + absolute_zero_in_c;
}
}  // namespace others

/**
* @brief Self-test implementations
* @returns void
*/
static void tests() {
assert(others::are_almost_equal(others::kelvin_to_celsius(230), -43.15));
assert(others::are_almost_equal(others::kelvin_to_celsius(512), 238.85));
assert(others::are_almost_equal(others::kelvin_to_celsius(55), -218.15));
assert(others::are_almost_equal(others::kelvin_to_celsius(77), -196.15));
assert(others::are_almost_equal(others::kelvin_to_celsius(9.78), -263.37));
assert(others::are_almost_equal(others::kelvin_to_celsius(15), -258.15));
assert(others::are_almost_equal(others::kelvin_to_celsius(273.15), 0));

std::cout << "All tests have successfully passed!\n";
}

/**
* @brief Main function
* @returns 0 on exit
*/
int main() {
tests();  // run self-test implementations
return 0;
}
```