even_time_difference_sol.cpp

// Changing variables included from headers (monkey patching) is not possible in
// C++. Instead we refactor the function to make it pure.
// Another way would be to use templates for dependency injection
#include <catch2/catch.hpp>

// Making it pure, probably renaming would be a good idea
bool check_time_to_now_even(int seconds, int current_time) {
  if (seconds < 0) {
    throw std::runtime_error('received negative input');
  }
  return (current_time - seconds) % 2 == 0;
}

// Normal invocation
auto now = std::chrono::system_clock::now().time_since_epoch();
int current_time_seconds =
    std::chrono::duration_cast<std::chrono::seconds>(now).count();
check_time_to_now_even(4, current_time_seconds);

TEST_CASE("Check time difference", "[reactor_state]") {
  REQUIRE(check_time_to_now_even(1, 3) == true);
  REQUIRE(check_time_to_now_even(4, 5) == false);
}

// using templates

template <class CLOCK> bool check_time_to_now_even(int seconds) {
  if (seconds < 0) {
    throw std::runtime_error('received negative input');
  }
  int current_time = CLOCK.getNowSeconds();
  return (current_time - seconds) % 2 == 0;
}

// Normal invocation
class ChronoClock {

  int getNowSeconds() {
    auto now = std::chrono::system_clock::now().time_since_epoch();
    return std::chrono::duration_cast<std::chrono::seconds>(now).count();
  }
};

check_time_to_now_even<ChronoClock>(3);

class TestClock {

  int getNowSeconds() { return 3; }
};

TEST_CASE("Check time difference", "[reactor_state]") {
  REQUIRE(check_time_to_now_even<TestClock>(3) == true);
  REQUIRE(check_time_to_now_even<TestClock>(4) == false);
}