Avoid overflow when converting time to microseconds

microsecs.c

// Taken from the Rust code base: https://github.com/rust-lang/rust/blob/3809bbf47c8557bd149b3e52ceb47434ca8378d5/src/libstd/sys_common/mod.rs#L124
// Computes (value*numer)/denom without overflow, as long as both
// (numer*denom) and the overall result fit into i64 (which is the case
// for our time conversions).
int64_t int64MulDiv(int64_t value, int64_t numer, int64_t denom) {
  int64_t q = value / denom;
  int64_t r = value % denom;
  // Decompose value as (value/denom*denom + value%denom),
  // substitute into (value*numer)/denom and simplify.
  // r < denom, so (denom*numer) is the upper bound of (r*numer)
  return q * numer + r * numer / denom;
}

int64_t timeInMicros() {
  // Get time and frequency
  LARGE_INTEGER time;
  QueryPerformanceCounter(&time);
  LARGE_INTEGER frequency;
  QueryPerformanceFrequency(&frequency);
  
  // Convert to microseconds
  const int64_t microsPerSecond = 1000000LL;
  int64_t micros = int64MulDiv(time.QuadPart, microsPerSecond, frequency.QuadPart);
  return micros;
}