nbdkit is_power_of_2

  /* Returns true if v is a power of 2.
   *
   * Uses the algorithm described at
   * http://graphics.stanford.edu/~seander/bithacks.html#DetermineIfPowerOf2
   */
  static inline bool
  is_power_of_2 (unsigned long v)
  {
    return v && ((v & (v - 1)) == 0);
  }