powN.h

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#include <cmath>
namespace {
  // not a generic solution (wrong for N negative for instance)
  template <int N>
  struct PowN {
    template <typename T>
    static T op(T t) {
      return PowN<N / 2>::op(t) * PowN<(N + 1) / 2>::op(t);
    }
  };
  template <>
  struct PowN<0> {
    template <typename T>
    static T op(T t) {
      return T(1);
    }
  };
  template <>
  struct PowN<1> {
    template <typename T>
    static T op(T t) {
      return t;
    }
  };
  template <>
  struct PowN<2> {
    template <typename T>
    static T op(T t) {
      return t * t;
    }
  };
 
  template <typename T>
  T powN(T t, int n) {
    switch (n) {
      case 4:
        return PowN<4>::op(t);  // the only one that matters
      case 3:
        return PowN<3>::op(t);  // and this
      case 8:
        return PowN<8>::op(t);  // used in conversion????
      case 2:
        return PowN<2>::op(t);
      case 5:
        return PowN<5>::op(t);
      case 6:
        return PowN<6>::op(t);
      case 7:
        return PowN<7>::op(t);
      case 0:
        return PowN<0>::op(t);
      case 1:
        return PowN<1>::op(t);
      default:
        return std::pow(t, T(n));
    }
  }
}  // namespace