c10/util/math_compat.h

#pragma once

#include <cmath>

// Android NDK platform < 21 with libstdc++ has spotty C++11 support.
// Various hacks in this header allow the rest of the codebase to use
// standard APIs.
#if (defined(__ANDROID__) && __ANDROID_API__ < 21 && defined(__GLIBCXX__)) || \
    defined(__NEWLIB__)
#include <stdexcept>

namespace std {
// Import double versions of these functions from the global namespace.
using ::acosh;
using ::asinh;
using ::atanh;
using ::erf;
using ::erfc;
using ::expm1;
using ::lgamma;
using ::log1p;
using ::nearbyint;
using ::round;
using ::tgamma;
using ::trunc;
using ::truncf;

// Define float versions the same way as more recent libstdc++
inline float acosh(float x) {
  return __builtin_acoshf(x);
}
inline float asinh(float x) {
  return __builtin_asinhf(x);
}
inline float atanh(float x) {
  return __builtin_atanhf(x);
}
inline float copysign(float x, float y) {
  return __builtin_copysignf(x, y);
}
inline float erf(float x) {
  return __builtin_erff(x);
}
inline float erfc(float x) {
  return __builtin_erfcf(x);
}
inline float expm1(float x) {
  return __builtin_expm1f(x);
}
inline float fmax(float x, float y) {
  return __builtin_fmaxf(x, y);
}
inline float fmin(float x, float y) {
  return __builtin_fminf(x, y);
}
inline float lgamma(float x) {
  return __builtin_lgammaf(x);
}
inline float log1p(float x) {
  return __builtin_log1pf(x);
}
inline float nearbyint(float x) {
  return __builtin_nearbyintf(x);
}
inline float remainder(float x, float y) {
  return __builtin_remainderf(x, y);
}
inline float round(float x) {
  return __builtin_roundf(x);
}
inline float tgamma(float x) {
  return __builtin_tgammaf(x);
}
inline float trunc(float x) {
  return __builtin_truncf(x);
}

// __builtin_nexttoward isn't doesn't work.  It appears to try to
// link against the global nexttoward function, which is not present
// prior to API 18.  Just bail for now.
inline float nexttoward(float x, long double y) {
  throw std::runtime_error("std::nexttoward is not present on older Android");
}
inline double nexttoward(double x, long double y) {
  throw std::runtime_error("std::nexttoward is not present on older Android");
}

#if !defined(__NEWLIB__)
// TODO: this function needs to be implemented and tested. Currently just throw
// an error.
inline float hypot(float x, float y) {
  throw std::runtime_error("std::hypot is not implemented on older Android");
}
inline double hypot(double x, double y) {
  throw std::runtime_error("std::hypot is not implemented on older Android");
}
#else
inline float hypot(float x, float y) {
  return hypot((double)x, (double)y);
}
#endif

// TODO: this function needs to be implemented and tested. Currently just throw
// an error.
inline float igamma(float x, float y) {
  throw std::runtime_error("igamma is not implemented on older Android");
}
inline double igamma(double x, double y) {
  throw std::runtime_error("igamma is not implemented on older Android");
}
inline float igammac(float x, float y) {
  throw std::runtime_error("igammac is not implemented on older Android");
}
inline double igammac(double x, double y) {
  throw std::runtime_error("igammac is not implemented on older Android");
}

// Note: std::signbit returns true for negative zero (-0), but this
// implementation returns false.
inline bool signbit(float x) {
  return x < 0;
}
inline bool signbit(double x) {
  return x < 0;
}
inline bool signbit(long double x) {
  return x < 0;
}

#if !defined(__NEWLIB__)
// TODO: this function needs to be implemented and tested. Currently just throw
// an error.
inline float nextafter(float x, float y) {
  throw std::runtime_error(
      "std::nextafter is not implemented on older Android");
}
inline double nextafter(double x, double y) {
  throw std::runtime_error(
      "std::nextafter is not implemented on older Android");
}
#else
inline float nextafter(float x, float y) {
  return nextafter((double)x, (double)y);
}
#endif

#if !defined(__NEWLIB__)
// TODO: this function needs to be implemented and tested. Currently just throw
// an error.
inline float exp2(float x) {
  throw std::runtime_error("std::exp2 is not implemented on older Android");
}
inline double exp2(double x) {
  throw std::runtime_error("std::exp2 is not implemented on older Android");
}
#else
inline float exp2(float x) {
  return exp2((double)x);
}
#endif

// Define integral versions the same way as more recent libstdc++
template <typename T>
typename std::enable_if<std::is_integral<T>::value, double>::type acosh(T x) {
  return __builtin_acosh(x);
}
template <typename T>
typename std::enable_if<std::is_integral<T>::value, double>::type asinh(T x) {
  return __builtin_asinh(x);
}
template <typename T>
typename std::enable_if<std::is_integral<T>::value, double>::type atanh(T x) {
  return __builtin_atanh(x);
}
template <typename T>
typename std::enable_if<std::is_integral<T>::value, double>::type erf(T x) {
  return __builtin_erf(x);
}
template <typename T>
typename std::enable_if<std::is_integral<T>::value, double>::type erfc(T x) {
  return __builtin_erfc(x);
}
template <typename T>
typename std::enable_if<std::is_integral<T>::value, double>::type expm1(T x) {
  return __builtin_expm1(x);
}
template <typename T>
typename std::enable_if<std::is_integral<T>::value, double>::type lgamma(T x) {
  return __builtin_lgamma(x);
}
template <typename T>
typename std::enable_if<std::is_integral<T>::value, double>::type log1p(T x) {
  return __builtin_log1p(x);
}
template <typename T>
typename std::enable_if<std::is_integral<T>::value, double>::type nearbyint(
    T x) {
  return __builtin_nearbyint(x);
}
template <typename T>
typename std::enable_if<std::is_integral<T>::value, double>::type round(T x) {
  return __builtin_round(x);
}
template <typename T>
typename std::enable_if<std::is_integral<T>::value, double>::type tgamma(T x) {
  return __builtin_tgamma(x);
}
template <typename T>
typename std::enable_if<std::is_integral<T>::value, double>::type trunc(T x) {
  return __builtin_trunc(x);
}

// Convoluted definition of these binary functions for overloads other than
// (float,float) and (double,double).  Using a template from __gnu_cxx
// is dirty, but this code is only enabled on a dead platform, so there
// shouldn't be any risk of it breaking due to updates.
template <typename T, typename U>
typename __gnu_cxx::__promote_2<T, U>::__type fmax(T x, U y) {
  typedef typename __gnu_cxx::__promote_2<T, U>::__type type;
  return fmax(type(x), type(y));
}
template <typename T, typename U>
typename __gnu_cxx::__promote_2<T, U>::__type fmin(T x, U y) {
  typedef typename __gnu_cxx::__promote_2<T, U>::__type type;
  return fmin(type(x), type(y));
}
template <typename T, typename U>
typename __gnu_cxx::__promote_2<T, U>::__type copysign(T x, U y) {
  typedef typename __gnu_cxx::__promote_2<T, U>::__type type;
  return copysign(type(x), type(y));
}
template <typename T, typename U>
typename __gnu_cxx::__promote_2<T, U>::__type remainder(T x, U y) {
  typedef typename __gnu_cxx::__promote_2<T, U>::__type type;
  return remainder(type(x), type(y));
}

// log2 is a macro on Android API < 21, so we need to define it ourselves.
inline float log2(float arg) {
  return ::log(arg) / ::log(2.0);
}
#if !defined(__NEWLIB__)
inline double log2(double arg) {
  return ::log(arg) / ::log(2.0);
}
#endif
inline long double log2(long double arg) {
  return ::log(arg) / ::log(2.0);
}
template <typename T>
typename std::enable_if<std::is_integral<T>::value, double>::type log2(T x) {
  return ::log(x) / ::log(2.0);
}
} // namespace std

#endif