Eigen/src/Core/util/IntegralConstant.h

// This file is part of Eigen, a lightweight C++ template library
// for linear algebra.
//
// Copyright (C) 2017 Gael Guennebaud <gael.guennebaud@inria.fr>
//
// This Source Code Form is subject to the terms of the Mozilla
// Public License v. 2.0. If a copy of the MPL was not distributed
// with this file, You can obtain one at http://mozilla.org/MPL/2.0/.

#ifndef EIGEN_INTEGRAL_CONSTANT_H
#define EIGEN_INTEGRAL_CONSTANT_H

// IWYU pragma: private
#include "../InternalHeaderCheck.h"

namespace Eigen {

namespace internal {

template <int N>
class FixedInt;
template <int N>
class VariableAndFixedInt;

/** \internal
 * \class FixedInt
 *
 * This class embeds a compile-time integer \c N.
 *
 * It is similar to c++11 std::integral_constant<int,N> but with some additional features
 * such as:
 *  - implicit conversion to int
 *  - arithmetic and some bitwise operators: -, +, *, /, %, &, |
 *  - c++98/14 compatibility with fix<N> and fix<N>() syntax to define integral constants.
 *
 * It is strongly discouraged to directly deal with this class FixedInt. Instances are expected to
 * be created by the user using Eigen::fix<N> or Eigen::fix<N>().
 * \code
 * internal::cleanup_index_type<T>::type
 * internal::cleanup_index_type<T,DynamicKey>::type
 * \endcode
 * where T can a FixedInt<N>, a pointer to function FixedInt<N> (*)(), or numerous other integer-like representations.
 * \c DynamicKey is either Dynamic (default) or DynamicIndex and used to identify true compile-time values.
 *
 * For convenience, you can extract the compile-time value \c N in a generic way using the following helper:
 * \code
 * internal::get_fixed_value<T,DefaultVal>::value
 * \endcode
 * that will give you \c N if T equals FixedInt<N> or FixedInt<N> (*)(), and \c DefaultVal if T does not embed any
 * compile-time value (e.g., T==int).
 *
 * \sa fix<N>, class VariableAndFixedInt
 */
template <int N>
class FixedInt {
 public:
  static constexpr int value = N;
  constexpr operator int() const { return N; }

  constexpr FixedInt() = default;
  constexpr FixedInt(std::integral_constant<int, N>) {}

  constexpr FixedInt(VariableAndFixedInt<N> other) {
#ifndef EIGEN_INTERNAL_DEBUGGING
    EIGEN_UNUSED_VARIABLE(other);
#endif
    eigen_internal_assert(int(other) == N);
  }

  constexpr FixedInt<-N> operator-() const { return FixedInt<-N>(); }

  template <int M>
  constexpr FixedInt<N + M> operator+(FixedInt<M>) const {
    return FixedInt<N + M>();
  }

  template <int M>
  constexpr FixedInt<N - M> operator-(FixedInt<M>) const {
    return FixedInt<N - M>();
  }

  template <int M>
  constexpr FixedInt<N * M> operator*(FixedInt<M>) const {
    return FixedInt<N * M>();
  }

  template <int M>
  constexpr FixedInt<N / M> operator/(FixedInt<M>) const {
    return FixedInt<N / M>();
  }

  template <int M>
  constexpr FixedInt<N % M> operator%(FixedInt<M>) const {
    return FixedInt<N % M>();
  }

  template <int M>
  constexpr FixedInt<N | M> operator|(FixedInt<M>) const {
    return FixedInt<N | M>();
  }

  template <int M>
  constexpr FixedInt<N & M> operator&(FixedInt<M>) const {
    return FixedInt<N & M>();
  }

  // Needed in C++14 to allow fix<N>():
  constexpr FixedInt operator()() const { return *this; }

  constexpr VariableAndFixedInt<N> operator()(int val) const { return VariableAndFixedInt<N>(val); }
};

/** \internal
 * \class VariableAndFixedInt
 *
 * This class embeds both a compile-time integer \c N and a runtime integer.
 * Both values are supposed to be equal unless the compile-time value \c N has a special
 * value meaning that the runtime-value should be used. Depending on the context, this special
 * value can be either Eigen::Dynamic (for positive quantities) or Eigen::DynamicIndex (for
 * quantities that can be negative).
 *
 * It is the return-type of the function Eigen::fix<N>(int), and most of the time this is the only
 * way it is used. It is strongly discouraged to directly deal with instances of VariableAndFixedInt.
 * Indeed, in order to write generic code, it is the responsibility of the callee to properly convert
 * it to either a true compile-time quantity (i.e. a FixedInt<N>), or to a runtime quantity (e.g., an Index)
 * using the following generic helper:
 * \code
 * internal::cleanup_index_type<T>::type
 * internal::cleanup_index_type<T,DynamicKey>::type
 * \endcode
 * where T can be a template instantiation of VariableAndFixedInt or numerous other integer-like representations.
 * \c DynamicKey is either Dynamic (default) or DynamicIndex and used to identify true compile-time values.
 *
 * For convenience, you can also extract the compile-time value \c N using the following helper:
 * \code
 * internal::get_fixed_value<T,DefaultVal>::value
 * \endcode
 * that will give you \c N if T equals VariableAndFixedInt<N>, and \c DefaultVal if T does not embed any compile-time
 * value (e.g., T==int).
 *
 * \sa fix<N>(int), class FixedInt
 */
template <int N>
class VariableAndFixedInt {
 public:
  static const int value = N;
  operator int() const { return m_value; }
  VariableAndFixedInt(int val) { m_value = val; }

 protected:
  int m_value;
};

template <typename T, int Default = Dynamic>
struct get_fixed_value {
  static const int value = Default;
};

template <int N, int Default>
struct get_fixed_value<FixedInt<N>, Default> {
  static const int value = N;
};

template <int N, int Default>
struct get_fixed_value<VariableAndFixedInt<N>, Default> {
  static const int value = N;
};

template <typename T, int N, int Default>
struct get_fixed_value<variable_if_dynamic<T, N>, Default> {
  static const int value = N;
};

template <typename T>
EIGEN_DEVICE_FUNC Index get_runtime_value(const T &x) {
  return x;
}

// Cleanup integer/FixedInt/VariableAndFixedInt/etc types:

// By default, no cleanup:
template <typename T, int DynamicKey = Dynamic, typename EnableIf = void>
struct cleanup_index_type {
  typedef T type;
};

// Convert any integral type (e.g., short, int, unsigned int, etc.) to Eigen::Index
template <typename T, int DynamicKey>
struct cleanup_index_type<T, DynamicKey, std::enable_if_t<internal::is_integral<T>::value>> {
  typedef Index type;
};

// If VariableAndFixedInt does not match DynamicKey, then we turn it to a pure compile-time value:
template <int N, int DynamicKey>
struct cleanup_index_type<VariableAndFixedInt<N>, DynamicKey> {
  typedef FixedInt<N> type;
};
// If VariableAndFixedInt matches DynamicKey, then we turn it to a pure runtime-value (aka Index):
template <int DynamicKey>
struct cleanup_index_type<VariableAndFixedInt<DynamicKey>, DynamicKey> {
  typedef Index type;
};

template <int N, int DynamicKey>
struct cleanup_index_type<std::integral_constant<int, N>, DynamicKey> {
  typedef FixedInt<N> type;
};

}  // end namespace internal

#ifndef EIGEN_PARSED_BY_DOXYGEN

template <int N>
constexpr internal::FixedInt<N> fix{};

#else  // EIGEN_PARSED_BY_DOXYGEN

/** \var fix<N>()
 * \ingroup Core_Module
 *
 * This \em identifier permits to construct an object embedding a compile-time integer \c N.
 *
 * \tparam N the compile-time integer value
 *
 * It is typically used in conjunction with the Eigen::seq and Eigen::seqN functions to pass compile-time values to
 * them: \code seqN(10,fix<4>,fix<-3>)   // <=> [10 7 4 1] \endcode
 *
 * See also the function fix(int) to pass both a compile-time and runtime value.
 *
 * In c++14, it is implemented as:
 * \code
 * template<int N> static const internal::FixedInt<N> fix{};
 * \endcode
 * where internal::FixedInt<N> is an internal template class similar to
 * <a href="http://en.cppreference.com/w/cpp/types/integral_constant">\c std::integral_constant </a><tt> <int,N> </tt>
 * Here, \c fix<N> is thus an object of type \c internal::FixedInt<N>.
 *
 * \sa fix<N>(int), seq, seqN
 */
template <int N>
static const auto fix();

/** \fn fix<N>(int)
 * \ingroup Core_Module
 *
 * This function returns an object embedding both a compile-time integer \c N, and a fallback runtime value \a val.
 *
 * \tparam N the compile-time integer value
 * \param  val the fallback runtime integer value
 *
 * This function is a more general version of the \ref fix identifier/function that can be used in template code
 * where the compile-time value could turn out to actually mean "undefined at compile-time". For positive integers
 * such as a size or a dimension, this case is identified by Eigen::Dynamic, whereas runtime signed integers
 * (e.g., an increment/stride) are identified as Eigen::DynamicIndex. In such a case, the runtime value \a val
 * will be used as a fallback.
 *
 * A typical use case would be:
 * \code
 * template<typename Derived> void foo(const MatrixBase<Derived> &mat) {
 *   const int N = Derived::RowsAtCompileTime==Dynamic ? Dynamic : Derived::RowsAtCompileTime/2;
 *   const int n = mat.rows()/2;
 *   ... mat( seqN(0,fix<N>(n) ) ...;
 * }
 * \endcode
 * In this example, the function Eigen::seqN knows that the second argument is expected to be a size.
 * If the passed compile-time value N equals Eigen::Dynamic, then the proxy object returned by fix will be dismissed,
 * and converted to an Eigen::Index of value \c n. Otherwise, the runtime-value \c n will be dismissed, and the
 * returned ArithmeticSequence will be of the exact same type as <tt> seqN(0,fix<N>) </tt>.
 *
 * \sa fix, seqN, class ArithmeticSequence
 */
template <int N>
static const auto fix(int val);

#endif  // EIGEN_PARSED_BY_DOXYGEN

}  // end namespace Eigen

#endif  // EIGEN_INTEGRAL_CONSTANT_H