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//
// Copyright (C) 1998 - 2016 by the deal.II authors
//
// This file is part of the deal.II library.
//
// The deal.II library is free software; you can use it, redistribute
// it, and/or modify it under the terms of the GNU Lesser General
// Public License as published by the Free Software Foundation; either
// version 2.1 of the License, or (at your option) any later version.
// The full text of the license can be found in the file LICENSE at
// the top level of the deal.II distribution.
//
// ---------------------------------------------------------------------
#ifndef dealii__tensor_accessors_h
#define dealii__tensor_accessors_h
#include <deal.II/base/config.h>
#include <deal.II/base/template_constraints.h>
#include <deal.II/base/table_indices.h>
DEAL_II_NAMESPACE_OPEN
/**
* This namespace is a collection of algorithms working on generic tensorial
* objects (of arbitrary rank).
*
* The rationale to implement such functionality in a generic fashion in a
* separate namespace is
* - to easy code reusability and therefore avoid code duplication.
* - to have a well-defined interface that allows to exchange the low
* level implementation.
*
*
* A tensorial object has the notion of a rank and allows a rank-times
* recursive application of the index operator, e.g., if <code>t</code> is a
* tensorial object of rank 4, the following access is valid:
* @code
* t[1][2][1][4]
* @endcode
*
* deal.II has its own implementation for tensorial objects such as
* dealii::Tensor<rank, dim, Number> and dealii::SymmetricTensor<rank, dim,
* Number>
*
* The methods and algorithms implemented in this namespace, however, are
* fully generic. More precisely, it can operate on nested c-style arrays, or
* on class types <code>T</code> with a minimal interface that provides a
* local typedef <code>value_type</code> and an index operator
* <code>operator[](unsigned int)</code> that returns a (const or non-const)
* reference of <code>value_type</code>:
* @code
* template<...>
* class T
* {
* typedef ... value_type;
* value_type & operator[](unsigned int);
* const value_type & operator[](unsigned int) const;
* };
* @endcode
*
* This namespace provides primitives for access, reordering and contraction
* of such objects.
*
* @ingroup geomprimitives
*
* @author Matthias Maier, 2015
*/
namespace TensorAccessors
{
// forward declarations
namespace internal
{
template <int index, int rank, typename T> class ReorderedIndexView;
template <int position, int rank> struct ExtractHelper;
template <int no_contr, int rank_1, int rank_2, int dim> class Contract;
template <int rank_1, int rank_2, int dim> class Contract3;
}
/**
* This class provides a local typedef @p value_type denoting the resulting
* type of an access with operator[](unsigned int). More precisely, @p
* value_type will be
* - <code>T::value_type</code> if T is a tensorial class providing a
* typedef <code>value_type</code> and does not have a const qualifier.
* - <code>const T::value_type</code> if T is a tensorial class
* providing a typedef <code>value_type</code> and does have a const
* qualifier.
* - <code>const T::value_type</code> if T is a tensorial class
* providing a typedef <code>value_type</code> and does have a const
* qualifier.
* - <code>A</code> if T is of array type <code>A[...]</code>
* - <code>const A</code> if T is of array type <code>A[...]</code> and
* does have a const qualifier.
*/
template <typename T>
struct ValueType
{
typedef typename T::value_type value_type;
};
template <typename T>
struct ValueType<const T>
{
typedef const typename T::value_type value_type;
};
template <typename T, std::size_t N>
struct ValueType<T[N]>
{
typedef T value_type;
};
template <typename T, std::size_t N>
struct ValueType<const T[N]>
{
typedef const T value_type;
};
/**
* This class provides a local typedef @p value_type that is equal to the
* typedef <code>value_type</code> after @p deref_steps recursive
* dereferences via ```operator[](unsigned int)```. Further, constness is
* preserved via the ValueType type trait, i.e., if T is const,
* ReturnType<rank, T>::value_type will also be const.
*/
template <int deref_steps, typename T>
struct ReturnType
{
typedef typename ReturnType<deref_steps - 1, typename ValueType<T>::value_type>::value_type value_type;
};
template <typename T>
struct ReturnType<0, T>
{
typedef T value_type;
};
/**
* Provide a "tensorial view" to a reference @p t of a tensor object of rank
* @p rank in which the index @p index is shifted to the end. As an example
* consider a tensor of 5th order in dim=5 space dimensions that can be
* accessed through 5 recursive <code>operator[]()</code> invocations:
* @code
* Tensor<5, dim> tensor;
* tensor[0][1][2][3][4] = 42.;
* @endcode
* Index 1 (the 2nd index, count starts at 0) can now be shifted to the end
* via
* @code
* auto tensor_view = reordered_index_view<1, 5>(tensor);
* tensor_view[0][2][3][4][1] == 42.; // is true
* @endcode
* The usage of the dealii::Tensor type was solely for the sake of an
* example. The mechanism implemented by this function is available for
* fairly general tensorial types @p T.
*
* The purpose of this reordering facility is to be able to contract over an
* arbitrary index of two (or more) tensors:
* - reorder the indices in mind to the end of the tensors
* - use the contract function below that contracts the _last_ elements of
* tensors.
*
* @note This function returns an internal class object consisting of an
* array subscript operator <code>operator[](unsigned int)</code> and a
* typedef <code>value_type</code> describing its return value.
*
* @tparam index The index to be shifted to the end. Indices are counted
* from 0, thus the valid range is $0\le\text{index}<\text{rank}$.
* @tparam rank Rank of the tensorial object @p t
* @tparam T A tensorial object of rank @p rank. @p T must provide a local
* typedef <code>value_type</code> and an index operator
* <code>operator[]()</code> that returns a (const or non-const) reference
* of <code>value_type</code>.
*
* @author Matthias Maier, 2015
*/
template <int index, int rank, typename T>
inline DEAL_II_ALWAYS_INLINE
internal::ReorderedIndexView<index, rank, T>
reordered_index_view(T &t)
{
#ifdef DEAL_II_WITH_CXX11
static_assert(0 <= index && index < rank,
"The specified index must lie within the range [0,rank)");
#endif
return internal::ReorderedIndexView<index, rank, T>(t);
}
/**
* Return a reference (const or non-const) to a subobject of a tensorial
* object @p t of type @p T, as described by an array type @p ArrayType
* object @p indices. For example: @code
* Tensor<5, dim> tensor;
* TableIndices<5> indices (0, 1, 2, 3, 4);
* TensorAccessors::extract(tensor, indices) = 42;
* @endcode
* This is equivalent to <code>tensor[0][1][2][3][4] = 42.</code>.
*
* @tparam T A tensorial object of rank @p rank. @p T must provide a local
* typedef <code>value_type</code> and an index operator
* <code>operator[]()</code> that returns a (const or non-const) reference
* of <code>value_type</code>. Further, its tensorial rank must be equal or
* greater than @p rank.
*
* @tparam ArrayType An array like object, such as std::array, or
* dealii::TableIndices that stores at least @p rank indices that can be
* accessed via operator[]().
*
* @author Matthias Maier, 2015
*/
template<int rank, typename T, typename ArrayType> typename
ReturnType<rank, T>::value_type &
extract(T &t, const ArrayType &indices)
{
return internal::ExtractHelper<0, rank>::template extract<T, ArrayType>(t, indices);
}
/**
* This function contracts two tensorial objects @p left and @p right and
* stores the result in @p result. The contraction is done over the _last_
* @p no_contr indices of both tensorial objects:
*
* @f[
* \text{result}_{i_1,..,i_{r1},j_1,..,j_{r2}}
* = \sum_{k_1,..,k_{\text{no\_contr}}}
* \text{left}_{i_1,..,i_{r1},k_1,..,k_{\text{no\_contr}}}
* \text{right}_{j_1,..,j_{r2},k_1,..,k_{\text{no\_contr}}}
* @f]
*
* Calling this function is equivalent of writing the following low level
* code:
* @code
* for(unsigned int i_0 = 0; i_0 < dim; ++i_0)
* ...
* for(unsigned int i_ = 0; i_ < dim; ++i_)
* for(unsigned int j_0 = 0; j_0 < dim; ++j_0)
* ...
* for(unsigned int j_ = 0; j_ < dim; ++j_)
* {
* result[i_0]..[i_][j_0]..[j_] = 0.;
* for(unsigned int k_0 = 0; k_0 < dim; ++k_0)
* ...
* for(unsigned int k_ = 0; k_ < dim; ++k_)
* result[i_0]..[i_][j_0]..[j_] += left[i_0]..[i_][k_0]..[k_] * right[j_0]..[j_][k_0]..[k_];
* }
* @endcode
* with r = rank_1 + rank_2 - 2 * no_contr, l = rank_1 - no_contr, l1 =
* rank_1, and c = no_contr.
*
* @note The Types @p T1, @p T2, and @p T3 must have rank rank_1 + rank_2 -
* 2 * no_contr, rank_1, or rank_2, respectively. Obviously, no_contr must
* be less or equal than rank_1 and rank_2.
*
* @author Matthias Maier, 2015
*/
template <int no_contr, int rank_1, int rank_2, int dim, typename T1, typename T2, typename T3>
inline DEAL_II_ALWAYS_INLINE
void contract(T1 &result, const T2 &left, const T3 &right)
{
#ifdef DEAL_II_WITH_CXX11
static_assert(rank_1 >= no_contr, "The rank of the left tensor must be "
"equal or greater than the number of "
"contractions");
static_assert(rank_2 >= no_contr, "The rank of the right tensor must be "
"equal or greater than the number of "
"contractions");
#endif
internal::Contract<no_contr, rank_1, rank_2, dim>::template contract<T1, T2, T3>
(result, left, right);
}
/**
* Full contraction of three tensorial objects:
*
* @f[
* \sum_{i_1,..,i_{r1},j_1,..,j_{r2}}
* \text{left}_{i_1,..,i_{r1}}
* \text{middle}_{i_1,..,i_{r1},j_1,..,j_{r2}}
* \text{right}_{j_1,..,j_{r2}}
* @f]
*
* Calling this function is equivalent of writing the following low level
* code:
* @code
* T1 result = T1();
* for(unsigned int i_0 = 0; i_0 < dim; ++i_0)
* ...
* for(unsigned int i_ = 0; i_ < dim; ++i_)
* for(unsigned int j_0 = 0; j_0 < dim; ++j_0)
* ...
* for(unsigned int j_ = 0; j_ < dim; ++j_)
* result += left[i_0]..[i_] * middle[i_0]..[i_][j_0]..[j_] * right[j_0]..[j_];
* @endcode
*
* @note The Types @p T2, @p T3, and @p T4 must have rank rank_1, rank_1 +
* rank_2, and rank_3, respectively. @p T1 must be a scalar type.
*
* @author Matthias Maier, 2015
*/
template <int rank_1, int rank_2, int dim, typename T1, typename T2, typename T3, typename T4>
T1 contract3(const T2 &left, const T3 &middle, const T4 &right)
{
return internal::Contract3<rank_1, rank_2, dim>::template contract3<T1, T2, T3, T4>
(left, middle, right);
}
namespace internal
{
// -------------------------------------------------------------------------
// Forward declarations and type traits
// -------------------------------------------------------------------------
template <int rank, typename S> class StoreIndex;
template <typename T> class Identity;
template <int no_contr, int dim> class Contract2;
/**
* An internally used type trait to allow nested application of the
* function reordered_index_view(T &t).
*
* The problem is that when working with the actual tensorial types, we
* have to return subtensors by reference - but sometimes, especially for
* StoreIndex and ReorderedIndexView that return rvalues, we have to
* return by value.
*/
template<typename T>
struct ReferenceType
{
typedef T &type;
};
template <int rank, typename S>
struct ReferenceType<StoreIndex<rank, S> >
{
typedef StoreIndex<rank, S> type;
};
template <int index, int rank, typename T>
struct ReferenceType<ReorderedIndexView<index, rank, T> >
{
typedef ReorderedIndexView<index, rank, T> type;
};
// TODO: Is there a possibility to just have the following block of
// explanation on an internal page in doxygen? If, yes. Doxygen
// wizards, your call!
// -------------------------------------------------------------------------
// Implementation of helper classes for reordered_index_view
// -------------------------------------------------------------------------
// OK. This is utterly brutal template magic. Therefore, we will not
// comment on the individual internal helper classes, because this is
// of not much value, but explain the general recursion procedure.
//
// (In order of appearance)
//
// Our task is to reorder access to a tensor object where a specified
// index is moved to the end. Thus we want to construct an object
// <code>reordered</code> out of a <code>tensor</code> where the
// following access patterns are equivalent:
// @code
// tensor [i_0]...[i_index-1][i_index][i_index+1]...[i_n]
// reordered [i_0]...[i_index_1][i_index+1]...[i_n][i_index]
// @endcode
//
// The first task is to get rid of the application of
// [i_0]...[i_index-1]. This is a classical recursion pattern - relay
// the task from <index, rank> to <index-1, rank-1> by accessing the
// subtensor object:
template <int index, int rank, typename T>
class ReorderedIndexView
{
public:
ReorderedIndexView(typename ReferenceType<T>::type t) : t_(t) {}
typedef ReorderedIndexView<index - 1, rank - 1, typename ValueType<T>::value_type>
value_type;
// Recurse by applying index j directly:
inline DEAL_II_ALWAYS_INLINE
value_type operator[](unsigned int j) const
{
return value_type(t_[j]);
}
private:
typename ReferenceType<T>::type t_;
};
// At some point we hit the condition index == 0 and rank > 1, i.e.,
// the first index should be reordered to the end.
//
// At this point we cannot be lazy any more and have to start storing
// indices because we get them in the wrong order. The user supplies
// [i_0][i_1]...[i_{rank - 1}]
// but we have to call the subtensor object with
// [i_{rank - 1}[i_0][i_1]...[i_{rank-2}]
//
// So give up and relay the task to the StoreIndex class:
template <int rank, typename T>
class ReorderedIndexView<0, rank, T>
{
public:
ReorderedIndexView(typename ReferenceType<T>::type t) : t_(t) {}
typedef StoreIndex<rank - 1, internal::Identity<T> > value_type;
inline DEAL_II_ALWAYS_INLINE
value_type operator[](unsigned int j) const
{
return value_type(Identity<T>(t_), j);
}
private:
typename ReferenceType<T>::type t_;
};
// Sometimes, we're lucky and don't have to do anything. In this case
// just return the original tensor.
template <typename T>
class ReorderedIndexView<0, 1, T>
{
public:
ReorderedIndexView(typename ReferenceType<T>::type t) : t_(t) {}
typedef typename ReferenceType<typename ValueType<T>::value_type>::type value_type;
inline DEAL_II_ALWAYS_INLINE
value_type operator[](unsigned int j) const
{
return t_[j];
}
private:
typename ReferenceType<T>::type t_;
};
// Here, Identity is a helper class to ground the recursion in
// StoreIndex. Its implementation is easy - we haven't stored any
// indices yet. So, we just provide a function apply that returns the
// application of an index j to the stored tensor t_:
template <typename T>
class Identity
{
public:
Identity(typename ReferenceType<T>::type t) : t_(t) {}
typedef typename ValueType<T>::value_type return_type;
inline DEAL_II_ALWAYS_INLINE
typename ReferenceType<return_type>::type apply(unsigned int j) const
{
return t_[j];
}
private:
typename ReferenceType<T>::type t_;
};
// StoreIndex is a class that stores an index recursively with every
// invocation of operator[](unsigned int j): We do this by recursively
// creating a new StoreIndex class of lower rank that stores the
// supplied index j and holds a copy of the current class (with all
// other stored indices). Again, we provide an apply member function
// that knows how to apply an index on the highest rank and all
// subsequently stored indices:
template <int rank, typename S>
class StoreIndex
{
public:
StoreIndex(S s, int i) : s_(s), i_(i) {}
typedef StoreIndex<rank - 1, StoreIndex<rank, S> > value_type;
inline DEAL_II_ALWAYS_INLINE
value_type operator[](unsigned int j) const
{
return value_type(*this, j);
}
typedef typename ValueType<typename S::return_type>::value_type return_type;
inline
typename ReferenceType<return_type>::type apply(unsigned int j) const
{
return s_.apply(j)[i_];
}
private:
const S s_;
const int i_;
};
// We have to store indices until we hit rank == 1. Then, upon the next
// invocation of operator[](unsigned int j) we have all necessary
// information available to return the actual object.
template <typename S>
class StoreIndex<1, S>
{
public:
StoreIndex(S s, int i) : s_(s), i_(i) {}
typedef typename ValueType<typename S::return_type>::value_type return_type;
typedef return_type value_type;
inline DEAL_II_ALWAYS_INLINE
return_type &operator[](unsigned int j) const
{
return s_.apply(j)[i_];
}
private:
const S s_;
const int i_;
};
// -------------------------------------------------------------------------
// Implementation of helper classes for extract
// -------------------------------------------------------------------------
// Straightforward recursion implemented by specializing ExtractHelper
// for position == rank. We use the type trait ReturnType<rank, T> to
// have an idea what the final type will be.
template<int position, int rank>
struct ExtractHelper
{
template<typename T, typename ArrayType>
inline
static
typename ReturnType<rank - position, T>::value_type &
extract(T &t,
const ArrayType &indices)
{
return ExtractHelper<position + 1, rank>::
template extract<typename ValueType<T>::value_type, ArrayType>
(t[indices[position]], indices);
}
};
// For position == rank there is nothing to extract, just return the
// object.
template<int rank>
struct ExtractHelper<rank, rank>
{
template<typename T, typename ArrayType>
inline
static
T &extract(T &t,
const ArrayType &)
{
return t;
}
};
// -------------------------------------------------------------------------
// Implementation of helper classes for contract
// -------------------------------------------------------------------------
// Straightforward recursive pattern:
//
// As long as rank_1 > no_contr, assign indices from the left tensor to
// result. This builds up the first part of the nested outer loops:
//
// for(unsigned int i_0; i_0 < dim; ++i_0)
// ...
// for(i_; i_ < dim; ++i_)
// [...]
// result[i_0]..[i_] ... left[i_0]..[i_] ...
template <int no_contr, int rank_1, int rank_2, int dim>
class Contract
{
public:
template<typename T1, typename T2, typename T3>
inline DEAL_II_ALWAYS_INLINE static
void contract(T1 &result, const T2 &left, const T3 &right)
{
for (unsigned int i = 0; i < dim; ++i)
Contract<no_contr, rank_1 - 1, rank_2, dim>::
contract(result[i], left[i], right);
}
};
// If rank_1 == no_contr leave out the remaining no_contr indices for
// the contraction and assign indices from the right tensor to the
// result. This builds up the second part of the nested loops:
//
// for(unsigned int i_0 = 0; i_0 < dim; ++i_0)
// ...
// for(unsigned int i_ = 0; i_ < dim; ++i_)
// for(unsigned int j_0 = 0; j_0 < dim; ++j_0)
// ...
// for(unsigned int j_ = 0; j_ < dim; ++j_)
// [...]
// result[i_0]..[i_][j_0]..[j_] ... left[i_0]..[i_] ... right[j_0]..[j_]
//
template <int no_contr, int rank_2, int dim>
class Contract<no_contr, no_contr, rank_2, dim>
{
public:
template<typename T1, typename T2, typename T3>
inline DEAL_II_ALWAYS_INLINE static
void contract(T1 &result, const T2 &left, const T3 &right)
{
for (unsigned int i = 0; i < dim; ++i)
Contract<no_contr, no_contr, rank_2 - 1, dim>::
contract(result[i], left, right[i]);
}
};
// If rank_1 == rank_2 == no_contr we have built up all of the outer
// loop. Now, it is time to do the actual contraction:
//
// [...]
// {
// result[i_0]..[i_][j_0]..[j_] = 0.;
// for(unsigned int k_0 = 0; k_0 < dim; ++k_0)
// ...
// for(unsigned int k_ = 0; k_ < dim; ++k_)
// result[i_0]..[i_][j_0]..[j_] += left[i_0]..[i_][k_0]..[k_] * right[j_0]..[j_][k_0]..[k_];
// }
//
// Relay this summation to another helper class.
template <int no_contr, int dim>
class Contract<no_contr, no_contr, no_contr, dim>
{
public:
template<typename T1, typename T2, typename T3>
inline DEAL_II_ALWAYS_INLINE static
void contract(T1 &result, const T2 &left, const T3 &right)
{
result = Contract2<no_contr, dim>::template contract2<T1>(left, right);
}
};
// Straightforward recursion:
//
// Contract leftmost index and recurse one down.
template <int no_contr, int dim>
class Contract2
{
public:
template<typename T1, typename T2, typename T3>
inline DEAL_II_ALWAYS_INLINE static
T1 contract2(const T2 &left, const T3 &right)
{
T1 result = T1();
for (unsigned int i = 0; i < dim; ++i)
result += Contract2<no_contr - 1, dim>::template contract2<T1>(left[i], right[i]);
return result;
}
};
// A contraction of two objects of order 0 is just a scalar
// multiplication:
template <int dim>
class Contract2<0, dim>
{
public:
template<typename T1, typename T2, typename T3>
inline DEAL_II_ALWAYS_INLINE static
T1 contract2(const T2 &left, const T3 &right)
{
return left * right;
}
};
// -------------------------------------------------------------------------
// Implementation of helper classes for contract3
// -------------------------------------------------------------------------
// Fully contract three tensorial objects
//
// As long as rank_1 > 0, recurse over left and middle:
//
// for(unsigned int i_0; i_0 < dim; ++i_0)
// ...
// for(i_; i_ < dim; ++i_)
// [...]
// left[i_0]..[i_] ... middle[i_0]..[i_] ... right
template <int rank_1, int rank_2, int dim>
class Contract3
{
public:
template<typename T1, typename T2, typename T3, typename T4>
static inline
T1 contract3(const T2 &left, const T3 &middle, const T4 &right)
{
T1 result = T1();
for (unsigned int i = 0; i < dim; ++i)
result += Contract3<rank_1 - 1, rank_2, dim>::template contract3<T1>(left[i], middle[i], right);
return result;
}
};
// If rank_1 ==0, continue to recurse over middle and right:
//
// for(unsigned int i_0; i_0 < dim; ++i_0)
// ...
// for(i_; i_ < dim; ++i_)
// for(unsigned int j_0; j_0 < dim; ++j_0)
// ...
// for(j_; j_ < dim; ++j_)
// [...]
// left[i_0]..[i_] ... middle[i_0]..[i_][j_0]..[j_] ... right[j_0]..[j_]
template <int rank_2, int dim>
class Contract3<0, rank_2, dim>
{
public:
template<typename T1, typename T2, typename T3, typename T4>
static inline
T1 contract3(const T2 &left, const T3 &middle, const T4 &right)
{
T1 result = T1();
for (unsigned int i = 0; i < dim; ++i)
result += Contract3<0, rank_2 - 1, dim>::template contract3<T1>(left, middle[i], right[i]);
return result;
}
};
// Contraction of three tensorial objects of rank 0 is just a scalar
// multiplication.
template <int dim>
class Contract3<0, 0, dim>
{
public:
template<typename T1, typename T2, typename T3, typename T4>
static inline
T1 contract3(const T2 &left, const T3 &middle, const T4 &right)
{
return left * middle * right;
}
};
// -------------------------------------------------------------------------
} /* namespace internal */
} /* namespace TensorAccessors */
DEAL_II_NAMESPACE_CLOSE
#endif /* dealii__tensor_accessors_h */
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