/**
* Reduces, or \em folds, a matrix into a scalar.
*
* Reduction takes place according a monoid \f$ (\oplus,1) \f$, where
* \f$ \oplus:\ D_1 \times D_2 \to D_3 \f$ with associated identities
* \f$ 1_k in D_k \f$. Usually, \f$ D_k \subseteq D_3, 1 \leq k < 3 \f$,
* though other more exotic structures may be envisioned (and used).
*
* Let \f$ x_0 = 1 \f$ and let
* \f$ x_{i+1} = \begin{cases}
* x_i \oplus y_i\text{ if }y_i\text{ is nonzero and }
* m_i\text{ evaluates true}x_i\text{ otherwise}
* \end{cases},\f$
* for all \f$ i \in \{ 0, 1, \ldots, n-1 \} \f$.
*
* \note Per this definition, the folding happens in a left-to-right
* direction. If another direction is wanted, which may have use in
* cases where \f$ D_1 \f$ differs from \f$ D_2 \f$, then either a
* monoid with those operator domains switched may be supplied, or
* #grb::foldr may be used instead.
*
* After a successfull call, \a x will be equal to \f$ x_n \f$.
*
* Note that the operator \f$ \oplus \f$ must be associative since it is
* part of a monoid. This algebraic property is exploited when parallelising
* the requested operation. The identity is required when parallelising over
* multiple user processes.
*
* \warning In so doing, the order of the evaluation of the reduction
* operation should not be expected to be a serial, left-to-right,
* evaluation of the computation chain.
*
* @tparam descr The descriptor to be used (descriptors::no_operation if
* left unspecified).
* @tparam Monoid The monoid to use for reduction.
* @tparam InputType The type of the elements in the supplied ALP/GraphBLAS
* matrix \a y.
* @tparam IOType The type of the output scalar \a x.
* @tparam MaskType The type of the elements in the supplied ALP/GraphBLAS
* matrix \a mask.
*
* @param[in, out] x The result of the reduction.
* Prior value will be considered.
* @param[in] A Any ALP/GraphBLAS matrix.
* @param[in] mask Any ALP/GraphBLAS matrix.
* @param[in] monoid The monoid under which to perform this reduction.
*
* @return grb::SUCCESS When the call completed successfully.
* @return grb::MISMATCH If a \a mask was not empty and does not have size
* equal to \a A.
* @return grb::ILLEGAL If the provided input matrix \a A was not dense,
* while #grb::descriptors::dense was given.
*
* @see grb::foldr provides similar in-place functionality.
* @see grb::eWiseApply provides out-of-place semantics.
*
* \parblock
* \par Valid descriptors
* - descriptors::no_operation: the default descriptor.
* - descriptors::no_casting: the first domain of
* \a monoid must match \a InputType, the second domain of \a op
* match \a IOType, the third domain must match \a IOType, and the
* element type of \a mask must be <tt>bool</tt>.
* - descriptors::transpose_left: A^T will be considered instead
* of \a A.
* - descriptors::transpose_right: mask^T will be considered
* instead of \a mask.
* - descriptors::invert_mask: Not supported yet.
*
* \note Invalid descriptors will be ignored.
*
* \endparblock
*
* \par Performance semantics
* Each backend must define performance semantics for this primitive.
*
* @see perfSemantics
*/
template<
Descriptor descr = descriptors::no_operation,
class Monoid,
typename InputType, typename IOType, typename MaskType,
typename RIT_A, typename CIT_A, typename NIT_A,
typename RIT_M, typename CIT_M, typename NIT_M,
Backend backend
>
RC foldl(
IOType &x,
const Matrix< InputType, backend, RIT_A, CIT_A, NIT_A > &A,
const Matrix< MaskType, backend, RIT_M, CIT_M, NIT_M > &mask,
const Monoid &monoid = Monoid(),
const typename std::enable_if< !grb::is_object< IOType >::value &&
!grb::is_object< InputType >::value &&
!grb::is_object< MaskType >::value &&
grb::is_monoid< Monoid >::value, void
>::type * const = nullptr
) {}
/**
* Reduces, or \em folds, a matrix into a scalar.
* Left-to-right unmasked variant.
*
* Please see the masked grb::foldl variant for a full description.
*
* @tparam descr The descriptor to be used (descriptors::no_operation if
* left unspecified).
* @tparam Operator The operator to use for reduction.
* @tparam InputType The type of the elements in the supplied ALP/GraphBLAS
* matrix \a y.
* @tparam IOType The type of the output scalar \a x.
*
* @param[in, out] x The result of the reduction.
* Prior value will be considered.
* @param[in] A Any ALP/GraphBLAS matrix.
* @param[in] operator The operator used for reduction.
*
* @return grb::SUCCESS When the call completed successfully.
* @return grb::ILLEGAL If the provided input matrix \a y was not dense, while
* #grb::descriptors::dense was given.
*
* \parblock
* \par Valid descriptors
* - descriptors::no_operation: the default descriptor.
* - descriptors::no_casting: the first domain of
* \a monoid must match \a InputType, the second domain of \a op
* match \a IOType, the third domain must match \a IOType.
* - descriptors::transpose_matrix: A^T will be considered instead
* of \a A.
*
* \note Invalid descriptors will be ignored.
*
* \endparblock
*
*/
template<
Descriptor descr = descriptors::no_operation,
class Monoid,
typename InputType, typename IOType,
typename RIT, typename CIT, typename NIT,
Backend backend
>
RC foldl(
IOType &x,
const Matrix< InputType, backend, RIT, CIT, NIT > &A,
const Monoid &monoid,
const typename std::enable_if<
!grb::is_object< IOType >::value &&
!grb::is_object< InputType >::value &&
grb::is_monoid< Monoid >::value, void
>::type * const = nullptr
) {}
This issue concerns the implementation for the reference+omp, hyperdags and distributed backends.
The foldl and foldr primitives, which are currently not implemented in any backend for Matrix reduction, appear to be crucial methods that need to be implemented.
I suggest that we keep the same API signature as the current blas1::foldl and blas1::foldr
Current documentation for foldl (foldr is implicit):