Commit f46a5405 authored by Tomáš Oberhuber's avatar Tomáš Oberhuber

Writing documentation for Scan.

parent 496d4e77
......@@ -245,7 +245,7 @@ and for exclusive segmented prefix sum it is
[0,1,4][0,2,6,12][0,3][0,3,9,18,30]
```
In addition to common scan, we need to encode the segments of the input sequence. It is done by auxiliary flags array (it can be array of booleans) having one at the begining of each segment and zeros on all other positions. In our example, it would be like this:
In addition to common scan, we need to encode the segments of the input sequence. It is done by auxiliary flags array (it can be array of booleans) having `1` at the begining of each segment and `0` on all other positions. In our example, it would be like this:
```
[1,0,0,1,0,0,0,1,0,1,0,0, 0, 0]
......
......@@ -19,15 +19,79 @@ namespace TNL {
namespace Containers {
namespace Algorithms {
/**
* \brief Scan (or prefix sum) type - inclusive or exclusive.
*
* See \ref TNL::Containers::Algorithms::Scan
*/
enum class ScanType {
Exclusive,
Inclusive
};
/**
* \brief Computes scan (or prefix sum) on a vector.
*
* [Scan (or prefix sum)](https://en.wikipedia.org/wiki/Prefix_sum) operation turns a sequence
* \f$a_1, \ldots, a_n\f$ into a sequence \f$s_1, \ldots, s_n\f$ defined as
*
* \f[
* s_i = \sum_{j=1}^i a_i.
* \f]
* Exclusive scan (or prefix sum) is defined as
*
* \f[
* \sigma_i = \sum_{j=1}^{i-1} a_i.
* \f]
*
* \tparam Device parameter says on what device the reduction is gonna be performed.
* \tparam Type parameter says if inclusive or exclusive is scan is to be computed.
*
* See \ref Scan< Devices::Host, Type > and \ref Scan< Devices::Cuda, Type >.
*/
template< typename Device,
ScanType Type = ScanType::Inclusive >
struct Scan;
/**
* \brief Computes segmented scan (or prefix sum) on a vector.
*
* Segmented scan is a modification of common scan. In this case the sequence of
* numbers in hand is divided into segments like this, for example
*
* ```
* [1,3,5][2,4,6,9][3,5],[3,6,9,12,15]
* ```
*
* and we want to compute inclusive or exclusive scan of each segment. For inclusive segmented prefix sum we get
*
* ```
* [1,4,9][2,6,12,21][3,8][3,9,18,30,45]
* ```
*
* and for exclusive segmented prefix sum it is
*
* ```
* [0,1,4][0,2,6,12][0,3][0,3,9,18,30]
* ```
*
* In addition to common scan, we need to encode the segments of the input sequence.
* It is done by auxiliary flags array (it can be array of booleans) having `1` at the
* beginning of each segment and `0` on all other positions. In our example, it would be like this:
*
* ```
* [1,0,0,1,0,0,0,1,0,1,0,0, 0, 0]
* [1,3,5,2,4,6,9,3,5,3,6,9,12,15]
*
* ```
*
* \tparam Device parameter says on what device the reduction is gonna be performed.
* \tparam Type parameter says if inclusive or exclusive is scan is to be computed.
*
* See \ref Scan< Devices::Host, Type > and \ref Scan< Devices::Cuda, Type >.
*
* **Note: Segmented scan is not implemented for CUDA yet.**
*/
template< typename Device,
ScanType Type = ScanType::Inclusive >
struct SegmentedScan;
......@@ -36,6 +100,33 @@ struct SegmentedScan;
template< ScanType Type >
struct Scan< Devices::Host, Type >
{
/**
* \brief Computes scan (prefix sum) on CPU.
*
* \tparam Vector type vector being used for the scan.
* \tparam Reduction lambda function defining the reduction operation
*
* \param v input vector, the result of scan is stored in the same vector
* \param begin the first element in the array to be scanned
* \param end the last element in the array to be scanned
* \param reduction lambda function implementing the reduction operation
* \param zero is the idempotent element for the reduction operation, i.e. element which
* does not change the result of the reduction.
*
* The reduction lambda function takes two variables which are supposed to be reduced:
*
* ```
* auto reduction = [] __cuda_callable__ ( const Result& a, const Result& b ) { return ... };
* ```
*
* \par Example
*
* \include ReductionAndScan/ScanExample.cpp
*
* \par Output
*
* \include ScanExample.out
*/
template< typename Vector,
typename Reduction >
static void
......@@ -45,30 +136,59 @@ struct Scan< Devices::Host, Type >
const Reduction& reduction,
const typename Vector::RealType zero );
template< typename Vector,
typename Reduction >
static auto
performFirstPhase( Vector& v,
const typename Vector::IndexType begin,
const typename Vector::IndexType end,
const Reduction& reduction,
const typename Vector::RealType zero );
protected:
template< typename Vector,
typename BlockShifts,
typename Reduction >
static void
performSecondPhase( Vector& v,
const BlockShifts& blockShifts,
const typename Vector::IndexType begin,
const typename Vector::IndexType end,
const Reduction& reduction,
const typename Vector::RealType shift );
template< typename Vector,
typename Reduction >
static auto
performFirstPhase( Vector& v,
const typename Vector::IndexType begin,
const typename Vector::IndexType end,
const Reduction& reduction,
const typename Vector::RealType zero );
template< typename Vector,
typename BlockShifts,
typename Reduction >
static void
performSecondPhase( Vector& v,
const BlockShifts& blockShifts,
const typename Vector::IndexType begin,
const typename Vector::IndexType end,
const Reduction& reduction,
const typename Vector::RealType shift );
};
template< ScanType Type >
struct Scan< Devices::Cuda, Type >
{
/**
* \brief Computes scan (prefix sum) on GPU.
*
* \tparam Vector type vector being used for the scan.
* \tparam Reduction lambda function defining the reduction operation
*
* \param v input vector, the result of scan is stored in the same vector
* \param begin the first element in the array to be scanned
* \param end the last element in the array to be scanned
* \param reduction lambda function implementing the reduction operation
* \param zero is the idempotent element for the reduction operation, i.e. element which
* does not change the result of the reduction.
*
* The reduction lambda function takes two variables which are supposed to be reduced:
*
* ```
* auto reduction = [] __cuda_callable__ ( const Result& a, const Result& b ) { return ... };
* ```
*
* \par Example
*
* \include ReductionAndScan/ScanExample.cpp
*
* \par Output
*
* \include ScanExample.out
*/
template< typename Vector,
typename Reduction >
static void
......@@ -102,6 +222,35 @@ struct Scan< Devices::Cuda, Type >
template< ScanType Type >
struct SegmentedScan< Devices::Host, Type >
{
/**
* \brief Computes segmented scan (prefix sum) on CPU.
*
* \tparam Vector type vector being used for the scan.
* \tparam Reduction lambda function defining the reduction operation
* \tparam Flags array type containing zeros and ones defining the segments begining
*
* \param v input vector, the result of scan is stored in the same vector
* \param flags is an array with zeros and ones defining the segments begining
* \param begin the first element in the array to be scanned
* \param end the last element in the array to be scanned
* \param reduction lambda function implementing the reduction operation
* \param zero is the idempotent element for the reduction operation, i.e. element which
* does not change the result of the reduction.
*
* The reduction lambda function takes two variables which are supposed to be reduced:
*
* ```
* auto reduction = [] __cuda_callable__ ( const Result& a, const Result& b ) { return ... };
* ```
*
* \par Example
*
* \include ReductionAndScan/SegmentedScanExample.cpp
*
* \par Output
*
* \include SegmentedScanExample.out
*/
template< typename Vector,
typename Reduction,
typename Flags >
......@@ -117,6 +266,37 @@ struct SegmentedScan< Devices::Host, Type >
template< ScanType Type >
struct SegmentedScan< Devices::Cuda, Type >
{
/**
* \brief Computes segmented scan (prefix sum) on GPU.
*
* \tparam Vector type vector being used for the scan.
* \tparam Reduction lambda function defining the reduction operation
* \tparam Flags array type containing zeros and ones defining the segments begining
*
* \param v input vector, the result of scan is stored in the same vector
* \param flags is an array with zeros and ones defining the segments begining
* \param begin the first element in the array to be scanned
* \param end the last element in the array to be scanned
* \param reduction lambda function implementing the reduction operation
* \param zero is the idempotent element for the reduction operation, i.e. element which
* does not change the result of the reduction.
*
* The reduction lambda function takes two variables which are supposed to be reduced:
*
* ```
* auto reduction = [] __cuda_callable__ ( const Result& a, const Result& b ) { return ... };
* ```
*
* \par Example
*
* \include ReductionAndScan/SegmentedScanExample.cpp
*
* \par Output
*
* \include SegmentedScanExample.out
*
* **Note: Segmented scan is not implemented for CUDA yet.**
*/
template< typename Vector,
typename Reduction,
typename Flags >
......
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