traverse a tiled Lattice optimally with a tile cursor More...

#include <TileStepper.h>

Inheritance diagram for casa::TileStepper:

Public Member Functions
	TileStepper (const IPosition &latticeShape, const IPosition &tileShape)
	Construct a TileStepper by specifying the Lattice shape, a tile shape, and an optional axis path (default is natural order).
	TileStepper (const IPosition &latticeShape, const IPosition &tileShape, const IPosition &axisPath)
	TileStepper (const TileStepper &other)
	Copy constructor (copy semantics).
	~TileStepper ()
TileStepper &	operator= (const TileStepper &other)
	Assignment (copy semantics).
virtual Bool	operator++ (int)
	Increment operator (postfix or prefix version) - move the cursor forward one step.
virtual Bool	operator-- (int)
	Decrement operator (postfix or prefix version) - move the cursor backwards one step.
virtual void	reset ()
	Function to move the cursor to the beginning of the Lattice.
virtual Bool	atStart () const
	Function which returns "True" if the cursor is at the beginning of the Lattice, otherwise, returns "False".
virtual Bool	atEnd () const
	Function which returns "True" if an attempt has been made to increment the cursor beyond the end of the Lattice.
virtual uInt	nsteps () const
	Function to return the number of steps (increments & decrements) taken since construction (or since last reset).
virtual IPosition	position () const
	Function which returns the current position of the beginning of the cursor.
virtual IPosition	endPosition () const
	Function which returns the current position of the end of the cursor.
virtual IPosition	latticeShape () const
	Functions which return the shape of the Lattice being iterated through.
virtual IPosition	subLatticeShape () const
virtual IPosition	cursorShape () const
	Function which returns the shape of the cursor.
virtual IPosition	cursorAxes () const
	Function which returns the axes of the cursor.
IPosition	tileShape () const
	Function which returns the shape of the "tile" the cursor will iterate through before moving onto the next tile.
virtual Bool	hangOver () const
	Function which returns "True" if the increment/decrement operators have moved the cursor position such that part of the cursor beginning or end is hanging over the edge of the Lattice.
virtual void	subSection (const IPosition &blc, const IPosition &trc)
	Functions to specify a "section" of the Lattice to step over.
virtual void	subSection (const IPosition &blc, const IPosition &trc, const IPosition &inc)
virtual IPosition	blc () const
	Return the bottom left hand corner (blc), top right corner (trc) or step size (increment) used by the current sub-Lattice.
virtual IPosition	trc () const
virtual IPosition	increment () const
virtual const IPosition &	axisPath () const
	Return the axis path.
virtual LatticeNavigator *	clone () const
	Function which returns a pointer to dynamic memory of an exact copy of this instance.
virtual Bool	ok () const
	Function which checks the internal data of this class for correct dimensionality and consistant values.
virtual uInt	calcCacheSize (const IPosition &cubeShape, const IPosition &tileShape, uInt maxCacheSize, uInt bucketSize) const
	Calculate the cache size (in tiles) for this type of access to a lattice in the given row of the tiled hypercube.
Private Member Functions
	TileStepper ()
	Prevent the default constructor from being used.
Private Attributes
IPosition	itsBlc
IPosition	itsTrc
IPosition	itsInc
LatticeIndexer	itsSubSection
LatticeIndexer	itsTiler
IPosition	itsTilerCursorPos
IPosition	itsTileShape
IPosition	itsAxisPath
IPosition	itsCurBlc
IPosition	itsCurTrc
uInt	itsNsteps
Bool	itsEnd
Bool	itsStart

Detailed Description

traverse a tiled Lattice optimally with a tile cursor

Intended use:

Public interface

Review Status

Reviewed By:: Peter Barnes
Date Reviewed:: 1999/10/30
Test programs:: tTileStepper

Prerequisite

LatticeNavigator

Etymology

TileStepper is used to step optimally through a tiled Lattice.

Synopsis

When you wish to traverse a Lattice (say, a PagedArray or an Image) you will usually create a LatticeIterator. Once created, you may attach a LatticeNavigator to the iterator. A TileStepper is a concrete class derived from the abstract LatticeNavigator that allows you to step through the Lattice in a way that will minimize the amount of cache memory consumed and maximize the speed.

Some Lattices (in particular PagedArrays) are stored (on disk) in tiles. For an N-dimensional Lattice a tile is an N-dimensional subsection with fewer elements along each axis. For example a Lattice of shape [512,512,4,32] may have a tile shape of [32,16,4,16], and there will be 16*32*1*2 (=1024) tiles in the entire Lattice. To allow efficient access of the data in a Lattice some tiles are cached in memory. As each tile may consume a fair bit of memory (in this example 128kBytes, assuming each element consumes 4 bytes), it is desirable to minimise the number of tiles held in the cache. But it is also desirable to minimise the number of times a tiles must be read into or written from the cache as this may require a time consuming operation like disk I/O.

TileStepper steps through a lattice in a tile-by-tile way. This means that the cache contains 1 tile only and that a tile is accessed only once. It should be clear that traversing a lattice in this way cannot be used if an entire vector or plane is needed. It is, however, very well suited for purposes like initialising a lattice, where the order in which the lattice pixels are accessed is not important.

In constructing a TileStepper, you specify the Lattice shape, the tile shape and optionally the axis path. The axis path defines the order in which the tiles are fetched from the lattice. Default is the natural order (thus x-axis in the inner loop).
It is possible to use the function subSection to traverse only a subsection of the lattice.

The cursor position can be incremented or decremented to retrieve the next or previous tile in the Lattice. The position of the next tile in the Lattice will depend on the tile shape, and is described above.
Note that the cursor shape does not need to be constant when iterating through the lattice. If the lattice shape is not an integer multiple of the tile shape, the cursor will be smaller on the edges of the lattice.

Example

This example initializes a lattice with the given value.

    void init (Lattice<Complex>& cArray, Complex value)
    {
      const IPosition latticeShape = cArray.shape();
      const IPosition tileShape = cArray.niceCursorShape();
      TileStepper tsx(latticeShape, tileShape);
      LatticeIterator<Complex> lix(cArray, tsx);
      for (lix.reset();!lix.atEnd();lix++)
        lix.woCursor() = value;
      }
    }

Note that a TileStepper is the default navigator for an iterator. So the code above could be made simpler like shown below. Also note that this example is a bit artificial, because the Lattice::set() function should be used to initialize a lattice.

    void init (Lattice<Complex>& cArray, Complex value)
    {
      LatticeIterator<Complex> lix(cArray);
      for (lix.reset();!lix.atEnd();lix++)
        lix.woCursor() = value;
      }
    }

Motivation

This class makes it possible to traverse a lattice in the optimal way.

Definition at line 138 of file TileStepper.h.

Constructor & Destructor Documentation

casa::TileStepper::TileStepper	(	const IPosition &	latticeShape,
		const IPosition &	tileShape
	)

Construct a TileStepper by specifying the Lattice shape, a tile shape, and an optional axis path (default is natural order).

Is is nearly always advisable to make the tileShape identical to the Lattice tileShape. This can be obtained by lat.niceCursorShape() where lat is a Lattice object.

casa::TileStepper::TileStepper	(	const IPosition &	latticeShape,
		const IPosition &	tileShape,
		const IPosition &	axisPath
	)

casa::TileStepper::TileStepper ( const TileStepper & other )

Copy constructor (copy semantics).

casa::TileStepper::~TileStepper ( )

casa::TileStepper::TileStepper ( ) [private]

Prevent the default constructor from being used.

Member Function Documentation

virtual Bool casa::TileStepper::atEnd ( ) const [virtual]

Function which returns "True" if an attempt has been made to increment the cursor beyond the end of the Lattice.

virtual uInt casa::TileStepper::calcCacheSize	(	const IPosition &	cubeShape,
		const IPosition &	tileShape,
		uInt	maxCacheSize,
		uInt	bucketSize
	)		const `[virtual]`

virtual void casa::TileStepper::subSection	(	const IPosition &	blc,
		const IPosition &	trc
	)		`[virtual]`

Public Member Functions

Private Member Functions

Private Attributes