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.
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;
}
}
Copy constructor (copy semantics).
Assignment (copy semantics).
Increment operator (postfix or prefix version) - move the cursor forward one step. Returns True if the cursor was moved.
Decrement operator (postfix or prefix version) - move the cursor backwards one step. Returns True if the cursor was moved.
Function to move the cursor to the beginning of the Lattice. Also resets the number of steps (nsteps function) to zero.
Function which returns "True" if the cursor is at the beginning of the Lattice, otherwise, returns "False"
Function which returns "True" if an attempt has been made to increment the cursor beyond the end of the Lattice.
Function to return the number of steps (increments & decrements) taken since construction (or since last reset). This is a running count of all cursor movement (operator++ or operator--), even though N-increments followed by N-decrements will always leave the cursor in the original position.
Function which returns the current position of the beginning of the cursor. The position function is relative to the origin in the main Lattice.
Function which returns the current position of the end of the cursor. The endPosition function is relative the origin in the main Lattice.
Functions which return the shape of the Lattice being iterated through. latticeShape always returns the shape of the main Lattice while subLatticeShape returns the shape of any sub-Lattice defined using the subSection function.
Function which returns the shape of the cursor. This always includes all axes (i.e. it includes degenerates axes)
Function which returns the axes of the cursor.
Function which returns the shape of the "tile" the cursor will iterate through before moving onto the next tile.
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. This always returns False.
Functions to specify a "section" of the Lattice to step over. A section is defined in terms of the Bottom Left Corner (blc), Top Right Corner (trc), and step size (inc), on ALL of its axes, including degenerate axes. The step size defaults to one if not specified.
Return the bottom left hand corner (blc), top right corner (trc) or step size (increment) used by the current sub-Lattice. If no sub-Lattice has been defined (with the subSection function) these functions return blc=0, trc=latticeShape-1, increment=1, ie. the entire Lattice.
Return the axis path.
Function which returns a pointer to dynamic memory of an exact copy of this instance. The pointer returned by this function must be deleted externally.
Function which checks the internal data of this class for correct dimensionality and consistant values. Returns True if everything is fine otherwise returns False