Geografic-Information-System/lib/segment/segmentlib.dox

/*! \page segmentlib GRASS Segment Library
<!-- doxygenized from "GRASS 5 Programmer's Manual" 
     by M. Neteler 8/2005
  -->

\section segmentintro Segment Library


<P>
Authors: CERL

<P>
Large data files which contain data in a matrix format often need to be 
accessed in a nonsequential or random manner. This requirement complicates 
the programming.

<P>
Methods for accessing the data are to:

<P>
(1) read the entire data file into memory and process the data as a 
two-dimensional matrix,

<P>
(2) perform direct access i/o to the data file for every data value to be 
accessed, or

<P>
(3) read only portions of the data file into memory as needed.

<P>
Method (1) greatly simplifies the programming effort since i/o is done once 
and data access is simple array referencing. However, it has the 
disadvantage that large amounts of memory may be required to hold the data. 
The memory may not be available, or if it is, system paging of the module 
may severely degrade performance. Method (2) is not much more complicated to 
code and requires no significant amount of memory to hold the data. But the 
i/o involved will certainly degrade performance. Method (3) is a mixture of 
(1) and (2) . Memory requirements are fixed and data is read from the data 
file only when not already in memory. However the programming is more 
complex.

<P>
The routines provided in this library are an implementation of method (3) . 
They are based on the idea that if the original matrix were segmented or 
partitioned into smaller matrices these segments could be managed to reduce 
both the memory required and the i/o. Data access along connected paths 
through the matrix, (i.e., moving up or down one row and left or right one 
column) should benefit.

<P>
In most applications, the original data is not in the segmented format. The 
data must be transformed from the nonsegmented format to the segmented 
format. This means reading the original data matrix row by row and writing 
each row to a new file with the segmentation organization. This step 
corresponds to the i/o step of method (1) .

<P>
Then data can be retrieved from the segment file through routines by 
specifying the row and column of the original matrix. Behind the scenes, the 
data is paged into memory as needed and the requested data is returned to 
the caller.

<P>
<B>Note:</B> All routines and global variables in this library, documented 
or undocumented, start with the prefix <B>segment_.</B> To avoid name 
conflicts, programmers should not create variables or routines in their own 
modules which use this prefix.


\section Segment_Routines Segment Routines

<P>
The routines in the <I>Segment Library</I> are described below, more or 
less in the order they would logically be used in a module. They use a data 
structure called SEGMENT which is defined in the header file 
<grass/segment.h> that must be included in any code using these 
routines: [footnote]

\verbatim
#include <grass/segment.h>
\endverbatim 

<P>
The first step is to create a file which is properly formatted for use by 
the <I>Segment Library</I> routines:


<P>
<I>int segment_format (int fd, int nrows, int ncols, int srows, int scols,
  int len)</I>, format a segment file
<P>
  The segmentation routines require a disk file to be used for paging
  segments in and out of memory. This routine formats the file open for
  write on file descriptor <B>fd</B> for use as a segment file. A segment
  file must be formatted before it can be processed by other segment
  routines. The configuration parameters <B>nrows, ncols, srows, scols</B>,
  and <B>len</B> are written to the beginning of the segment file which is
  then filled with zeros.

<P>
The corresponding nonsegmented data matrix, which is to be transferred to the
  segment file, is <B>nrows</B> by <B>ncols.</B> The segment file is to be
  formed of segments which are <B>srows</B> by <B>scols.</B> The data items
  have length <B>len</B> bytes. For example, if the <I>data type is int</I>,
  <B><I>len</I> </B><I>is sizeof(int) .</I>

<P>
Return codes are: 1 ok; else -1 could not seek or write <I>fd</I>, or -3
  illegal configuration parameter(s) .

<P>
The next step is to initialize a SEGMENT structure to be associated with a
segment file formatted by <I>segment_format.</I>

<P>
<I>int segment_init (SEGMENT *seg, int fd, int nsegs)</I>, initialize segment
  structure
<P>
  Initializes the <B>seg</B> structure. The file on <B>fd</B> is
  a segment file created by <I>segment_format</I> and must be open for
  reading and writing. The segment file configuration parameters <I>nrows,
    ncols, srows, scols</I>, and <I>len</I>, as written to the file by
  <I>segment_format</I>, are read from the file and stored in the
  <B>seg</B> structure. <B>Nsegs</B> specifies the number of segments that
  will be retained in memory. The minimum value allowed is 1.

<P>
<B>Note.</B> The size of a segment is <I>scols*srows*len</I> plus a few
  bytes for managing each segment.

<P>
Return codes are:  1 if ok; else -1 could not seek or read segment file,  or -2 out of memory.

<P>
Then data can be written from another file to the segment file row by row:

<P>
<I>int segment_put_row (SEGMENT *seg, char *buf, int row)</I>, write row to
  segment file
<P>
  Transfers nonsegmented matrix data, row by row, into a segment
  file.  <B>Seg</B> is the segment structure that was configured from a call
  to <I>segment_init.</I> <B>Buf</B> should contain <I>ncols*len</I>
  bytes of data to be transferred to the segment file. <B>Row</B> specifies
  the row from the data matrix being transferred.

<P>
Return codes are: 1 if ok;   else -1 could not seek or write segment file.

<P>
Then data can be read or written to the segment file randomly:

<P>
<I>int segment_get (SEGMENT *seg, char *value, int row, int col)</I>, get value
  from segment file
<P>
  Provides random read access to the segmented data. It gets
  <I>len</I> bytes of data into <B>value</B> from the segment file
  <B>seg</B> for the corresponding <B>row</B> and <B>col</B> in the
  original data matrix.

<P>
Return codes are:  1 if ok;  else -1 could not seek or read segment file.

<P>
<I>int segment_put (SEGMENT *seg, char *value, int row, int col)</I>, put
  value to segment file
<P>
  Provides random write access to the segmented data. It
  copies <I>len</I> bytes of data from <B>value</B> into the segment
  structure <B>seg</B> for the corresponding <B>row</B> and <B>col</B> in
  the original data matrix.

<P>
The data is not written to disk immediately. It is stored in a memory segment
  until the segment routines decide to page the segment to disk.

<P>
Return codes are: 1 if ok; else -1 could not seek or write segment file.

<P>
After random reading and writing is finished, the pending updates must be 
flushed to disk:

<P>
<I>int segment_flush (SEGMENT *seg)</I>, flush pending updates to disk
<P>
  Forces all pending updates generated by <I>segment_put()</I> to be
  written to the segment file <B>seg.</B> Must be called after the final
  segment_put() to force all pending updates to disk. Must also be called
  before the first call to <I>segment_get_row.</I>

<P>
Now the data in segment file can be read row by row and transferred to a normal
sequential data file:

<P>
<I>int segment_get_row (SEGMENT *seg, char *buf, int row)</I>, read row from
  segment file
<P>
  Transfers data from a segment file, row by row, into memory
  (which can then be written to a regular matrix file) . <B>Seg</B> is the
  segment structure that was configured from a call to <I>segment_init.</I>
  <B>Buf</B> will be filled with <I>ncols*len</I> bytes of data
  corresponding to the <B>row</B> in the data matrix.

<P>
Return codes are: 1 if ok; else -1 could not seek or read segment file.

<P>
Finally, memory allocated in the SEGMENT structure is freed:

<P>
<I>int segment_release (SEGMENT *seg)</I>, free allocated memory
<P>
  Releases the allocated memory associated with the segment file
  <B>seg.</B> Does not close the file. Does not flush the data which may
  be pending from previous <I>segment_put()</I> calls.

<P>

\section How_to_Use_the_Library_Routines How to Use the Library Routines


The following should provide the programmer with a good idea of how to use the
<I>Segment Library</I> routines. The examples assume that the data is integer.
The first step is the creation and formatting of a segment file. A file is
created, formatted and then closed:


\verbatim 
fd = creat (file, 0666);

segment_format (fd, nrows, ncols, srows, scols, sizeof(int));

close(fd);
\endverbatim

<P>
The next step is the conversion of the nonsegmented matrix data into segment
file format. The segment file is reopened for read and write, initialized, and
then data read row by row from the original data file and put into the segment
file:


\verbatim
#include <fcntl.h>

int buf[NCOLS];

SEGMENT seg;

fd = open (file, O_RDWR);
segment_init (&seg, fd, nseg);

for (row = 0; row < nrows; row++)
{
    <code to get original matrix data for row into buf>

    segment_put_row (&seg, buf, row);
}
\endverbatim


<P>
Of course if the intention is only to add new values rather than update existing
values, the step which transfers data from the original matrix to the segment
file, using segment_put_row() , could be omitted, since
<I>segment_format</I> will fill the segment file with zeros.

<P>
The data can now be accessed directly using <I>segment_get.</I> For example,
to get the value at a given row and column:

\verbatim
int value;

SEGMENT seg;

segment_get (&seg, &value, row, col);
\endverbatim

<P>
Similarly <I>segment_put()</I> can be used to change data values in the 
segment file:


\verbatim
int value;

SEGMENT seg;

value = 10;

segment_put (&seg, &value, row, col);
\endverbatim

<P>
<B>WARNING:</B> It is an easy mistake to pass a value directly to 
segment_put(). The following should be avoided:


\verbatim
segment_put (&seg, 10, row, col); /* this will not work */
\endverbatim

<P>
Once the random access processing is complete, the data would be extracted 
from the segment file and written to a nonsegmented matrix data file as 
follows:


\verbatim
segment_flush (&seg);

for (row = 0; row < nrows; row++) 
{
    segment_get_row (&seg, buf, row);

    <code to put buf into a matrix data file for row>
}
\endverbatim

<P>
Finally, the memory allocated for use by the segment routines would be 
released and the file closed:


\verbatim
segment_release (&seg);

close (fd);
\endverbatim

<P>
<B>Note:</B> The <I>Segment Library</I> does not know the name of the 
segment file. It does not attempt to remove the file. If the file is only 
temporary, the programmer should remove the file after closing it.

<P>

\section Segment_Library_Performance Segment Library Performance

Performance of the <I>Segment Library</I> routines can be improved by
about 10% if <B>srows, scols</B> are each powers of 2; in this case a
faster alternative is used to access the segment file. An additional
improvement can be achieved if <B>len</B> is also a power of 2. For
highly random and scattered access to a large dataset, smaller segments,
i.e. values for <B>srows, scols</B> of 32, 64, or 128 seem to provide
better performance than e.g. srows = nrows / 4 + 1.

\section Loading_the_Segment_Library Loading the Segment Library

<P>
The library is loaded by specifying
\verbatim
$(SEGMENTLIB)
\endverbatim
in the Makefile.

<P>
See \ref Compiling_and_Installing_GRASS_Modules for a complete 
discussion of Makefiles.

*/