NAME

       PDL::DiskCache -- Non-memory-resident array object

SYNOPSIS

       NON-OO:

          use PDL::DiskCache;
          tie @a,'PDL::DiskCache', \@files, \%options;
          imag $a[3];

       OO:

          use PDL::DiskCache;
          $a = diskcache(\@files,\%options);
          imag $a->[3];

       or

          use PDL::DiskCache;
          $a = new PDL::DiskCache(\@files,\%options);
          imag $a->[4];

       \@files
          an array ref containing a list of file names

       \%options
          a hash ref containing options for the PDL::DiskCache object (see "TIEARRAY" below for
          details)

DESCRIPTION

       A PDL::DiskCache object is a perl "tied array" that is useful for operations where you
       have to look at a large collection of PDLs  one or a few at a time (such as tracking
       features through an image sequence).  You can write prototype code that uses a perl list
       of a few PDLs, then scale up to to millions of PDLs simply by handing the prototype code a
       DiskCache tied array instead of a native perl array.  The individual PDLs are stored on
       disk and a few of them are swapped into memory on a FIFO basis.  You can set whether the
       data are read-only or writeable.

       By default, PDL::DiskCache uses FITS files to represent the PDLs, but you can use any sort
       of file at all -- the read/write routines are the only place where it examines the
       underlying data, and you can specify the routines to use at construction time (or, of
       course, subclass PDL::DiskCache).

       Items are swapped out on a FIFO basis, so if you have 10 slots and an expression with 10
       items in it then you're OK (but you probably want more slots than that); but if you use
       more items in an expression than there are slots, thrashing will occur!

       The hash ref interface is kept for historical reasons; you can access the sync() and
       purge() method calls directly from the returned array ref.

Shortcomings & caveats

       There's no file locking, so you could really hose yourself by having two of these things
       going at once on the same files.

       Since this is a tied array, things like Dumper traverse it transparently.  That is sort-of
       good but also sort-of dangerous.  You wouldn't want to PDL::Dumper::sdump() a large
       PDL::DiskCache, for example -- that would defeat the purpose of using a PDL::DiskCache in
       the first place.

Author, license, no warranty

       Copyright 2001, Craig DeForest

       This code may be distributed under the same terms as Perl itself (license available at
       <http://www.perl.org>).  Copying, reverse engineering, distribution, and modification are
       explicitly allowed so long as this notice is preserved intact and modified versions are
       clearly marked as such.

       If you modify the code and it's useful, please send a copy of the modified version to
       cdeforest@solar.stanford.edu.

       This package comes with NO WARRANTY.

FUNCTIONS

   diskcache
       Object constructor.

         $a = diskcache(\@f,\%options);

       Options

       •  See the TIEARRAY options,below.

   TIEARRAY
       Tied-array constructor; invoked by perl during object construction.

         TIEARRAY(class,\@f,\%options)

       Options

       ro (default 0)
          If set, treat the files as read-only (modifications to the tied array will only persist
          until the changed elements are swapped out)

       rw (default 1)
          If set, allow reading and writing to the files.  Because there's currently no way to
          determine reliably whether a PDL has been modified, rw files are always written to disk
          when they're swapped out -- this causes a slight performance hit.

       mem (default 20)
          Number of files to be cached in memory at once.

       read (default \&rfits)
          A function ref pointing to code that will read list objects from disk.  The function
          must have the same syntax as rfits: $object = rfits(filename).

       write (default \&wfits)
          A function ref pointing to code that will write list objects to disk.  The function
          must have the same syntax as wfits: func(object,filename).

       bless (default 0)
          If set to a nonzero value, then the array ref gets blessed into the DiskCache class for
          for easier access to the "purge" and "sync" methods.  This means that you can say
          "$a->sync" instead of the more complex "(%{tied @$a})->sync", but "ref $a" will return
          "PDL::DiskCache" instead of "ARRAY", which could break some code.

       verbose (default 0)
          Get chatty.

   purge
       Remove an item from the oldest slot in the cache, writing to disk as necessary.  You also
       send in how many slots to purge (default 1; sending in -1 purges everything.)

       For most uses, a nice MODIFIED flag in the data structure could save some hassle here.
       But PDLs can get modified out from under us with slicing and .= -- so for now we always
       assume everything is tainted and must be written to disk.

   sync
       In a rw cache, flush all items out to disk but retain them in the cache.  This is useful
       primarily for cache protection and could be slow.  Because we have no way of knowing
       what's modified and what's not in the cache, all elements are always flushed from an rw
       cache.  For ro caches, this is a not-too-slow (but safe) no-op.

   DESTROY
       This is the perl hook for object destruction.  It just makes a call to "sync", to flush
       the cache out to disk.  Destructor calls from perl don't happen at a guaranteed time, so
       be sure to call "sync" if you need to ensure that the files get flushed out, e.g. to use
       'em somewhere else.