File Coverage

blib/lib/Cache/Mmap.pm

Criterion	Covered	Total	%
statement	331	388	85.3
branch	106	160	66.2
condition	24	41	58.5
subroutine	40	42	95.2
pod	14	14	100.0
total	515	645	79.8

# $Id: Mmap.pm,v 1.15 2008/04/15 09:41:26 pmh Exp $

=head1 NAME

Cache::Mmap - Shared data cache using memory mapped files

=head1 SYNOPSIS

  use Cache::Mmap;

  $cache=Cache::Mmap->new($filename,\%options);

  $val1=$cache->read($key1);
  $cache->write($key2,$val2);
  $cache->delete($key3);

=head1 DESCRIPTION

This module implements a shared data cache, using memory mapped files.
If routines are provided which interact with the underlying data, access to
the cache is completely transparent, and the module handles all the details of
refreshing cache contents, and updating underlying data, if necessary.

Cache entries are assigned to "buckets" within the cache file, depending on
the key. Within each bucket, entries are stored approximately in order of last
access, so that frequently accessed entries will move to the head of the
bucket, thus decreasing access time. Concurrent accesses to the same bucket are
prevented by file locking of the relevant section of the cache file.

=cut

package Cache::Mmap;

# Do we need to worry about UTF-8?
use constant has_utf8 => has_utf8 => $] >= 5.006_000;

use Carp qw(croak);
use DynaLoader();
use Exporter;
use Fcntl;
use IO::Seekable qw(SEEK_SET SEEK_END);
use Storable qw(freeze thaw);
use Symbol();
use integer;
use strict;
use vars qw(
  $VERSION @ISA
  @EXPORT_OK
);

$VERSION='0.11';
@ISA=qw(DynaLoader Exporter);
@EXPORT_OK=qw(CMM_keep_expired CMM_keep_expired_refresh);

__PACKAGE__->bootstrap($VERSION);

# Default cache options
my %def_options=(
  buckets => 13,	# Number of buckets
  bucketsize => 1024,	# Size of each bucket
  pagesize => 1024,	# Bucket alignment
  strings => 0,		# Store strings, rather than refs
  expiry => 0,		# Number of seconds to hold values, 0==forever
  context => undef,	# Context to pass to read and write subs
  permissions => 0600,	# Permissions for new file creation
# read => sub called as ($found,$val)/$val=$read->($key,$context)
  cachenegative  => 0,	# true:  Cache not-found values
			# false: Don't cache not-found values
# write => sub called as $write->($key,$oval,$context)
			# Leave out for no writing to underlying data
  writethrough => 1,	# true:  Write when value is added to cache
			# false: Write when value expires or is pushed out
# delete => sub called as $delete->($key,$oval,$context)
			# Leave out for no deleting of underlying data
);

# Bit positions for cache-level flags
use constant flag_strings => 0x0001;
# Names for cache-level flags
my %bool_opts=(
  strings => flag_strings,
);

# Bit positions for element flags
use constant elem_dirty => 0x0001;

use constant magic => 0x15ACACE;# Cache file magic number
use constant filevers => 1;	# File format version number supported


my $headsize=4*10;  # File: magic, buckets, bucketsize, pagesize, flags,
		    #       file format version
my $bheadsize=4*10; # Bucket: filled
my $eheadsize=4*10; # Element: size, time, klen, vlen, flags
my $maxheadsize=$headsize > $bheadsize ? $headsize : $bheadsize;
$maxheadsize=$eheadsize if $eheadsize > $maxheadsize;

# While these look random, the low word could be a bitmask
use constant CMM_keep_expired =>         0xCACE0001; # Keep the expired value
use constant CMM_keep_expired_refresh => 0xCACE0003; # Keep the expired value, and unexpire it


=head1 CLASS METHODS

=over

=item new($filename,\%options)

Creates a new cache object. If the file named by C<$filename> does not already
exist, it will be created. If the cache object cannot be created for any
reason, an exception will be thrown. Various options may be set in C<%options>,
which affect the behaviour of the cache (defaults in parentheses):

=over 4

=item permissions (0600)

Sets the file permissions for the cache file if it doesn't already exist.

=item buckets (13)

Sets the number of buckets inside the cache file. A larger number of buckets
will give better performance for a cache with many accesses, as there will be
less chance of concurrent access to the same bucket.

=item bucketsize (1024)

Sets the size of each bucket, in bytes. A larger bucket size will be needed to
store large cache entries. If the bucketsize is not large enough to hold a
particular entry, it will still be passed between the underlying data and the
application in its entirety, but will not be stored in the cache.

=item pagesize (1024)

Sets the alignment of buckets within the file. The file header will be extended
to this size, and bucket sizes will be rounded up to the nearest multiple.
Choosing a pagesize equal to the virtual memory page size of the host system
should improve performance.

=item strings (0)

If true, cache entries are treated as strings, rather than references. This
will help performance for string-only caches, as no time will be taken to
serialize cache entries.

=item expiry (0)

If non-zero, sets the length of time, in seconds, which cache entries are
considered valid. A new entry will be fetched from the underlying data if
an expired cache entry would otherwise have been returned.

=item context (undef)

This value is passed to the read/write/delete routines below, to provide
context. This will typically be a database handle, used to fetch data from.

=item read (undef)

Provides a code reference to a routine which will fetch entries from the
underlying data. Called as C<$read-E($key,$context)>, this routine should
return a list C<($found,$value)>, where C<$found> is true if the entry could
be found in the underlying data, and C<$value> is the value to cache.

If the routine only returns a single scalar, that will be taken as
the value, and C<$found> will be set to true if the value is defined.

If this routine is not provided, only values already in the cache will ever
be returned.

There are currently two special values of C<$found> which cause slightly
different behaviour. These are constants which may be imported in the
C statement.

=over 4

=item C

Use the previously cached value, even if it has expired. This is useful if
the underlying data source has become unavailable for some reason. Note that
even though the value returned will be ignored in this case, it must be
returned to avoid C<$found> being interpreted as a single scalar:

  return (Cache::Mmap::CMM_keep_expired, undef);

=item C

This causes the same behaviour as C, but the cache entry's
expiry time will be reset as if a value had been successfully read from the
underlying data.

=back

=item cachenegative (0)

If true, even unsuccessful fetches from the underlying data are cached. This
can be useful to only search the underlying data once for each required key.

=item write (undef)

Provides a code reference to a routine which will write cache entries into the
underlying data. This routine will be called by write(), to synchronise the
underlying data with the cache. Called as C<$write-E($key,$val,$context)>.
If the routine is not provided, the underlying data will not be synchronised
after cache writes.

=item writethrough (1)

If true, the C routine above will be called as soon as
write() is called. This provides immediate synchronisation of
underlying data and cache contents.

If false, the C routine will
be called for each cache entry which no longer fits in its bucket after a
cache read or write. This provides a write-as-necessary behaviour, which may
be more efficient than the writethrough behaviour. However, only data fetched
through the cache will reflect these changes.

=item delete (undef)

Provides a code reference to a routine which will delete items from the
underlying data. This routine will be called by delete(),
to synchronise the underlying data with the cache. Called as
C<$delete-E($key,$cval,$context)>, where C<$cval> is the value
currently stored in the cache. If this routine is not provided, entries
deleted from the cache have no effect on the underlying data.

=back

An alternative to supplying a C routine, is to call
delete() after updating the underlying data. Note however, that
in the case of databases, this should be done after committing the update, so
that a concurrent process doesn't reload the cache between being the entry
being deleted, and the database updates being committed.

=cut

sub new{
  my($class,$filename,$options)=@_;
  my $self={
    %def_options,
    %{$options || {}},
  };

  # Check options for sensible values
  foreach(qw(buckets bucketsize pagesize permissions)){
    defined($self->{$_}) && $self->{$_}=~/^[1-9]\d*$/s
      or croak "'$_' option for $class must be a positive integer";
  }
  $self->{pagesize}>=$maxheadsize
    or croak "'pagesize' option for $class must be at least $maxheadsize";
  foreach(qw(read write delete)){
    !$self->{$_} || ref $self->{$_} eq 'CODE'
      or croak "'$_' option for $class must be a CODE ref or empty";
  }

  # Align bucketsize
  {
    no integer;
    my $n_pages=$self->{bucketsize}/$self->{pagesize};
    if((my $i_pages=int $n_pages)!=$n_pages){
      $self->{bucketsize}=($i_pages+1)*$self->{pagesize};
    }
  }

  # Try to open a file
  my $fh=Symbol::gensym;
  sysopen($fh,$filename,O_RDWR|O_CREAT,$self->{permissions})
    or croak "Can't open cache file $filename: $!";

  # Create cache object
  bless $self,$class;
  $self->{_filename}=$filename;
  $self->{_fh}=$fh;

  # Set options
  $self->_set_options;

  $self;
}

=back

=head1 METHODS

=head2 CACHE DATA METHODS

These are the everyday methods used to access the data stored by the cache.

=over 4

=item read($key)

Reads an entry from the cache, or from the underlying data if not cached.
Returns the value in scalar context, and C<($found,$value)> in list context,
where C<$found> is true if the item was found in either the cache or the
underlying data.

=cut

sub read{
  my($self,$key)=@_;
  my $bucket=$self->_bucket($key);
  my $ekey=$self->_encode($key,1);

  # Lock the bucket. This is a write lock, even for reading, since we may
  # move items within the bucket
  $self->_lock($bucket);

  my($found,$val,$err);
  eval{
    local $SIG{__DIE__};

    ($found,my($expired,$poff,$off,$size,$klen,$vlen,$flags))
      =$self->_find($bucket,$key);

    # We need to read a new value if we don't have a value,
    # or if that value is expired.
    my ($new_found, $new_val);
    if (!$found or $expired) {
      my @_read=$self->{read}
	? $self->{read}->($key,$self->{context}) : ();
      ($new_found,$new_val)=@_read==1 ? (defined($_read[0]),$_read[0]) : @_read;
      $new_found=0 if !defined $new_found;
      undef $new_val unless $new_found;

      if($new_found==CMM_keep_expired){
	# Use the old value, even though it's expired
        $found=$expired;
	$expired=0;
	$new_found=0;
      }elsif($new_found==CMM_keep_expired_refresh){
	# Use the old value, and update its time so it's not expired anymore
        $found=$expired;
	$expired=0;
	$new_found=0;

	# Modify the time field in a hideously unmaintainable way
	substr($self->{_mmap},$off+4,4)=pack 'l',time
	  if $found;
      }
    }

    if($found){{
      # Remove expired item, and pretend we didn't find it
      # XXX What about dirty expired items???
      if($expired && !($flags & elem_dirty)){
	# No need to write underlying data, because it's not dirty
	my $b_end=$bucket+$self->{bucketsize};
	substr($self->{_mmap},$off,$b_end-$off)
	  =substr($self->{_mmap},$off+$size,$b_end-$off-$size).("\0" x $size);
	my($filled)=unpack 'l',substr($self->{_mmap},$bucket,$bheadsize);
	$filled-=$size;
	substr($self->{_mmap},$bucket,$bheadsize)
	  =substr(pack("lx$bheadsize",$filled),0,$bheadsize);
	$found=0; # it's expired, so pretend we didn't find anything
	last;
      }
      # Swap with previous item unless at head of bucket
      if($poff){
	my $psize=$off-$poff;
	substr($self->{_mmap},$poff,$psize+$size)
	  =substr($self->{_mmap},$off,$size)
	  .substr($self->{_mmap},$poff,$psize);
	$off=$poff;
      }
      $val=$self->_decode(substr($self->{_mmap},$off+$eheadsize+$klen,$vlen),0);
    }}
    if(!$found){
      # go ahead and use the new data, read above
      ($found,$val)=($new_found,$new_val);

      # Store value in cache
      if($found || $self->{cachenegative}){
	my $eval=$self->_encode($val,0);

	$self->_insert($bucket,$ekey,$eval,0);
      }
    }

    1;
  } or $err=1;
  $self->_unlock;

  # Propagate errors
  die $@ if $err;

  return ($found,$val);
}

=item write($key,$val)

Writes an entry into the cache, and depending on the configuration, into the
underlying data.

=cut

sub write{
  my($self,$key,$val)=@_;
  my $ekey=$self->_encode($key,1);
  my $klen=length $ekey;
  my $eval=$self->_encode($val,0);
  my $vlen=length $eval;
  my $size=$eheadsize+$klen+$vlen;
  my $bsize=$self->{bucketsize}-$bheadsize;

  if($size<=$bsize){
    # The item will fit in a cache bucket, so store it
    my $bucket=$self->_bucket($key);
    $self->_lock($bucket);
    my $err;
    eval{
      local $SIG{__DIE__};
      my($found,$expired,$poff,$off,$_size,$_klen,$_vlen,$flags)
	=$self->_find($bucket,$key);

      # Remove the old version
      if($found){
	my($filled)=unpack 'l',substr($self->{_mmap},$bucket,$bheadsize);
	my $pre=substr $self->{_mmap},
	    $bucket+$bheadsize,$off-($bucket+$bheadsize);
	my $post=substr $self->{_mmap},
	    $off+$_size,$bucket+$bheadsize+$filled-$off-$_size;
        my $new_filled=length($pre.$post);
	my $bhead=substr(pack("lx$bheadsize",$new_filled),0,$bheadsize);

	substr($self->{_mmap},$bucket,$bheadsize+$new_filled)
	  =$bhead.$pre.$post;
      }

      # Generate new bucket contents
      $self->_insert($bucket,$ekey,$eval,1);

      # Write to underlying data
      if($self->{writethrough} and my $write=$self->{write}){
	$write->($key,$val,$self->{context});
      }

      1;
    } or $err=1;
    $self->_unlock;
    die $@ if $err;
  }elsif(my $wsub=$self->{write}){
    # It won't fit in a cache bucket, but we can update the underlying data
    $self->delete($key);
    $wsub->($key,$val,$self->{context});
  }else{
    # It won't fit, and we can't update the data
    # XXX Should we blow up here?
    # Probably not, since an item may be removed from the cache at any time
  }

  1;
}

=item delete($key)

Deletes an entry from the cache, and depending on C options, from the
underlying data.
Returns the value in scalar context, and C<($found,$value)> in list context,
where C<$found> is true if the item was found in the cache.

=cut

sub delete{
  my($self,$key)=@_;
  my $bucket=$self->_bucket($key);

  # Lock the bucket
  $self->_lock($bucket);

  my($found,$val,$err);
  eval{
    local $SIG{__DIE__};

    ($found,my($expired,$poff,$off,$size,$klen,$vlen,$flags))
      =$self->_find($bucket,$key);

    if($found){
      $val=$self->_decode(substr($self->{_mmap},$off+$eheadsize+$klen,$vlen),0);
      if(my $dsub=$self->{delete} and !($flags & elem_dirty)){
	$dsub->($key,$val,$self->{context});
      }
      my($filled)=unpack 'l',substr($self->{_mmap},$bucket,$bheadsize);
      my $new_filled=$filled-$size;
      substr($self->{_mmap},$bucket,$bheadsize)
	=substr(pack("lx$bheadsize",$new_filled),0,$bheadsize);

      my $fill_end=$bucket+$bheadsize+$filled;
      my $elem_end=$off+$size;
      substr($self->{_mmap},$off,$fill_end-$elem_end)
        =substr($self->{_mmap},$elem_end,$fill_end-$elem_end);
    }

    1;
  } or $err=1;
  $self->_unlock;

  # Propagate errors
  die $@ if $err;

  return ($found,$val);
}

=item entries()

=item entries(0)

Returns a list of the keys of entries held in the cache. Note that this list
may be immediately out of date, due to the shared nature of the cache. Entries
may be added or removed by other processes between this list being generated
and when it is used.

=item entries(1)

Returns a list of hashrefs representing entries held in the cache. The
following keys are present in each hashref:

  key    The key used to identify the entry
  time   The time the entry was stored (seconds since the epoch)
  dirty  Whether the entry needs writing to the underlying data

The same caveat applies to the currency of this information as above.

=item entries(2)

As C, with the addition of a C element in each
hashref, holding the value stored in the cache entry.

=cut

sub entries{
  my($self,$details)=@_;
  $details=defined($details) && $details+0;

  my $buckets=$self->buckets;
  my $bucketsize=$self->bucketsize;
  my $pagesize=$self->pagesize;
  my $expiry=$self->expiry;

  my @entries;
  for(0..$buckets-1){
    my $bucket=$pagesize+$bucketsize*$_;
    $self->_lock($bucket);

    my $err;
    eval{
      local $SIG{__DIE__};

      my($filled)=unpack 'l',substr($self->{_mmap},$bucket,$bheadsize);
      my $off=$bucket+$bheadsize;
      my $end=$off+$filled;
      my $size;
      while($off<$end){
	($size,my($time,$klen,$vlen,$flags))
	  =unpack 'l5',substr $self->{_mmap},$off,$eheadsize;
	if(!$size){
	  my $part=substr($self->{_mmap},$off,$end-$off);
	  $part=~s/\\/\\\\/g;
	  $part=~s/([^\040-\176])/sprintf '\\%02x',ord $1/ge;
	  die "Zero-sized entry in $self->{_filename}, offset $off! Remaining bucket contents: $part";
	}
	next if $expiry && time()-$time > $expiry;

	my $key=$self->_decode(substr($self->{_mmap},$off+$eheadsize,$klen),1);
        if($details){
	  push @entries,{
	    key => $key,
	    'time' => $time,
	    dirty => $flags & elem_dirty,
	    $details>1 ? (
	      value => $self->_decode(
		substr($self->{_mmap},$off+$eheadsize+$klen,$vlen),0
	      ),
	    ) : (),
	  };
	}else{
	  push @entries,$key;
	}
      }continue{
	$off+=$size;
      }

      1;
    } or $err=1;
    $self->_unlock;

    die $@ if $err;
  }

  @entries;
}

=item quick_clear()

Forcefully delete the cache, with prejudice. Unwritten dirty elements are B
written back to the underlying data source; they are simply thrown away.

=cut

sub quick_clear{
  my($self)=@_;

  $self->_lock(0)
    or croak "Can't lock cache file: $!";

  my $err;
  eval{
    local $SIG{__DIE__};

    my $buckets=$self->buckets;
    my $bucketsize=$self->bucketsize;
    my $pagesize=$self->pagesize;
    my $empty="\0" x $bucketsize;

    for(0..$buckets-1){
      substr($self->{_mmap},$pagesize+$bucketsize*$_,$bucketsize)=$empty;
    }

    1;
  } or $err=1;

  $self->_unlock;

  die $@ if $err;
}

=back

=head2 CONFIGURATION METHODS

These methods are used to examine/update the configuration of a cache.
Most of these methods are read-only, and the value returned may be different
to that passed to the constructor, since the cache may have been created by
an earlier process which specified different parameters.

=over

=item buckets()

Returns the number of buckets in the cache file.

=cut

sub buckets{
  my($self)=@_;

  $self->{buckets};
}

=item bucketsize()

Returns the size of buckets (in bytes) in the cache file.

=cut

sub bucketsize{
  my($self)=@_;

  $self->{bucketsize};
}

=item cachenegative()

Returns true if items not found in the underlying data are cached anyway.

=cut

sub cachenegative{
  my($self)=@_;

  $self->{cachenegative};
}

=item context()

Returns the context data for reads and writes to the underlying data.

=item context($context)

Provides new context data for reads and writes to the underlying data.

=cut

sub context{
  my $self=shift;

  @_ ? ($self->{context}=$_[0]) : $self->{context};
}

=item expiry()

Returns the time in seconds cache entries are considered valid for, or zero
for indefinite validity.

=cut

sub expiry{
  my($self)=@_;

  $self->{expiry};
}

=item pagesize()

Returns the page size (in bytes) of the cache file.

=cut

sub pagesize{
  my($self)=@_;

  $self->{pagesize};
}

=item strings()

Returns true if the cache stores strings rather than references.

=cut

sub strings{
  my($self)=@_;

  $self->{strings};
}

=item writethrough()

Returns true if items written to the cache are immediately written to the
underlying data.

=cut

sub writethrough{
  my($self)=@_;

  $self->{writethrough};
}

=back

=begin private

=head1 PRIVATE METHODS

These methods are for internal use only, and are not for general consumption.

=over

=item _set_options()

If the cache already exists, read its options. Otherwise, set them according
to the values passed to the constructor.

This method should only be called by the constructor.

=cut

sub _set_options{
  my($self)=@_;

  # Lock file, so only one process sets the size
  $self->_lock(0)
    or croak "Can't lock cache file: $!";

  my $err;
  eval{
    local $SIG{__DIE__};

    # If the file is big enough to contain a header, attempt to read one
    my $size_cur= -s $self->{_fh};
    my $magic_ok;
    if($size_cur>=$headsize){
      my $head;
      if((my $bytes=sysread($self->{_fh},$head,$headsize))!=$headsize){
	croak "Expecting $headsize bytes, read $bytes from cache header\n";
      }
      my($mg,$buckets,$bucketsize,$pagesize,$flags,$format)=unpack('l6',$head);
      $mg==magic
        or croak "$self->{_filename} is not a Cache::Mmap file";
      ($format+=0)==filevers
        or croak "$self->{_filename} uses v$format data structures. Cache::Mmap $VERSION only supports v".filevers." data structures";

      $self->{buckets}=$buckets;
      $self->{bucketsize}=$bucketsize;
      $self->{pagesize}=$pagesize;
      while(my($opt,$bit)=each %bool_opts){
	$self->{$opt}=!!($flags&$bit);
      }
      $magic_ok=1;
    }

    # Make sure the file is big enough for the whole cache
    my $size=$self->{pagesize}+$self->{buckets}*$self->{bucketsize};
    if($size_cur < $size){
      my $pad="\0" x 1024;
      sysseek $self->{_fh},SEEK_END,0
	or croak "Can't seek to end of file: $!\n";
      while($size_cur < $size){
	my $len=syswrite($self->{_fh},$pad,1024)
	  or croak "Can't pad file: $!";
	$size_cur+=$len;
      }
      -s $self->{_fh} >= $size
	or croak "Failed to set correct file size\n";
    }

    # Write file header if it's not already done
    if(!$magic_ok){
      my $flags=0;
      while(my($opt,$bit)=each %bool_opts){
	$flags|=$bit if $self->{$opt};
      }
      my $head=pack("l6x$headsize",
	magic,@$self{'buckets','bucketsize','pagesize'},$flags,filevers
      );
      sysseek $self->{_fh},SEEK_SET,0
	or croak "Can't seek to beginning: $!";
      syswrite($self->{_fh},$head,$headsize)==$headsize
	or croak "Can't write file header: $!";
    }

    # mmap() isn't supposed to work on locked files, so unlock
    $self->_unlock;

    mmap($self->{_mmap}='',$size,$self->{_fh})
      or do{
	delete $self->{_mmap};
	croak "Can't mmap $self->{_filename}: $!";
      };
    length($self->{_mmap}) eq $size
      or do{
        delete $self->{_mmap};
	croak "mmap() failed silently: $!";
      };

    1;
  } or $err=1;

  # Unlock file before returning
  $self->_unlock;

  # Propagate caught error if there was one
  die $@ if $err;
}

=item DESTROY()

Unmap and close the file.

=cut

sub DESTROY{
  my($self)=@_;

  munmap($self->{_mmap}) if exists $self->{_mmap};
  close $self->{_fh};
}

=item _lock($offset)

Lock the cache file. If $offset is zero, the file header is locked.
Otherwise, the bucket starting at $offset is locked.

XXX This also needs to create an internal lock if threading is enabled.

=cut

sub _lock{
  my($self,$offset)=@_;
  my $length=$offset ? $self->{bucketsize} : $headsize;

  _lock_xs($self->{_fh},$offset,$length,1);
}

=item _unlock()

Unlocks the entire cache file.

XXX This needs to unlock internal lock and take an offset arg if threading

=cut

sub _unlock{
  my($self)=@_;

  _lock_xs($self->{_fh},0,0,0);
}

=item _insert($bucket,$ekey,$eval,$write)

Inserts the key/value pair into the bucket. The item will be marked as dirty
if $write is true, and writethrough() is false.

=cut

sub _insert{
  my($self,$bucket,$ekey,$eval,$write)=@_;
  my $klen=length $ekey;
  my $vlen=length $eval;
  my $size=$eheadsize+$klen+$vlen;
  my $bsize=$self->{bucketsize}-$bheadsize;
  return if $size>$bsize;

  my $ehead=substr(pack("l5x$eheadsize",
    $size,time(),$klen,$vlen,($write && !$self->{writethrough} && elem_dirty),
  ),0,$eheadsize);
  my($filled)=unpack 'l',substr($self->{_mmap},$bucket,4);
  my $content=$ehead.$ekey.$eval
    .substr($self->{_mmap},$bucket+$bheadsize,$filled);
  $filled=length $content;

  # Trim down to fit into bucket
  if($filled > $bsize){
    # Find all items which fit in the bucket
    my $poff=my $off=$size;
    while($off<=$bsize){
      $poff=$off;
      last if $poff>=$filled;
      my($size)=unpack 'l',substr($content,$off,4);
      $off+=$size;
    }

    # Write remaining items back to underlying data if dirty
    if(my $wsub=$self->{write} && !$self->{writethrough}){
      for($off=$poff;$off<$filled;){
	my($size,$time,$vlen,$klen,$flags)
	  =unpack 'l5',substr($content,$off,$eheadsize);
	if(!$size){
	  my $part=substr($content,$off,length($content)-$off);
	  my $off=$bucket+$off;
	  $part=~s/\\/\\\\/g;
	  $part=~s/([^\040-\176])/sprintf '\\%02x',ord $1/ge;
	  die "Zero-size entry in $self->{_filename}, offset $off! [ekey=$ekey] Remaining bucket contents: $part";
	  return;
	}
	if($flags & elem_dirty){
	  my $key=$self->_decode(substr($content,$off+$eheadsize,$klen),1);
	  my $val=$self->_decode(
	    substr($content,$off+$eheadsize+$klen,$vlen),0);
	  $wsub->($key,$val,$self->{content});
	}
	$off+=$size;
      }
    }

    # Remove dead items
    $filled=$poff;
    substr($content,$filled)=''; # Chop off the end of the string
  }

  # Write the bucket
  my $bhead=substr(pack("lx$bheadsize",$filled),0,$bheadsize);
  substr($self->{_mmap},$bucket,$bheadsize+$filled)=$bhead.$content;
}

=item _bucket($key)

Returns the offset of the bucket which would hold $key.

=cut

sub _bucket{
  my($self,$key)=@_;

  my $hash=0;
  while($key=~/(.)/gs){
    $hash*=33;
    $hash+=ord $1;
  }

  my $bucket=do{ no integer; $hash % $self->{buckets}; };
  return $self->{pagesize}+$bucket*$self->{bucketsize};
}

=item _find($bucket,$key)

Locate the item keyed by $key in the bucket starting at $bucket.

Returns: ($found,$expired,$poff,$off,$size,$klen,$vlen,$flags)

=cut

sub _find{
  my($self,$bucket,$key)=@_;
  my($filled)=unpack 'l',substr($self->{_mmap},$bucket,$bheadsize);
  my $off=$bucket+$bheadsize;
  my $end=$off+$filled;
  my $b_end=$bucket+$self->bucketsize;

  my($found,$size,$time,$klen,$vlen,$flags,$poff);
  while($off<$end){
    if($off>=$b_end){
      die "Super-sized entry in $self->{_filename}, offset $poff! [size=$size, finding key=$key]";
    }
    ($size,$time,$klen,$vlen,$flags)
      =unpack 'l5',substr $self->{_mmap},$off,$eheadsize;
    if(!$size){
      my $part=substr($self->{_mmap},$off,$end-$off);
      $part=~s/\\/\\\\/g;
      $part=~s/([^\040-\176])/sprintf '\\%02x',ord $1/ge;
      my $prev;
      if($poff){
	$prev=" [poff=$poff]";
      }
      local $^W;
      die "Zero-sized entry in $self->{_filename}, offset $off! [bucket=$bucket][key=$key]$prev Remaining bucket contents: $part";
    }
    if($self->_decode(substr($self->{_mmap},$off+$eheadsize,$klen),1) eq $key){
      $found=1;
      last;
    }
    $poff=$off;
    $off+=$size;
  }

  return unless $found;

  my $expired;
  if($found and my $exp=$self->expiry){
    $expired=time-$time>$exp;
  }

  return ($found,$expired,$poff,$off,$size,$klen,$vlen,$flags);
}

=item _encode($value,$is_key)

Encodes the given value into a string

=cut

sub _encode{
  my($self,$value,$is_key)=@_;

  if(!defined $value){
    return '';
  }elsif($self->{strings} || $is_key){
    if(has_utf8){
      my $eval=pack 'a*',$value;
      if($eval eq $value){
        return " $eval";
      }else{
        return "U$eval";
      }
    }else{
      return " $value";
    }
  }else{
    return ' '.freeze($value);
  }
}

=item _decode($value,$is_key)

Decodes the given string value

=cut

sub _decode{
  my($self,$value,$is_key)=@_;

  if($value eq ''){
    return undef;
  }else{
    $value=~s/^(.)//s;
    my $code=$1;
    if($code eq 'U'){
      if(has_utf8){
        utf8::decode($value);
	return $value;
      }else{
        croak "UTF8 encoded value in $self->{_filename} detected\n";
      }
    }elsif($self->{strings} || $is_key){
      return $value;
    }else{
      return thaw($value);
    }
  }
}



# Return true to require
1;


=back

=end private

=head1 AUTHOR

Copyright (C) Institute of Physics Publishing 2002-2008

	Peter Haworth

You may distribute under the terms of the GPL or the Artistic License,
as distributed with Perl.