path: root/lib/Tie/File.pm

package Tie::File;
use Carp;
use POSIX 'SEEK_SET';
use Fcntl 'O_CREAT', 'O_RDWR';
require 5.005;

$VERSION = "0.12";

# Idea: The object will always contain an array of byte offsets
# this will be filled in as is necessary and convenient.
# fetch will do seek-read.
# There will be a cache parameter that controls the amount of cached *data*
# Also an LRU queue of cached records
# store will read the relevant record into the cache
# If it's the same length as what is being written, it will overwrite it in 
#   place; if not, it will do a from-to copying write.
# The record separator string is also a parameter

# Record numbers start at ZERO.

my $DEFAULT_CACHE_SIZE = 1<<21;    # 2 megabytes

sub TIEARRAY {
  if (@_ % 2 != 0) {
    croak "usage: tie \@array, $_[0], filename, [option => value]...";
  }
  my ($pack, $file, %opts) = @_;

  # transform '-foo' keys into 'foo' keys
  for my $key (keys %opts) {
    my $okey = $key;
    if ($key =~ s/^-+//) {
      $opts{$key} = delete $opts{$okey};
    }
  }

  $opts{cachesize} ||= $DEFAULT_CACHE_SIZE;

  # the cache is a hash instead of an array because it is likely to be
  # sparsely populated
  $opts{cache} = {}; 
  $opts{cached} = 0;   # total size of cached data
  $opts{lru} = [];     # replace with heap in later version

  $opts{offsets} = [0];
  $opts{filename} = $file;
  $opts{recsep} = $/ unless defined $opts{recsep};
  $opts{recseplen} = length($opts{recsep});
  if ($opts{recseplen} == 0) {
    croak "Empty record separator not supported by $pack";
  }

  my $mode = defined($opts{mode}) ? $opts{mode} : O_CREAT|O_RDWR;

  my $fh = \do { local *FH };   # only works in 5.005 and later
  sysopen $fh, $file, $mode, 0666 or return;
  binmode $fh;
  { my $ofh = select $fh; $| = 1; select $ofh } # autoflush on write
  $opts{fh} = $fh;

  bless \%opts => $pack;
}

sub FETCH {
  my ($self, $n) = @_;

  # check the record cache
  { my $cached = $self->_check_cache($n);
    return $cached if defined $cached;
  }

  unless ($#{$self->{offsets}} >= $n) {
    my $o = $self->_fill_offsets_to($n);
    # If it's still undefined, there is no such record, so return 'undef'
    return unless defined $o;
  }

  my $fh = $self->{FH};
  $self->_seek($n);             # we can do this now that offsets is populated
  my $rec = $self->_read_record;
  $self->_cache_insert($n, $rec) if defined $rec;
  $rec;
}

sub STORE {
  my ($self, $n, $rec) = @_;

  $self->_fixrecs($rec);

  # TODO: what should we do about the cache?  Install the new record
  # in the cache only if the old version of the same record was
  # already there?

  # We need this to decide whether the new record will fit
  # It incidentally populates the offsets table 
  # Note we have to do this before we alter the cache
  my $oldrec = $self->FETCH($n);

  # _check_cache promotes record $n to MRU.  Is this correct behavior?
  $self->{cache}{$n} = $rec if $self->_check_cache($n);

  if (not defined $oldrec) {
    # We're storing a record beyond the end of the file
    $self->_extend_file_to($n);
    $oldrec = $self->{recsep};
  }
  my $len_diff = length($rec) - length($oldrec);

  $self->_twrite($rec, $self->{offsets}[$n], length($oldrec));

  # now update the offsets
  # array slice goes from element $n+1 (the first one to move)
  # to the end
  for (@{$self->{offsets}}[$n+1 .. $#{$self->{offsets}}]) {
    $_ += $len_diff;
  }
}

sub FETCHSIZE {
  my $self = shift;
  my $n = $#{$self->{offsets}};
  while (defined ($self->_fill_offsets_to($n+1))) {
    ++$n;
  }
  $n;
}

sub STORESIZE {
  my ($self, $len) = @_;
  my $olen = $self->FETCHSIZE;
  return if $len == $olen;      # Woo-hoo!

  # file gets longer
  if ($len > $olen) {
    $self->_extend_file_to($len-1);  # record numbers from 0 .. $len-1
    return;
  }

  # file gets shorter
  $self->_seek($len);
  $self->_chop_file;
  $#{$self->{offsets}} = $len-1;
  my @cached = grep $_ > $len, keys %{$self->{cache}};
  delete @{$self->{cache}}{@cached} if @cached;
}

sub SPLICE {
  my ($self, $pos, $nrecs, @data) = @_;
  my @result;

  $pos += $self->FETCHSIZE if $pos < 0;

  $self->_fixrecs(@data);
  my $data = join '', @data;
  my $datalen = length $data;
  my $oldlen = 0;

  # compute length of data being removed
  for ($pos .. $pos+$nrecs-1) {
    my $rec = $self->FETCH($_);
    last unless defined $rec;
    push @result, $rec;
    $oldlen += length($rec);
  }

  $self->_fill_offsets_to($pos);
  $self->_twrite($data, $self->{offsets}[$pos], $oldlen);

  # update the offsets table part 1
  # compute the offsets of the new records:
  my @new_offsets;
  if (@data) {
    push @new_offsets, $self->{offsets}[$pos];
    for (0 .. $#data-1) {
      push @new_offsets, $new_offsets[-1] + length($data[$_]);
    }
  }
  splice(@{$self->{offsets}}, $pos, $nrecs, @new_offsets);

  # update the offsets table part 2
  # adjust the offsets of the following old records
  for ($pos+@data .. $#{$self->{offsets}}) {
    $self->{offsets}[$_] += $datalen - $oldlen;
  }
  # If we scrubbed out all known offsets, regenerate the trivial table
  # that knows that the file does indeed start at 0.
  $self->{offsets}[0] = 0 unless @{$self->{offsets}};

  # update the read cache, part 1
  # modified records
  # Consider this carefully for correctness
  for ($pos .. $pos+$nrecs-1) {
    my $cached = $self->{cache}{$_};
    next unless defined $cached;
    my $new = $data[$_-$pos];
    if (defined $new) {
      $self->{cached} += length($new) - length($cached);
      $self->{cache}{$_} = $new;
    } else {
      delete $self->{cache}{$_};
      $self->{cached} -= length($cached);
    }
  }
  # update the read cache, part 2
  # moved records - records past the site of the change
  # need to be renumbered
  # Maybe merge this with the previous block?
  for (keys %{$self->{cache}}) {
    next unless $_ >= $pos + $nrecs;
    $self->{cache}{$_-$nrecs+@data} = delete $self->{cache}{$_};
  }

  # fix the LRU queue
  my(@new, @changed);
  for (@{$self->{lru}}) {
    if ($_ >= $pos + $nrecs) {
      push @new, $_ + @data - $nrecs;
    } elsif ($_ >= $pos) {
      push @changed, $_ if $_ < $pos + @data;
    } else {
      push @new, $_;
    }
  }
  @{$self->{lru}} = (@new, @changed);

  @result;
}

# write data into the file
# $data is the data to be written. 
# it should be written at position $pos, and should overwrite
# exactly $len of the following bytes.  
# Note that if length($data) > $len, the subsequent bytes will have to 
# be moved up, and if length($data) < $len, they will have to
# be moved down
sub _twrite {
  my ($self, $data, $pos, $len) = @_;

  unless (defined $pos) {
    die "\$pos was undefined in _twrite";
  }

  my $len_diff = length($data) - $len;

  if ($len_diff == 0) {          # Woo-hoo!
    my $fh = $self->{fh};
    $self->_seekb($pos);
    $self->_write_record($data);
    return;                     # well, that was easy.
  }

  # the two records are of different lengths
  # our strategy here: rewrite the tail of the file,
  # reading ahead one buffer at a time
  # $bufsize is required to be at least as large as the data we're overwriting
  my $bufsize = _bufsize($len_diff);
  my ($writepos, $readpos) = ($pos, $pos+$len);

  # Seems like there ought to be a way to avoid the repeated code
  # and the special case here.  The read(1) is also a little weird.
  # Think about this.
  do {
    $self->_seekb($readpos);
    my $br = read $self->{fh}, my($next_block), $bufsize;
    my $more_data = read $self->{fh}, my($dummy), 1;
    $self->_seekb($writepos);
    $self->_write_record($data);
    $readpos += $br;
    $writepos += length $data;
    $data = $next_block;
    unless ($more_data) {
      $self->_seekb($writepos);
      $self->_write_record($next_block);
    }
  } while $more_data;

  # There might be leftover data at the end of the file
  $self->_chop_file if $len_diff < 0;
}

# If a record does not already end with the appropriate terminator
# string, append one.
sub _fixrecs {
  my $self = shift;
  for (@_) {
    $_ .= $self->{recsep}
      unless substr($_, - $self->{recseplen}) eq $self->{recsep};
  }
}

# seek to the beginning of record #$n
# Assumes that the offsets table is already correctly populated
#
# Note that $n=-1 has a special meaning here: It means the start of
# the last known record; this may or may not be the very last record
# in the file, depending on whether the offsets table is fully populated.
#
sub _seek {
  my ($self, $n) = @_;
  my $o = $self->{offsets}[$n];
  defined($o)
    or confess("logic error: undefined offset for record $n");
  seek $self->{fh}, $o, SEEK_SET
    or die "Couldn't seek filehandle: $!";  # "Should never happen."
}

sub _seekb {
  my ($self, $b) = @_;
  seek $self->{fh}, $b, SEEK_SET
    or die "Couldn't seek filehandle: $!";  # "Should never happen."
}

# populate the offsets table up to the beginning of record $n
# return the offset of record $n
sub _fill_offsets_to {
  my ($self, $n) = @_;
  my $fh = $self->{fh};
  local *OFF = $self->{offsets};
  my $rec;

  until ($#OFF >= $n) {
    my $o = $OFF[-1];
    $self->_seek(-1);           # tricky -- see comment at _seek
    $rec = $self->_read_record;
    if (defined $rec) {
      push @OFF, $o+length($rec);
    } else {
      return;                   # It turns out there is no such record
    }
  }

  # we have now read all the records up to record n-1,
  # so we can return the offset of record n
  return $OFF[$n];
}

# assumes that $rec is already suitably terminated
sub _write_record {
  my ($self, $rec) = @_;
  my $fh = $self->{fh};
  print $fh $rec
    or die "Couldn't write record: $!";  # "Should never happen."

}

sub _read_record {
  my $self = shift;
  my $rec;
  { local $/ = $self->{recsep};
    my $fh = $self->{fh};
    $rec = <$fh>;
  }
  $rec;
}

sub _cache_insert {
  my ($self, $n, $rec) = @_;

  # Do not cache records that are too big to fit in the cache.
  return unless length $rec <= $self->{cachesize};

  $self->{cache}{$n} = $rec;
  $self->{cached} += length $rec;
  push @{$self->{lru}}, $n;     # most-recently-used is at the END

  $self->_cache_flush if $self->{cached} > $self->{cachesize};
}

sub _check_cache {
  my ($self, $n) = @_;
  my $rec;
  return unless defined($rec = $self->{cache}{$n});

  # cache hit; update LRU queue and return $rec
  # replace this with a heap in a later version
  @{$self->{lru}} = ((grep $_ ne $n, @{$self->{lru}}), $n);
  $rec;
}

sub _cache_flush {
  my ($self) = @_;
  while ($self->{cached} > $self->{cachesize}) {
    my $lru = shift @{$self->{lru}};
    $self->{cached} -= length $lru;
    delete $self->{cache}{$lru};
  }
}

# We have read to the end of the file and have the offsets table
# entirely populated.  Now we need to write a new record beyond
# the end of the file.  We prepare for this by writing
# empty records into the file up to the position we want
# $n here is the record number of the last record we're going to write
sub _extend_file_to {
  my ($self, $n) = @_;
  $self->_seek(-1);             # position after the end of the last record
  my $pos = $self->{offsets}[-1];

  # the offsets table has one entry more than the total number of records
  $extras = $n - ($#{$self->{offsets}} - 1);

  # Todo : just use $self->{recsep} x $extras here?
  while ($extras-- > 0) {
    $self->_write_record($self->{recsep});
    $pos += $self->{recseplen};
    push @{$self->{offsets}}, $pos;
  }
}

# Truncate the file at the current position
sub _chop_file {
  my $self = shift;
  truncate $self->{fh}, tell($self->{fh});
}

# compute the size of a buffer suitable for moving
# all the data in a file forward $n bytes
# ($n may be negative)
# The result should be at least $n.
sub _bufsize {
  my $n = shift;
  return 8192 if $n < 0;
  my $b = $n & ~8191;
  $b += 8192 if $n & 8191;
  $b;
}


# Given a file, make sure the cache is consistent with the
# file contents
sub _check_integrity {
  my ($self, $file, $warn) = @_;
  my $good = 1; 
  local *F;
  open F, $file or die "Couldn't open file $file: $!";
  local $/ = $self->{recsep};
  unless ($self->{offsets}[0] == 0) {
    $warn && print STDERR "# rec 0: offset <$self->{offsets}[0]> s/b 0!\n";
    $good = 0;
  }
  while (<F>) {
    my $n = $. - 1;
    my $cached = $self->{cache}{$n};
    my $offset = $self->{offsets}[$.];
    my $ao = tell F;
    if (defined $offset && $offset != $ao) {
      $warn && print STDERR "# rec $n: offset <$offset> actual <$ao>\n";
    }
    if (defined $cached && $_ ne $cached) {
      $good = 0;
      chomp $cached;
      chomp;
      $warn && print STDERR "# rec $n: cached <$cached> actual <$_>\n";
    }
  }

  my $cachesize = 0;
  while (my ($n, $r) = each %{$self->{cache}}) {
    $cachesize += length($r);
    next if $n+1 <= $.;         # checked this already
    $warn && print STDERR "# spurious caching of record $n\n";
    $good = 0;
  }
  if ($cachesize != $self->{cached}) {
    $warn && print STDERR "# cache size is $self->{cached}, should be $cachesize\n";
    $good = 0;
  }

  my (%seen, @duplicate);
  for (@{$self->{lru}}) {
    $seen{$_}++;
    if (not exists $self->{cache}{$_}) {
      print "# $_ is mentioned in the LRU queue, but not in the cache\n";
      $good = 0;
    }
  }
  @duplicate = grep $seen{$_}>1, keys %seen;
  if (@duplicate) {
    my $records = @duplicate == 1 ? 'Record' : 'Records';
    my $appear  = @duplicate == 1 ? 'appears' : 'appear';
    print "# $records @duplicate $appear multiple times in LRU queue: @{$self->{lru}}\n";
    $good = 0;
  }
  for (keys %{$self->{cache}}) {
    unless (exists $seen{$_}) {
      print "# $record $_ is in the cache but not the LRU queue\n";
      $good = 0;
    }
  }

  $good;
}

=head1 NAME

Tie::File - Access the lines of a disk file via a Perl array

=head1 SYNOPSIS

	# This file documents Tie::File version 0.12

	tie @array, 'Tie::File', filename or die ...;

	$array[13] = 'blah';     # line 13 of the file is now 'blah'
	print $array[42];        # display line 42 of the file

	$n_recs = @array;        # how many records are in the file?
	$#array = $n_recs - 2;   # chop records off the end

	# As you would expect
	@old_recs = splice @array, 3, 7, new recs...;

	untie @array;            # all finished

=head1 DESCRIPTION

C<Tie::File> represents a regular text file as a Perl array.  Each
element in the array corresponds to a record in the file.  The first
line of the file is element 0 of the array; the second line is element
1, and so on.

The file is I<not> loaded into memory, so this will work even for
gigantic files.

Changes to the array are reflected in the file immediately.

=head2 C<recsep>

What is a 'record'?  By default, the meaning is the same as for the
C<E<lt>...E<gt>> operator: It's a string terminated by C<$/>, which is
probably C<"\n"> or C<"\r\n">.  You may change the definition of
"record" by supplying the C<recsep> option in the C<tie> call:

	tie @array, 'Tie::File', $file, recsep => 'es';

This says that records are delimited by the string C<es>.  If the file contained the following data:

	Curse these pesky flies!\n

then the C<@array> would appear to have four elements: 

	"Curse thes"
	"e pes"
	"ky flies"
	"!\n"

An undefined value is not permitted as a record separator.  Perl's
special "paragraph mode" semantics (E<agrave> la C<$/ = "">) are not
emulated.

Records read from the tied array will have the record separator string
on the end, just as if they were read from the C<E<lt>...E<gt>>
operator.  Records stored into the array will have the record
separator string appended before they are written to the file, if they
don't have one already.  For example, if the record separator string
is C<"\n">, then the following two lines do exactly the same thing:

	$array[17] = "Cherry pie";
	$array[17] = "Cherry pie\n";

The result is that the contents of line 17 of the file will be
replaced with "Cherry pie"; a newline character will separate line 17
from line 18.  This means that inparticular, this will do nothing:

	chomp $array[17];

Because the C<chomp>ed value will have the separator reattached when
it is written back to the file.  There is no way to create a file
whose trailing record separator string is missing.

Inserting records that I<contain> the record separator string will
produce a reasonable result, but if you can't foresee what this result
will be, you'd better avoid doing this.

=head2 C<mode>

Normally, the specified file will be opened for read and write access,
and will be created if it does not exist.  (That is, the flags
C<O_RDWR | O_CREAT> are supplied in the C<open> call.)  If you want to
change this, you may supply alternative flags in the C<mode> option.
See L<Fcntl> for a listing of available flags.
For example:

	# open the file if it exists, but fail if it does not exist
	use Fcntl 'O_RDWR';
	tie @array, 'Tie::File', $file, mode => O_RDWR;

	# create the file if it does not exist
	use Fcntl 'O_RDWR', 'O_CREAT';
	tie @array, 'Tie::File', $file, mode => O_RDWR | O_CREAT;

	# open an existing file in read-only mode
	use Fcntl 'O_RDONLY';
	tie @array, 'Tie::File', $file, mode => O_RDONLY;

Opening the data file in write-only or append mode is not supported.

=head2 C<cachesize>

Records read in from the file are cached, to avoid having to re-read
them repeatedly.  If you read the same record twice, the first time it
will be stored in memory, and the second time it will be fetched from
memory.

The cache has a bounded size; when it exceeds this size, the
least-recently visited records will be purged from the cache.  The
default size is 2Mib.  You can adjust the amount of space used for the
cache by supplying the C<cachesize> option.  The argument is the desired cache size, in bytes.

	# I have a lot of memory, so use a large cache to speed up access
	tie @array, 'Tie::File', $file, cachesize => 20_000_000;

Setting the cache size to 0 will inhibit caching; records will be
fetched from disk every time you examine them.

=head2 Option Format

C<-mode> is a synonym for C<mode>.  C<-recsep> is a synonym for
C<recsep>.  C<-cachesize> is a synonym for C<cachesize>.  You get the
idea.

=head1 Public Methods

The C<tie> call returns an object, say C<$o>.  You may call 

	$rec = $o->FETCH($n);
	$o->STORE($n, $rec);

to fetch or store the record at line C<$n>, respectively.  There are
no other public methods in this package.

=head1 CAVEATS

(That's Latin for 'warnings'.)

=head2 Efficiency Note

Every effort was made to make this module efficient.  Nevertheless,
changing the size of a record in the middle of a large file will
always be slow, because everything after the new record must be move.

In particular, note that:

	# million-line file
	for (@file_array) {
	  $_ .= 'x';
	}

is likely to be very slow, because the first iteration must relocate
lines 1 through 999,999; the second iteration must relocate lines 2
through 999,999, and so on.  The relocation is done using block
writes, however, so it's not as slow as it might be.

A future version of this module will provide some mechanism for
getting better performance in such cases, by deferring the writing
until it can be done all at once.

=head2 Efficiency Note 2

Not every effort was made to make this module as efficient as
possible.  C<FETCHSIZE> should use binary search instead of linear
search.  The cache's LRU queue should be a heap instead of a list.
These defects are probably minor; in any event, they will be fixed in
a later version of the module.

=head2 Efficiency Note 3

The author has supposed that since this module is concerned with file
I/O, almost all normal use of it will be heavily I/O bound, and that
the time to maintain complicated data structures inside the module
will be dominated by the time to actually perform the I/O.  This
suggests, for example, that and LRU read-cache is a good tradeoff,
even if it requires substantial adjustment following a C<splice>
operation.

=head2 Missing Methods

The tied array does not yet support C<push>, C<pop>, C<shift>,
C<unshift>, C<splice>, or size-setting via C<$#array = $n>.  I will
put these in soon.

=head1 AUTHOR

Mark Jason Dominus

To contact the author, send email to: C<mjd-perl-tiefile+@plover.com>

To receive an announcement whenever a new version of this module is
released, send a blank email message to
C<mjd-perl-tiefile-subscribe@plover.com>.

=head1 LICENSE

C<Tie::File> version 0.12 is copyright (C) 2002 Mark Jason Dominus.

This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or (at
your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; it should be in the file C<COPYING>.  If not,
write to the Free Software Foundation, Inc., 59 Temple Place, Suite
330, Boston, MA 02111 USA

For licensing inquiries, contact the author at:

	Mark Jason Dominus
	255 S. Warnock St.
	Philadelphia, PA 19107

=head1 WARRANTY

C<Tie::File> version 0.12 comes with ABSOLUTELY NO WARRANTY.
For details, see the license.

=head1 TODO

C<push>, C<pop>, C<shift>, C<unshift>.

More tests.  (Configuration options, cache flushery.  _twrite shoule
be tested separately, because there are a lot of weird special cases
lurking in there.)

More tests.  (Stuff I didn't think of yet.)

File locking.

Deferred writing. (!!!)

Paragraph mode?

More tests.

Fixed-length mode.

=cut