package FileCache;

our $VERSION = 1.04;

=head1 NAME

FileCache - keep more files open than the system permits

=head1 SYNOPSIS

    use FileCache;
    # or
    use FileCache maxopen => 16;

    cacheout $mode, $path;
    # or
    cacheout $path;
    print $path @data;

    $fh = cacheout $mode, $path;
    # or
    $fh = cacheout $path;
    print $fh @data;

=head1 DESCRIPTION

The C<cacheout> function will make sure that there's a filehandle open
for reading or writing available as the pathname you give it. It
automatically closes and re-opens files if you exceed your system's
maximum number of file descriptors, or the suggested maximum I<maxopen>.

=over

=item cacheout EXPR

The 1-argument form of cacheout will open a file for writing (C<< '>' >>)
on it's first use, and appending (C<<< '>>' >>>) thereafter.

Returns EXPR on success for convenience. You may neglect the
return value and manipulate EXPR as the filehandle directly if you prefer.

=item cacheout MODE, EXPR

The 2-argument form of cacheout will use the supplied mode for the initial
and subsequent openings. Most valid modes for 3-argument C<open> are supported
namely; C<< '>' >>, C<< '+>' >>, C<< '<' >>, C<< '<+' >>, C<<< '>>' >>>,
C< '|-' > and C< '-|' >

To pass supplemental arguments to a program opened with C< '|-' > or C< '-|' >
append them to the command string as you would system EXPR.

Returns EXPR on success for convenience. You may neglect the
return value and manipulate EXPR as the filehandle directly if you prefer.

=head1 CAVEATS

While it is permissible to C<close> a FileCache managed file,
do not do so if you are calling C<FileCache::cacheout> from a package other
than which it was imported, or with another module which overrides C<close>.
If you must, use C<FileCache::cacheout_close>.

=head1 BUGS

F<sys/param.h> lies with its C<NOFILE> define on some systems,
so you may have to set I<maxopen> yourself.

=head1 NOTES

FileCache installs localized signal handlers for CHLD (a.k.a. CLD) and PIPE
to handle deceased children from 2-arg C<cacheout> with C<'|-'> or C<'-|'>
I<expediently>. The children would otherwise be reaped eventually, unless you
terminated before repeatedly calling cacheout.

=cut

require 5.006;
use Carp;
use Config;
use strict;
no strict 'refs';
# These are not C<my> for legacy reasons.
# Previous versions requested the user set $cacheout_maxopen by hand.
# Some authors fiddled with %saw to overcome the clobber on initial open.
use vars qw(%saw $cacheout_maxopen);
my %isopen;
my $cacheout_seq = 0;

sub import {
    my ($pkg,%args) = @_;
    $pkg = caller(1);
    *{$pkg.'::cacheout'} = \&cacheout;
    *{$pkg.'::close'}    = \&cacheout_close;

    # Truth is okay here because setting maxopen to 0 would be bad
    return $cacheout_maxopen = $args{maxopen} if $args{maxopen};
    foreach my $param ( '/usr/include/sys/param.h' ){
      if (open($param, '<', $param)) {
	local ($_, $.);
	while (<$param>) {
	  if( /^\s*#\s*define\s+NOFILE\s+(\d+)/ ){
	    $cacheout_maxopen = $1 - 4;
	    close($param);
	    last;
	  }
	}
	close $param;
      }
    }
    $cacheout_maxopen ||= 16;
}

# Open in their package.
sub cacheout_open {
  # Reap our children
  local $SIG{CLD}  ||= 'IGNORE'if $Config{sig_name} =~ /\bCLD\b/;
  local $SIG{CHLD} ||= 'IGNORE'if $Config{sig_name} =~ /\bCHLD\b/;
  local $SIG{PIPE} ||= 'IGNORE'if $Config{sig_name} =~ /\bPIPE\b/;

  return open(*{caller(1) . '::' . $_[1]}, $_[0], $_[1]) && $_[1];
}

# Close in their package.
sub cacheout_close {
  # Short-circuit in case the filehandle disappeared
  my $pkg = caller($_[1]||0);
  fileno(*{$pkg . '::' . $_[0]}) &&
    CORE::close(*{$pkg . '::' . $_[0]});
  delete $isopen{$_[0]};
}

# But only this sub name is visible to them.
sub cacheout {
    my($mode, $file, $class, $ret, $ref, $narg);
    croak "Not enough arguments for cacheout"  unless $narg = scalar @_;
    croak "Too many arguments for cacheout"    if $narg > 2;

    ($mode, $file) = @_;
    ($file, $mode) = ($mode, $file) if $narg == 1;
    croak "Invalid mode for cacheout" if $mode &&
      ( $mode !~ /^\s*(?:>>|\+?>|\+?<|\|\-|)|\-\|\s*$/ );

    # Mode changed?
    if( $isopen{$file} && ($mode||'>') ne $isopen{$file}->[2] ){
      &cacheout_close($file, 1);
    }
    
    if( $isopen{$file}) {
      $ret = $file;
      $isopen{$file}->[0]++;
    }
    else{
      if( scalar keys(%isopen) > $cacheout_maxopen -1 ) {
	my @lru = sort{ $isopen{$a}->[0] <=> $isopen{$b}->[0] } keys(%isopen);
	$cacheout_seq = 0;
	$isopen{$_}->[0] = $cacheout_seq++ for
	  splice(@lru, int($cacheout_maxopen / 3)||$cacheout_maxopen);
	&cacheout_close($_, 1) for @lru;
      }

      unless( $ref ){
	$mode ||= $saw{$file} ? '>>' : ($saw{$file}=1, '>');
      }
      #XXX should we just return the value from cacheout_open, no croak?
      $ret = cacheout_open($mode, $file) or croak("Can't create $file: $!");
      
      $isopen{$file} = [++$cacheout_seq, $mode];
    }
    return $ret;
}
1;