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package App::FatPacker::Trace;

use strict;
use warnings FATAL => 'all';
use B ();

my $trace_file;
my %initial_inc;

sub import {
  my (undef, $file, @extras) = @_;

  $trace_file = $file || '>>fatpacker.trace';
  # For filtering out our own deps later.
  # (Not strictly required as these are core only and won't have packlists, but 
  # looks neater.)
  %initial_inc = %INC;

  # Use any extra modules specified
  eval "use $_" for @extras;

  B::minus_c;
}

CHECK {
  return unless $trace_file; # not imported

  open my $trace, $trace_file
      or die "Couldn't open $trace_file to trace to: $!";

  for my $inc (keys %INC) {
    next if exists $initial_inc{$inc};
    next unless defined($INC{$inc}) and $INC{$inc} =~ /\Q${inc}\E\Z/;
    print $trace "$inc\n";
  }
}

1;

__END__

=head1 NAME

App::FatPacker::Trace - Tracing module usage using compilation checking

=head1 SYNOPSIS

    # open STDERR for writing
    # will be like: open my $fh, '>', '&STDERR'...
    perl -MApp::FatPacker::Trace=>&STDERR myscript.pl

    # open a file for writing
    # will be like: open my $fh, '>>', 'fatpacker.trace'
    perl -MApp::FatPacker::Trace=>>fatpacker.trace myscript.pl

=head1 DESCRIPTION

This module allows tracing the modules being used by your code. It does that
using clever trickery using the C<import> method, the C<CHECK> block and
L<B>'s C<minus_c> function.

When App::FatPacker::Trace is being used, the import() method will call
C<B::minus_c> in order to set up the global compilation-only flag perl
(the interpreter) has. This will prevent any other code from being run.

Then in the C<CHECK> block which is reached at the end of the compilation
phase (see L<perlmod>), it will gather all modules that have been loaded,
using C<%INC>, and will write it to a file or to STDERR, determined by
parameters sent to the C<import> method.

=head1 METHODS

=head2 import

This method gets run when you just load L<App::FatPacker::Trace>. It will
note the current C<%INC> and will set up the output to be written to, and