package PM::FixedWidthFile;

use strict;
use warnings;
use autodie;

use Exporter qw{import};
our @EXPORT_OK = qw{populate_file};

use Carp;

sub populate_file {
    my ($fh, $record_length, $field_data) = @_;

    for my $fields (@$field_data) {
        my $line = pack 'A' . $record_length;
        my $offset = 0;
        _add_field(\$line, \$offset, @$_) for @$fields;
        print {$fh} $line, "\n";
    }   

    return;
}

sub _add_field {
    my ($line_ref, $offset_ref, $length, $data, $r_align) = @_;

    if ($length < length $data) {
        croak "Data [$data] too large for field of length [$length]";
    } 

    my @dat_pad = ($data, ' ' x ($length - length $data));
    substr($$line_ref, $$offset_ref, $length)
        = join '' => @dat_pad[$r_align ? (1, 0) : (0, 1)];
    $$offset_ref += $length;

    return;
}   

1;

=head1 NAME

PM::FixedWidthFile - TODO (for Jake): module documentation in POD format

##</code><code>##

use PM::FixedWidthFile qw{populate_file};
...
my $record_length = ...;
my $outfile = ...;
my $file_data = ...;
open my $fh, '>', $outfile;
populate_file($fh, $record_length, $file_data);

##</code><code>##

#!/usr/bin/env perl

use strict;
use warnings;
use autodie;

use PM::FixedWidthFile qw{populate_file};

my $fixed_width_file_base = './pm_fixed_width_file.out_';

my $record_length = 32; # not including line terminator

my @multi_file_data = (
    [
        [ [10, ''], [10, 123], [10, 456] ],
    ],
    [
        [ [10, ''], [10, 123], [10, 456] ],
        [ [10, ''], [10, 123, 1], [10, 456] ],
        [ [10, ''], [10, 123], [10, 456], [2, 78] ],
    ],
    [
        [ [10, ''], [10, 123], [10, 456], [2, 78] ],
        [ [10, ''], [10, 123], [10, 456], [2, 789] ],
    ],
);

for my $i (0 .. $#multi_file_data) {
    my $outfile = $fixed_width_file_base . $i;
    print "Populating: $outfile\n";
    open my $fh, '>', $outfile;
    populate_file($fh, $record_length, $multi_file_data[$i]);
    close $fh;
    system qw{cat -vet}, $outfile;
    unlink $outfile; # my housekeeping
}

##</code><code>##

Populating: ./pm_fixed_width_file.out_0
          123       456         $
Populating: ./pm_fixed_width_file.out_1
          123       456         $
                 123456         $
          123       456       78$
Populating: ./pm_fixed_width_file.out_2
Data [789] too large for field of length [2] at ./pm_fixed_width_file.pl line 32.