sub init_input_filenames {
    my ($type) = @_;
    my $dir = ".";
    my %fileNames;

    $type ||= 'tokenized';
    
    $fileNames{plant}               ="$dir/plant.$type";
    $fileNames{plant_hist}          ="$dir/plant.$type.hist";
    $fileNames{plant_titles}        ="$dir/plant.titles";
    $fileNames{tank}                ="$dir/tank.$type";
    $fileNames{tank_hist}           ="$dir/tank.$type.hist";
    $fileNames{tank_titles}         ="$dir/tank.titles";
    $fileNames{perplace}            ="$dir/perplace.$type";
    $fileNames{perplace_hist}       ="$dir/perplace.$type.hist";
    $fileNames{perplace_titles}     ="$dir/perplace.titles";
    $fileNames{common_words}        ="$dir/common_words.$type";

    return \%fileNames;
}