sub truncate_string($$) {
    my ( $string, $max ) = @_;

    # always do nothing if already short enough 
    ( length( $string ) <= $max ) and return $string;

    # issue warning if forced to chop a word anyway
    if ( $string =~ /\s/ ) {
        warn "cannot truncate string on word boundary";
        return substr( $string, 0, $max );
    }

    # truncate pre-existing trailing whitespace
    $string =~ /^(.*)\s+$/ and return $1;

    # otherwise truncate on word boundary 
    $string =~ s/\S+$// and return $string;
    
    die; # unreachable
}