annotate_ifdef_directives: generate paren-balanced expressions

class Problem(Exception):

    pass

def close_parens_needed(expr):

    """Return the number of left-parentheses needed to make 'expr'

       balanced.

    """

    return expr.count("(") - expr.count(")")

def truncate_expression(expr, new_width):

    """Given a parenthesized C expression in 'expr', try to return a new

       expression that is similar to 'expr', but no more than 'new_width'

       characters long.

       Try to return an expression with balanced parentheses.

    """

    if len(expr) <= new_width:

        # The expression is already short enough.

        return expr

    ellipsis = "..."

    # Start this at the minimum that we might truncate.

    n_to_remove = len(expr) + len(ellipsis) - new_width

    # Try removing characters, one by one, until we get something where

    # re-balancing the parentheses still fits within the limit.

    while n_to_remove < len(expr):

        truncated = expr[:-n_to_remove] + ellipsis

        truncated += ")" * close_parens_needed(truncated)

        if len(truncated) <= new_width:

            return truncated

        n_to_remove += 1

    return ellipsis

def commented_line(fmt, argument, maxwidth=LINE_WIDTH):

    """

    Return fmt%argument, for use as a commented line.  If the line would

    be longer than maxwidth, truncate argument.

    if len(result) <= maxwidth:

        return result

    else:

        # figure out how much we need to truncate by to fit the argument,

        # plus an ellipsis.

        ellipsis = "..."

        result = fmt % (argument + ellipsis)

        overrun = len(result) - maxwidth

        truncated_argument = argument[:-overrun] + ellipsis

        result = fmt % truncated_argument

        # How long can we let the argument be?  Try filling in the

        # format with an empty argument to find out.

        max_arg_width = maxwidth - len(fmt % "")

        result = fmt % truncate_expression(argument, max_arg_width)

        assert len(result) <= maxwidth

        return result