def xpath_nth_child_function(self, xpath, function, last=False,
add_name_test=True):
try:
a, b = parse_series(function.arguments)
except ValueError:
raise ExpressionError("Invalid series: '%r'" % function.arguments)
if add_name_test:
xpath.add_name_test()
xpath.add_star_prefix()
if a == 0:
if last:
b = 'last() - %s' % b
return xpath.add_condition('position() = %s' % b)
if last:
# FIXME: I'm not sure if this is right
a = -a
b = -b
if b > 0:
b_neg = str(-b)
else:
b_neg = '+%s' % (-b)
if a != 1:
expr = ['(position() %s) mod %s = 0' % (b_neg, a)]
else:
expr = []
if b >= 0:
expr.append('position() >= %s' % b)
elif b < 0 and last:
expr.append('position() < (last() %s)' % b)
expr = ' and '.join(expr)
if expr:
xpath.add_condition(expr)
return xpath
