def load(filename):
	with open(filename) as f:
		for l in f:
			s = l.rstrip()
			n = len(s)
			offset = int(n/2)
			c1 = s[0:offset]
			c2 = s[offset:]
			yield c1, c2


def overlap(c1, c2):
	return list(set(c1).intersection(set(c2)))[0]

def priority(c):
	n = ord(c)

	if ord("a") <= n <= ord("z"):
		return n - ord("a") + 1
	if ord("A") <= n <= ord("Z"):
		return n - ord("A") + 27

	raise Exception()

def calcsum(l):
	l = [overlap(*x) for x in l]
	t = sum([priority(*x) for x in l])
	return t


l = list(load("demo.txt"))
assert "".join(l[0]) == "vJrwpWtwJgWrhcsFMMfFFhFp"
assert l[0][0] == "vJrwpWtwJgWr"
assert l[0][1] == "hcsFMMfFFhFp"
assert overlap(*l[0]) == "p"

assert l[1] == ("jqHRNqRjqzjGDLGL", "rsFMfFZSrLrFZsSL")
assert overlap(*l[1]) == "L"

assert l[2] == ("PmmdzqPrV", "vPwwTWBwg")
assert overlap(*l[2]) == "P"

assert overlap(*l[3]) == "v"
assert overlap(*l[4]) == "t"
assert overlap(*l[5]) == "s"

assert priority("a") == 1
assert priority("z") == 26
assert priority("A") == 27
assert priority("Z") == 52

assert calcsum(l) == 157

l = load("data.txt")
print("Sum is:", calcsum(l))