day19.py

class ParseError(Exception):
    pass

def parse_rule(alts):
    def parse(inp):
        for rule in alts:
            s = inp
            for el in rule:
                try:
                    s = rules[el](s)
                except ParseError:
                    break
            else:
                return s
        raise ParseError
    return parse

def parse_str(st):
    def parse(inp):
        if inp and inp[0] == st:
            return inp[1:]
        else:
            raise ParseError
    return parse

rules = {}
teststr = []

for l in open('inputs/day19.txt').read().splitlines():
    if l.startswith("a") or l.startswith("b"):
        teststr.append(l)
        continue
    if not l.strip():
        break
    n, rule = l.split(": ")
    if rule.startswith('"'):
        rules[int(n)] = parse_str(rule.strip('"'))
    else:
        alts = []
        for alt in rule.split(" | "):
            alts.append(ints(alt))
        rules[int(n)] = parse_rule(alts)

def parse_modified(inp):
    i = 0
    s = rules[42](inp)
    while s:
        s2 = s
        try:
            for j in range(i):
                s2 = rules[31](s2)
                if s2 == "":
                    return ""
        except ParseError:
            pass
        s = rules[42](s)
        i += 1
    raise ParseError

PART1 = False
count = 0
for s in teststr:
    try:
        if (rules[0](s) if PART1 else parse_modified(s)) == "":
            count += 1
    except ParseError:
        pass

print(count)