https://adventofcode.com/2023/day/20

day20.py

from collections import *
from math import *

O, I = defaultdict(list), defaultdict(list)
FLIP, CONJ = set(), set()
for l in open(0):
    a, bs = l.strip().split(' -> ')
    a = a.strip()
    if a[0]=='%':
        a = a[1:]
        FLIP.add(a)
    if a[0]=='&':
        a = a[1:]
        CONJ.add(a)
    for b in bs.split(', '):
        b = b.strip()
        O[a].append(b)
        I[b].append(a)
O['button'] = ['broadcaster']
print(O, I)

P2 = True
if P2:
    assert 'rx' not in FLIP and 'rx' not in CONJ
    assert len(I['rx'])==1
    assert I['rx'][0] in CONJ
L, H = 0, 0
S = defaultdict(lambda: 'low')
cycles = {i: 0 for i in I[I['rx'][0]]}  # input specific lcm luck
for i in range(10**9 if P2 else 1000):
    if P2 and all(cycles.values()): break
    PS = [('button', 'low')]  # pulses
    while PS:
        if P2 and all(cycles.values()): break
        NPS = []
        for c, p in PS:
            if c not in O: continue
            if c in FLIP and p=='high': continue
            s, outs = S[c], O[c]
            if c in FLIP and p=='low':
                S[c] = s = 'low' if s=='high' else 'high'
            elif c in CONJ:
                S[c] = s = 'low' if all(S[b]=='high' for b in I[c]) else 'high'
            if s=='low': L += len(outs)
            if s=='high': H += len(outs)
            if P2 and c in cycles and s=='high' and cycles[c]==0:
                cycles[c] = i+1  # cannot be 0
                if all(cycles.values()): break
            # for ch in chs:
            #     print(c, f'-{s}->', ch)
            for p in outs:
                NPS += [(p, s)]
        else:
            PS = NPS

if not P2: print(L, H, L*H)
if P2: print(lcm(*cycles.values()))