day18.py

def convert(l, d=0):
    for i, e in enumerate(l):
        if isinstance(e, list):
            yield from convert(e, d + 1)
        else:
            yield (e, d)


def reduce(l):
    c = list(l)
    for i, (e, d) in enumerate(l):
        if d == 4 and l[i + 1][1] == 4:
            if i > 0:
                c[i - 1] = (c[i - 1][0] + e, c[i - 1][1])
            if i < len(l) - 2:
                c[i + 2] = (c[i + 2][0] + l[i + 1][0], c[i + 2][1])
            c[i : i + 2] = [(0, d - 1)]
            return c
    for i, (e, d) in enumerate(l):
        if e >= 10:
            c[i : i + 1] = [(e // 2, d + 1), (math.ceil(e / 2), d + 1)]
            return c
    return c


def add(a, b):
    result = [*[(i, j + 1) for i, j in a], *[(i, j + 1) for i, j in b]]
    while True:
        b = reduce(result)
        if b == result:
            return b
        result = b


def unconvert(l):
    result = []
    depth = 0
    q = result
    for (e, d) in l:
        q = result
        for i in range(depth):
            q = q[-1]
        while depth < d:
            q.append([])
            q = q[-1]
            depth += 1
        q.append(e)
        if len(q) >= 2:
            depth -= 1
    return result


def mag(l):
    if isinstance(l, int):
        return l
    if len(l) == 2:
        return 3 * mag(l[0]) + 2 * mag(l[1])
    return sum((mag(e) if isinstance(e, list) else e) for e in l)


tot = convert(eval(LINES[0]))

for l in LINES[1:]:
    l = convert(eval(l))
    tot = add(tot, l)

print(
    mag(
        [
            [
                [[tot[0][0], tot[1][0]], [tot[2][0], tot[3][0]]],
                [[tot[4][0], tot[5][0]], [tot[6][0], tot[7][0]]],
            ],
            [
                [[tot[8][0], tot[9][0]], [tot[10][0], tot[11][0]]],
                [[tot[12][0], tot[13][0]], [tot[14][0], tot[15][0]]],
            ],
        ]
    )
)

most = 0
for a, b in itertools.combinations(LINES, 2):
    res = add(convert(eval(a)), convert(eval(b)))
    if all(x[1] == 3 for x in res):
        best = mag(
            [
                [
                    [[res[0][0], res[1][0]], [res[2][0], res[3][0]]],
                    [[res[4][0], res[5][0]], [res[6][0], res[7][0]]],
                ],
                [
                    [[res[8][0], res[9][0]], [res[10][0], res[11][0]]],
                    [[res[12][0], res[13][0]], [res[14][0], res[15][0]]],
                ],
            ]
        )
        if best > most:
            most = best
    res = add(convert(eval(b)), convert(eval(a)))
    if all(x[1] == 3 for x in res):
        best = mag(
            [
                [
                    [[res[0][0], res[1][0]], [res[2][0], res[3][0]]],
                    [[res[4][0], res[5][0]], [res[6][0], res[7][0]]],
                ],
                [
                    [[res[8][0], res[9][0]], [res[10][0], res[11][0]]],
                    [[res[12][0], res[13][0]], [res[14][0], res[15][0]]],
                ],
            ]
        )
        if best > most:
            most = best

print(most)