adventofcode2022/day17/sol.py

import itertools
rocks = [
    0b00011110,
    0b00001000_00011100_00001000,
    0b00000100_00000100_00011100,
    0b00010000_00010000_00010000_00010000,
    0b00011000_00011000,
]

wall = 0b10000000_10000000_10000000_10000000_10000000

jets = ">>><<><>><<<>><>>><<<>>><<<><<<>><>><<>>"
jets = open("input").read().strip()


def ceil(n):
    if n%8 != 0:
        n += 8 - n%8
    return n

def drop(a, r, jets):
    h = ceil(a.bit_length()) + 8*3
    numjets = 0
    while True:
        # push left or right
        j = next(jets)
        if j == '<':
            x = (r<<1)
            if x & wall == 0:
                if x & (a>>h) == 0:
                    r = x
        elif j == '>':
            x = (r<<7)
            if x & wall == 0:
                x >>= 8
                if x & (a>>h) == 0:
                    r = x
        numjets += 1

        # continue falling?
        if r & (a>>(h-8)) == 0:
            h -= 8
            assert h > 0
        else:
            # stop
            a |= (r<<h)
            return a, numjets

def head(a):
    # technically this isn't sufficient.
    # you could imagine a stack of overhangs like
    #
    #  |#     #|
    #  |####  #|
    #  |#     #|
    #  |#     #|
    #  |#   ###|
    #  |#     #|
    #
    # that a rock might be able to navigate through indefinitely,
    # but our stacks are much messier than that, so no need to worry.
    #
    # we could probably just take the top 10 or so rows and call it good.
    h = ceil(a.bit_length())
    x = 0x80
    while h >= 8:
        h -= 8
        x |= (a>>h)&0xff
        if x == 0xff:
            return a>>h
    return a

def height(a):
    return ceil(a.bit_length())//8


# Part 1
a = 0b11111111
jeti = itertools.cycle(jets)
rocki = itertools.cycle(rocks)
for _ in range(2022):
    r = next(rocki)
    a, _ = drop(a,r,jeti)
print(height(a>>8))


# Part 2
a = 0b11111111
stopped = 0
jeti = itertools.cycle(jets)
rocki = itertools.cycle(rocks)
i = 0
j = 0
seen = dict()

try:
    while True:
        r = next(rocki)
        a, n = drop(a,r,jeti)
        stopped += 1
        i = (i+1) % len(rocks)
        j = (j+n) % len(jets)
        if (i,j,head(a)) in seen:
            break
        seen[(i,j,head(a))] = (a,stopped)
        #for i in reversed(range(0,a.bit_length(),8)):
        #    print("{:08b}".format((a>>i)&0xff))
        #print()
except:
    print(i, j, stopped)
    raise

b, old = seen[(i,j,head(a))]
sdiff = stopped - old
hdiff = height(a) - height(b)

goal = 10**12
rounds, extra = divmod(goal - stopped, sdiff)
for _ in range(extra):
    r = next(rocki)
    a, n = drop(a,r,jeti)
print(rounds*hdiff + height(a)-1)