from adventlib import *
from collections import defaultdict, deque
import copy
import re

def step(g, xr, yr):
    ng = {}
    moved = False
    for pos, v in g.items():
        # only consider east facing
        if v != '>':
            continue

        # move this guy and do bounds check
        l = pos[0] + 1, pos[1]
        if l[0] >= xr:
            l = 0, l[1]

        # is someone in this slot?
        if g.get(l, '.') in '>v':
            ng[pos] = '>'
        else:
            moved = True
            ng[l] = '>'

    for pos, v in g.items():
        # only consider south facing
        if v != 'v':
            continue

        # move, bounds check in y
        l = pos[0], pos[1] + 1
        if l[1] >= yr:
            l = l[0], 0

        # we need to see if any unmoved vs are below us (since they will move)
        # or see if an east one moved into our spot in the east pass (check ng instead of grid)
        if g.get(l, '.') == 'v' or ng.get(l, '.') == '>':
            ng[pos] = 'v'
        else:
            moved = True
            ng[l] = 'v'

    return moved, ng

def main(f):
    data = lines(f)
    xr = len(data[0])
    yr = len(data)

    grid = gridify(f)
    i = 0
    for _ in range(1000):
        moved, grid = step(grid, xr, yr)
        i += 1
        if not moved:
            print(i)
            break

run_main(main)