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# adapted from day 16
L = {
(y, x): c
for y, l in enumerate(open("input").readlines())
for x, c in enumerate(l.strip())
if c != "#"
}
def oob(y, x, d):
return (y, x) not in L
def next(y, x, d):
if d == 0:
return (y - 1, x, d)
if d == 1:
return (y, x + 1, d)
if d == 2:
return (y + 1, x, d)
if d == 3:
return (y, x - 1, d)
def neighbours(y, x, d):
ch = L[(y, x)]
if ch == "^":
return [next(y, x, 0)]
if ch == ">":
return [next(y, x, 1)]
if ch == "v":
return [next(y, x, 2)]
if ch == "<":
return [next(y, x, 3)]
return [next(y, x, k) for k in range(4) if abs(d - k) != 2]
def dfs(y, x, d):
end = list(L.keys())[-1]
longest = 0
ps = []
st = []
st.append((y, x, d, set(), 1))
while st:
y, x, d, v, p = st.pop()
v.add((y, x))
ns = neighbours(y, x, d)
for n in ns:
if n[:2] not in v and not oob(*n):
np = p + 1
st.append((*n, set(v), np))
if n[:2] == end: # reached end
ps.append(np)
return ps
ps = dfs(0, 1, 2)
mx = max(p for p in ps)
print(mx - 1)
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