https://adventofcode.com/2023/day/10 Pick's theorem

day10.py

G = open(0).read().splitlines()
N, M = len(G), len(G[0])
# dest needs to be valid and the dest direction change is predetermined
# (dx, dy, valid_dest, next MS index)
MS = [(1, 0, "|LJS", [0, 1, 3, 2]), (0, 1, "-J7S", [1, 2, 0, 3]),
      (-1, 0, "|7FS", [2, 3, 1, 0]), (0, -1, "-FLS", [3, 0, 2, 1])]
sj, si = max((l.find('S'), i) for i, l in enumerate(G))

best = 0, 0
for d in range(3):  # trying 3 directions is enough
    x, y, l, area = si, sj, 1, 0
    ps = x, y, x, y  # area calculation
    while True:
        dx, dy, v, nxtds = MS[d]
        nx, ny = x+dx, y+dy
        if 0 <= nx < N and 0 <= ny < M and (nxti := v.find(G[nx][ny])) != -1:
            x, y, l, d = nx, ny, l+1, nxtds[nxti]
            x1, y1 = ps[:2]
            area += x1*y-x*y1
            ps = x, y, x1, y1
        else:
            l = 0  # unreachable
            break
        if nx==si and ny==sj: break
    best = max(best, (l, area//2))
ans, A = best
print(ans//2)  # Part 1
print(A-ans//2+1)  # Part 2: Pick's theorem