path = "day_16.txt"
# path = "test.txt"

with open(path, 'r') as file:
    grid = [[c for c in line.strip()] for line in file]

directions = {
    'U': (-1, 0),
    'D': (1, 0),
    'R': (0, 1),
    'L': (0, -1)
}
reflections = {
    'R': {'/': 'U', '\\': 'D'},
    'L': {'/': 'D', '\\': 'U'},
    'U': {'/': 'R', '\\': 'L'},
    'D': {'/': 'L', '\\': 'R'},
}


def beam_deflect(beam, char):
    if char == '-':
        return beam[2] if beam[2] in 'RL' else 'RL'

    if char == '|':
        return beam[2] if beam[2] in 'UD' else 'UD'

    return reflections[beam[2]][char]

def propagate_beam(initial, grid):
    beams = [initial]
    R = len(grid)
    C = len(grid)

    energized = set()
    seen = set()
    while len(beams) > 0:
        beam = beams.pop()
        i, j = (beam[0] + directions[beam[2]][0], beam[1] + directions[beam[2]][1])

        # Beam got to the end of the grid or is a beam we already saw before
        if i >= R or j >= C or i < 0 or j < 0 or beam in seen:

            continue

        seen.add(beam)
        energized.add((i,j))
        if grid[i][j] == '.':
            beam = (i,j,beam[2])
            beams.append(beam)
            continue

        # If we got here, the beam will be deflected, so we can delete it

        for d in beam_deflect(beam, grid[i][j]):
            beam = (i,j,d)
            beams.append(beam)

    return len(energized)

answer = 0
# from above or below
for j in range(len(grid[0])):
    # From up going down
    answer = max(propagate_beam((-1, j, 'D'), grid), answer)
    # From below going up
    answer = max(propagate_beam((len(grid), j, 'U'), grid), answer)

# from left or right
for i in range(len(grid)):
    # From left going right
    answer = max(propagate_beam((i, -1, 'R'), grid), answer)
    # From right going left
    answer = max(propagate_beam((i, len(grid[0]), 'L'), grid), answer)

print(answer)