problem10.py

import sys

def first_pos(start, grid, m, n):
    x, y = start
    if x > 0 and grid[x-1][y] in ("|", "F", "7"):
        return (x-1, y), "N"
    if x < m-1 and grid[x+1][y] in ("|", "J", "L"):
        return (x+1, y), "S"
    if y > 0 and grid[x][y-1] in ("-", "F", "L"):
        return (x, y-1), "W"
    if y < n-1 and grid[x][y+1] in ("-", "7", "J"):
        return (x, y+1), "E"

def next_pos(pos, grid, m, n, prev_dir):
    x, y = pos
    match grid[x][y], prev_dir:
        case "F", "N":
            return (x, y+1), "E"
        case "F", "W":
            return (x+1, y), "S"
        case "|", "N":
            return (x-1, y), "N"
        case "|", "S":
            return (x+1, y), "S"
        case "-", "W":
            return (x, y-1), "W"
        case "-", "E":
            return (x, y+1), "E"
        case "L", "S":
            return (x, y+1), "E"
        case "L", "W":
            return (x-1, y), "N"
        case "7", "E":
            return (x+1, y), "S"
        case "7", "N":
            return (x, y-1), "W"
        case "J", "E":
            return (x-1, y), "N"
        case "J", "S":
            return (x, y-1), "W"

def follow(start, grid, m, n):
    loop = [start]
    pos, direction = first_pos(start, grid, m, n)
    while True:
        print(pos)
        pos, direction = next_pos(pos, grid, m, n, direction)
        if pos == start:
            break
        else:
            loop.append(pos)
    return loop

with open(sys.argv[1]) as fh:
    grid = [line.rstrip() for line in fh]
    m, n = len(grid), len(grid[0])
    for i, r in enumerate(grid):
        if (j := r.find("S")) != -1:
            break
    start = (i, j)
    loop = follow(start, grid, m, n)