advent_5_part2.py

from collections import defaultdict

def key(x, y):
  return "{0}-{1}".format(x, y)

def walk(grid, from_x, from_y, to_x, to_y):
  grid[key(from_x, from_y)] += 1
  while from_x != to_x or from_y != to_y:
    if from_x < to_x:
      from_x += 1
    elif from_x > to_x:
      from_x -= 1
    if from_y < to_y:
      from_y += 1
    elif from_y > to_y:
      from_y -= 1
    grid[key(from_x, from_y)] += 1

if __name__ == '__main__':
  grid = defaultdict(lambda: 0)
  with open('input.txt') as data:
    for line in data:
      line = line.strip()
      from_str, to_str = line.split(' -> ')
      from_x, from_y = [int(value) for value in from_str.split(',')]
      to_x, to_y = [int(value) for value in to_str.split(',')]
      walk(grid, from_x, from_y, to_x, to_y)
    count = 0
    for key in grid:
      if grid[key] > 1:
        count += 1
    print(count)