day6.py

#!/usr/bin/python

filepath = "input.txt"

def max_bounds(pts):
  max_x = 0
  max_y = 0
  for x, y in pts:
    if x > max_x: max_x = x
    if y > max_y: max_y = y
  return (x,y)

def mk_pairs(lines):
  for idx, l in enumerate(lines):
    xy = l.split(',')
    lines[idx] = (int(xy[0]), int(xy[1]))

def distance(pt0, pt1):
  return abs(pt0[0] - pt1[0]) + abs(pt0[1] - pt1[1])

# closest idx. -1 if multiple closest.
def closest(max_xy, xy, index_points):
  closest_pt = max_xy[0] + max_xy[1]
  closest_idx = -1
  # This loop runs once in python3
  for idx, pt in index_points:
    d = distance(pt, xy)
    if d < closest_pt:
      closest_pt = d
      closest_idx = idx
    elif d == closest_pt:
      closest_idx = -1
  return closest_idx

def map_points(pt_dict, max_xy, points):
  index_points = zip(range(0, len(points)), points)
  for x in range(0, max_xy[0]+1):
    for y in range(0, max_xy[1]+1):
      pt_dict[(x,y)] = closest(max_xy, (x,y), index_points)

# Walk the border, collecting indexes. These are infinite.
def filter_infinite(max_xy, pt_dict):
  filter_set = set()
  for x in range(0, max_xy[0]+1):
    filter_set.add(pt_dict[(x,0)])
  for x in range(0, max_xy[0]+1):
    filter_set.add(pt_dict[(x,max_xy[1])])
  for y in range(0, max_xy[1]+1):
    filter_set.add(pt_dict[(0,y)])
  for x in range(0, max_xy[1]+1):
    filter_set.add(pt_dict[(max_xy[0],y)])
  for pt, idx in pt_dict.items():
    if idx in filter_set:
      pt_dict[pt] = -1

def max_area(pt_dict):
  # From idxs to the number of times they appear
  cnt_dict = {}
  for pt, idx in pt_dict.items():
    if idx not in cnt_dict:
      cnt_dict[idx] = 1
    else:
      cnt_dict[idx] += 1
  max_cnt = 0
  for idx, cnt in cnt_dict.items():
    if cnt > max_cnt and idx != -1:
      max_cnt = cnt
  return max_cnt

def main():
    points = []
    with open(filepath) as fp:
        lines = fp.readlines()
        points = [x for x in lines if x != '\n']
    mk_pairs(points)
    max_xy = max_bounds(points)

    # Map from points in the grid to their closest designated point.
    pt_dict = {}
    map_points(pt_dict, max_xy, points)
    filter_infinite(max_xy, pt_dict)

    res = max_area(pt_dict)
    print(res)

if __name__ == "__main__":
    main()