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def load_data(file):
with open(file) as f:
data = f.readlines()
coords = [line.strip("\n").split(",") for line in data]
# swap i, j because this is easier for my brain at this point
coords = [(int(j), int(i)) for i,j in coords]
return coords
def build_grid(M, N, coords):
grid = []
for i in range(M):
row = []
for j in range(N):
c = "." if (i,j) not in coords else "#"
row.append(c)
grid.append(row)
return grid
def pprint(grid):
grid_str = "\n".join(["".join(l) for l in grid])
print(grid_str)
def pprint2(grid):
new_grid = copy.deepcopy(grid)
for i in range(M):
for j in range(N):
if isinstance(grid[i][j], tuple):
new_grid[i][j] = "O"
grid_str = "\n".join(["".join(l) for l in new_grid])
print(grid_str)
def get_neighbours(pos, grid):
directions = [(0,1), (1,0), (-1,0), (0, -1)]
M = len(grid)
N = len(grid[0])
ns = []
i, j = pos
for dx, dy in directions:
ni, nj = (i+dx, j+dy)
if ni in range(M) and nj in range(N):
if grid[ni][nj] != "#":
ns.append((ni, nj))
return ns
import copy
def bfs(grid):
parents = copy.deepcopy(grid)
start = (0, 0)
q = []
q.append(start)
visited = set()
count = 0
while len(q) > 0:
# # Visualize grid filling up
# # So much fun!
# if count % 5 == 0:
# print()
# pprint2(parents)
# print()
pos = q.pop(0)
if pos in visited:
continue
ns = get_neighbours(pos, grid)
for n in ns:
if n not in visited:
q.append(n)
ni, nj = n
parents[ni][nj] = (pos)
visited.add(pos)
# print(len(visited))
count += 1
return parents
# M, N = 7, 7
# n_bytes = 12
# file = "test.txt"
M, N = 71, 71
n_bytes = 1024
file = "input.txt"
coords = load_data(file)
grid = build_grid(M, N, coords[:n_bytes])
# Run bfs, collect parents info
parents = bfs(grid)
shortest_grid = copy.deepcopy(grid)
shortest_path = []
next_ = (M-1,N-1)
while next_ != (0, 0):
shortest_path.append(next_)
i, j = next_
shortest_grid[i][j] = "O"
next_ = parents[i][j]
print(len(shortest_path))
# Visualize shortest path
# pprint(shortest_grid)
## Part 2
def is_dead_end(coords, n_bytes, M, N):
grid = build_grid(M, N, coords[:n_bytes])
# Run bfs, collect parents info
parents = bfs(grid)
return parents[M-1][N-1] == "."
def euler(coords, n_bytes):
"""Returns coord of first cause of dead end, use mid-point to swap out"""
mid = len(n_bytes) // 2
left = n_bytes[:mid]
right = n_bytes[mid:]
if len(n_bytes) == 1:
return n_bytes[0] - 1 # Off by one because the last one left is still a dead end
if is_dead_end(coords, left[-1], M, N):
return euler(coords, left)
else:
return euler(coords, right)
M, N = 71, 71
n_bytes = 1024
file = "input.txt"
coords = load_data(file)
grid = build_grid(M, N, coords[:n_bytes])
n_bytes = list(range(len(coords)))
max_n_bytes = euler(coords, n_bytes)
i, j = coords[max_n_bytes]
print(f"{j},{i}") # Reverse it because we read coordinates in reverse at loading time
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