Solution to first part of problem 10 in Python

src/Year_2023/Day10.py

@@ -0,0 +1,186 @@
+# --- Day 10: Pipe Maze ---
+
+# You use the hang glider to ride the hot air from Desert Island all the way up
+# to the floating metal island. This island is surprisingly cold and there
+# definitely aren't any thermals to glide on, so you leave your hang glider
+# behind.
+
+# You wander around for a while, but you don't find any people or animals.
+# However, you do occasionally find signposts labeled "Hot Springs" pointing in
+# a seemingly consistent direction; maybe you can find someone at the hot
+# springs and ask them where the desert-machine parts are made.
+
+# The landscape here is alien; even the flowers and trees are made of metal. As
+# you stop to admire some metal grass, you notice something metallic scurry away
+# in your peripheral vision and jump into a big pipe! It didn't look like any
+# animal you've ever seen; if you want a better look, you'll need to get ahead
+# of it.
+
+# Scanning the area, you discover that the entire field you're standing on is
+# densely packed with pipes; it was hard to tell at first because they're the
+# same metallic silver color as the "ground". You make a quick sketch of all of
+# the surface pipes you can see (your puzzle input).
+
+# The pipes are arranged in a two-dimensional grid of tiles:
+
+#     | is a vertical pipe connecting north and south.
+#     - is a horizontal pipe connecting east and west.
+#     L is a 90-degree bend connecting north and east.
+#     J is a 90-degree bend connecting north and west.
+#     7 is a 90-degree bend connecting south and west.
+#     F is a 90-degree bend connecting south and east.
+#     . is ground; there is no pipe in this tile.
+#     S is the starting position of the animal; there is a pipe on this tile,
+# but your sketch doesn't show what shape the pipe has.
+
+# Based on the acoustics of the animal's scurrying, you're confident the pipe
+# that contains the animal is one large, continuous loop.
+
+# For example, here is a square loop of pipe:
+
+# .....
+# .F-7.
+# .|.|.
+# .L-J.
+# .....
+
+# If the animal had entered this loop in the northwest corner, the sketch would
+# instead look like this:
+
+# .....
+# .S-7.
+# .|.|.
+# .L-J.
+# .....
+
+# In the above diagram, the S tile is still a 90-degree F bend: you can tell
+# because of how the adjacent pipes connect to it.
+
+# Unfortunately, there are also many pipes that aren't connected to the loop!
+# This sketch shows the same loop as above:
+
+# -L|F7
+# 7S-7|
+# L|7||
+# -L-J|
+# L|-JF
+
+# In the above diagram, you can still figure out which pipes form the main loop:
+# they're the ones connected to S, pipes those pipes connect to, pipes those
+# pipes connect to, and so on. Every pipe in the main loop connects to its two
+# neighbors (including S, which will have exactly two pipes connecting to it,
+# and which is assumed to connect back to those two pipes).
+
+# Here is a sketch that contains a slightly more complex main loop:
+
+# ..F7.
+# .FJ|.
+# SJ.L7
+# |F--J
+# LJ...
+
+# Here's the same example sketch with the extra, non-main-loop pipe tiles also
+# shown:
+
+# 7-F7-
+# .FJ|7
+# SJLL7
+# |F--J
+# LJ.LJ
+
+# If you want to get out ahead of the animal, you should find the tile in the
+# loop that is farthest from the starting position. Because the animal is in the
+# pipe, it doesn't make sense to measure this by direct distance. Instead, you
+# need to find the tile that would take the longest number of steps along the
+# loop to reach from the starting point - regardless of which way around the
+# loop the animal went.
+
+# In the first example with the square loop:
+
+# .....
+# .S-7.
+# .|.|.
+# .L-J.
+# .....
+
+# You can count the distance each tile in the loop is from the starting point
+# like this:
+
+# .....
+# .012.
+# .1.3.
+# .234.
+# .....
+
+# In this example, the farthest point from the start is 4 steps away.
+
+# Here's the more complex loop again:
+
+# ..F7.
+# .FJ|.
+# SJ.L7
+# |F--J
+# LJ...
+
+# Here are the distances for each tile on that loop:
+
+# ..45.
+# .236.
+# 01.78
+# 14567
+# 23...
+
+# Find the single giant loop starting at S. How many steps along the loop does
+# it take to get from the starting position to the point farthest from the
+# starting position?
+
+with open("files/P10.txt") as f:
+    p = [row for row in f.read().strip().split("\n")]
+
+
+def bfs(grid, directions, connections):
+    start = [pos for pos, c in grid.items() if c == "S"][0]
+    queue, seen = [(0, start)], set()
+    while queue:
+        count, (x, y) = queue.pop(0)
+        seen.add((x, y))
+        for dx, dy in directions[grid[x, y]]:
+            new_x, new_y = x + dx, y + dy
+            if (new_x, new_y) not in grid or (new_x, new_y) in seen:
+                continue
+            if grid[new_x, new_y] not in connections[dx, dy]:
+                continue
+            queue.append((count + 1, (new_x, new_y)))
+    return count, seen
+
+
+def part1():
+    N, S, W, E = (0, -1), (0, 1), (-1, 0), (1, 0)
+    directions = {
+        "S": [N, S, W, E],
+        "|": [N, S],
+        "-": [E, W],
+        "L": [N, E],
+        "J": [N, W],
+        "7": [S, W],
+        "F": [S, E],
+    }
+    connections = {
+        N: {"|", "7", "F"},
+        S: {"|", "L", "J"},
+        E: {"-", "J", "7"},
+        W: {"-", "L", "F"},
+    }
+    grid = {
+        (x, y): c
+        for y, row in enumerate(p)
+        for x, c in enumerate(row)
+        if c != "."
+    }
+    total, _ = bfs(grid, directions, connections)
+
+    print(f"You need {total} steps")
+
+
+if __name__ == "__main__":
+    part1()