From 1a7f6304ee325f44f665f0653b9d718c647d3111 Mon Sep 17 00:00:00 2001
From: Kees van Kempen <git@keesvankempen.nl>
Date: Thu, 26 Dec 2024 11:35:43 +0100
Subject: [PATCH] 2024(6): try part 2

* My code is way too slow.
* I should put the obstacle in the path before starting the walk, as it
  could be that the obstacle is ran into earlier.
---
 2024/06/part1.py | 60 +++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 54 insertions(+), 6 deletions(-)

diff --git a/2024/06/part1.py b/2024/06/part1.py
index ac918bf..674b0d0 100644
--- a/2024/06/part1.py
+++ b/2024/06/part1.py
@@ -1,12 +1,16 @@
 import numpy as np
 from enum import Enum
+import copy
 
 
 verbose = False
 
 class Map:
+	"""
+	Handles a map with obstacles and how a guard walks through it.
+	"""
+
 	directions = ["^", ">", "v", "<"]
-	#step_per_direction = [(0, -1), (1, 0), (0, 1), (-1, 0)]
 	step_per_direction = [(-1, 0), (0, 1), (1, 0), (0, -1)]
 
 	class Tiles(Enum):
@@ -37,7 +41,7 @@ class Map:
 		if verbose:
 			print("pos_in_map", pos) 
 		return pos[0] >= 0 and pos[0] < self.map.shape[0] \
-		   and pos[1] >= 0 and pos[0] < self.map.shape[1]
+		   and pos[1] >= 0 and pos[1] < self.map.shape[1]
 
 	def look_ahead(self):
 		next_step = self.next_pos()
@@ -97,9 +101,16 @@ class Map:
 		self.pos = guards[0]
 		self.trace.append(self.pos)
 		self.direction = self.map[self.pos]
-		# TODO: Use enum.
-		#self.map[self.pos] = self.Tiles.FREE
-		self.map[self.pos] = "."
+		self.map[self.pos] = self.Tiles.FREE.value
+
+	class LoopException(Exception):
+		"""
+		Gets thrown when the map results in the guard walking in an
+		infinite loop.
+		"""
+
+		pass
+
 
 	def step(self):
 		"""
@@ -110,6 +121,9 @@ class Map:
 		if verbose:
 			print("step.next_tile", next_tile)
 
+		if self.is_stuck_in_loop():
+			raise self.LoopException()
+
 		if next_tile == self.Tiles.FREE:
 			# TODO: Really take a step! Do not only turn.
 			self.pos = self.next_pos()
@@ -134,14 +148,48 @@ class Map:
 					print(self.map[i][j], end="")
 			print()
 
+	def is_stuck_in_loop(self):
+		"""
+		Returns whether the guard is stuck in loop.
+
+		This only happens when enough steps are taken. Currently, it
+		checks whether the current position is already part of the trace
+		at least four times, so that it has travelled the position at
+		least twice from at least one direction.
+		"""
+
+		return self.trace.count(self.pos) > 4
+
+	def copy(self):
+		if verbose:
+			print(f"Deep copying")
+		return copy.deepcopy(self)
+
+
 if __name__ == "__main__":
 	m = Map()
 	#m.load_map("testinput")
 	m.load_map("input")
 
+	loop_obstacles = []
+
 	while m.step():
 		if verbose:
 			print(m.pos, m.direction)
+
+		# Create a copy
+		forked_map = m.copy()
+		forked_map.map[forked_map.next_pos()] = Map.Tiles.OBSTACLE.value
+		print(f"Forking at {forked_map.next_pos()}... ", end="")
+		try:
+			while forked_map.step():
+				continue
+		except Map.LoopException:
+			print("loop.")
+			loop_obstacles.append(m.next_pos())
+		else:
+			print("no loop")
 		#m.show()
 	#print(len(m.trace), m.trace)
-	print(len(set(m.trace)))
+	#print(len(set(m.trace)))
+	print(len(loop_obstacles))