feat: add 2023 day 08 part 02

src/adventofcode/year_2023/day_08_2023.py

@@ -1,3 +1,6 @@
+from functools import partial, reduce
+
+import math
 import re
 from collections import deque
 
@@ -9,7 +12,7 @@
 def parse_input(data: list[str]) -> tuple[deque, dict[str, tuple[str, str]]]:
     instructions = deque(data[0])
     paths: dict[str, tuple[str, str]] = {}
-    pattern = re.compile("[A-Z]{3}")
+    pattern = re.compile("[A-Z0-9]{3}")
 
     for line in data[2:]:
         key, left, right = pattern.findall(line)
@@ -36,6 +39,41 @@ def follow_instructions(data: list[str]) -> int:
     return steps
 
 
+def highest_common_multiple(num_a: int, num_b: int) -> int:
+    return num_a * num_b // math.gcd(num_a, num_b)
+
+
+def find_end_as_ghost(starting_position: str, instructions: deque[str], paths: dict[str, tuple[str, str]]) -> int:
+    """
+    Count how many staps it will take to arrive at a location that ends with Z
+    """
+    steps = 0
+    current_location = starting_position
+
+    while not current_location.endswith("Z"):
+        direction = 0 if instructions[0] == "L" else 1
+        current_location = paths[current_location][direction]
+        steps += 1
+        instructions.rotate(-1)
+
+    return steps
+
+
+def follow_instructions_as_a_ghost(data: list[str]) -> int:
+    """
+    The solution for part one will not work here, it will take too much time.
+    Find the steps to the end for all starting position and then use the highest common multiple
+    """
+    instructions, paths = parse_input(data)
+    starting_positions = list(filter(lambda k: k.endswith("A"), paths.keys()))
+    steps_required: list[int] = []
+
+    for position in starting_positions:
+        steps_required.append(find_end_as_ghost(position, instructions, paths))
+
+    return reduce(highest_common_multiple, steps_required)
+
+
 @register_solution(2023, 8, 1)
 def part_one(input_data: list[str]):
     answer = follow_instructions(input_data)
@@ -48,7 +86,7 @@ def part_one(input_data: list[str]):
 
 @register_solution(2023, 8, 2)
 def part_two(input_data: list[str]):
-    answer = ...
+    answer = follow_instructions_as_a_ghost(input_data)
 
     if not answer:
         raise SolutionNotFoundError(2023, 8, 2)

tests/adventofcode/year_2023/test_day_08_2023.py

@@ -22,6 +22,19 @@
     "ZZZ = (ZZZ, ZZZ)",
 ]
 
+test_input_3 = [
+    "LR",
+    "",
+    "11A = (11B, XXX)",
+    "11B = (XXX, 11Z)",
+    "11Z = (11B, XXX)",
+    "22A = (22B, XXX)",
+    "22B = (22C, 22C)",
+    "22C = (22Z, 22Z)",
+    "22Z = (22B, 22B)",
+    "XXX = (XXX, XXX)",
+]
+
 
 def test_parse_input():
     assert parse_input(test_input_2) == (
@@ -44,4 +57,4 @@ def test_part_one():
 
 
 def test_part_two():
-    assert part_two(test_input) == "x"
+    assert part_two(test_input_3) == 6