121 lines
2.8 KiB
Python
121 lines
2.8 KiB
Python
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from pathlib import Path
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from typing import NamedTuple
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from itertools import cycle
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import math
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class Node(NamedTuple):
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id: str
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left: str
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right: str
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def parse(input: Path) -> tuple[str, dict[str, Node]]:
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with input.open() as f:
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steps = f.readline().strip()
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nodes: dict[str, Node] = {}
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_ = f.readline()
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for line in f:
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node = Node(
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line[0:3],
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line[7:10],
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line[12:15],
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)
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nodes[node.id] = node
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return steps, nodes
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def part2(input: Path) -> int:
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steps, nodes = parse(input)
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start_nodes: list[str] = [
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node for node in nodes.keys() if node.endswith("A")
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]
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periods: dict[str, int] = {}
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for start_node in start_nodes:
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current = start_node
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num_steps = 0
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for step in cycle(steps):
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if current.endswith("Z"):
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print(f"Found a node for {start_node} at {current} in {num_steps}")
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periods[start_node] = num_steps
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# This assumes there is only one possible end and that it begins a
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# cycle after that. Apparently true for this data set.
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break
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num_steps += 1
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if step == "L":
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current = nodes[current].left
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elif step == "R":
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current = nodes[current].right
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else:
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raise ValueError(f"Unknown step {step}")
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return math.lcm(*[p for p in periods.values()])
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def part2_brute(input: Path) -> int:
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steps, nodes = parse(input)
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current: list[str] = [
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node for node in nodes.keys() if node.endswith("A")
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]
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num_steps = 0
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for step in steps:
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if all(node.endswith("Z") for node in current):
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break
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num_steps += 1
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for i, node in enumerate(current):
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# if node.endswith("Z"):
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# print(f"Node {initial[i]} reached {node} in {num_steps-1} steps")
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if step == "L":
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current[i] = nodes[node].left
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elif step == "R":
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current[i] = nodes[node].right
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else:
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raise ValueError(f"Unknown step {step}")
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return num_steps
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def part1(input: Path) -> int:
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steps_s, nodes = parse(input)
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steps = cycle(steps_s)
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current = "AAA"
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num_steps = 0
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for step in steps:
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if current == "ZZZ":
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break
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num_steps += 1
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if step == "L":
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current = nodes[current].left
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elif step == "R":
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current = nodes[current].right
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else:
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raise ValueError(f"Unknown step {step}")
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return num_steps
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if __name__ == "__main__":
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# input = Path("sample.txt")
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input = Path("input.txt")
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result1 = part1(input)
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print("part1", result1)
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result2 = part2(input)
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print("part2", result2)
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