From ea329a70e524306e1ce5a7f3953d74dfe31c7973 Mon Sep 17 00:00:00 2001
From: Daniel Lin <ephemient@gmail.com>
Date: Fri, 8 Dec 2023 02:14:07 -0500
Subject: [PATCH] Day 8: Haunted Wasteland

---
 README.md          |   2 +-
 py/aoc2023/day8.py | 103 +++++++++++++++++++++++++++++++++++++++++++++
 py/pyproject.toml  |   1 +
 3 files changed, 105 insertions(+), 1 deletion(-)
 create mode 100644 py/aoc2023/day8.py

diff --git a/README.md b/README.md
index 213cbdf9..b8cd5e3e 100644
--- a/README.md
+++ b/README.md
@@ -12,4 +12,4 @@ Development occurs in language-specific directories:
 |[Day5.hs](hs/src/Day5.hs)|[Day5.kt](kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day5.kt)|[day5.py](py/aoc2023/day5.py)|[day5.rs](rs/src/day5.rs)|
 |[Day6.hs](hs/src/Day6.hs)|[Day6.kt](kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day6.kt)|[day6.py](py/aoc2023/day6.py)|[day6.rs](rs/src/day6.rs)|
 |[Day7.hs](hs/src/Day7.hs)|[Day7.kt](kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day7.kt)|[day7.py](py/aoc2023/day7.py)|[day7.rs](rs/src/day7.rs)|
-|[Day8.hs](hs/src/Day8.hs)|[Day8.kt](kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day8.kt)|||
+|[Day8.hs](hs/src/Day8.hs)|[Day8.kt](kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day8.kt)|[day8.py](py/aoc2023/day8.py)||
diff --git a/py/aoc2023/day8.py b/py/aoc2023/day8.py
new file mode 100644
index 00000000..5a5ab2a6
--- /dev/null
+++ b/py/aoc2023/day8.py
@@ -0,0 +1,103 @@
+"""
+Day 8: Haunted Wasteland
+"""
+
+import re
+from functools import reduce
+from itertools import accumulate, repeat
+from math import lcm
+
+SAMPLE_INPUT_1, SAMPLE_INPUT_2, SAMPLE_INPUT_3 = (
+    """
+RL
+
+AAA = (BBB, CCC)
+BBB = (DDD, EEE)
+CCC = (ZZZ, GGG)
+DDD = (DDD, DDD)
+EEE = (EEE, EEE)
+GGG = (GGG, GGG)
+ZZZ = (ZZZ, ZZZ)
+""",
+    """
+LLR
+
+AAA = (BBB, BBB)
+BBB = (AAA, ZZZ)
+ZZZ = (ZZZ, ZZZ)
+""",
+    """
+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)
+""",
+)
+
+INSTRUCTIONS = re.compile(r"[LR]+")
+NODE = re.compile(r"(\w+) = \((\w+), (\w+)\)")
+
+
+def _parse(data):
+    instructions = INSTRUCTIONS.search(data).group()
+    table = {node: (left, right) for node, left, right in NODE.findall(data)}
+
+    def step(node, instruction):
+        match instruction:
+            case "L":
+                return table[node][0]
+            case "R":
+                return table[node][1]
+
+    return (
+        lambda start: reduce(step, instructions, start),
+        len(instructions),
+        table.keys(),
+    )
+
+
+def part1(data):
+    """
+    >>> part1(SAMPLE_INPUT_1)
+    2
+    >>> part1(SAMPLE_INPUT_2)
+    6
+    """
+    step, n, _ = _parse(data)
+    return n * next(
+        i
+        for i, node in enumerate(
+            accumulate(repeat(()), lambda node, _: step(node), initial="AAA")
+        )
+        if node == "ZZZ"
+    )
+
+
+def part2(data):
+    """
+    >>> part2(SAMPLE_INPUT_3)
+    6
+    """
+    step, n, nodes = _parse(data)
+
+    def find_cycle(start):
+        i, end = next(
+            (i, node)
+            for i, node in enumerate(
+                accumulate(repeat(()), lambda node, _: step(node), initial=start)
+            )
+            if node.endswith("Z")
+        )
+        assert step(start) == step(end)
+        return i
+
+    return n * reduce(lcm, (find_cycle(node) for node in nodes if node.endswith("A")))
+
+
+parts = (part1, part2)
diff --git a/py/pyproject.toml b/py/pyproject.toml
index d99ca4de..d0d8be4e 100644
--- a/py/pyproject.toml
+++ b/py/pyproject.toml
@@ -29,6 +29,7 @@ day4 = "aoc2023.day4:parts"
 day5 = "aoc2023.day5:parts"
 day6 = "aoc2023.day6:parts"
 day7 = "aoc2023.day7:parts"
+day8 = "aoc2023.day8:parts"
 
 [tool.black]
 target_version = ["py312"]