Day 23: Unstable Diffusion

README.md

@@ -28,4 +28,4 @@ Development occurs in language-specific directories:
 |[Day20.hs](hs/src/Day20.hs)|[Day20.kt](kt/src/commonMain/kotlin/com/github/ephemient/aoc2022/Day20.kt)|[day20.py](py/aoc2022/day20.py)|[day20.rs](rs/src/day20.rs)|
 |[Day21.hs](hs/src/Day21.hs)|[Day21.kt](kt/src/commonMain/kotlin/com/github/ephemient/aoc2022/Day21.kt)|[day21.py](py/aoc2022/day21.py)|[day21.rs](rs/src/day21.rs)|
 |[Day22.hs](hs/src/Day22.hs)|[Day22.kt](kt/src/commonMain/kotlin/com/github/ephemient/aoc2022/Day22.kt)|[day22.py](py/aoc2022/day22.py)|[day22.rs](rs/src/day22.rs)|
-|[Day23.hs](hs/src/Day23.hs)|[Day23.kt](kt/src/commonMain/kotlin/com/github/ephemient/aoc2022/Day23.kt)|
+|[Day23.hs](hs/src/Day23.hs)|[Day23.kt](kt/src/commonMain/kotlin/com/github/ephemient/aoc2022/Day23.kt)|[day23.py](py/aoc2022/day23.py)|

py/aoc2022/day23.py

@@ -0,0 +1,107 @@
+"""
+Day 23: Unstable Diffusion
+"""
+
+from collections import defaultdict
+from itertools import cycle, islice
+
+SAMPLE_INPUT = [
+    "....#..",
+    "..###.#",
+    "#...#.#",
+    ".#...##",
+    "#.###..",
+    "##.#.##",
+    ".#..#..",
+]
+
+
+def _directions():
+    return cycle(("NSWE", "SWEN", "WENS", "ENSW"))
+
+
+def _sides(direction, x, y):
+    if direction == "N":
+        return [(x, y - 1) for x in range(x - 1, x + 2)]
+    if direction == "S":
+        return [(x, y + 1) for x in range(x - 1, x + 2)]
+    if direction == "W":
+        return [(x - 1, y) for y in range(y - 1, y + 2)]
+    if direction == "E":
+        return [(x + 1, y) for y in range(y - 1, y + 2)]
+    raise ValueError()
+
+
+def _move(direction, x, y):
+    if direction == "N":
+        return x, y - 1
+    if direction == "S":
+        return x, y + 1
+    if direction == "W":
+        return x - 1, y
+    if direction == "E":
+        return x + 1, y
+    raise ValueError()
+
+
+def _neighbors(x, y):
+    return [
+        (x2, y2)
+        for x2 in range(x - 1, x + 2)
+        for y2 in range(y - 1, y + 2)
+        if x2 != x or y2 != y
+    ]
+
+
+def _parse(lines):
+    return {
+        (x, y) for y, line in enumerate(lines) for x, c in enumerate(line) if c == "#"
+    }
+
+
+def _step(state, directions):
+    proposals = defaultdict(list)
+    for x, y in state:
+        if not any(neighbor in state for neighbor in _neighbors(x, y)):
+            continue
+        for direction in directions:
+            if not any(neighbor in state for neighbor in _sides(direction, x, y)):
+                proposals[_move(direction, x, y)].append((x, y))
+                break
+    state = set(state)
+    for new, old in proposals.items():
+        if len(old) != 1:
+            continue
+        state.remove(old[0])
+        state.add(new)
+    return state
+
+
+def part1(lines):
+    """
+    >>> part1(SAMPLE_INPUT)
+    110
+    """
+    state = _parse(lines)
+    for directions in islice(_directions(), 10):
+        state = _step(state, directions)
+    min_x, max_x = min(x for x, _ in state), max(x for x, _ in state)
+    min_y, max_y = min(y for _, y in state), max(y for _, y in state)
+    return (max_y - min_y + 1) * (max_x - min_x + 1) - len(state)
+
+
+def part2(lines):
+    """
+    >>> part2(SAMPLE_INPUT)
+    20
+    """
+    state, previous = _parse(lines), None
+    for i, directions in enumerate(_directions()):
+        state = _step(state, directions)
+        if state == previous:
+            return i + 1
+        previous = state
+    raise LookupError()
+
+
+parts = (part1, part2)

py/pyproject.toml

@@ -54,6 +54,7 @@ day19 = "aoc2022.day19:parts"
 day20 = "aoc2022.day20:parts"
 day21 = "aoc2022.day21:parts"
 day22 = "aoc2022.day22:parts"
+day23 = "aoc2022.day23:parts"
 
 [tool.black]
 target_version = ["py311"]