2023 day 22.

src/holt59/aoc/2023/day22.py

@@ -1,13 +1,111 @@
+import itertools
+import logging
+import os
+import string
 import sys
 from collections import defaultdict
-from dataclasses import dataclass
+
+VERBOSE = os.getenv("AOC_VERBOSE") == "True"
+logging.basicConfig(level=logging.INFO if VERBOSE else logging.WARNING)
+
 
 lines = sys.stdin.read().splitlines()
 
+
+def _name(i: int) -> str:
+    if len(lines) < 26:
+        return string.ascii_uppercase[i]
+    return f"B{i:04d}"
+
+
+def build_supports(
+    bricks: list[tuple[tuple[int, int, int], tuple[int, int, int]]]
+) -> tuple[dict[int, set[int]], dict[int, set[int]]]:
+    # 1. compute locations where a brick of sand will land after falling by processing
+    # them in sorted order of bottom z location
+    levels: dict[tuple[int, int, int], int] = defaultdict(lambda: -1)
+    for i_brick, ((sx, sy, sz), (ex, ey, ez)) in enumerate(bricks):
+        assert sx <= ex and sy <= ey and sz <= ez
+
+        xs, ys = range(sx, ex + 1), range(sy, ey + 1)
+
+        for z in range(sz - 1, 0, -1):
+            if any(levels[x, y, z] >= 0 for x, y in itertools.product(xs, ys)):
+                break
+            sz, ez = sz - 1, ez - 1
+
+        bricks[i_brick] = ((sx, sy, sz), (ex, ey, ez))
+        zs = range(sz, ez + 1)
+
+        for x, y, z in itertools.product(xs, ys, zs):
+            levels[x, y, z] = i_brick
+
+    # 2. compute the bricks that supports any brick
+    supported_by: dict[int, set[int]] = {}
+    supports: dict[int, set[int]] = {i_brick: set() for i_brick in range(len(bricks))}
+    for i_brick, ((sx, sy, sz), (ex, ey, ez)) in enumerate(bricks):
+        name = _name(i_brick)
+
+        supported_by[i_brick] = {
+            v
+            for x, y in itertools.product(range(sx, ex + 1), range(sy, ey + 1))
+            if (v := levels[x, y, sz - 1]) != -1
+        }
+        logging.info(
+            f"{name} supported by {', '.join(map(_name, supported_by[i_brick]))}"
+        )
+
+        for support in supported_by[i_brick]:
+            supports[support].add(i_brick)
+
+    return supported_by, supports
+
+
+bricks: list[tuple[tuple[int, int, int], tuple[int, int, int]]] = []
+for line in lines:
+    bricks.append(
+        (
+            tuple(int(c) for c in line.split("~")[0].split(",")),  # type: ignore
+            tuple(int(c) for c in line.split("~")[1].split(",")),  # type: ignore
+        )
+    )
+
+# sort bricks by bottom z position to compute supports
+bricks = sorted(bricks, key=lambda b: b[0][-1])
+supported_by, supports = build_supports(bricks)
+
 # part 1
-answer_1 = ...
+answer_1 = len(bricks) - sum(
+    any(len(supported_by[supported]) == 1 for supported in supports_to)
+    for supports_to in supports.values()
+)
 print(f"answer 1 is {answer_1}")
 
 # part 2
-answer_2 = ...
+falling_in_chain: dict[int, set[int]] = {}
+for i_brick in range(len(bricks)):
+    to_disintegrate: set[int] = {
+        supported
+        for supported in supports[i_brick]
+        if len(supported_by[supported]) == 1
+    }
+
+    supported_by_copy = dict(supported_by)
+
+    falling_in_chain[i_brick] = set()
+    while to_disintegrate:
+        falling_in_chain[i_brick].update(to_disintegrate)
+
+        to_disintegrate_v: set[int] = set()
+
+        for d_brick in to_disintegrate:
+            for supported in supports[d_brick]:
+                supported_by_copy[supported] = supported_by_copy[supported] - {d_brick}
+
+                if not supported_by_copy[supported]:
+                    to_disintegrate_v.add(supported)
+
+        to_disintegrate = to_disintegrate_v
+
+answer_2 = sum(len(falling) for falling in falling_in_chain.values())
 print(f"answer 2 is {answer_2}")

src/holt59/aoc/inputs/tests/2023/day22.txt

@@ -0,0 +1,7 @@
+1,0,1~1,2,1
+0,0,2~2,0,2
+0,2,3~2,2,3
+0,0,4~0,2,4
+2,0,5~2,2,5
+0,1,6~2,1,6
+1,1,8~1,1,9