From 36a4c8a21813c4d2e9c0d54b26ec8ddf6be0ea53 Mon Sep 17 00:00:00 2001
From: Daniel Lin <ephemient@gmail.com>
Date: Fri, 22 Dec 2023 23:45:18 -0500
Subject: [PATCH] Day 22: Sand Slabs

---
 README.md           |  2 +-
 py/aoc2023/day22.py | 88 +++++++++++++++++++++++++++++++++++++++++++++
 py/pyproject.toml   |  1 +
 3 files changed, 90 insertions(+), 1 deletion(-)
 create mode 100644 py/aoc2023/day22.py

diff --git a/README.md b/README.md
index 810fd69d..66e277ad 100644
--- a/README.md
+++ b/README.md
@@ -26,4 +26,4 @@ Development occurs in language-specific directories:
 |[Day19.hs](hs/src/Day19.hs)|[Day19.kt](kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day19.kt)|[day19.py](py/aoc2023/day19.py)|[day19.rs](rs/src/day19.rs)|
 |[Day20.hs](hs/src/Day20.hs)|[Day20.kt](kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day20.kt)|[day20.py](py/aoc2023/day20.py)|[day20.rs](rs/src/day20.rs)|
 |[Day21.hs](hs/src/Day21.hs)|[Day21.kt](kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day21.kt)|[day21.py](py/aoc2023/day21.py)|[day21.rs](rs/src/day21.rs)|
-|[Day22.hs](hs/src/Day22.hs)|[Day22.kt](kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day22.kt)|||
+|[Day22.hs](hs/src/Day22.hs)|[Day22.kt](kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day22.kt)|[day22.py](py/aoc2023/day22.py)||
diff --git a/py/aoc2023/day22.py b/py/aoc2023/day22.py
new file mode 100644
index 00000000..964527e9
--- /dev/null
+++ b/py/aoc2023/day22.py
@@ -0,0 +1,88 @@
+"""
+Day 22: Sand Slabs
+"""
+
+from collections import defaultdict
+
+SAMPLE_INPUT = """
+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
+"""
+
+
+def _parse(data):
+    bricks = [
+        ([int(num) for num in first], [int(num) for num in second])
+        for line in data.splitlines()
+        if len(parts := line.split("~", maxsplit=1)) == 2
+        and len(first := parts[0].split(",", maxsplit=2)) == 3
+        and len(second := parts[1].split(",", maxsplit=2)) == 3
+        and all(num.isdecimal() for num in first + second)
+    ]
+    bricks.sort(key=lambda brick: brick[0][2])
+    heights = defaultdict(int)
+    for first, second in bricks:
+        (x0, y0, z0), (x1, y1, z1) = first, second
+        dz = (
+            max(heights[(x, y)] for x in range(x0, x1 + 1) for y in range(y0, y1 + 1))
+            + 1
+            - z0
+        )
+        first[2], second[2] = z0, z1 = z0 + dz, z1 + dz
+        for x in range(x0, x1 + 1):
+            for y in range(y0, y1 + 1):
+                heights[(x, y)] = z1
+    bricks.sort(key=lambda brick: brick[0][2])
+    rdeps, deps = defaultdict(set), defaultdict(set)
+    for i, ((x0, y0, _), (x1, y1, z1)) in enumerate(bricks):
+        for j, ((x2, y2, z2), (x3, y3, _)) in enumerate(bricks[i + 1 :]):
+            if z2 <= z1:
+                continue
+            if z2 > z1 + 1:
+                break
+            if x0 <= x3 and x2 <= x1 and y0 <= y3 and y2 <= y1:
+                rdeps[i].add(i + j + 1)
+                deps[i + j + 1].add(i)
+            j += 1
+    return len(bricks), rdeps, deps
+
+
+def part1(data):
+    """
+    >>> part1(SAMPLE_INPUT)
+    5
+    """
+    n, _, deps = _parse(data)
+    return n - len({next(iter(below)) for below in deps.values() if len(below) == 1})
+
+
+def part2(data):
+    """
+    >>> part2(SAMPLE_INPUT)
+    7
+    """
+    _, rdeps, deps = _parse(data)
+    acc = 0
+    for item in rdeps.items():
+        _deps = {above: set(belows) for above, belows in deps.items()}
+        stack = [item]
+        while stack:
+            below, aboves = stack.pop()
+            for above in aboves:
+                belows = _deps[above]
+                if below not in belows:
+                    continue
+                belows.remove(below)
+                if belows:
+                    continue
+                acc += 1
+                stack.append((above, rdeps.get(above, ())))
+    return acc
+
+
+parts = (part1, part2)
diff --git a/py/pyproject.toml b/py/pyproject.toml
index 5a68ffee..e0c6321d 100644
--- a/py/pyproject.toml
+++ b/py/pyproject.toml
@@ -43,6 +43,7 @@ day18 = "aoc2023.day18:parts"
 day19 = "aoc2023.day19:parts"
 day20 = "aoc2023.day20:parts"
 day21 = "aoc2023.day21:parts"
+day22 = "aoc2023.day22:parts"
 
 [tool.black]
 target_version = ["py312"]