From ce08d39469720115cfc416ece0f5049968bc9d10 Mon Sep 17 00:00:00 2001
From: Daniel Lin <ephemient@gmail.com>
Date: Tue, 26 Dec 2023 17:54:38 -0500
Subject: [PATCH] Day 23: A Long Walk

---
 README.md           |   2 +-
 py/aoc2023/day23.py | 136 ++++++++++++++++++++++++++++++++++++++++++++
 py/pyproject.toml   |   1 +
 3 files changed, 138 insertions(+), 1 deletion(-)
 create mode 100644 py/aoc2023/day23.py

diff --git a/README.md b/README.md
index 2707d8bb..0ef577f7 100644
--- a/README.md
+++ b/README.md
@@ -27,6 +27,6 @@ Development occurs in language-specific directories:
 |[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.py](py/aoc2023/day22.py)|[day22.rs](rs/src/day22.rs)|
-|[Day23.hs](hs/src/Day23.hs)|[Day23.kt](kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day23.kt)||[day23.rs](rs/src/day23.rs)|
+|[Day23.hs](hs/src/Day23.hs)|[Day23.kt](kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day23.kt)|[day23.py](py/aoc2023/day23.py)|[day23.rs](rs/src/day23.rs)|
 |[Day24.hs](hs/src/Day24.hs)|[Day24.kt](kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day24.kt)|[day24.py](py/aoc2023/day24.py)|[day24.rs](rs/src/day24.rs) ½|
 |[Day25.hs](hs/src/Day25.hs)|[Day25.kt](kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day25.kt)|[day25.py](py/aoc2023/day25.py)|[day25.rs](rs/src/day25.rs)|
diff --git a/py/aoc2023/day23.py b/py/aoc2023/day23.py
new file mode 100644
index 00000000..d972401c
--- /dev/null
+++ b/py/aoc2023/day23.py
@@ -0,0 +1,136 @@
+"""
+Day 23: A Long Walk
+"""
+
+SAMPLE_INPUT = """
+#.#####################
+#.......#########...###
+#######.#########.#.###
+###.....#.>.>.###.#.###
+###v#####.#v#.###.#.###
+###.>...#.#.#.....#...#
+###v###.#.#.#########.#
+###...#.#.#.......#...#
+#####.#.#.#######.#.###
+#.....#.#.#.......#...#
+#.#####.#.#.#########v#
+#.#...#...#...###...>.#
+#.#.#v#######v###.###v#
+#...#.>.#...>.>.#.###.#
+#####v#.#.###v#.#.###.#
+#.....#...#...#.#.#...#
+#.#########.###.#.#.###
+#...###...#...#...#.###
+###.###.#.###v#####v###
+#...#...#.#.>.>.#.>.###
+#.###.###.#.###.#.#v###
+#.....###...###...#...#
+#####################.#
+"""
+
+
+# pylint: disable=too-many-locals
+def _parse(data):
+    data = {
+        (y, x): c
+        for y, line in enumerate(data.splitlines())
+        for x, c in enumerate(line)
+        if c in ".<>^v"
+    }
+    start, end = min(data), max(data)
+    gr = {
+        (y, x): {
+            dst: (
+                c.strip(".") in f and d.strip(".") in f,
+                c.strip(".") in b and d.strip(".") in b,
+                1,
+            )
+            for dst, f, b in zip(
+                [(y, x - 1), (y, x + 1), (y - 1, x), (y + 1, x)], "<>^v", "><v^"
+            )
+            if (d := data.get(dst, "#")) in ".<>^v"
+        }
+        for (y, x), c in data.items()
+    }
+    done = False
+    while not done:
+        done = True
+        for key in list(gr):
+            if key in (start, end):
+                continue
+            edges = gr[key]
+            if not edges:
+                del gr[key]
+            elif len(edges) == 1:
+                (key2,) = edges
+                del gr[key]
+                del gr[key2][key]
+            elif len(edges) == 2:
+                (key1, (f01, b01, w01)), (key2, (f02, b02, w02)) = edges.items()
+                f1, b1, w1 = gr[key1][key]
+                f2, b2, w2 = gr[key2][key]
+                del gr[key]
+                del gr[key1][key]
+                del gr[key2][key]
+                gr[key1][key2] = (f1 & f02, b1 & b02, w1 + w02)
+                gr[key2][key1] = (f2 & f01, b2 & b01, w2 + w01)
+            else:
+                continue
+            done = False
+    return (
+        start,
+        end,
+        {
+            key: weights
+            for key, edges in gr.items()
+            if (weights := {dst: w for dst, (f, _, w) in edges.items() if f})
+        },
+    )
+
+
+def part1(data):
+    """
+    >>> part1(SAMPLE_INPUT)
+    94
+    """
+    start, end, gr = _parse(data)
+
+    def go(pos, used, distance, best):
+        if pos == end:
+            return distance if best is None or distance > best else best
+
+        reachable = {pos}
+
+        def dfs(pos):
+            for dst in gr.get(pos, {}):
+                if dst not in used and dst not in reachable:
+                    reachable.add(dst)
+                    dfs(dst)
+
+        dfs(pos)
+        potential = distance + sum(
+            max((w for dst, w in gr.get(dst, {}).items() if dst not in used), default=0)
+            for dst in reachable
+        )
+        if best is not None and potential <= best or end not in reachable:
+            return best
+
+        used = used | {pos}
+        for dst, w in gr[pos].items():
+            if dst not in used:
+                best = go(dst, used, distance + w, best)
+
+        return best
+
+    return go(start, set(), 0, None)
+
+
+def part2(data):
+    """
+    >>> part2(SAMPLE_INPUT)
+    154
+    """
+    return part1(data.translate(data.maketrans("<>^v", "....")))
+
+
+parts = (part1, part2)
diff --git a/py/pyproject.toml b/py/pyproject.toml
index 75bbf5d6..6f459e13 100644
--- a/py/pyproject.toml
+++ b/py/pyproject.toml
@@ -45,6 +45,7 @@ day19 = "aoc2023.day19:parts"
 day20 = "aoc2023.day20:parts"
 day21 = "aoc2023.day21:parts"
 day22 = "aoc2023.day22:parts"
+day23 = "aoc2023.day23:parts"
 day24 = "aoc2023.day24:parts"
 day25 = "aoc2023.day25:parts"