Add partially complete 2021-15 solution in Python

Signed-off-by: AKU <tom@tdpain.net>

challenges/2021/15-chiton/README.md

@@ -0,0 +1,2 @@
+# [Day 15: Chiton](https://adventofcode.com/2021/day/15)
+

challenges/2021/15-chiton/info.json

@@ -0,0 +1,17 @@
+{
+    "inputFile": "input.txt",
+    "testCases": {
+        "one": [
+            {
+                "input": "1163751742\n1381373672\n2136511328\n3694931569\n7463417111\n1319128137\n1359912421\n3125421639\n1293138521\n2311944581\n",
+                "expected": "40"
+            }
+        ],
+        "two": [
+            {
+                "input": "1163751742\n1381373672\n2136511328\n3694931569\n7463417111\n1319128137\n1359912421\n3125421639\n1293138521\n2311944581\n",
+                "expected": "315"
+            }
+        ]
+    }
+}

challenges/2021/15-chiton/py/__init__.py

@@ -0,0 +1,83 @@
+import heapq
+import queue
+from typing import Any, Tuple, List, Dict, Optional
+from aocpy import BaseChallenge
+from dataclasses import dataclass
+
+Point = Tuple[int, int]
+Nodes = Dict[Point, int]
+
+
+@dataclass
+class Graph:
+    nodes: Nodes
+    edges: Dict[Point, List[Point]]
+
+
+def get_adjacent_points(point: Point) -> List[Point]:
+    x, y = point
+    return [
+        (x, y - 1),
+        (x - 1, y),
+        (x + 1, y),
+        (x, y + 1),
+    ]
+
+
+def parse(instr: str) -> Graph:
+    nodes = {}
+    edges = {}
+
+    for y, line in enumerate(instr.strip().splitlines()):
+        for x, char in enumerate(line):
+            nodes[(x, y)] = int(char)
+
+    for node in nodes:
+        edges[node] = edges.get(node, []) + [
+            p for p in get_adjacent_points(node) if p in nodes
+        ]
+
+    return Graph(nodes, edges)
+
+
+def get_shortest_path_len(graph: Graph) -> int:
+    # ty https://old.reddit.com/r/adventofcode/comments/rgqzt5/2021_day_15_solutions/hooca55/
+
+    start = (0, 0)
+    end = (
+        max(x for x, _ in graph.nodes),
+        max(y for _, y in graph.nodes),
+    )
+
+    dist: Dict[Point, int] = {start: 0}
+    pq: List[Tuple[int, Point]] = []
+
+    for v in graph.nodes:
+        if v != start:
+            dist[v] = int(1e9)
+        heapq.heappush(pq, (dist[v], v))
+
+    while pq:
+        _, u = heapq.heappop(pq)
+        if u == end:
+            return dist[end]
+        for v in graph.edges[u]:
+            if v not in graph.nodes:
+                continue
+            alt = dist[u] + graph.nodes[v]
+            if alt < dist[v]:
+                dist[v] = alt
+                heapq.heappush(pq, (alt, v))
+
+    return dist[end]
+
+
+class Challenge(BaseChallenge):
+    @staticmethod
+    def one(instr: str) -> int:
+        g = parse(instr)
+        return get_shortest_path_len(g)
+
+    @staticmethod
+    def two(instr: str) -> Any:
+        raise NotImplementedError

challenges/2021/README.md

@@ -26,6 +26,7 @@ Solutions to the [2021 Advent of Code](https://adventofcode.com/2021).
 | 12 - Passage Pathing                | ★ ★                | [Go](12-passagePathing/go)                                                                              | I couldn't tell you how it works, but it does kinda work and I think I have a vague idea (external help was used).                  |
 | 13 - Transparent Origami            | ★ ★                | [Python](13-transparentOrigami/py), [Nim](13-transparentOrigami/nim)                                    | I got stuck for hours on an intermittent off-by-one error. :(                                                                       |
 | 14 - Extended Polymerization        | ★ ★                | [Python](14-extendedPolymerization/py)                                                                  | Another off-by-one error, but this time it was because of dodgy division. Wonderful.                                                |
+| 15 - Chiton                         | ★ ☆                | [Python](15-chiton/py)                                                                                  | Pathfinding is hard                                                                                                                 |
 
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