Solve 2019 day 20

2019/20_donutmaze.py

@@ -0,0 +1,72 @@
+#!/usr/bin/env python3
+import sys
+from collections import deque
+from itertools import chain
+
+def read_grid(f):
+    rows = [line.replace('\n', '') for line in f]
+    return list(chain.from_iterable(rows)), len(rows[0])
+
+def find_labels(grid, w):
+    def get(i):
+        return grid[i] if 0 <= i < len(grid) else ' '
+
+    def try_label(i, spacediff, outer):
+        if get(i + spacediff) == '.':
+            labels.setdefault(label, []).append((i, i + spacediff, outer))
+
+    labels = {}
+    h = len(grid) // w
+
+    for i, cell in enumerate(grid):
+        if cell.isalpha():
+            y, x = divmod(i, w)
+
+            label = get(i) + get(i + 1)
+            if label.isalpha():
+                try_label(i, -1, x > w // 2)
+                try_label(i + 1, 1, x < w // 2)
+
+            label = get(i) + get(i + w)
+            if label.isalpha():
+                try_label(i, -w, y > h // 2)
+                try_label(i + w, w, y < h // 2)
+
+    return labels
+
+def find_portals(labels):
+    assert all(len(v) == 2 for v in labels.values())
+    inner = {}
+    outer = {}
+    for (l1, s1, o1), (l2, s2, o2) in labels.values():
+        assert o1 ^ o2
+        (outer if o1 else inner)[l1] = s2
+        (outer if o2 else inner)[l2] = s1
+    return inner, outer
+
+def shortest_path(grid, w, leveldiff):
+    labels = find_labels(grid, w)
+    src = labels.pop('AA')[0][1]
+    dst = labels.pop('ZZ')[0][1]
+    inner, outer = find_portals(labels)
+    work = deque([(src, 0, 0)])
+    visited = set()
+
+    while work:
+        i, dist, level = work.popleft()
+        if i == dst and level == 0:
+            return dist
+        if (i, level) in visited:
+            continue
+        visited.add((i, level))
+        for nb in (i - 1, i + 1, i - w, i + w):
+            if grid[nb] == '.':
+                work.append((nb, dist + 1, level))
+            elif nb in inner:
+                work.append((inner[nb], dist + 1, level + leveldiff))
+            elif nb in outer and level > 0:
+                work.append((outer[nb], dist + 1, level - leveldiff))
+
+grid, w = read_grid(sys.stdin)
+print(shortest_path(grid, w, 0))
+print(shortest_path(grid, w, 1))

2019/input/20

@@ -0,0 +1,121 @@
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+                                             U   J       S         D         C   G                                           
+                                             E   W       I         I         Z   O