Solve 2020 day 24

2020/24_hexes.py

@@ -0,0 +1,44 @@
+#!/usr/bin/env python3
+import sys
+
+DIFFS = {'e': (1, 0), 'w': (-1, 0), 'se': (0, -1),
+         'sw': (-1, -1), 'ne': (1, 1), 'nw': (0, 1)}
+
+def walk(line):
+    x = y = 0
+    vert = ''
+    for char in line.rstrip():
+        if char in 'sn':
+            vert = char
+        else:
+            dx, dy = DIFFS[vert + char]
+            x += dx
+            y += dy
+            vert = ''
+    return x, y
+
+def flip(coords):
+    tiles = {}
+    for xy in coords:
+        tiles[xy] = not tiles.get(xy, False)
+    return list(xy for xy, black in tiles.items() if black)
+
+def evolve(black, days):
+    black = set(black)
+    for day in range(days):
+        makewhite = set()
+        white = {}
+        for x, y in black:
+            nbs = [(x + dx, y + dy) for dx, dy in DIFFS.values()]
+            black_nbs = sum(nb in black for nb in nbs)
+            if black_nbs == 0 or black_nbs > 2:
+                makewhite.add((x, y))
+            white.update({nb: white.get(nb, 0) + 1
+                          for nb in nbs if nb not in black})
+        black -= makewhite
+        black |= {xy for xy, black_nbs in white.items() if black_nbs == 2}
+    return black
+
+black = flip(map(walk, sys.stdin))
+print(len(black))
+print(len(evolve(black, 100)))

2020/input/24

@@ -0,0 +1,377 @@
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