1 files changed, 47 insertions, 0 deletions
diff --git a/day21.py b/day21.py
new file mode 100644
index 0000000..5904366
--- /dev/null
+++ b/day21.py
@@ -0,0 +1,47 @@
+import numpy as np
+
+
+with open('day21.txt') as f:
+    instructions = f.readlines()
+
+mappings = {}
+start = '.#./..#/###'
+
+
+def translate_to_np(s):
+    return np.array([[c == '#' for c in l]
+                     for l in s.split('/')])
+
+for line in instructions:
+    k, v = map(translate_to_np, line.strip().split(' => '))
+    for a in (k, np.fliplr(k)):
+        for r in range(4):
+            mappings[np.rot90(a, r).tobytes()] = v
+
+
+def enhance(grid):
+    size = len(grid)
+    by = 2 if size % 2 == 0 else 3
+    resize = lambda x: x * (by+1) // by
+    new_size = resize(size)
+    solution = np.empty((new_size, new_size), dtype=bool)
+    squares = range(0, size, by)
+    new_squares = range(0, new_size, by+1)
+
+    for i, ni in zip(squares, new_squares):
+        for j, nj in zip(squares, new_squares):
+            square = grid[i:i+by, j:j+by]
+            enhanced = mappings[square.tobytes()]
+            solution[ni:ni+by+1, nj:nj+by+1] = enhanced
+    return solution
+
+def solve(part):
+    grid = translate_to_np(start)
+    iterations = 5 if part == 1 else 18
+    for _ in range(iterations):
+        grid = enhance(grid)
+    return int(grid.sum())
+
+
+print(solve(part=1))
+print(solve(part=2))