Day 23

23.input

@@ -0,0 +1,75 @@
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23.py

@@ -0,0 +1,84 @@
+from queue import deque
+from itertools import product
+
+lines = [line.strip() for line in open("23.input")]
+
+elves = set()
+
+for y, line in enumerate(lines):
+    for x, c in enumerate(line):
+        if c == "#":
+            elves.add((x, y))
+
+SURROUND = set(product([-1, 0, 1], repeat=2)) - set([(0, 0)])
+DIRS = deque([(0, -1), (0, 1), (-1, 0), (1, 0)])
+
+def add(a, b):
+    return (a[0] + b[0], a[1] + b[1])
+
+def show(elves):
+    xs = [a[0] for a in elves]
+    ys = [a[1] for a in elves]
+
+    for y in range(min(ys), max(ys)+1):
+        for x in range(min(xs), max(xs)+1):
+            if (x, y) in elves:
+                print("#", end="")
+            else:
+                print(".", end="")
+        print("")
+    print("-" * 20)
+    print("Empty space:", (max(ys) + 1 - min(ys)) * (max(xs) + 1 - min(xs)) - len(elves))
+
+for i in range(1000000):
+    proposals = {}
+
+    for elf in elves:
+        for d in SURROUND:
+            if add(elf, d) in elves:
+                break
+        else:
+            proposals[elf] = [elf]
+            continue
+        for d in DIRS:
+            zero_axis = 0 if d[0] == 0 else 1
+
+            for dz in [-1, 0, 1]:
+                xd = list(d)
+                xd[zero_axis] = dz
+                if add(elf, xd) in elves:
+                    break
+            else:
+                prop = add(d, elf)
+
+                total = proposals.get(prop, [])
+                proposals[prop] = total + [elf]
+                break
+        else:
+            proposals[elf] = [elf]
+            continue
+
+    new_elves = set()
+
+    for pos, candidates in proposals.items():
+        if len(candidates) > 1:
+            for c in candidates:
+                new_elves.add(c)
+        else:
+            new_elves.add(pos)
+
+
+    if i == 9:
+        print("Part 1:")
+        show(new_elves)
+
+    assert len(elves) == len(new_elves)
+
+    if new_elves == elves:
+        print("Part 2:", i + 1)
+        break
+
+
+    head = DIRS.popleft()
+    DIRS.append(head)
+    elves = new_elves