Finish 2016.

2016/d24.py

@@ -0,0 +1,61 @@
+from collections import deque, defaultdict
+from itertools import permutations
+from lib import Grid2D, INF
+
+
+def shortest_path(grid, start, end):
+    seen = set()
+    open = deque([(start, 0)])
+
+    while open:
+        current, steps = open.popleft()
+        if current in seen:
+            continue
+        seen.add(current)
+        if current == end:
+            return steps
+        for nb in grid.neighbors_ort(current):
+            if grid[nb] == "#":
+                continue
+            open.append((nb, steps + 1))
+    return None
+
+
+def solve(data, part_2=False):
+    g = Grid2D(data)
+    # g.print()
+    start = g.find("0")[0]
+    points = g.find_not("#.")
+
+    shortest = defaultdict(defaultdict)
+    for i in range(len(points)):
+        for j in range(i + 1, len(points)):
+            a, b = points[i], points[j]
+            s = shortest_path(g, a, b)
+            assert s is not None
+            shortest[a][b] = s
+            shortest[b][a] = s
+
+    # Brute force all combinations.
+    points.remove(start)
+    dmin = INF
+    for p in permutations(points):
+        d = 0
+        d += shortest[start][p[0]]
+        for i in range(len(p) - 1):
+            d += shortest[p[i]][p[i + 1]]
+        if part_2:
+            d += shortest[p[-1]][start]
+        dmin = min(d, dmin)
+    print(dmin)
+    return
+
+
+def main():
+    data = open(0).read().strip()
+    solve(data)
+    solve(data, True)
+
+
+if __name__ == "__main__":
+    main()