Add dot file to 2021 day 12.

src/holt59/aoc/2021/day12.py

@@ -46,5 +46,19 @@ class Solver(BaseSolver):
             if b != "end" and a != "start":
                 neighbors[b].append(a)
 
+        if self.files:
+            graph = "graph {\n"
+            for node, neighbors_of in neighbors.items():
+                graph += (
+                    " ".join(
+                        f"{node} -- {neighbor};"
+                        for neighbor in neighbors_of
+                        if node <= neighbor or node == "start" or neighbor == "end"
+                    )
+                    + "\n"
+                )
+            graph += "}\n"
+            self.files.create("graph.dot", graph.encode(), False)
+
         yield len(list(enumerate_paths(neighbors)))
         yield len(list(enumerate_paths(neighbors, 1)))