Solve day 20

20_regex.py

@@ -0,0 +1,49 @@
+#!/usr/bin/env python3
+import sys
+
+def parse(regex):
+    graph = {}
+    branches = []
+    dirs = {'N': (0, -1), 'W': (-1, 0), 'S': (0, 1), 'E': (1, 0)}
+    cur = (0, 0)
+
+    assert regex[0] == '^' and regex[-1] == '$'
+    for c in regex[1:-1]:
+        if c == '(':
+            branches.append(cur)
+        elif c == ')':
+            cur = branches.pop()
+        elif c == '|':
+            cur = branches[-1]
+        else:
+            dx, dy = dirs[c]
+            x, y = cur
+            vertex = x + dx, y + dy
+            graph.setdefault(cur, []).append(vertex)
+            graph.setdefault(vertex, [])
+            cur = vertex
+
+    return graph
+
+def shortest_paths(graph, source):
+    Q = set(graph.keys())
+    inf = 1 << 32
+    dist = {v: inf for v in graph}
+    dist[source] = 0
+
+    while Q:
+        u = min(Q, key=lambda x: dist[x])
+        Q.remove(u)
+        for v in graph[u]:
+            if v in Q:
+                alt = dist[u] + 1
+                if alt < dist[v]:
+                    dist[v] = alt
+
+    return dist
+
+regex = sys.stdin.readline().rstrip()
+graph = parse(regex)
+dist = shortest_paths(graph, (0, 0))
+print(max(dist.values()))
+print(sum(int(d >= 1000) for d in dist.values()))