day 20

20/a.py

@@ -0,0 +1,99 @@
+#!/usr/bin/env python3
+
+
+from collections import defaultdict
+
+
+def cmp2dir(char):
+    # Return (X,Y) translations corresponding to compass directions
+    return {"N": (0, -1),
+            "E": (1, 0),
+            "S": (0, 1),
+            "W": (-1, 0)}[char]
+
+
+def addpts(a, b):
+    return (a[0] + b[0], a[1] + b[1])
+
+
+def parseregex(regex):
+    ptr = 0
+
+    def parseblock():
+        nonlocal ptr
+        # recurses the () sections of a regex
+        # we want to return the length of the longest sub path
+        children = []
+        curchild = []
+        while True:
+            if ptr == len(regex):
+                children.append(curchild)
+                return max(children)
+            char = regex[ptr]
+            ptr += 1
+            if char == "(":
+                curchild.append(parseblock())
+            elif char == "|":
+                children.append(curchild)
+                curchild = []
+            elif char == ")":
+                children.append(curchild)
+                curchild = []
+                return children
+            else:  # Some NESW direction
+                curchild.append(char)
+
+    return parseblock()
+
+
+def parselinks(res):
+
+    links = defaultdict(set)  # mapping of roomA -> (roomB, ...) AND roomB -> (roomA, ...)
+
+    def parseres(sub, coords):
+        nonlocal links
+        for unit in sub:
+            if isinstance(unit, str):
+                # Step into another room
+                # Add link from previous room to this room
+                newroom = addpts(coords, cmp2dir(unit))
+                links[newroom].update([coords])
+                links[coords].update([newroom])
+                coords = newroom
+            else:
+                parseres(unit, coords)
+
+    parseres(res, (0, 0))
+    return links
+
+
+def main():
+    with open("input.txt") as f:
+        regex = f.read().strip()[1:-1]
+
+    # Parse a regex like 'NE(S|WW)' into a structure like [N, E, [[S], [W, W]]]
+    res = parseregex(regex)
+    # Parse the above output into coordinate links
+    # E.g. the initial N creates a link between 0,0 and 0,-1 and vice-versa
+    links = parselinks(res)
+
+    # Explore the links until all rooms have been visited
+    frontier = [(0, 0)]
+    room_distance = dict()
+    dist = 0
+    while frontier:
+        new_frontier = []
+        for room in frontier:
+            if room in room_distance:  # already found room
+                continue
+            room_distance[room] = dist
+            new_frontier.extend(links[room])
+        frontier = new_frontier
+        dist += 1
+
+    print(max(room_distance.values()))
+    print(len([i for i in filter(lambda x: x>=1000, room_distance.values())]))
+
+
+if __name__ == '__main__':
+    main()