From b1108e98a41e2c87523d7cd87033206b84334099 Mon Sep 17 00:00:00 2001
From: Adrien Abraham <adrien.abraham.external@stormshield.eu>
Date: Fri, 20 Dec 2019 13:39:21 +0100
Subject: [PATCH] Day 20

---
 2019/day20.py | 109 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 109 insertions(+)
 create mode 100644 2019/day20.py

diff --git a/2019/day20.py b/2019/day20.py
new file mode 100644
index 0000000..4809728
--- /dev/null
+++ b/2019/day20.py
@@ -0,0 +1,109 @@
+from collections import defaultdict
+from string import ascii_uppercase
+
+from grid import Grid
+
+
+def main():
+    with open("day20.txt") as input_file:
+        lines = [l.rstrip() for l in input_file.readlines()]
+    maze = Maze(lines, value_factory=str)
+
+    maze.find_portals()
+    maze.dumb_print()
+    min_steps = maze.count_min_steps(maze.entry, maze.exit)
+    print("Min steps:", min_steps)
+
+
+class Maze(Grid):
+    
+    TILE_PATH = "."
+    TILE_WALL = "#"
+
+    ENTRY_DG = "AA"
+    EXIT_DG = "ZZ"
+
+    def __init__(self, lines, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.entry = None
+        self.exit = None
+        self.portals = {}
+        for y, line in enumerate(lines):
+            for x, value in enumerate(line):
+                self.setv(x, y, value)
+
+    def find_portals(self):
+        portals = defaultdict(list)
+        for x, y, v in list(self.values_gen()):
+            if v in ascii_uppercase:
+                rv = self.getv(x + 1, y)
+                if rv and rv in ascii_uppercase:
+                    rrv = self.getv(x + 2, y)
+                    portal_pos = (x + 2, y) if rrv == Maze.TILE_PATH else (x - 1, y)
+                    portals[v + rv].append(portal_pos)
+                    continue
+                dv = self.getv(x, y + 1)
+                if dv and dv in ascii_uppercase:
+                    ddv = self.getv(x, y + 2)
+                    portal_pos = (x, y + 2) if ddv == Maze.TILE_PATH else (x, y - 1)
+                    portals[v + dv].append(portal_pos)
+        for n, p in portals.items():
+            if n == "AA":
+                self.entry = p[0]
+            elif n == "ZZ":
+                self.exit = p[0]
+            else:
+                self.portals[p[0]] = p[1]
+                self.portals[p[1]] = p[0]
+    
+    def dumb_print(self):
+        for y, row in self.g.items():
+            for x, v in row.items():
+                if (x, y) in self.portals:
+                    v = "░"
+                if (x, y) in (self.entry, self.exit):
+                    v = "▒"
+                print(v, end="")
+            print()
+
+    def bfs(self, start, end):
+        discovered = Grid(value_factory=bool)
+        discovered.setv(start[0], end[1], True)
+        parents = {}
+        q = [start]
+        while q:
+            pos = q.pop(0)
+            if pos == end:
+                return parents
+            nears = self.near_objects(pos)
+            for near_pos, near_tile in nears.items():
+                if near_tile != Maze.TILE_PATH:
+                    continue
+                if discovered.getv(near_pos[0], near_pos[1]):
+                    continue
+                discovered.setv(near_pos[0], near_pos[1], True)
+                parents[near_pos] = pos
+                q.append(near_pos)
+        return None
+    
+    def count_min_steps(self, start, end):
+        parents = self.bfs(start, end)
+        count = 0
+        pos = end
+        while pos != start:
+            pos = parents[pos]
+            count += 1
+            if pos in self.portals:  # One more step to traverse portal.
+                count += 1
+        return count
+    
+    def near_objects(self, p):
+        nears = super().near_objects(p)
+        for near in list(nears):
+            if near in self.portals:
+                nears[self.portals[near]] = Maze.TILE_PATH
+        return nears
+
+
+if __name__ == "__main__":
+    main()