Solve 2016 day 18.

2016/d18.py

@@ -0,0 +1,32 @@
+from lib import *
+
+row = open(0).read().strip()
+
+def is_trap(s):
+    assert len(s) == 3
+    match s:
+        case "^^.":
+            return "^"
+        case ".^^":
+            return "^"
+        case "^..":
+            return "^"
+        case "..^":
+            return "^"
+        case _:
+            return "."
+
+# 400_000 is still easily bruteforcible. If it was a much larget number, we
+# would have to cache the rows till we get a repeated row. We would then
+# memorize the number of safe tiles from repeated row to repeated row and jump
+# forward by multiples of that amount to reach much higher numbers.
+r = 0
+for _ in range(400_000):
+    r += row.count(".")
+    nrow = is_trap("." + row[:2])
+    for i in range(1, len(row) - 1):
+        nrow += is_trap(row[i-1:i+2])
+    nrow += is_trap(row[-2:] + ".")
+    row = nrow
+
+print(r)

README.md

@@ -29,7 +29,7 @@ written in Python.
 - Day 22: That was bad. Did not know how to choose between dfs/bfs and logic errors.
 - Day 23: 10:00
 - Day 24: 20:00 ugly - recursive solution would be more elegant
-- Day 25: 
+- Day 25: 9:34
 
 # 2016
 
@@ -50,7 +50,9 @@ written in Python.
 - Day 15: Trial and error. Should use CRT instead.
 - Day 16: 14:11
 - Day 17: 45:00 didn't follow instructions... focus!
-- Day 18:
+- Day 18: 9:43
+- Day 19: 
+
 
 # 2017