Day 18

18.py

@@ -0,0 +1,34 @@
+#!/usr/bin/python3
+
+"""Solution for day 18 of Advent of Code 2016.
+
+Each line of the maze is represented as a list of booleans, with True for safe spaces and False for traps.
+
+The four separate trap rules can be simplified to just "left is different to right", which is handled in the line
+method (with some annoying fudging to handle the first and last elements whose parents are 'walls').
+
+The solution is obtained by repeatedly generating lines and summing them (Python booleans are a subclass of ints,
+with True being equal to 1 and False to 0, so a simple call to sum counts the number of safe squares). Only the most
+recent line is kept, everything else is discarded once counted.
+"""
+
+seed = [x == '.' for x in
+        '^^.^..^.....^..^..^^...^^.^....^^^.^.^^....^.^^^...^^^^.^^^^.^..^^^^.^^.^.^.^.^.^^...^^..^^^..^.^^^^']
+
+
+def line(previous):
+    return [(i == 0 or previous[i - 1]) == (i == len(previous) - 1 or previous[i + 1]) for i in range(len(previous))]
+
+
+def calculate(num):
+    last_line = seed
+    safe_spaces = sum(last_line)
+    lines = 1
+    while lines < num:
+        last_line = line(last_line)
+        safe_spaces += sum(last_line)
+        lines += 1
+    return safe_spaces
+
+print('Part one: %s' % calculate(40))
+print('Part two: %s' % calculate(400000))