Day 20

20.py

@@ -0,0 +1,35 @@
+#!/usr/bin/python3
+
+"""Solution for day 20 of Advent of Code 2016.
+
+This solution first compresses the blacklist by merging overlapping sections, using functools.reduce and the merge
+function below. For example, [(0, 100), (50, 150)] becomes (0, 150). The blacklist is then inverted to become a
+whitelist, which gives both answers pretty easily.
+"""
+
+import functools
+
+def merge(x, y):
+    if y[0] <= x[-1][1] + 1:
+        # This entry overlaps with the last one, combine them
+        x[-1] = (x[-1][0], max(y[1], x[-1][1]))
+    else:
+        # New, non-overlapping entry, just append it to our list
+        x.append(y)
+    return x
+
+with open('data/20.txt', 'r') as file:
+    ranges = sorted(list(map(lambda x: tuple(map(int, x.strip().split('-'))), file.readlines())))
+    blacklist = functools.reduce(merge, ranges[1:], [ranges[0]])
+
+    whitelist = []
+    last = 0
+    for pair in blacklist:
+        if pair[0] > last:
+            whitelist.append((last, pair[0] - 1))
+        last = pair[1] + 1
+    if last < 4294967295:
+        whitelist.append((last, 4294967295))
+
+    print("Part 1: %s" % whitelist[0][0])
+    print("Part 2: %s" % sum(map(lambda p: 1 + p[1] - p[0], whitelist)))