2019: Day 4 Part 1

2019/day4/part1.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+input = '' # TODO
+range = Range.new(*input.split('-').map(&:to_i))
+
+def pairwise(arr)
+  arr[0...-1].zip(arr[1..])
+end
+
+count = range.count do |n|
+  # 1. All numbers in this range are six digits
+  # 2. All numbers in this range are in this range
+
+  digits = n.digits.reverse
+
+  # 3. Two adjacent digits are the same
+  double = pairwise(digits).any? { |pair| pair.first == pair.last }
+
+  # 4. Going from left to right, the digits *never decrease*
+  monotonic = pairwise(digits).none? { |pair| pair.last < pair.first }
+
+  double && monotonic
+end
+pp count