2019: Day 16 Part 2

2019/day16/part2.rb

@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+def run(text)
+  numbers = text.strip.split('').map(&:to_i) * 10_000
+  offset = numbers[0, 7].join.to_i
+  arr = numbers[offset..-1]
+
+  100.times do
+    # All coefficients are
+    # * 0: when before output index
+    # * 1: when at or after output index
+    # The pattern is long enough that there are no -1's or later 0's.
+    updated = []
+    total = arr.sum
+    arr.each do |elt|
+      updated << total.abs % 10
+      total -= elt
+    end
+    arr = updated
+  end
+  arr[0, 8].join
+end
+
+input = File.read(File.join(__dir__, 'input'))
+puts run(input)