From c8452594c8b993099e3a263edbed1080aa300d13 Mon Sep 17 00:00:00 2001
From: Jeremy Kaplan <jdkaplan@metagram.net>
Date: Sun, 15 Dec 2019 21:52:41 -0800
Subject: [PATCH] 2019: Day 16 Part 1

---
 2019/day16/part1.rb | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)
 create mode 100644 2019/day16/part1.rb

diff --git a/2019/day16/part1.rb b/2019/day16/part1.rb
new file mode 100644
index 0000000..491a049
--- /dev/null
+++ b/2019/day16/part1.rb
@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+def fft(input)
+  base = [0, 1, 0, -1]
+  input.each_with_index.map do |_n, i|
+    pattern = base.flat_map { |e| [e] * (i + 1) }
+    pattern *= (input.length.fdiv pattern.length).ceil + 1
+    pattern.shift
+    cross(input, pattern[0...input.length])
+  end
+end
+
+def cross(input, pattern)
+  total = input.zip(pattern).sum { |a, b| a * b }
+  total.abs % 10
+end
+
+input = File.read(File.join(__dir__, 'input'))
+numbers = input.strip.split('').map(&:to_i)
+
+100.times do |_i|
+  numbers = fft(numbers)
+end
+puts numbers[0, 8].join