solved part 1

day15/solution.rb

@@ -0,0 +1,18 @@
+#!/bin/env ruby
+require 'pry'
+numbers = File.read('./input').split(',')
+numbers.map!(&:to_i)
+
+2020.times do |i|
+    next if i < numbers.size
+    num = numbers[-1]
+    prev_turn = numbers[..-2].reverse.find_index(num)
+    if prev_turn == nil
+        numbers.push(0)
+    else
+        new_num = (i - 1) - (numbers.size - 2 - prev_turn)
+        numbers.push(new_num)
+    end
+end
+
+puts numbers[-1]