Solution for Day 13

day_13/first.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+input = File.readlines('./input')
+
+$earliest = Integer(input.first)
+$ids = input[1].split(',')
+$ids.delete 'x'
+$ids = $ids.map(&:to_i)
+
+out = $ids.map do |id|
+  b = $earliest / id
+  nearest = b * id
+  nearest < $earliest ? nearest + id : nearest
+end
+
+busid = $ids[out.index(out.min)]
+
+puts busid * (out.min - $earliest)

day_13/input

@@ -0,0 +1,2 @@
+1000186
+17,x,x,x,x,x,x,x,x,x,x,37,x,x,x,x,x,907,x,x,x,x,x,x,x,x,x,x,x,19,x,x,x,x,x,x,x,x,x,x,23,x,x,x,x,x,29,x,653,x,x,x,x,x,x,x,x,x,41,x,x,13

day_13/second.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+input = File.readlines('./input')
+
+$ids = input[1].split(',')
+$ids = $ids.map(&:to_i)
+
+$first = 0
+$period = 1
+
+$ids.each_with_index do |bus_period, id|
+  next if bus_period == 0
+
+  0.step do |i|
+    cand = $first + $period * i
+    if (cand + id) % bus_period == 0
+      $first = cand
+      $period = $period.lcm bus_period
+      break
+    end
+  end
+end
+
+puts $first

day_13/test

@@ -0,0 +1,2 @@
+939
+7,13,x,x,59,x,31,19