Day thirteen

13/main.rb

@@ -0,0 +1,50 @@
+#!/usr/bin/env ruby
+
+require "debug"
+require "matrix"
+
+input = (ARGV.first.nil? ? DATA : ARGF)
+  .read
+  .scan(/(\d+)/m)
+  .each_slice(6)
+  .to_a
+  .map { _1.flatten.map(&:to_i).each_slice(2).to_a }
+  .map { [:a, :b, :prize].zip(_1).to_h }
+
+def solve(input, offset = 0)
+  input.map do |machine|
+    a = Matrix.columns([ # why does rotating matrix work?
+      [machine[:a].first, machine[:a].last],
+      [machine[:b].first, machine[:b].last]
+    ])
+    b = Matrix[
+      [machine[:prize].first + offset],
+      [machine[:prize].last + offset]
+    ]
+
+    (a.inverse * b).to_a.flatten
+  end
+    .select { _1.denominator == 1 && _2.denominator == 1 }
+    .map { 3 * _1 + _2 }
+    .sum
+end
+
+p solve(input).to_i
+p solve(input, 10_000_000_000_000).to_i
+
+__END__
+Button A: X+94, Y+34
+Button B: X+22, Y+67
+Prize: X=8400, Y=5400
+
+Button A: X+26, Y+66
+Button B: X+67, Y+21
+Prize: X=12748, Y=12176
+
+Button A: X+17, Y+86
+Button B: X+84, Y+37
+Prize: X=7870, Y=6450
+
+Button A: X+69, Y+23
+Button B: X+27, Y+71
+Prize: X=18641, Y=10279