Day 13

1 files changed, 61 insertions, 0 deletions

diff --git a/src/day13.exs b/src/day13.exs
new file mode 100644
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+++ b/src/day13.exs
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+defmodule Day13 do
+  defmodule Game do
+    @derive Inspect
+    defstruct [:a, :b, :prize]
+
+    def from_data(data, extra \\ 0) do
+      Regex.scan(~r/\d+/, data)
+      |> Enum.map(&String.to_integer(hd(&1)))
+      |> Enum.chunk_every(2)
+      |> Enum.map(&List.to_tuple/1)
+      |> then(fn [a, b, prize] ->
+        %Game{
+          a: a,
+          b: b,
+          prize: {elem(prize, 0) + extra, elem(prize, 1) + extra},
+        }
+      end)
+    end
+
+    def cheapest(game) do
+      cheapest(game.a, game.b, game.prize)
+    end
+
+    defp cheapest({a1, a2}, {b1, b2}, {s1, s2}) do
+      c = s2*a1 - a2*s1
+      d = a1*b2 - a2*b1
+      if rem(c, d) == 0 do
+        y = div(c, d)
+        e = s1 - b1 * y
+        f = a1
+        if rem(e, f) == 0 do
+          x = div(e, f)
+          x*3 + y
+        end
+      end
+    end
+  end
+
+  def part1(data, extra \\ 0) do
+    data
+    |> String.split("\n\n", trim: true)
+    |> Enum.map(&Game.from_data(&1, extra))
+    |> Enum.map(&Game.cheapest/1)
+    |> Enum.reject(& &1 == nil)
+    |> Enum.sum()
+  end
+
+  def part2(data) do
+    part1(data, 10000000000000)
+  end
+end
+
+data = IO.read(:stdio, :eof)
+
+{time1 , ans1} = :timer.tc(fn -> Day13.part1(data) end)
+IO.puts("Time  : #{time1 / 1000000}")
+IO.puts("Answer: #{ans1}")
+
+{time2 , ans2} = :timer.tc(fn -> Day13.part2(data) end)
+IO.puts("Time  : #{time2 / 1000000}")
+IO.puts("Answer: #{ans2}")