Day 13

lib/day13/day13.ex

@@ -0,0 +1,67 @@
+defmodule Day13 do
+  @example """
+  939
+  7,13,x,x,59,x,31,19
+  """
+
+  def part1(example \\ false) do
+    {earliest_time, busses} = parse_input(example)
+
+    bus_id =
+      busses
+      |> Enum.reject(&(&1 == "x"))
+      |> Enum.min_by(fn bus ->
+        bus - rem(earliest_time, bus)
+      end)
+
+    bus_id * (bus_id - rem(earliest_time, bus_id))
+  end
+
+  def part2(example \\ false) do
+    {_, busses} = parse_input(example)
+
+    busses =
+      busses
+      |> Enum.with_index()
+      |> Enum.reject(&(elem(&1, 0) == "x"))
+
+    find_time(0, busses)
+  end
+
+  def parse_input(example) do
+    [l1, l2] =
+      if(example, do: @example, else: File.read!("lib/day13/input.txt"))
+      |> String.trim()
+      |> String.split()
+
+    busses =
+      l2
+      |> String.split(",")
+      |> Enum.map(fn
+        "x" -> "x"
+        s -> String.to_integer(s)
+      end)
+
+    {String.to_integer(l1), busses}
+  end
+
+  def find_time(time, busses) do
+    {valid, rest} =
+      Enum.split_while(busses, fn {id, index} ->
+        rem(time + index, id) == 0
+      end)
+
+    case rest do
+      [] ->
+        time
+
+      _ ->
+        if valid == [] do
+          raise "uh oh"
+        end
+
+        step = Enum.reduce(valid, 1, &(elem(&1, 0) * &2))
+        find_time(time + step, busses)
+    end
+  end
+end

lib/day13/input.txt

@@ -0,0 +1,2 @@
+1002618
+19,x,x,x,x,x,x,x,x,41,x,x,x,37,x,x,x,x,x,367,x,x,x,x,x,x,x,x,x,x,x,x,13,x,x,x,17,x,x,x,x,x,x,x,x,x,x,x,29,x,373,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,23