Day 13 Part 1 and WIP Part 2

README.md

@@ -2,7 +2,7 @@
 
 My (attempted) solutions to [Advent of Code 2020](https://adventofcode.com/2020) in Elixir.
 
-<img width="975" alt="image" src="https://user-images.githubusercontent.com/498229/101977622-1a5b1280-3c92-11eb-8dc5-73c0998623e0.png">
+<img width="979" alt="image" src="https://user-images.githubusercontent.com/498229/102020090-57222900-3dba-11eb-9916-4b90c0107dfa.png">
 
 ## Strategy
 

day13/README

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+Day 13 Notes
+
++--------+
+| Part 1 |
++--------+
+
+$ elixir day13part1.exs
+<will add later>
+
+Thoughts:
+
+Pretty easy.
+Filter out the x items, divide the current time by the bus ID, and the remainder is how many
+minutes to wait. Take the minimum.
+
+
++--------+
+| Part 2 |
++--------+
+
+$ elixir day13part2.exs
+*runs forever*
+
+Thoughts:
+
+Another one that's impossible to brute force. Currently stuck.
+Have created a correct algorithm that produces the correct answer for the examples.
+However, it will never complete for the real input.
+
+There must be some other relationship that I'm missing. Maybe something to do with common factors?
+
+Attempted to improve by starting with the multiple of all the ids and searching downwards, but
+that doesn't complete either (I'm not interested in brute-force, so won't let it run for a long
+time).
+
+Wondering if the fact that three of the examples use the same IDs and differ only in offset
+is important.
+
+Going to come back to this later with a clear mind. Not sure I'll get it without a hint.
+Seems like I have a knowledge gap.
+
+
++------------------+
+| Overall Thoughts |
++------------------+
+
+Doesn't really feel like a "coding" problem, because I implemented the algorithm but there's
+obviously another trick. But will judge that once I know the answer.
+
+Working out things from first principles is hard! But I think the thought process is more
+valuable than just getting the right answer. So far I've been a bit competitive due to the
+private leaderboards, but I guess that ends now.

day13/day13part1.exs

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+defmodule Day13Part1 do
+  def run do
+    File.read!("input")
+    |> String.split(["\n", ","], trim: true)
+    |> Enum.reject(&(&1 == "x"))
+    |> Enum.map(&String.to_integer/1)
+    |> earliest_bus()
+    |> (fn {id, time} -> id * time end).()
+    |> IO.puts()
+  end
+
+  def earliest_bus([now | busses]) do
+    busses
+    |> Enum.map(fn bus -> {bus, bus - rem(now, bus)} end)
+    |> Enum.min(fn {_, time1}, {_, time2} -> time1 <= time2 end)
+  end
+end
+
+Day13Part1.run()

day13/day13part2.exs

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+defmodule Day13Part2 do
+  def run do
+    schedule =
+      File.read!("input")
+      |> String.split(["\n", ","], trim: true)
+      |> Enum.drop(1)
+      |> Enum.map(&parse_schedule/1)
+      |> Enum.with_index()
+      |> Enum.reject(&match?({:any, _}, &1))
+      |> IO.inspect()
+
+    {max_id, max_offset} =
+      Enum.max(schedule, fn {id1, _}, {id2, _} -> id1 >= id2 end)
+      |> IO.inspect()
+
+    start_time =
+      Enum.reduce(schedule, 1, fn {id, _}, acc -> id * acc end)
+      |> Kernel.-(max_offset)
+      |> IO.inspect(label: "start time")
+
+    find_sequential_departures(start_time, schedule, max_id)
+    |> IO.inspect()
+  end
+
+  def parse_schedule(id) do
+    case Integer.parse(id) do
+      {id, ""} -> id
+      _ -> :any
+    end
+  end
+
+  def find_sequential_departures(t, busses, period) do
+    if Enum.all?(busses, fn {id, offset} -> departs_at_offset?(t, id, offset) end) do
+      t
+    else
+      find_sequential_departures(t - period, busses, period)
+    end
+  end
+
+  def departs_at_offset?(t, id, offset), do: rem(t + offset, id) == 0
+end
+
+Day13Part2.run()