Day 15 Parts 1 and 2

README.md

@@ -2,7 +2,7 @@
 
 My (attempted) solutions to [Advent of Code 2020](https://adventofcode.com/2020) in Elixir.
 
-<img width="978" alt="image" src="https://user-images.githubusercontent.com/498229/102170372-8c706900-3ed7-11eb-8bdd-3b0f28b95fde.png">
+<img width="978" alt="image" src="https://user-images.githubusercontent.com/498229/102176995-f132c000-3ee5-11eb-9dc5-7eb450c4d68f.png">
 
 ## Strategy
 

day15/README

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+Day 15 Notes
+
++--------+
+| Part 1 |
++--------+
+
+$ elixir day15.exs 2020
+253
+
+Thoughts:
+
+Separate function to set the initial state.
+Store the last two round numbers for each answer.
+If it's the first time, store the same round number twice, which results in 0
+when subtracted.
+
+
++--------+
+| Part 2 |
++--------+
+
+$ elixir day15.exs 30000000
+13710
+
+Thoughts:
+
+I'm not sure if this was supposed to be brute-forcible.
+
+I think it is and that I designed the algorithm efficiently in the first part.
+The answer is a bit slow, but that's probably due to all the map updates. Using
+a more efficient data structure would probably speed it up - guessing this is
+faster in mutable languages.
+
+Anyway, no code changes here, so for this excercise I decided to combine the
+two answers, and just specify the last round via the command-line.
+
+
++------------------+
+| Overall Thoughts |
++------------------+
+
+Did this one pretty quickly. May even have time to go back to day 13 and learn
+about the Chinese Remainder Theorem...

day15/day15.exs

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+defmodule Day15Part1 do
+  @input [18, 8, 0, 5, 4, 1, 20]
+
+  def run do
+    [stop] = System.argv()
+
+    start(@input, nil, 1, String.to_integer(stop), %{})
+    |> IO.puts()
+  end
+
+  def start([], last, round, stop, state), do: play(last, round, stop, state)
+
+  def start([num | rest], _last, round, stop, state) do
+    start(rest, num, round + 1, stop, Map.put(state, num, {round, round}))
+  end
+
+  def play(num, stop, stop, state) do
+    {a, b} = state[num]
+    b - a
+  end
+
+  def play(num, round, stop, state) do
+    {a, b} = state[num]
+    say = b - a
+    state = Map.update(state, say, {round, round}, fn {_a, b} -> {b, round} end)
+    play(say, round + 1, stop, state)
+  end
+end
+
+Day15Part1.run()