Day 15 Part 2

2020/day15/day15.ex

@@ -21,7 +21,7 @@ defmodule Day15 do
     {_, idx} =
       rest
       |> Enum.with_index()
-      |> Enum.find({previous, -1}, fn {num, idx} -> num == previous end)
+      |> Enum.find({previous, -1}, fn {num, _idx} -> num == previous end)
 
     idx + 1
   end
@@ -34,12 +34,55 @@ defmodule Day15 do
     end
   end
 
-  def part1 do
+  def part1(limit) do
     read_input()
     |> parse_numbers()
     |> Enum.reverse()
-    |> run(2020)
+    |> run(limit)
+  end
+
+  defp setup(initial) do
+    cache =
+      initial
+      |> Enum.slice(0..-2)
+      |> Enum.with_index()
+      |> Enum.into(%{})
+
+    {cache, Enum.at(initial, -1)}
+  end
+
+  defp next2(cache, said, now) do
+    now - Map.get(cache, said, now)
+  end
+
+  defp run2(cache, said, now, stop) do
+    say = next2(cache, said, now)
+
+    if now + 1 == stop do
+      said
+    else
+      run2(Map.put(cache, said, now), say, now + 1, stop)
+    end
+  end
+
+  def part2(limit) do
+    {cache, said} =
+      read_input()
+      |> parse_numbers()
+      |> setup()
+
+    run2(cache, said, map_size(cache), limit)
+  end
+
+  def debug(limit) do
+    10..limit
+    |> Enum.filter(fn i ->
+      p1 = part1(i)
+      p2 = part2(i)
+      p1 != p2
+    end)
   end
 end
 
-Day15.part1() |> IO.inspect()
+Day15.part1(2020) |> IO.inspect()
+Day15.part2(30_000_000) |> IO.inspect()