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-<!-- vim: set syntax=markdown: -->
-<!-- livebook:{"persist_outputs":true} -->
-
-# Advent of Code 2021, Day 6
-
-## Short problem summary
-
-A school of fish reproduce according to the following rules:
-
-* Every fish has an "internal timer"
-* The timer decrements by one every day
-* If the timer is at 0, the timer is instead reset to 6, 
-  and a new fish with an internal timer of 8 is added to the school
-
-Questions:
-
-1. How many fish are in the school after 80 days?
-2. How many fish are in the school after 256 days?
-
-## Setting up
-
-The initial input is a list of fish, represented by the initial value of their internal timer:
-
-```elixir
-school =
-  with {:ok, data} <- File.read("day-06/input.txt") do
-    data
-    |> String.trim()
-    |> String.split(",")
-    |> Enum.map(&String.to_integer/1)
-  end
-```
-
-```output
-[5, 4, 3, 5, 1, 1, 2, 1, 2, 1, 3, 2, 3, 4, 5, 1, 2, 4, 3, 2, 5, 1, 4, 2, 1, 1, 2, 5, 4, 4, 4, 1, 5,
- 4, 5, 2, 1, 2, 5, 5, 4, 1, 3, 1, 4, 2, 4, 2, 5, 1, ...]
-```
-
-Every fish with the same starting internal timer will reproduce at the same time, 
-as will all of the children of those fish and their children, and so forth,
-so we don't need to track individual fish; we just need to group the fish based on
-their starting internal timer and track those groups throughout the simulation.
-
-```elixir
-starting_state = Enum.frequencies(school)
-```
-
-```output
-%{1 => 88, 2 => 45, 3 => 54, 4 => 52, 5 => 61}
-```
-
-Every time a day passes, the following things happen:
-
-* All the internal timers decrement by 1
-* The group of fish with an internal timer of -1 is reset to 6
-  (added to any existing fish whose timers are already at 6),
-  and an equal-sized group of fish with internal timer 8 is added
-
-```elixir
-defmodule Day06 do
-  def next_day(state) do
-    with one_day_older <- Enum.into(state, %{}, fn {k, v} -> {k - 1, v} end),
-         {n, s} <- Map.pop(one_day_older, -1, 0) do
-      Map.update(s, 6, n, &(&1 + n))
-      |> Map.put(8, n)
-    end
-  end
-end
-
-day1 = Day06.next_day(starting_state)
-```
-
-```output
-%{0 => 88, 1 => 45, 2 => 54, 3 => 52, 4 => 61, 6 => 0, 8 => 0}
-```
-
-After the first day there's not any fish old enough to reproduce yet, but after the second day,
-
-```elixir
-day2 = Day06.next_day(day1)
-```
-
-```output
-%{0 => 45, 1 => 54, 2 => 52, 3 => 61, 5 => 0, 6 => 88, 7 => 0, 8 => 88}
-```
-
-The 88 fish whose timers were at 0 have rolled over to 6 and created 88 more fish with timers at 8.
-
-## Solution
-
-Now we just need to apply the transformation function the necessary number
-of times and sum up the total population in the end:
-
-```elixir
-part1_state =
-  Enum.reduce(
-    Enum.to_list(1..80),
-    starting_state,
-    fn _, acc -> Day06.next_day(acc) end
-  )
-  |> IO.inspect()
-  |> Enum.reduce(0, fn {_, v}, acc -> v + acc end)
-```
-
-```output
-%{
-  0 => 24572,
-  1 => 43660,
-  2 => 30525,
-  3 => 48458,
-  4 => 41318,
-  5 => 47697,
-  6 => 57731,
-  7 => 23218,
-  8 => 33738
-}
-```
-
-```output
-350917
-```
-
-Identically for part 2,
-
-```elixir
-part2_state =
-  Enum.reduce(
-    Enum.to_list(1..256),
-    starting_state,
-    fn _, acc -> Day06.next_day(acc) end
-  )
-  |> IO.inspect()
-  |> Enum.reduce(0, fn {_, v}, acc -> v + acc end)
-```
-
-```output
-%{
-  0 => 139170477178,
-  1 => 162618979933,
-  2 => 169389497028,
-  3 => 188231720546,
-  4 => 207908029672,
-  5 => 217769615201,
-  6 => 252681772250,
-  7 => 117023886952,
-  8 => 138124736869
-}
-```
-
-```output
-1592918715629
-```