# Day11
```elixir
Mix.install([
{:kino_aoc, git: "https://github.com/ljgago/kino_aoc"}
# {:queue, "~> 0.1.0"}
# {:heap, "~> 3.0"}
])
```
## Setup
```elixir
{:ok, data} = KinoAOC.download_puzzle("2024", "11", System.fetch_env!("LB_AOC_SECRET"))
```
### Pebble patterns
* Any number will be eventually converted to number with even num of digits
* even number will be eventually split into single digit numbers or 16192
* single digit numbers or 16192 follow next patterns
* we can use cache to catch this cycles
* digit: "0"
* round 1: ["1"]
* round 2: ["2024"]
* round 3: ["20", "24"]
* round 4: ["2", "0", "2", "4"]
* digit: "1"
* round 1: ["2024"]
* round 2: ["20", "24"]
* round 3: ["2", "0", "2", "4"]
* digit: "2"
* round 1: ["4048"]
* round 2: ["40", "48"]
* round 3: ["4", "0", "4", "8"]
* digit: "3"
* round 1: ["6072"]
* round 2: ["60", "72"]
* round 3: ["6", "0", "7", "2"]
* digit: "4"
* round 1: ["8096"]
* round 2: ["80", "96"]
* round 3: ["8", "0", "9", "6"]
* digit: "5"
* round 1: ["10120"]
* round 2: ["20482880"]
* round 3: ["2048", "2880"]
* round 4: ["20", "48", "28", "80"]
* round 5: ["2", "0", "4", "8", "2", "8", "8", "0"]
* digit: "6"
* round 1: ["12144"]
* round 2: ["24579456"]
* round 3: ["2457", "9456"]
* round 4: ["24", "57", "94", "56"]
* round 5: ["2", "4", "5", "7", "9", "4", "5", "6"]
* digit: "7"
* round 1: ["14168"]
* round 2: ["28676032"]
* round 3: ["2867", "6032"]
* round 4: ["28", "67", "60", "32"]
* round 5: ["2", "8", "6", "7", "6", "0", "3", "2"]
* digit: "8"
* round 1: ["16192"]
* round 2: ["32772608"]
* round 3: ["3277", "2608"]
* round 4: ["32", "77", "26", "8"]
* round 5: ["3", "2", "7", "7", "2", "6", "16192"]
* digit: "9"
* round 1: ["18216"]
* round 2: ["36869184"]
* round 3: ["3686", "9184"]
* round 4: ["36", "86", "91", "84"]
* round 5: ["3", "6", "8", "6", "9", "1", "8", "4"]
* num: "16192"
* round 1: ["32772608"]
* round 2: ["3277", "2608"]
* round 3: ["32", "77", "26", "8"]
* round 4: ["3", "2", "7", "7", "2", "6", "16192"]
## Solve
```elixir
defmodule Day11 do
import Integer, only: [is_even: 1]
# usual %{} for cache will be slow
# moving the map between calls and updating it is expensive
def parse(data) do
data |> String.trim() |> String.split(" ")
end
def solve(data, n) do
data
|> parse()
|> Enum.reduce(0, fn st, sum -> sum + count(st, n) end)
end
def count(_st, 0), do: 1
def count(st, n) do
case :ets.lookup(:cache, {st, n}) do
[{_key, res}] ->
res
[] ->
res = next_stone(st) |> handle_next(n)
:ets.insert(:cache, {{st, n}, res})
res
end
end
def next_stone("0"), do: "1"
def next_stone(st) do
len = String.length(st)
if is_even(len) do
{a, b} = String.split_at(st, div(len, 2))
b = b |> String.to_integer() |> to_string()
{a, b}
else
to_string(String.to_integer(st) * 2024)
end
end
def handle_next({a, b}, n), do: count(a, n-1) + count(b, n-1)
def handle_next(st, n), do: count(st, n-1)
end
tdata = """
125 17
"""
:ets.new(:cache, [:set, :protected, :named_table])
Day11.solve(tdata, 25) |> IO.inspect(label: "t1 test")
Day11.solve(tdata, 75) |> IO.inspect(label: "t2 test")
Day11.solve(data, 25) |> IO.inspect(label: "t1")
Day11.solve(data, 75) |> IO.inspect(label: "t2") # 202_019, 239_321_955_280_205
Day11.solve(tdata, 500) |> IO.inspect(label: "test 500")
:ets.delete(:cache) && :ok
```
