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hauleth.dev
1<!-- vim:ft=markdown -->
2
3<!-- livebook:{"persist_outputs":true} -->
4
5# Day 21
6
7## Section
8
9```elixir
10input = File.read!("day21.txt")
11
12[[_, p1], [_, p2]] = Regex.scan(~r/: (\d)/, input)
13
14p1 = String.to_integer(p1)
15p2 = String.to_integer(p2)
16
17{p1, p2}
18```
19
20```output
21{3, 5}
22```
23
24```elixir
25round =
26 Stream.unfold(0, &{&1, &1 + 1})
27 |> Stream.scan({{0, p1}, {0, p2}}, fn n, {{s, p}, other} ->
28 p = rem(p - 1 + 6 + n * 9, 10) + 1
29
30 {other, {s + p, p}}
31 end)
32 |> Enum.take_while(fn {{s, _}, {_, _}} -> s < 1000 end)
33
34{rounds, {{loser, _}, _}} = {length(round), List.last(round)}
35
36rounds * 3 * loser
37```
38
39```output
40720750
41```
42
43```elixir
44defmodule Day21.Task2 do
45 def play(p1, p2) do
46 tid = :ets.new(__MODULE__, [])
47 {a, b} = cached_round({p1 - 1, 0}, {p2 - 1, 0}, tid)
48 :ets.delete(tid)
49 max(a, b)
50 end
51
52 defp cached_round(a, b, tid) do
53 case :ets.lookup(tid, {a, b}) do
54 [{_, v}] ->
55 v
56
57 [] ->
58 value = round(a, b, tid)
59
60 :ets.insert(tid, {{a, b}, value})
61
62 value
63 end
64 end
65
66 defp round({_, s}, _, _) when s >= 21, do: {1, 0}
67 defp round(_, {_, s}, _) when s >= 21, do: {0, 1}
68
69 defp round({pos, score}, p2, tid) do
70 for a <- 1..3, b <- 1..3, c <- 1..3, reduce: {0, 0} do
71 {wins1, wins2} ->
72 next = rem(pos + a + b + c, 10)
73
74 {nscore2, nscore1} =
75 cached_round(
76 p2,
77 {next, score + next + 1},
78 tid
79 )
80
81 {wins1 + nscore1, wins2 + nscore2}
82 end
83 end
84end
85```
86
87```output
88{:module, Day21.Task2, <<70, 79, 82, 49, 0, 0, 12, ...>>, {:round, 3}}
89```
90
91```elixir
92Day21.Task2.play(p1, p2)
93```
94
95```output
96275067741811212
97```