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hauleth.dev
1<!-- vim:set ft=markdown: -->
2
3<!-- livebook:{"persist_outputs":true} -->
4
5# Day 3
6
7## Input
8
9```elixir
10stream =
11 File.stream!("day3.txt")
12 |> Enum.map(&String.trim/1)
13 |> Enum.map(&String.to_charlist/1)
14
15defmodule Day3 do
16 def count(list) do
17 Enum.reduce(list, List.duplicate(0, 12), fn input, acc ->
18 for {value, counter} <- Enum.zip(input, acc) do
19 case value do
20 ?1 -> counter + 1
21 ?0 -> counter
22 end
23 end
24 end)
25 end
26end
27```
28
29```output
30{:module, Day3, <<70, 79, 82, 49, 0, 0, 7, ...>>, {:count, 1}}
31```
32
33## Task 1
34
35```elixir
36half = div(length(stream), 2)
37
38{a, b} =
39 stream
40 |> Day3.count()
41 |> Enum.reduce({0, 0}, fn elem, {a, b} ->
42 if elem > half do
43 {a * 2 + 1, b * 2}
44 else
45 {a * 2, b * 2 + 1}
46 end
47 end)
48
49a * b
50```
51
52```output
533847100
54```
55
56## Task 2
57
58```elixir
59defmodule Day3.Task2 do
60 def reduce(list, cb), do: reduce(list, 0, cb)
61
62 defp reduce([elem], _, _), do: elem
63
64 defp reduce(list, at, cb) do
65 counts = Day3.count(list)
66
67 half = div(length(list), 2)
68 count = Enum.at(counts, at)
69
70 bit =
71 cond do
72 count == half and cb.(count + 1, half) -> ?1
73 count != half and cb.(count, half) -> ?1
74 true -> ?0
75 end
76
77 reduce(Enum.filter(list, &(Enum.at(&1, at) == bit)), at + 1, cb)
78 end
79end
80
81co2 = List.to_integer(Day3.Task2.reduce(stream, &</2), 2)
82o2 = List.to_integer(Day3.Task2.reduce(stream, &>/2), 2)
83
84co2 * o2
85```
86
87```output
884105235
89```