From c09fcc61667e921a5ff651fd3127d0f6db578509 Mon Sep 17 00:00:00 2001
From: Sloane Perrault <sloane.perrault@gmail.com>
Date: Wed, 21 Sep 2022 09:19:53 -0400
Subject: [PATCH] solve day 3
---
2021/README.md | 3 +-
2021/lib/2021/3.ex | 89 ++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 91 insertions(+), 1 deletion(-)
create mode 100644 2021/lib/2021/3.ex
diff --git a/2021/README.md b/2021/README.md
index c61a9c9..d662ecb 100644
--- a/2021/README.md
+++ b/2021/README.md
@@ -14,7 +14,7 @@
| S | M | T | W | T | F | S |
| :-: | :-: | :-: | :-: | :-: | :-: | :-: |
-| | | | [1] | [2] | 3 | 4 |
+| | | | [1] | [2] | [3] | 4 |
| 5 | 6 | 7 | 8 | 9 | 10 | 11 |
| 12 | 13 | 14 | 15 | 16 | 17 | 18 |
| 19 | 20 | 21 | 22 | 23 | 24 | 25 |
@@ -23,3 +23,4 @@
[1]: ./lib/2021/1.ex
[2]: ./lib/2021/2.ex
+[3]: ./lib/2021/3.ex
diff --git a/2021/lib/2021/3.ex b/2021/lib/2021/3.ex
new file mode 100644
index 0000000..610abda
--- /dev/null
+++ b/2021/lib/2021/3.ex
@@ -0,0 +1,89 @@
+import AOC
+
+aoc 2021, 3 do
+ def p1 do
+ [first] = Stream.take(input_stream(), 1) |> Enum.to_list()
+
+ bit_length = String.length(first)
+
+ tallies = :atomics.new(bit_length, [])
+
+ Stream.concat([first], input_stream())
+ |> Stream.each(fn value ->
+ for {v, i} <- Enum.with_index(String.split(value, "", trim: true), 1) do
+ :atomics.add(tallies, i, String.to_integer(v))
+ end
+ end)
+ |> Stream.run()
+
+ gamma_str =
+ for b <- 1..bit_length, into: "" do
+ if :atomics.get(tallies, b) > 500 do
+ "1"
+ else
+ "0"
+ end
+ end
+
+ epsilon_str =
+ for b <- String.split(gamma_str, "", trim: true), into: "" do
+ if b == "1", do: "0", else: "1"
+ end
+
+ gamma = String.to_integer(gamma_str, 2)
+ epsilon = String.to_integer(epsilon_str, 2)
+ gamma * epsilon
+ end
+
+ def p2 do
+ input_list =
+ input_stream()
+ |> Enum.map(fn word ->
+ String.split(word, "", trim: true) |> Enum.map(&String.to_integer/1)
+ end)
+
+ word_size = input_list |> hd() |> length()
+
+ [o2_gen_rating] =
+ Enum.reduce(0..word_size, input_list, fn
+ _bit, [value] ->
+ [value]
+
+ bit_index, list ->
+ total_values = length(list)
+
+ bit_sum =
+ for word <- list, reduce: 0 do
+ acc -> acc + Enum.at(word, bit_index)
+ end
+
+ matching_bit = if bit_sum >= total_values / 2, do: 1, else: 0
+
+ Enum.filter(list, fn word -> Enum.at(word, bit_index) == matching_bit end)
+ end)
+
+ o2_gen_rating = Enum.join(o2_gen_rating) |> String.to_integer(2)
+
+ [co2_scrub_rating] =
+ Enum.reduce(0..word_size, input_list, fn
+ _bit, [value] ->
+ [value]
+
+ bit_index, list ->
+ total_values = length(list)
+
+ bit_sum =
+ for word <- list, reduce: 0 do
+ acc -> acc + Enum.at(word, bit_index)
+ end
+
+ matching_bit = if bit_sum >= total_values / 2, do: 0, else: 1
+
+ Enum.filter(list, fn word -> Enum.at(word, bit_index) == matching_bit end)
+ end)
+
+ co2_scrub_rating = Enum.join(co2_scrub_rating) |> String.to_integer(2)
+
+ o2_gen_rating * co2_scrub_rating
+ end
+end