2021.03

.gitignore

@@ -1 +1,2 @@
 node_modules
+.vscode/settings.json

challenges/2021/03.exs

@@ -0,0 +1,47 @@
+Code.compile_file(Path.join([__ENV__.file], ["../../../utils/utils.exs"]))
+
+defmodule Challenge do
+  @moduledoc false
+
+  def first_result(input) do
+    bit_counts =
+      input
+      |> Enum.reduce(
+        %{},
+        fn line, acc ->
+          line
+          |> String.split("", trim: true)
+          |> Enum.with_index()
+          |> Enum.reduce(acc, fn y, acc2 ->
+            idx = elem(y, 1)
+            bit = String.to_integer(elem(y, 0))
+
+            Map.update(
+              acc2,
+              idx,
+              [0, 0],
+              fn counts -> List.update_at(counts, bit, &(&1 + 1)) end
+            )
+          end)
+        end
+      )
+
+    gamma =
+      Enum.map(0..(map_size(bit_counts) - 1), fn idx ->
+        [count_zero, count_one] = get_in(bit_counts, [idx])
+        if count_zero > count_one, do: "0", else: "1"
+      end)
+
+    epsilon =
+      Enum.map(gamma, fn
+        "0" -> "1"
+        "1" -> "0"
+      end)
+
+    int(gamma) * int(epsilon)
+  end
+
+  defp int(bits), do: bits |> Enum.join() |> String.to_integer(2)
+end
+
+Utils.run(&Challenge.first_result/1, __ENV__.file, "Q1")

challenges/2021/03.js

@@ -0,0 +1,27 @@
+// Part 2
+const solvePuzzle = (input) => {
+  input = input.split("\n");
+  return find(input, "O2") * find(input, "CO2");
+};
+
+const find = (input, measure, currIdx = 0) => {
+  const mostFrequentBits = getMostFrequentBits(input);
+
+  input = input.filter((str) =>
+    measure === "O2" ? str[currIdx] === mostFrequentBits[currIdx] : str[currIdx] !== mostFrequentBits[currIdx]
+  );
+
+  return input.length <= 1 ? Number.parseInt(input[0], 2) : find(input, measure, currIdx + 1);
+};
+
+const getMostFrequentBits = (input) => {
+  const frequencies = Array.from({ length: input[0].length }, () => [0, 0]);
+
+  input.forEach((line) =>
+    line.split("").forEach((bit, bitidx) => (frequencies[bitidx][bit] = frequencies[bitidx][bit] + 1))
+  );
+
+  return frequencies.map(([count0, count1]) => (count0 > count1 ? "0" : "1"));
+};
+
+require(__dirname + "/../../utils/test.js").test(__filename, __dirname, solvePuzzle);