Day 15

lib/day15.ex

@@ -0,0 +1,32 @@
+defmodule Aoc2023.Day15 do
+  use Aoc2023
+
+  def parse(input) do
+    input |> String.split(",")
+  end
+
+  defp hash(str) do
+    for <<char <- str>>, reduce: 0 do
+      acc -> Bitwise.band((acc + char) * 17, 255) 
+    end
+  end
+
+  def part1(input) do
+    input |> Enum.map(&hash/1) |> Enum.sum
+  end
+
+  def part2(input) do
+    for instr <- input, reduce: 0..255 |> Enum.map(&{&1, []}) |> Enum.into(%{}) do
+      boxes ->
+        [_, label, op, lens] = Regex.run(~r/([a-z]+)([-=])(\d*)/, instr)
+        box = hash(label)
+        label = String.to_atom(label)
+        case op do
+          "=" -> boxes |> Map.update!(box, &Keyword.update(&1, label, String.to_integer(lens), fn _ -> String.to_integer(lens) end))
+          "-" -> boxes |> Map.update!(box, &Keyword.delete(&1, label))
+        end
+    end
+    |> Enum.flat_map(fn {box, lenses} -> lenses |> Enum.with_index(1) |> Enum.map(fn {{_, focus}, i} -> focus * (box + 1) * i end) end)
+    |> Enum.sum
+  end
+end