2023 day 4

lib/2023_day4.ex

@@ -0,0 +1,67 @@
+defmodule Card do
+  defp to_list_numbers(str) do
+    str
+    |> String.split(" ")
+    |> Enum.filter(fn s -> String.length(s) !== 0 end)
+    |> Enum.map(fn u ->
+      {i, _} = Integer.parse(u)
+      i
+    end)
+  end
+
+  defp count_winning_cards(player_card, winning_numbers) do
+    player_card |> Enum.filter(fn n -> Enum.any?(winning_numbers, &(&1 == n)) end) |> Enum.count()
+  end
+
+  defp parse_card("Card " <> rest) do
+    [_, numbers] = rest |> String.split(":")
+    [winning, player] = numbers |> String.split("|")
+
+    [winning |> String.trim() |> to_list_numbers(), player |> String.trim() |> to_list_numbers()]
+  end
+
+  def parse(input) do
+    for card <- input |> String.split("\n"),
+        [winning_numbers, our_numbers] = parse_card(card) do
+      [winning_numbers, our_numbers]
+    end
+  end
+
+  def part1(cards) do
+    for [winning_numbers, our_numbers] <- cards do
+      case count_winning_cards(our_numbers, winning_numbers) do
+        n when n > 0 -> 2 ** (n - 1)
+        0 -> 0
+      end
+    end
+    |> Enum.sum()
+  end
+
+  def part2(cards) do
+    copies_of_each_card =
+      cards
+      |> Enum.with_index()
+      |> Enum.map(fn {_, _} ->
+        1
+      end)
+      |> Enum.into([])
+
+    cards
+    |> Enum.with_index()
+    |> Enum.reduce(
+      copies_of_each_card,
+      fn {[winning, player], idx}, copies ->
+        self_copies = Enum.at(copies, idx)
+        won = count_winning_cards(player, winning)
+        slice = if won > 0, do: (idx + 1)..(idx + won), else: (..)
+
+        copies
+        |> Enum.with_index()
+        |> Enum.map(fn {count, idx} ->
+          if idx in slice, do: count + self_copies, else: count
+        end)
+      end
+    )
+    |> Enum.sum()
+  end
+end