Solve day 5

lib/aoc2024/day_05.ex

@@ -1,13 +1,47 @@
 defmodule Aoc2024.Day05 do
   def parse(input) do
-    input |> String.split("\n", trim: true)
+    [rules_raw, updates_raw] = input |> String.trim() |> String.split("\n\n")
+    rules = Aoc2024.Utils.parse_rows_of_integers(rules_raw, "|")
+    updates = Aoc2024.Utils.parse_rows_of_integers(updates_raw, ",")
+    {rules, updates}
   end
 
-  def part1(_lines) do
-    :todo1
+  defp sort([], _rules), do: %{was_sorted?: true, list: []}
+
+  defp sort([head | tail], rules) do
+    result =
+      tail
+      |> Enum.reduce(%{sorted?: true, revlist: [], previous: head}, fn elem, acc ->
+        if rules |> Enum.member?([elem, acc.previous]) do
+          %{sorted?: false, revlist: [elem | acc.revlist], previous: acc.previous}
+        else
+          %{sorted?: acc.sorted?, revlist: [acc.previous | acc.revlist], previous: elem}
+        end
+      end)
+
+    partially_sorted = [result.previous | result.revlist] |> Enum.reverse()
+
+    if result.sorted? do
+      %{was_sorted?: true, list: partially_sorted}
+    else
+      %{was_sorted?: false, list: sort(partially_sorted, rules).list}
+    end
   end
 
-  def part2(_lines) do
-    :todo2
+  def part1({rules, updates}) do
+    updates
+    |> Enum.filter(fn update -> sort(update, rules).was_sorted? end)
+    |> Enum.map(fn update -> update |> Enum.at(length(update) |> div(2)) end)
+    |> Enum.sum()
+  end
+
+  def part2({rules, updates}) do
+    updates
+    |> Enum.map(fn update -> sort(update, rules) end)
+    |> Enum.filter(fn sort_result -> not sort_result.was_sorted? end)
+    |> Enum.map(fn sort_result ->
+      sort_result.list |> Enum.at(length(sort_result.list) |> div(2))
+    end)
+    |> Enum.sum()
   end
 end

lib/aoc2024/utils.ex

@@ -1,16 +1,20 @@
 defmodule Aoc2024.Utils do
-  def parse_rows_of_integers(input) do
+  def parse_integers(input, separator \\ ~r/\s+/) do
+    input
+    |> String.split(separator)
+    |> Enum.map(fn int_str ->
+      case int_str |> Integer.parse() do
+        {int, ""} -> int
+        _ -> raise "Invalid integer #{int_str}"
+      end
+    end)
+  end
+
+  def parse_rows_of_integers(input, separator \\ ~r/\s+/) do
     input
     |> String.split("\n", trim: true)
     |> Enum.map(fn line ->
-      line
-      |> String.split(~r/\s+/)
-      |> Enum.map(fn int_str ->
-        case int_str |> Integer.parse() do
-          {int, ""} -> int
-          _ -> raise "Invalid integer #{int_str}"
-        end
-      end)
+      parse_integers(line, separator)
     end)
   end
 end

test/aoc2024_test.exs

@@ -70,4 +70,41 @@ defmodule Aoc2024Test do
     assert Aoc2024.Day04.part1(parsed_input) == 18
     assert Aoc2024.Day04.part2(parsed_input) == 9
   end
+
+  test "Day 5" do
+    input = """
+    47|53
+    97|13
+    97|61
+    97|47
+    75|29
+    61|13
+    75|53
+    29|13
+    97|29
+    53|29
+    61|53
+    97|53
+    61|29
+    47|13
+    75|47
+    97|75
+    47|61
+    75|61
+    47|29
+    75|13
+    53|13
+
+    75,47,61,53,29
+    97,61,53,29,13
+    75,29,13
+    75,97,47,61,53
+    61,13,29
+    97,13,75,29,47
+    """
+
+    parsed_input = Aoc2024.Day05.parse(input)
+    assert Aoc2024.Day05.part1(parsed_input) == 143
+    assert Aoc2024.Day05.part2(parsed_input) == 123
+  end
 end