Day 7 Part 1

2020/day7/day7.ex

@@ -0,0 +1,66 @@
+defmodule Day7 do
+  defp read_input do
+    Path.expand('input', Path.dirname(__ENV__.file))
+    |> File.read!()
+  end
+
+  defp parse_rules(text) do
+    String.split(String.trim(text), "\n")
+    |> Enum.map(&parse_rule/1)
+    |> Enum.into(%{}, fn %{color: color, contents: contents} -> {color, contents} end)
+  end
+
+  @line_pattern ~r/^(?<color>.*?) bags contain (?<contents>.*?)\.$/
+  @clause_pattern ~r/^(?<count>\d+) (?<color>.*?) bags?$/
+
+  defp parse_rule(line) do
+    match = Regex.named_captures(@line_pattern, line)
+
+    %{
+      color: Map.fetch!(match, "color"),
+      contents: parse_contents(Map.fetch!(match, "contents"))
+    }
+  end
+
+  defp parse_contents(text) do
+    if text == "no other bags" do
+      []
+    else
+      String.split(text, ", ")
+      |> Enum.map(fn clause ->
+        match = Regex.named_captures(@clause_pattern, clause)
+        count = String.to_integer(Map.fetch!(match, "count"))
+        color = Map.fetch!(match, "color")
+        %{count: count, color: color}
+      end)
+    end
+  end
+
+  defp contain_fixpoint(rules, targets) do
+    new_targets =
+      Enum.reduce(rules, targets, fn {outer, inners}, working_set ->
+        inner_set = MapSet.new(Enum.map(inners, fn %{color: color} -> color end))
+
+        if MapSet.size(MapSet.intersection(targets, inner_set)) > 0 do
+          MapSet.put(working_set, outer)
+        else
+          working_set
+        end
+      end)
+
+    if targets == new_targets do
+      new_targets
+    else
+      contain_fixpoint(rules, new_targets)
+    end
+  end
+
+  def part1 do
+    rules = parse_rules(read_input())
+    containing_colors = contain_fixpoint(rules, MapSet.new(["shiny gold"]))
+    # No recursion :sweat_smile:
+    MapSet.size(MapSet.delete(containing_colors, "shiny gold"))
+  end
+end
+
+Day7.part1() |> IO.inspect()