Add solutions to day21 and day22.

day21.cr

@@ -0,0 +1,31 @@
+require "advent"
+input = input(2020, 21).lines.map do |line|
+  data = line.match(/^([a-z ]+) \(contains (.+)\)$/).not_nil!
+  {data[1].split(" ").to_set, data[2].split(", ").to_set}
+end
+
+allergens = input.flat_map(&.last.to_a).to_set
+ingredients = input.flat_map(&.first.to_a).to_set
+
+allergen_sets = {} of String => Set(String)
+allergens.each do |a|
+  input.each do |ings, als|
+    next unless als.includes? a
+    allergen_sets[a] ||= ings
+    allergen_sets[a] &= ings
+  end
+end
+
+safe = ingredients.reject { |i| allergen_sets.any? &.last.includes?(i) }
+puts "Part 1: #{input.sum &.first.count { |i| safe.includes? i }}"
+
+known_allergens = {} of String => String
+while allergen_sets.size > known_allergens.size
+  allergen_sets.each do |a, s|
+    next unless s.size == 1
+    new_known = s.first
+    known_allergens[a] = new_known
+    allergen_sets.each &.last.delete(new_known)
+  end
+end
+puts "Part 2: #{known_allergens.to_a.sort_by(&.first).map(&.last).join(",")}"