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(",")}"

day22.cr

@@ -0,0 +1,66 @@
+require "advent"
+first, second = input(2020, 22).split("\n\n")
+first = first.lines[1..].map &.to_i32
+second = second.lines[1..].map &.to_i32
+
+class Array(T)
+  def pop_left
+    delete_at(0)
+  end
+
+  def score
+    total = 0
+    s = size
+    each_with_index do |n, i|
+      total += n * (s - i)
+    end
+    total
+  end
+end
+
+def no_rec(deck1, deck2)
+  while !deck1.empty? && !deck2.empty?
+    f, s = deck1.delete_at(0), deck2.delete_at(0)
+    if f > s
+      deck1 << f << s
+    else
+      deck2 << s << f
+    end
+  end
+  return !deck1.empty?
+end
+
+def rec(deck1, deck2)
+  seen = Set({Int32, Int32}).new
+  while !deck1.empty? && !deck2.empty?
+    key = {deck1.score, deck2.score}
+    return true if seen.includes?(key)
+    seen << key
+
+    f = deck1.delete_at(0)
+    s = deck2.delete_at(0)
+
+    p1wins = (f <= deck1.size) && (s <= deck2.size) ? rec(deck1[0..f-1], deck2[0..s-1]) : f > s
+    if p1wins
+      deck1 << f << s
+    else
+      deck2 << s << f
+    end
+  end
+  return !deck1.empty?
+end
+
+def run(first, second, proc)
+  ((proc.call(first, second)) ? first : second).score
+end
+
+def part1(input)
+  run(input[0], input[1], ->no_rec(Array(Int32), Array(Int32)))
+end
+
+def part2(input)
+  run(input[0], input[1], ->rec(Array(Int32), Array(Int32)))
+end
+
+puts part1({first, second}.clone)
+puts part2({first, second}.clone)