Add solutions to day21 and day22.

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)