Optimize day 5 solution.

common.cr

@@ -20,3 +20,25 @@ class Rectangle
     end
   end
 end
+
+class String
+  def select(&block)
+    String.build do |s|
+      each_char do |c|
+        s << c if yield c
+      end
+    end
+  end
+
+  def select!(&block)
+    i = 0
+    s = size
+    while i < s
+      if yield [i]
+        i += 1
+      else
+        s -= 1
+      end
+    end
+  end
+end

day5.cr

@@ -0,0 +1,34 @@
+require "./common.cr"
+
+lines = File.read("day5_input").split("\n")
+
+struct Char
+  def inverse?(other)
+    return (other != self) && (other.downcase == self.downcase)
+  end
+end
+
+def react(string)
+  stack = [] of Char
+
+  string.each_char do |char|
+    last_char = stack.last?
+    if !last_char || !last_char.inverse? char
+      stack << char
+    else
+      stack.pop
+    end
+  end
+
+  return stack
+end
+
+puts react(lines[0]).size
+
+pairs = {} of Tuple(Char, Char) => Int32
+('a'..'z').to_a.zip(('A'..'Z').to_a).each do |pair|
+  new_string = lines[0].select { |c| c != pair[0] && c != pair[1] }
+  pairs[pair] = react(new_string).size
+end
+
+puts pairs.values.min