Use extra functions and stop using downcase array.

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