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

day5.cr

@@ -3,7 +3,6 @@ require "./common.cr"
 lines = File.read("day5_input").split("\n")
 
 def react(string)
-  lower = string.downcase.chars
   string = string.chars
 
   changed = true
@@ -13,11 +12,9 @@ def react(string)
     while i < string.size - 1
       next_i = i + 1
       if string[i] != string[next_i] &&
-          lower[i] == lower[next_i]
+          string[i].downcase == string[next_i].downcase
         string.delete_at i
         string.delete_at i
-        lower.delete_at i
-        lower.delete_at i
 
         i -= 1
         i = 0 if i < 0
@@ -35,7 +32,7 @@ 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].delete(pair[0]).delete(pair[1])
+  new_string = lines[0].select { |c| c != pair[0] && c != pair[1] }
   pairs[pair] = react(new_string).size
 end