Speed up Levenshtein by 50% and reduce 97% of memory usage

Calculating -------------------------------------
             each_char   924.000  i/100ms
        each_codepoint     1.381k i/100ms
  -------------------------------------------------
             each_char      9.320k (¡Þ 5.1%) i/s -     47.124k
        each_codepoint     13.857k (¡Þ 3.6%) i/s -     70.431k

  Comparison:
        each_codepoint:    13857.4 i/s
             each_char:     9319.5 i/s - 1.49x slower

The full report can be found here:
  https://gist.github.com/yuki24/a80988f35aceac76f1d5

guides/rails_guides/levenshtein.rb

@@ -14,10 +14,13 @@ module RailsGuides
       d = (0..m).to_a
       x = nil
 
-      str1.each_char.each_with_index do |char1,i|
+      # avoid duplicating an enumerable object in the loop
+      str2_codepoint_enumerable = str2.each_codepoint
+
+      str1.each_codepoint.with_index do |char1, i|
         e = i+1
 
-        str2.each_char.each_with_index do |char2,j|
+        str2_codepoint_enumerable.with_index do |char2, j|
           cost = (char1 == char2) ? 0 : 1
           x = [
                d[j+1] + 1, # insertion

railties/lib/rails/generators.rb

@@ -267,10 +267,13 @@ module Rails
         d = (0..m).to_a
         x = nil
 
-        str1.each_char.each_with_index do |char1,i|
+        # avoid duplicating an enumerable object in the loop
+        str2_codepoint_enumerable = str2.each_codepoint
+
+        str1.each_codepoint.with_index do |char1, i|
           e = i+1
 
-          str2.each_char.each_with_index do |char2,j|
+          str2_codepoint_enumerable.with_index do |char2, j|
             cost = (char1 == char2) ? 0 : 1
             x = [
                  d[j+1] + 1, # insertion