Partly revert #32289 to provide Rails' custom fallback in case

`DidYouMean::SpellChecker` is not defined.
`did_you_mean` is bundled in Ruby but can be uninstalled, and is not always
available, sometimes even on our CI:

https://travis-ci.org/rails/rails/jobs/372638523#L2405
https://travis-ci.org/rails/rails/jobs/372638523#L2416
https://travis-ci.org/rails/rails/jobs/372638523#L2427
...

1 files changed, 49 insertions, 2 deletions

diff --git a/railties/lib/rails/command/spellchecker.rb b/railties/lib/rails/command/spellchecker.rb
index 154358cd45..04485097fa 100644
--- a/railties/lib/rails/command/spellchecker.rb
+++ b/railties/lib/rails/command/spellchecker.rb
@@ -3,8 +3,55 @@
 module Rails
   module Command
     module Spellchecker # :nodoc:
-      def self.suggest(word, from:)
-        DidYouMean::SpellChecker.new(dictionary: from.map(&:to_s)).correct(word).first
+      class << self
+        def suggest(word, from:)
+          if defined?(DidYouMean::SpellChecker)
+            DidYouMean::SpellChecker.new(dictionary: from.map(&:to_s)).correct(word).first
+          else
+            from.sort_by { |w| levenshtein_distance(word, w) }.first
+          end
+        end
+
+        private
+
+          # This code is based directly on the Text gem implementation.
+          # Copyright (c) 2006-2013 Paul Battley, Michael Neumann, Tim Fletcher.
+          #
+          # Returns a value representing the "cost" of transforming str1 into str2.
+          def levenshtein_distance(str1, str2) # :doc:
+            s = str1
+            t = str2
+            n = s.length
+            m = t.length
+
+            return m if (0 == n)
+            return n if (0 == m)
+
+            d = (0..m).to_a
+            x = nil
+
+            # 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_codepoint_enumerable.with_index do |char2, j|
+                cost = (char1 == char2) ? 0 : 1
+                x = [
+                  d[j + 1] + 1, # insertion
+                  e + 1,        # deletion
+                  d[j] + cost   # substitution
+                ].min
+                d[j] = e
+                e = x
+              end
+
+              d[m] = x
+            end
+
+            x
+          end
       end
     end
   end