1 files changed, 69 insertions, 16 deletions

diff --git a/railties/lib/rails/generators.rb b/railties/lib/rails/generators.rb
index dce734b54e..e25c364178 100644
--- a/railties/lib/rails/generators.rb
+++ b/railties/lib/rails/generators.rb
@@ -153,13 +153,23 @@ module Rails
     def self.invoke(namespace, args=ARGV, config={})
       names = namespace.to_s.split(':')
       if klass = find_by_namespace(names.pop, names.any? && names.join(':'))
-        args << "--help" if args.empty? && klass.arguments.any? { |a| a.required? }
+        args << "--help" if args.empty? && klass.arguments.any?(&:required?)
         klass.start(args, config)
       else
-        puts "Could not find generator #{namespace}."
+        options     = sorted_groups.map(&:last).flatten
+        suggestions = options.sort_by {|suggested| levenshtein_distance(namespace.to_s, suggested) }.first(3)
+        msg =  "Could not find generator '#{namespace}'. "
+        msg << "Maybe you meant #{ suggestions.map {|s| "'#{s}'"}.join(" or ") }\n"
+        msg << "Run `rails generate --help` for more options."
+        puts msg
       end
     end
 
+    # Returns an array of generator namespaces that are hidden.
+    # Generator namespaces may be hidden for a variety of reasons.
+    # Some are aliased such as "rails:migration" and can be
+    # invoked with the shorter "migration", others are private to other generators
+    # such as "css:scaffold".
     def self.hidden_namespaces
       @hidden_namespaces ||= begin
         orm      = options[:rails][:orm]
@@ -199,17 +209,6 @@ module Rails
 
     # Show help message with available generators.
     def self.help(command = 'generate')
-      lookup!
-
-      namespaces = subclasses.map{ |k| k.namespace }
-      namespaces.sort!
-
-      groups = Hash.new { |h,k| h[k] = [] }
-      namespaces.each do |namespace|
-        base = namespace.split(':').first
-        groups[base] << namespace
-      end
-
       puts "Usage: rails #{command} GENERATOR [args] [options]"
       puts
       puts "General options:"
@@ -222,20 +221,74 @@ module Rails
       puts "Please choose a generator below."
       puts
 
-      # Print Rails defaults first.
+      print_generators
+    end
+
+    def self.public_namespaces
+      lookup!
+      subclasses.map(&:namespace)
+    end
+
+    def self.print_generators
+      sorted_groups.each { |b, n| print_list(b, n) }
+    end
+
+    def self.sorted_groups
+      namespaces = public_namespaces
+      namespaces.sort!
+      groups = Hash.new { |h,k| h[k] = [] }
+      namespaces.each do |namespace|
+        base = namespace.split(':').first
+        groups[base] << namespace
+      end
       rails = groups.delete("rails")
       rails.map! { |n| n.sub(/^rails:/, '') }
       rails.delete("app")
       rails.delete("plugin")
-      print_list("rails", rails)
 
       hidden_namespaces.each { |n| groups.delete(n.to_s) }
 
-      groups.sort.each { |b, n| print_list(b, n) }
+      [["rails", rails]] + groups.sort.to_a
     end
 
     protected
 
+      # This code is based directly on the Text gem implementation
+      # Returns a value representing the "cost" of transforming str1 into str2
+      def self.levenshtein_distance str1, str2
+        s = str1
+        t = str2
+        n = s.length
+        m = t.length
+        max = n/2
+
+        return m if (0 == n)
+        return n if (0 == m)
+        return n if (n - m).abs > max
+
+        d = (0..m).to_a
+        x = nil
+
+        str1.each_char.each_with_index do |char1,i|
+          e = i+1
+
+          str2.each_char.each_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
+
+        return x
+      end
+
       # Prints a list of generators.
       def self.print_list(base, namespaces) #:nodoc:
         namespaces = namespaces.reject do |n|