Suggest alternatives when typoing gem names

eg
$ bundle open thinking_sphinx
Could not find gem 'thinking_sphinx'.
Did you mean thinking-sphinx?

lib/bundler/cli.rb

@@ -1,6 +1,7 @@
 require 'bundler/vendored_thor'
 require 'rubygems/user_interaction'
 require 'rubygems/config_file'
+require 'bundler/similarity_detector'
 
 module Bundler
   class CLI < Thor
@@ -681,7 +682,7 @@ def have_groff?
 
     def locate_gem(name)
       spec = Bundler.load.specs.find{|s| s.name == name }
-      raise GemNotFound, "Could not find gem '#{name}' in the current bundle." unless spec
+      raise GemNotFound, not_found_message(name, Bundler.load.specs) unless spec
       if spec.name == 'bundler'
         return File.expand_path('../../../', __FILE__)
       end
@@ -690,9 +691,20 @@ def locate_gem(name)
 
     def gem_dependency_with_name(name)
       dep = Bundler.load.dependencies.find{|d| d.name == name }
-      raise GemNotFound, "Could not find gem '#{name}'." unless dep
+      raise GemNotFound, not_found_message(name, Bundler.load.dependencies) unless dep
       dep
     end
 
+    def not_found_message(missing_gem_name, alternatives)
+      message = "Could not find gem '#{missing_gem_name}'."
+
+      # This is called as the result of a GemNotFound, let's see if
+      # there's any similarly named ones we can propose instead
+      alternate_names = alternatives.map{|a| a.name}
+      suggestions = SimilarityDetector.new(alternate_names).similar_word_list(missing_gem_name)
+      message += "\nDid you mean #{suggestions}?" if suggestions
+      message
+    end
+
   end
 end

lib/bundler/similarity_detector.rb

@@ -0,0 +1,63 @@
+module Bundler
+  class SimilarityDetector
+    SimilarityScore = Struct.new(:string, :distance)
+
+    # initialize with an array of words to be matched against
+    def initialize(corpus)
+      @corpus = corpus
+    end
+
+    # return an array of words similar to 'word' from the corpus
+    def similar_words(word, limit=3)
+      words_by_similarity = @corpus.map{|w| SimilarityScore.new(w, levenshtein_distance(word, w))}
+      words_by_similarity.select{|s| s.distance<=limit}.sort_by(&:distance).map(&:string)
+    end
+
+    # return the result of 'similar_words', concatenated into a list
+    # (eg "a, b, or c")
+    def similar_word_list(word, limit=3)
+      words = similar_words(word,limit)
+      if words.length==1
+        words[0]
+      elsif words.length>1
+        [words[0..-2].join(', '), words[-1]].join(' or ')
+      end
+    end
+
+
+  protected
+    # http://www.informit.com/articles/article.aspx?p=683059&seqNum=36
+    def levenshtein_distance(this, that, ins=2, del=2, sub=1)
+      # ins, del, sub are weighted costs
+      return nil if this.nil?
+      return nil if that.nil?
+      dm = []        # distance matrix
+
+      # Initialize first row values
+      dm[0] = (0..this.length).collect { |i| i * ins }
+      fill = [0] * (this.length - 1)
+
+      # Initialize first column values
+      for i in 1..that.length
+        dm[i] = [i * del, fill.flatten]
+      end
+
+      # populate matrix
+      for i in 1..that.length
+        for j in 1..this.length
+          # critical comparison
+          dm[i][j] = [
+               dm[i-1][j-1] +
+                 (this[j-1] == that[i-1] ? 0 : sub),
+                   dm[i][j-1] + ins,
+               dm[i-1][j] + del
+         ].min
+        end
+      end
+
+      # The last value in matrix is the Levenshtein distance between the strings
+      dm[that.length][this.length]
+    end
+
+  end
+end

spec/other/open_spec.rb

@@ -32,4 +32,9 @@
     bundle "open missing", :env => {"EDITOR" => "echo editor", "VISUAL" => "", "BUNDLER_EDITOR" => ""}
     out.should match(/could not find gem 'missing'/i)
   end
+
+  it "suggests alternatives for similar-sounding gems" do
+    bundle "open Rails", :env => {"EDITOR" => "echo editor", "VISUAL" => "", "BUNDLER_EDITOR" => ""}
+    out.should match(/did you mean rails\?/i)
+  end
 end

spec/update/gems_spec.rb

@@ -61,6 +61,10 @@
       bundle "update halting-problem-solver", :expect_err=>true
       out.should include "Could not find gem 'halting-problem-solver'"
     end
+    it "should suggest alternatives" do
+      bundle "update active-support", :expect_err=>true
+      out.should include "Did you mean activesupport?"
+    end
   end
 
   describe "with --local option" do