in guides generator, broken link warning now suggests the existing anchor with min Levenshtein distance

railties/guides/rails_guides.rb

@@ -22,6 +22,7 @@ module RailsGuides
   autoload :Indexer, "rails_guides/indexer"
   autoload :Helpers, "rails_guides/helpers"
   autoload :TextileExtensions, "rails_guides/textile_extensions"
+  autoload :Levenshtein, "rails_guides/levenshtein"
 end
 
 RedCloth.send(:include, RailsGuides::TextileExtensions)

railties/guides/rails_guides/generator.rb

@@ -146,7 +146,10 @@ module RailsGuides
       html.scan(/<a\s+href="#([^"]+)/).flatten.each do |fragment_identifier|
         next if fragment_identifier == 'mainCol' # in layout, jumps to some DIV
         unless anchors.member?(fragment_identifier)
-          puts "BROKEN LINK: ##{fragment_identifier}"
+          guess = anchors.min { |a, b|
+            Levenshtein.distance(fragment_identifier, a) <=> Levenshtein.distance(fragment_identifier, b)
+          }
+          puts "*** BROKEN LINK: ##{fragment_identifier}, perhaps you meant ##{guess}."
         end
       end
     end

railties/guides/rails_guides/levenshtein.rb

@@ -0,0 +1,112 @@
+#
+# Levenshtein distance algorithm implementation for Ruby, with UTF-8 support
+#
+# Author::  Paul BATTLEY (pbattley @ gmail.com)
+# Version:: 1.3
+# Date::    2005-04-19
+#
+# == About
+#
+# The Levenshtein distance is a measure of how similar two strings s and t are,
+# calculated as the number of deletions/insertions/substitutions needed to
+# transform s into t.  The greater the distance, the more the strings differ.
+#
+# The Levenshtein distance is also sometimes referred to as the
+# easier-to-pronounce-and-spell 'edit distance'.
+#
+# == Revision history
+#
+# * 2005-05-19 1.3 Repairing an oversight, distance can now be called via
+#   Levenshtein.distance(s, t)
+# * 2005-05-04 1.2 Now uses just one 1-dimensional array.  I think this is as
+#   far as optimisation can go.
+# * 2005-05-04 1.1 Now storing only the current and previous rows of the matrix
+#   instead of the whole lot.
+#
+# == Licence
+#
+# Copyright (c) 2005 Paul Battley
+#
+# Usage of the works is permitted provided that this instrument is retained
+# with the works, so that any entity that uses the works is notified of this
+# instrument.
+#
+# DISCLAIMER: THE WORKS ARE WITHOUT WARRANTY.  
+#
+
+module Levenshtein
+
+    #
+    # Calculate the Levenshtein distance between two strings +str1+ and +str2+.
+    # +str1+ and +str2+ should be ASCII or UTF-8.
+    #
+    def distance(str1, str2)
+        s = str1.unpack('U*')
+        t = str2.unpack('U*')
+        n = s.length
+        m = t.length
+        return m if (0 == n)
+        return n if (0 == m)
+        
+        d = (0..m).to_a
+        x = nil
+
+        (0...n).each do |i|
+            e = i+1
+            (0...m).each do |j|
+                cost = (s[i] == t[j]) ? 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
+
+    extend self
+end
+
+if (__FILE__ == $0)
+    require 'test/unit'
+
+    class LevenshteinTest < Test::Unit::TestCase
+        include Levenshtein
+
+        EXPECTED = [
+            # Easy ones
+            ['test', 'test', 0],
+            ['test', 'tent', 1],
+            ['gumbo', 'gambol', 2],
+            ['kitten', 'sitting', 3],
+            # Empty strings
+            ['foo', '', 3],
+            ['', '', 0],
+            ['a', '', 1],
+            # UTF-8
+            ["f\303\266o", 'foo', 1],
+            ["fran\303\247ais", 'francais', 1],
+            ["fran\303\247ais", "fran\303\246ais", 1],
+            ["\347\247\201\343\201\256\345\220\215\345\211\215\343\201\257"<<
+             "\343\203\235\343\203\274\343\203\253\343\201\247\343\201\231",
+             "\343\201\274\343\201\217\343\201\256\345\220\215\345\211\215\343\201"<<
+             "\257\343\203\235\343\203\274\343\203\253\343\201\247\343\201\231", 
+             2], # Japanese
+            # Edge cases
+            ['a', 'a', 0],
+            ['0123456789', 'abcdefghijklmnopqrstuvwxyz', 26]
+        ]
+
+        def test_known_distances
+            EXPECTED.each do |a,b,x|
+                assert_equal(x, distance(a, b))
+                assert_equal(x, distance(b, a))
+            end
+        end
+    end
+end