class String
  # Finds the longest commong substring betweent the two supplied strings
  def self.lcs(s1, s2)
    num=Array.new(s1.size){Array.new(s2.size)}
    len,ans=0

    longest_substr = ""
    substr = ""
    s1.scan(/./).each_with_index do |l1,i |
      s2.scan(/./).each_with_index do |l2,j |

        unless l1==l2
          num[i][j]=[0,""]
        else
          if (i==0 || j==0)
            num[i][j]=[1,l1] 
          else
            num[i][j] = [1 + num[i-1][j-1][0], num[i-1][j-1][1] +l1]
          end
          if num[i][j][0] > len
            len = ans = num[i][j][0] 
            longest_substr = num[i][j][1]
          end
        end
      end
    end
    longest_substr
  end
  
  # Performs multiple gsub operations in a single pass.
  # +key_value_pairs+ is an array of [key,value] pairs where each key is a regular expression 
  # and the value is the value to substitute when that regexp is matched.
  def mgsub(key_value_pairs=[].freeze)
    regexp_fragments = key_value_pairs.collect { |k,v| k }
    gsub(Regexp.union(*regexp_fragments)) do |match|
      key_value_pairs.detect{|k,v| k =~ match}[1]
    end
  end
  
  # Performs multiple gsub operations in a single pass.
  # +key_value_pairs+ is an array of [key,value] pairs where each key is a regular expression 
  # and the value is the value to substitute when that regexp is matched.
  #
  # Unlike mgsub, this method allows parenthesized subsections of the match string from the regexp 
  # to be embedded in the replace string.  The normal variables are available in the replacement
  # string.
  #
  # eg.
  #
  # >> "this is that".mgpsub( [ [/(that)/, 'really $1!'], [/this/, 'that'] ] ) 
  # => that is really that!"
  #
  def mgpsub(key_value_pairs=[].freeze)
    regexp_fragments = key_value_pairs.collect { |k,v| k }
    gsub(Regexp.union(*regexp_fragments)) do |match|
      replacement_term = key_value_pairs.detect{|k,v| k=~match}[1]
      vars = ["$1", "$2", "$3", "$4", "$5", "$6", "$7", "$8", "$9", "$`", "$&", "$'"]
      vars.each do |var|
        replacement_term.gsub!( Regexp.compile("\\"+var), eval(var)||'' )
      end
      replacement_term
    end
  end
  
  # trims the string to the nearest word boundary ensuring that the resulting length is less than max_length
  def trim_to_word( max_length )
    string = self
    if string.length > max_length
      string = string[0..max_length-4]
      while string[-1,1] =~ /[\w]/ && string.length > 1
        string = string[0..string.length-2]
      end
      string += "..."
    end
    string
  end
  
end