gistfile1.txt

equire 'benchmark'

class String
  def ending
    length = self.length
    self[length-2].chr + self[length-1].chr
  end

  def base
    self[0..(self.length-3)]
  end
end

class EndingFinder

  attr_reader :results

  def initialize(filename)
    @results = []
    @cache   = {}

    # For cache expiration.
    @letter  = "a"
    File.open(filename).each do |word|
      w = word.chomp.downcase.strip
      process_word(w) if word.length > 2
    end
  end

  def process_word(word)
    base = word.base
    if @cache[base]
      @cache[base].each do |ending|
        if word.ending == ending.reverse
          @results << [base + ending, word]
        end
      end
    else
      cache_word(base, word)
    end
  end


  def cache_word(base, word) 
    if @current_letter != (letter = word[0].chr) 
      @cache = {} 
      @current_letter = letter 
    end 
    @cache[base] ||= [] 
    @cache[base] << word.ending 
  end 
end 
 
Benchmark.bm do |x| 
  x.report do 
    EndingFinder.new('web2down').results.each do |result| 
      puts result.join(' ')
    end
  end
end