It's a bloom filter! http://codekata.com/kata/kata05-bloom-filters/

bloom.rb

require 'minitest/autorun'

class Bloom

  attr_accessor :array

  def initialize(array_size=10_000_000, hashes_per_word=4, words_file="/usr/share/dict/words")
    @array_size = array_size
    @hashes_per_word = hashes_per_word
    @words_file = words_file
    initialize_array!
    populate_array!
  end

  def hash(word)
    result = []
    @hashes_per_word.times do |n|
      srand((word+n.to_s).hash)
      result << rand(@array_size)
    end
    result
  end

  def populate_array!
    io = IO.new(IO.sysopen(@words_file),"r")
    while !io.eof?
      indices = hash(io.readline.strip)
      indices.each{|n| array[n] = 1}
    end
    return nil
  end

  def initialize_array!
    self.array = Array.new(@array_size){0}
  end

  def in_dictionary? word
    indices = hash(word)
    indices.map{|n| array[n] == 1}.all?{|el| el == true} 
  end

end

class BloomTest < MiniTest::Test
  def setup
    @bloom = Bloom.new(1000000, 4)
  end

  def test_real_word
    assert @bloom.in_dictionary?("house")
    assert @bloom.in_dictionary?("garage")
  end

  def test_not_real
    refute @bloom.in_dictionary?("sadgsgdf")
  end
end