Simple recommendation system written in Ruby based on Jaccard index.

## jaccard_recommendation.rb
#!/usr/bin/env ruby

# Please read http://otobrglez.opalab.com for more information about this code.

class Book < Struct.new(:title)

  def words
    @words ||= self.title.gsub(/[a-zA-Z]{3,}/).map(&:downcase).uniq.sort
  end

end

class BookRecommender

  def initialize book, books
    @book, @books = book, books
  end

  def recommendations
    @books.map! do |this_book|
      this_book.define_singleton_method(:jaccard_index) do @jaccard_index;  end

      this_book.define_singleton_method("jaccard_index=") do |index|
        @jaccard_index = index || 0.0
      end

      intersection = (@book.words & this_book.words).size
      union = (@book.words | this_book.words).size

      this_book.jaccard_index = (intersection.to_f / union.to_f) rescue 0.0
      this_book

    end.sort_by { |book| 1 - book.jaccard_index }

  end

end


# Usage

# Load books from this file
BOOKS = DATA.read.split("\n").map { |l| Book.new(l) }

# Create initial book
this_book = Book.new("Ruby programming language")

# Load recommender
recommender = BookRecommender.new(this_book, BOOKS)

# Find recommendations
recommended_books = recommender.recommendations

# Show books that match the most
recommended_books.each do |book|
  puts "#{book.title} (#{'%.2f' % book.jaccard_index})"
end


__END__
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