module Semantic
  module VectorSpace
    # A algebraic model for representing text documents as vectors of identifiers.
    # A document is represented as a vector. Each dimension of the vector corresponds to a
    # separate term. If a term occurs in the document, then the value in the vector is non-zero.
    class Builder

      def initialize(options = {})
        @parser = Parser.new(:filter_stop_words => options[:filter_stop_words])
        @parsed_document_cache = []
      end

      def build_document_matrix(documents)
        @vector_keyword_index = build_vector_keyword_index(documents)

        document_vectors = documents.enum_for(:each_with_index).map{|document,document_id| build_vector(document, document_id)}

        n = document_vectors.size
        m = document_vectors.first.size

        # TODO check where else we use document_vectors and if we can directly use column based ones
        document_matrix = GSL::Matrix.alloc(*document_vectors.map {|v| v.transpose})

        Model.new(document_matrix, @vector_keyword_index)
      end

      def build_query_vector(term_list)
        build_vector(term_list.join(" "))
      end

      private
      def build_vector_keyword_index(documents)
        parse_and_cache(documents)
        vocabulary_list = find_unique_vocabulary
        map_vocabulary_to_vector_positions(vocabulary_list)
      end

      def parse_and_cache(documents)
        documents.each_with_index do |document, index|
          @parsed_document_cache[index] = @parser.tokenise_and_filter(document)
        end
      end

      def find_unique_vocabulary
        @parsed_document_cache.flatten.reverse.uniq
      end

      def map_vocabulary_to_vector_positions(vocabulary_list)
        vector_index={}
        column = 0
        vocabulary_list.each do |word|
          vector_index[word] = column
          column += 1
        end
        vector_index
      end

      def build_vector(word_string, document_id = nil)
        if document_id.nil?
          word_list = @parser.tokenise_and_filter(word_string)
        else
          word_list = @parsed_document_cache[document_id]
        end

        vector = GSL::Vector.alloc(@vector_keyword_index.length)
        word_list.each { |word|
          if @vector_keyword_index.has_key?(word)
            vector[@vector_keyword_index[word]] += 1
          end
        }

        vector
      end
    end
  end
end