# Author:: Eyal Oren
# Copyright:: (c) 2005-2006
# License:: LGPL

# require 'active_rdf'
require 'queryengine/query2jars2'
require 'net/http'
require 'cgi'

# Read-only adapter to jars2
# (experimental YARS branch for SWSE engine)
class Jars2Adapter < ActiveRdfAdapter
  ActiveRdfLogger::log_info "Loading Jars2 adapter", self
  ConnectionPool.register_adapter(:jars2, self)

  # initialises connection to jars2 datastore
  # available parameters are:
  # * :host (default 'm3pe.org')
  # * :port (default 2020)
  def initialize(params = {})
    super()
    @reads = true
    @writes = false

    @host = params[:host] || 'm3pe.org'
    @port = params[:port] || 2020
    ActiveRdfLogger::log_info(self) { "Initializing new instance with host: #{@host} port: #{@port}" }
    @yars = Net::HTTP.new(@host, @port)
  end

  def translate query
    Query2Jars2.translate(query)
  end

  # executes query on jars2 datastore
  def query(query)
    qs = Query2Jars2.translate(query)
    header = { 'Accept' => 'application/rdf+n3' }

    # querying Jars2, adding 'eyal' parameter to get all variable bindings in 
    # the result
    response = @yars.get("/?q=#{CGI.escape(qs)}&eyal", header)

    ActiveRdfLogger::log_debug(self) { "Jars2Adapter: query executed: #{qs}" }

    # return empty array if no content
    return [] if response.is_a?(Net::HTTPNoContent)

    # return false unless HTTP OK returned
    return false unless response.is_a?(Net::HTTPOK)

    # parse the result
    results = parse_result(response.body, query)

    # remove duplicates if asked for distinct results
    if query.distinct?
      final_results = results.uniq
    else
      final_results = results
    end

    ActiveRdfLogger::log_debug_pp "Query returned %s", final_results if ActiveRdfLogger::log_level == Logger::DEBUG
    final_results
  end

  private
  Resource = /<[^>]*>/
  Literal = /"[^"]*"/
  Node = Regexp.union(Resource,Literal)

  # parses Jars2 results into array of ActiveRDF objects
  def parse_result(response, query)
    # Jars2 responses contain one result per line
    results = response.split("\n")

    # the first line of the response contains the variable bindings of the 
    # results: we look at that line to figure out which column contains the 
    # data we are looking for (which is the variables mentioned in the select 
    # clauses of the query
    bindings = results[0].split(' ')

    # array of found answers, will be filled by iterating over the results and 
    # only including the requested (i.e. selected) clauses
    answers = []

    # we iterate over the real results, and extract the clauses that we're 
    # looking for (i.e. the select clauses from the query)
    results[1..-1].each do |result|

      # scan row for occurence of nodes (either resources or literals)
      row = result.scan(Node)

      # for each select clause, we find its index, and add the value at that 
      # location in the result row to our answer
      row = query.select_clauses.collect do |clause|
        clause_index = bindings.index(clause)
        convert_into_activerdf(row[clause_index], query.resource_class)
      end
      answers << row
    end

    answers
  end

  # converts ntriples serialisation of resource or literal into ActiveRDF object.
  #
  # resource_type is the class to be used for "resource" objects.
  def convert_into_activerdf(string, resource_type)
    case string
    when /<(.*)>/
      # <http://foaf/Person> is a resource
      resource_type.new($1)
    when /"(.*)"/
      # "30" is a literal
      # TODO: handle datatypes
      String.new($1)
    end
  end
end