require 'active_rdf'

# Maintains pool of adapter instances that are connected to datasources. Returns 
# right adapter for a given datasource, by either reusing an
# existing adapter-instance or creating new a adapter-instance.

class ConnectionPool
	class << self
		attr_accessor :write_adapter

		# sets automatic flushing of data from adapters to original datasources 
		# (e.g. redland on-file database). If disabled, changes to an adapter are 
		# not written back into the original source: you need to invoke 
		# ConnectionPool.flush manually
		bool_accessor :auto_flush
	end
  # pool of all adapters
  @@adapter_pool = Array.new

  # pool of connection parameters to all adapter
  @@adapter_parameters = Array.new

  # currently active write-adapter (we can only write to one at a time)
  self.write_adapter = nil

	# default setting for auto_flush
	self.auto_flush = true

  # adapters-classes known to the pool, registered by the adapter-class
  # itself using register_adapter method, used to select new
  # adapter-instance for requested connection type
  @@registered_adapter_types = Hash.new

  # clears the pool: removes all registered data sources
  def ConnectionPool.clear
    $activerdflog.info "ConnectionPool: clear called"
    @@adapter_pool = []
    @@adapter_parameters = []
    self.write_adapter = nil
  end

  def ConnectionPool.adapters
    @@adapter_pool.dup
  end

	# flushes all openstanding changes into the original datasource.
	def ConnectionPool.flush
		write_adapter.flush
	end

	def ConnectionPool.adapter_types
		@@registered_adapter_types.keys
	end

  # returns the set of currently registered read-access datasources
  def ConnectionPool.read_adapters
    @@adapter_pool.select {|adapter| adapter.reads? }
  end

  def ConnectionPool.write_adapters
    @@adapter_pool.select {|adapter| adapter.writes? }
  end

  # returns adapter-instance for given parameters (either existing or new)
  def ConnectionPool.add_data_source(connection_params)
    $activerdflog.info "ConnectionPool: add_data_source with params: #{connection_params.inspect}"

    # either get the adapter-instance from the pool
    # or create new one (and add it to the pool)
    index = @@adapter_parameters.index(connection_params)
    if index.nil?
      # adapter not in the pool yet: create it,
      # register its connection parameters in parameters-array
      # and add it to the pool (at same index-position as parameters)
      $activerdflog.debug("Create a new adapter for parameters #{connection_params.inspect}")
      adapter = create_adapter(connection_params)
      @@adapter_parameters << connection_params
      @@adapter_pool << adapter
    else
      # if adapter parametrs registered already,
      # then adapter must be in the pool, at the same index-position as its parameters
      $activerdflog.debug("Reusing existing adapter")
      adapter = @@adapter_pool[index]
    end

    # sets the adapter as current write-source if it can write
    self.write_adapter = adapter if adapter.writes?

    return adapter
  end
  
  # remove one adapter from activerdf
  def ConnectionPool.remove_data_source(adapter)
    $activerdflog.info "ConnectionPool: remove_data_source with params: #{adapter.to_s}"
    
    index = @@adapter_pool.index(adapter)

    # remove_data_source mit be called repeatedly, e.g because the adapter object is stale
    unless index.nil?
      @@adapter_parameters.delete_at(index)
      @@adapter_pool.delete_at(index)
      if self.write_adapters.empty?
        self.write_adapter = nil
      else
        self.write_adapter = self.write_adapters.first
      end
    end
    
  end

	# sets adapter-instance for connection parameters (if you want to re-enable an existing adapter)
	def ConnectionPool.set_data_source(adapter, connection_params = {})
		index = @@adapter_parameters.index(connection_params)
		if index.nil?
      @@adapter_parameters << connection_params
      @@adapter_pool << adapter
		else
			@@adapter_pool[index] = adapter
		end
		self.write_adapter = adapter if adapter.writes?
		adapter
	end

	# aliasing add_data_source as add
	# (code bit more complicad since they are class methods)
	class << self
		alias add add_data_source
	end

  # adapter-types can register themselves with connection pool by
  # indicating which adapter-type they are
  def ConnectionPool.register_adapter(type, klass)
    $activerdflog.info "ConnectionPool: registering adapter of type #{type} for class #{klass}"
    @@registered_adapter_types[type] = klass
  end

  # create new adapter from connection parameters
  def ConnectionPool.create_adapter connection_params
    # lookup registered adapter klass
    klass = @@registered_adapter_types[connection_params[:type]]

    # raise error if adapter type unknown
    raise(ActiveRdfError, "unknown adapter type #{connection_params[:type]}") if klass.nil?

    # create new adapter-instance
    klass.send(:new,connection_params)
  end
  
  private_class_method :create_adapter
end