writer.rb

# -*- encoding: utf-8 -*-
module RDF::NTriples
  ##
  # N-Triples serializer.
  #
  # Output is serialized for UTF-8, to serialize as ASCII
  # (with) unicode escapes, set encoding: Encoding::ASCII as
  # an option to {RDF::NTriples::Writer#initialize}.
  #
  # @example Obtaining an NTriples writer class
  #   RDF::Writer.for(:ntriples)     #=> RDF::NTriples::Writer
  #   RDF::Writer.for("etc/test.nt")
  #   RDF::Writer.for(file_name:      "etc/test.nt")
  #   RDF::Writer.for(file_extension: "nt")
  #   RDF::Writer.for(content_type:   "application/n-triples")
  #
  # @example Serializing RDF statements into an NTriples file
  #   RDF::NTriples::Writer.open("etc/test.nt") do |writer|
  #     graph.each_statement do |statement|
  #       writer << statement
  #     end
  #   end
  #
  # @example Serializing RDF statements into an NTriples string
  #   RDF::NTriples::Writer.buffer do |writer|
  #     graph.each_statement do |statement|
  #       writer << statement
  #     end
  #   end
  #
  # @example Serializing RDF statements into an NTriples string with escaped UTF-8
  #   RDF::NTriples::Writer.buffer(encoding: Encoding::ASCII) do |writer|
  #     graph.each_statement do |statement|
  #       writer << statement
  #     end
  #   end
  #
  # @see http://www.w3.org/TR/rdf-testcases/#ntriples
  # @see http://www.w3.org/TR/n-triples/
  class Writer < RDF::Writer
    include RDF::Util::Logger
    format RDF::NTriples::Format

    # @see http://www.w3.org/TR/rdf-testcases/#ntrip_strings
    ESCAPE_PLAIN = /\A[\x20-\x21\x23-\x26\x28#{Regexp.escape '['}#{Regexp.escape ']'}-\x7E]*\z/m.freeze
    ESCAPE_PLAIN_U = /\A(?:#{Reader::IRI_RANGE}|#{Reader::UCHAR})*\z/.freeze

    ##
    # Escape Literal and URI content. If encoding is ASCII, all unicode
    # is escaped, otherwise only ASCII characters that must be escaped are
    # escaped.
    #
    # @param  [String] string
    # @param  [Encoding] encoding
    # @return [String]
    # @see    http://www.w3.org/TR/rdf-testcases/#ntrip_strings
    def self.escape(string, encoding = nil)
      ret = case
        when string.match?(ESCAPE_PLAIN) # a shortcut for the simple case
          string
        when string.ascii_only?
          StringIO.open do |buffer|
            buffer.set_encoding(Encoding::ASCII)
            string.each_byte { |u| buffer << escape_ascii(u, encoding) }
            buffer.string
          end
        when encoding && encoding != Encoding::ASCII
          # Not encoding UTF-8 characters
          StringIO.open do |buffer|
            buffer.set_encoding(encoding)
            string.each_char do |u|
              buffer << case u.ord
              when (0x00..0x7F)
                escape_ascii(u, encoding)
              else
                u
              end
            end
            buffer.string
          end
        else
          # Encode ASCII && UTF-8 characters
          StringIO.open do |buffer|
            buffer.set_encoding(Encoding::ASCII)
            string.each_codepoint { |u| buffer << escape_unicode(u, encoding) }
            buffer.string
          end
      end
      encoding ? ret.encode(encoding) : ret
    end

    ##
    # Escape ascii and unicode characters.
    # If encoding is UTF_8, only ascii characters are escaped.
    #
    # @param  [Integer, #ord] u
    # @param  [Encoding] encoding
    # @return [String]
    # @raise  [ArgumentError] if `u` is not a valid Unicode codepoint
    # @see    http://www.w3.org/TR/rdf-testcases/#ntrip_strings
    def self.escape_unicode(u, encoding)
      case (u = u.ord)
        when (0x00..0x7F)        # ASCII 7-bit
          escape_ascii(u, encoding)
        when (0x80..0xFFFF)      # Unicode BMP
          escape_utf16(u)
        when (0x10000..0x10FFFF) # Unicode
          escape_utf32(u)
        else
          raise ArgumentError.new("expected a Unicode codepoint in (0x00..0x10FFFF), but got 0x#{u.to_s(16)}")
      end
    end

    ##
    # Standard ASCII escape sequences. If encoding is ASCII, use Test-Cases
    # sequences, otherwise, assume the test-cases escape sequences. Otherwise,
    # the N-Triples recommendation includes `\b` and `\f` escape sequences.
    #
    # Within STRING_LITERAL_QUOTE, only the characters `U+0022`, `U+005C`, `U+000A`, `U+000D` are encoded using `ECHAR`. `ECHAR` must not be used for characters that are allowed directly in STRING_LITERAL_QUOTE.
    #
    # @param  [Integer, #ord] u
    # @return [String]
    # @raise  [ArgumentError] if `u` is not a valid Unicode codepoint
    # @see    http://www.w3.org/TR/rdf-testcases/#ntrip_strings
    # @see    http://www.w3.org/TR/n-triples/
    def self.escape_ascii(u, encoding)
      case (u = u.ord)
      when (0x08)       then "\\b"
      when (0x09)       then "\\t"
      when (0x0A)       then "\\n"
      when (0x0C)       then "\\f"
      when (0x0D)       then "\\r"
      when (0x22)       then "\\\""
      when (0x5C)       then "\\\\"
      when (0x00..0x1F) then escape_utf16(u)
      when (0x7F)       then escape_utf16(u)
      when (0x20..0x7E) then u.chr
      else
        raise ArgumentError.new("expected an ASCII character in (0x00..0x7F), but got 0x#{u.to_s(16)}")
      end
    end

    ##
    # @param  [Integer, #ord] u
    # @return [String]
    # @see    http://www.w3.org/TR/rdf-testcases/#ntrip_strings
    def self.escape_utf16(u)
      sprintf("\\u%04X", u.ord)
    end

    ##
    # @param  [Integer, #ord] u
    # @return [String]
    # @see    http://www.w3.org/TR/rdf-testcases/#ntrip_strings
    def self.escape_utf32(u)
      sprintf("\\U%08X", u.ord)
    end

    ##
    # Returns the serialized N-Triples representation of the given RDF
    # value.
    #
    # @param  [RDF::Value] value
    # @return [String]
    # @raise  [ArgumentError] if `value` is not an `RDF::Statement` or `RDF::Term`
    def self.serialize(value)
      writer = (@serialize_writer_memo ||= self.new)
      case value
        when nil then nil
        when FalseClass then value.to_s
        when RDF::Statement
          writer.format_statement(value) + "\n"
        when RDF::Term
          writer.format_term(value)
        else
          raise ArgumentError, "expected an RDF::Statement or RDF::Term, but got #{value.inspect}"
      end
    end

    ##
    # Initializes the writer.
    #
    # @param  [IO, File] output
    #   the output stream
    # @param [Boolean]  validate (true)
    #   whether to validate terms when serializing
    # @param  [Hash{Symbol => Object}] options ({})
    #   any additional options. See {RDF::Writer#initialize}
    # @yield  [writer] `self`
    # @yieldparam  [RDF::Writer] writer
    # @yieldreturn [void]
    def initialize(output = $stdout, validate: true, **options, &block)
      super
    end

    ##
    # Outputs an N-Triples comment line.
    #
    # @param  [String] text
    # @return [void]
    def write_comment(text)
      puts "# #{text.chomp}" # TODO: correctly output multi-line comments
    end

    ##
    # Outputs the N-Triples representation of a triple.
    #
    # @param  [RDF::Resource] subject
    # @param  [RDF::URI]      predicate
    # @param  [RDF::Term]     object
    # @return [void]
    def write_triple(subject, predicate, object)
      puts format_triple(subject, predicate, object, **@options)
    end

    ##
    # Returns the N-Triples representation of a statement.
    #
    # @param  [RDF::Statement] statement
    # @param  [Hash{Symbol => Object}] options ({})
    # @return [String]
    def format_statement(statement, **options)
      format_triple(*statement.to_triple, **options)
    end

    ##
    # Returns the N-Triples representation of an RDF 1.2 triple term.
    #
    # @param  [RDF::Statement] statement
    # @param  [Hash{Symbol => Object}] options ({})
    # @return [String]
    def format_tripleTerm(statement, **options)
      "<<(%s %s %s)>>" % statement.to_a.map { |value| format_term(value, **options) }
    end

    ##
    # Returns the N-Triples representation of an RDF-star quoted triple.
    #
    # @param  [RDF::Statement] statement
    # @param  [Hash{Symbol => Object}] options ({})
    # @return [String]
    # @deprecated Quoted triples are now deprecated
    def format_quotedTriple(statement, **options)
      # FIXME: quoted triples are now deprecated
      "<<%s %s %s>>" % statement.to_a.map { |value| format_term(value, **options) }
    end

    ##
    # Returns the N-Triples representation of a triple.
    #
    # @param  [RDF::Resource] subject
    # @param  [RDF::URI]      predicate
    # @param  [RDF::Term]     object
    # @param  [Hash{Symbol => Object}] options ({})
    # @return [String]
    def format_triple(subject, predicate, object, **options)
      "%s %s %s ." % [subject, predicate, object].map { |value| format_term(value, **options) }
    end

    ##
    # Returns the N-Triples representation of a blank node.
    #
    # @param  [RDF::Node] node
    # @param [Boolean] unique_bnodes (false)
    #   Serialize node using unique identifier, rather than any used to create the node.
    # @param  [Hash{Symbol => Object}] options ({})
    # @return [String]
    def format_node(node, unique_bnodes: false, **options)
      unique_bnodes ? node.to_unique_base : node.to_s
    end

    ##
    # Returns the N-Triples representation of a URI reference using write encoding.
    #
    # @param  [RDF::URI] uri
    # @param  [Hash{Symbol => Object}] options ({})
    # @return [String]
    def format_uri(uri, **options)
      string = uri.to_s
      iriref = case
        when string.match?(ESCAPE_PLAIN_U) # a shortcut for the simple case
          string
        when string.ascii_only? || (encoding && encoding != Encoding::ASCII)
          StringIO.open do |buffer|
            buffer.set_encoding(encoding)
            string.each_char do |u|
              buffer << case u.ord
                when (0x00..0x20) then self.class.escape_utf16(u)
                when 0x22, 0x3c, 0x3e, 0x5c, 0x5e, 0x60, 0x7b, 0x7c, 0x7d # "<>\^`{|}
                  self.class.escape_utf16(u)
                else u
              end
            end
            buffer.string
          end
        else
          # Encode ASCII && UTF-8/16 characters
          StringIO.open do |buffer|
            buffer.set_encoding(Encoding::ASCII)
            string.each_byte do |u|
              buffer << case u
                when (0x00..0x20) then self.class.escape_utf16(u)
                when 0x22, 0x3c, 0x3e, 0x5c, 0x5e, 0x60, 0x7b, 0x7c, 0x7d # "<>\^`{|}
                  self.class.escape_utf16(u)
                when (0x80..0xFFFF)                then self.class.escape_utf16(u)
                when (0x10000..0x10FFFF)           then self.class.escape_utf32(u)
                else u
              end
            end
            buffer.string
          end
      end
      encoding ? "<#{iriref}>".encode(encoding) : "<#{iriref}>"
    end

    ##
    # Returns the N-Triples representation of a literal.
    #
    # @param  [RDF::Literal, String, #to_s] literal
    # @param  [Hash{Symbol => Object}] options ({})
    # @return [String]
    def format_literal(literal, **options)
      case literal
        when RDF::Literal
          # Note, escaping here is more robust than in Term
          text = quoted(escaped(literal.value))
          text << "@#{literal.language}" if literal.language?
          text << "--#{literal.direction}" if literal.direction?
          text << "^^<#{uri_for(literal.datatype)}>" if literal.datatype?
          text
        else
          quoted(escaped(literal.to_s))
      end
    end

    ##
    # @private
    def escaped(string)
      self.class.escape(string, encoding)
    end
  end
end