module SPARQL; module Algebra
  class Operator
    ##
    # The SPARQL `langMatches` operator.
    #
    # @example
    #   (prefix ((: <http://example.org/#>))
    #     (filter (langMatches (lang ?v) "en-GB")
    #       (bgp (triple :x ?p ?v))))
    #
    # @see https://www.w3.org/TR/sparql11-query/#func-langMatches
    # @see https://tools.ietf.org/html/rfc4647#section-3.3.1
    class LangMatches < Operator::Binary
      include Evaluatable

      NAME = :langMatches

      ##
      # Returns `true` if the language tag (the first operand) matches the
      # language range (the second operand).
      #
      # @param  [RDF::Literal] language_tag
      #   a simple literal containing a language tag
      # @param  [RDF::Literal] language_range
      #   a simple literal containing a language range, per
      #   [RFC 4647 section 2.1](https://tools.ietf.org/html/rfc4647#section-2.1)
      # @return [RDF::Literal::Boolean] `true` or `false`
      # @raise  [TypeError] if either operand is unbound
      # @raise  [TypeError] if either operand is not a simple literal
      def apply(language_tag, language_range)
        raise TypeError, "expected a plain RDF::Literal for language_tag, but got #{language_tag.inspect}" unless language_tag.is_a?(RDF::Literal) && language_tag.simple?
        language_tag = language_tag.to_s.downcase

        raise TypeError, "expected a plain RDF::Literal for language_range, but got #{language_range.inspect}" unless language_range.is_a?(RDF::Literal) && language_range.simple?
        language_range = language_range.to_s.downcase

        case
          # A language range of "*" matches any non-empty language tag.
          when language_range.eql?('*')
            RDF::Literal(!(language_tag.empty?))
          # A language range matches a particular language tag if, in a
          # case-insensitive comparison, it exactly equals the tag, ...
          when language_tag.eql?(language_range)
            RDF::Literal::TRUE
          # ... or if it exactly equals a prefix of the tag such that the
          # first character following the prefix is "-".
          else
            RDF::Literal(language_tag.start_with?(language_range + '-'))
        end
      end
    end # LangMatches
  end # Operator
end; end # SPARQL::Algebra