;;;; Copyright (C) 2020 Julien Lepiller <julien@lepiller.eu>

;;;; 

;;;; This library is free software; you can redistribute it and/or

;;;; modify it under the terms of the GNU Lesser General Public

;;;; License as published by the Free Software Foundation; either

;;;; version 3 of the License, or (at your option) any later version.

;;;; 

;;;; This library is distributed in the hope that it will be useful,

;;;; but WITHOUT ANY WARRANTY; without even the implied warranty of

;;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

;;;; Lesser General Public License for more details.

;;;; 

;;;; You should have received a copy of the GNU Lesser General Public

;;;; License along with this library; if not, write to the Free Software

;;;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA

;;;; 

(define-module (rdf rdf)

  #:use-module (srfi srfi-9)

  #:use-module (ice-9 match)

  #:export (rdf-datatype

            make-rdf-datatype

            rdf-datatype?

            rdf-datatype-iris

            rdf-datatype-description

            rdf-datatype-lexical?

            rdf-datatype-value?

            rdf-datatype-lexical->value

            rdf-datatype-value->lexical

            rdf-dataset

            make-rdf-dataset

            rdf-dataset?

            rdf-dataset-default-graph

            rdf-dataset-named-graphs

            rdf-triple

            make-rdf-triple

            rdf-triple?

            rdf-triple-subject

            rdf-triple-predicate

            rdf-triple-object

            rdf-literal

            make-rdf-literal

            rdf-literal?

            rdf-literal-lexical-form

            rdf-literal-type

            rdf-literal-langtag

            blank-node?

            rdf-graph?

            merge-graphs

            ))

;; From the specification:

;;   Datatypes are used with RDF literals to represent values such as strings,

;;   numbers and dates.  A datatype consists of a lexical space, a value space

;;   and a lexical-to-value mapping, and is denoted by one or more IRIs.

;;

;;   The lexical space of a datatype is a set of Unicode [UNICODE] strings.

;;

;;   The lexical-to-value mapping of a datatype is a set of pairs whose first

;;   element belongs to the lexical space, and the second element belongs to the

;;   value space of the datatype.  Each member of the lexical space is paired

;;   with exactly one value, and is a lexical representation of that value.  The

;;   mapping can be seen as a function from the lexical space to the value space.

;;

;; In addition to the specification, we introduce value->lexical, a canonical

;; function to map values to the lexical space.  An important property is that

;;   for any val, (value? val) implies:

;;           (equal? (lexical->value (value->lexical val)) val)

;;

;; We also introduce a list of IRIs that denote this type, as more than one

;; IRI can denote a type.  This is set to a list of IRIs, but may be changed

;; to a function to denote a set in the future.

;;

;; We also introduce a description, a text that helps humans understand the

;; purpose of the datatype.

(define-record-type rdf-datatype

  (make-rdf-datatype iris description lexical? value? lexical->value value->lexical)

  rdf-datatype?

  (iris           rdf-datatype-iris)

  (description    rdf-datatype-description)

  (lexical?       rdf-datatype-lexical?)

  (value?         rdf-datatype-value?)

  (lexical->value rdf-datatype-lexical->value)

  (value->lexical rdf-datatype-value->lexical))

;; From the specification:

;;   An RDF dataset is a collection of RDF graphs, and comprises:

;;

;;   * Exactly one default graph, being an RDF graph.  The default graph does

;;     not have a name and MAY be empty.

;;   * Zero or more named graphs.  Each named graph is a pair consisting of an

;;     IRI or a blank node (the graph name), and an RDF graph.  Graph names are

;;     unique within an RDF dataset.

;;

;; We represent named graphs with a association list whose keys are IRIs or

;; blank nodes, and values are RDF graphs.

(define-record-type rdf-dataset

  (make-rdf-dataset default-graph named-graphs)

  rdf-dataset?

  (default-graph rdf-dataset-default-graph)

  (named-graphs  rdf-dataset-named-graphs))

;; From the specification:

;;   An RDF triple consists of three components:

;;

;;   * the subject, which is an IRI or a blank node

;;   * the predicate, which is an IRI

;;   * the object, which is an IRI, a literal or a blank node

(define-record-type rdf-triple

  (make-rdf-triple subject predicate object)

  rdf-triple?

  (subject   rdf-triple-subject)

  (predicate rdf-triple-predicate)

  (object    rdf-triple-object))

;; From the specification:

;;   A literal in an RDF graph consists of two or three elements:

;;

;;   * a lexical form, being a Unicode [UNICODE] string, which SHOULD be in

;;     Normal Form C [NFC],

;;   * a datatype IRI, being an IRI identifying a datatype that determines how

;;     the lexical form maps to a literal value, and

;;   * if and only if the datatype IRI is `http://www.w3.org/1999/02/22-rdf-syntax-ns#langString`,

;;     a non-empty language tag as defined by [BCP47].  The language tag MUST

;;     be well-formed according to section 2.2.9 of [BCP47].

(define-record-type rdf-literal

  (make-rdf-literal lexical-form datatype language-tag)

  rdf-literal?

  (lexical-form rdf-literal-lexical-form)

  (datatype     rdf-literal-datatype)

  (language-tag rdf-literal-language-tag))

;; From the specification:

;;   Blank nodes are disjoint from IRIs and literals.  Otherwise, the set of

;;   possible blank nodes is arbitrary.  RDF makes no reference to any internal

;;   structure of blank nodes.

;;

;; Here, we will use integers as blank nodes

(define blank-node? integer?)

;; From the specification:

;;   An RDF graph is a set of RDF triples.

;;

;; We represent a graph as a list of RDF triples

(define (rdf-graph? graph)

  (and (list? graph) (null? (filter (lambda (t) (not (rdf-triple? t))) graph))))

(define (last-blank g)

  "Retun the biggest blank node identifier in g"

  (let loop ((g g) (m 0))

    (match g

      ('() m)

      ((triple g ...)

       (loop g (max m

                   (if (blank-node? (rdf-triple-subject triple))

                       (rdf-triple-subject triple)

                       0)

                   (if (blank-node? (rdf-triple-object triple))

                       (rdf-triple-object triple)

                       0)))))))

(define (rename-blanks g num)

  "Return the same graph, but blank nodes are renamed from num"

  (let loop ((g g) (renamings '()) (num num) (result '()))

    (match g

      ('() result)

      ((triple g ...)

       (let* ((subject (rdf-triple-subject triple))

              (num (if (and (blank-node? subject)

                            (assoc-ref renamings subject))

                       num

                       (+ num 1)))

              (renamings

                (if (and (blank-node? subject)

                         (assoc-ref renamings subject))

                    renamings

                    (cons (cons subject num) renamings)))

              (subject

                (if (blank-node? subject)

                    (assoc-ref renamings subject)

                    subject))

              (predicate (rdf-triple-predicate triple))

              (object (rdf-triple-object triple))

              (num (if (and (blank-node? object)

                            (assoc-ref renamings object))

                       num

                       (+ num 1)))

              (renamings

                (if (and (blank-node? object)

                         (assoc-ref renamings object))

                    renamings

                    (cons (cons object num) renamings)))

              (object

                (if (blank-node? object)

                    (assoc-ref renamings object)

                    object)))

           (loop g renamings num (cons (make-rdf-triple subject predicate object)

                                       result)))))))

(define (merge-graphs g1 g2)

  "Merge two graphs g1 and g2.  This is the same as append, but we need to make

sure we rename blank nodes, or some nodes will be merged when they shouldn't."

  (append g1 (rename-blanks g2 (last-blank g1))))