TurtleFragmentWriter.js

/*! @license ©2013 Ruben Verborgh - Multimedia Lab / iMinds / Ghent University */

/** A TurtleFragmentWriter represents a basic Linked Data Fragment as Turtle. */

var N3Writer = require('n3').Writer,
    N3Util = require('n3').Util,
    UriTemplate = require('uritemplate');

var dcterms = 'http://purl.org/dc/terms/',
    rdf = 'http://www.w3.org/1999/02/22-rdf-syntax-ns#',
    rdfs = 'http://www.w3.org/2000/01/rdf-schema#',
    rdfForms = 'http://www.markbaker.ca/2003/rdfforms/',
    xsd = 'http://www.w3.org/2001/XMLSchema',
    voID = 'http://rdfs.org/ns/void#';

// Creates a new TurtleFragmentWriter
function TurtleFragmentWriter(datasetName, dataset, fragment, pattern) {
  this._datasetName = datasetName;
  this._dataset = dataset;
  this._fragment = fragment;
  this._pattern = pattern;
}

TurtleFragmentWriter.prototype = {
  // Write the representation of the fragment results to the output stream
  write: function (outputStream, results) {
    var writer = new N3Writer(outputStream),
        addTriple = function (s, p, o) { writer.addTriple({ subject: s, predicate: p, object: o }); },
        // Dataset, fragment and pattern metadata
        datasetName = this._datasetName, dataset = this._dataset,
        fragment = this._fragment, pattern = this._pattern,
        subject = pattern.subject, predicate = pattern.predicate, object = pattern.object,
        subjectURI = subject && ('<' + subject + '>'),
        predicateURI = predicate && ('<' + predicate + '>'),
        objectIsLiteral = N3Util.isLiteral(object),
        objectURI = object && (objectIsLiteral ? object : '<' + object + '>'),
        patternString = ['{', subjectURI || '?s', predicateURI || '?p', objectURI || '?o', '}'].join(' '),
        // Controls metadata
        formTemplateUri = dataset + '{?subject,predicate,object}',
        formTemplate = UriTemplate.parse(formTemplateUri);

    // Add metadata about the dataset
    addTriple(dataset, voID + 'subset', fragment);
    addTriple(dataset, voID + 'uriLookupEndpoint', '"' + formTemplateUri + '"');

    // Add query form
    addTriple(dataset, rdf + 'type', rdfForms + 'indexable');
    addTriple(dataset, rdfForms + 'indexedBy', dataset + '#Subject');
    addTriple(dataset, rdfForms + 'indexedBy', dataset + '#Predicate');
    addTriple(dataset, rdfForms + 'indexedBy', dataset + '#Object');
    addTriple(dataset + '#Subject',   rdfForms + 'shortName',      '"subject"');
    addTriple(dataset + '#Subject',   rdf      + 'predicate', rdf + 'subject');
    addTriple(dataset + '#Predicate', rdfForms + 'shortName',      '"predicate"');
    addTriple(dataset + '#Predicate', rdf      + 'predicate', rdf + 'predicate');
    addTriple(dataset + '#Object',    rdfForms + 'shortName',      '"object"');
    addTriple(dataset + '#Object',    rdf      + 'predicate', rdf + 'object');

    // Add fragment metadata
    addTriple(fragment, dcterms + 'title', '"A \'' + datasetName + '\' Linked Data Fragment"@en');
    addTriple(fragment, dcterms + 'description',
              '"Basic Linked Data Fragment of the \'' + datasetName + '\' dataset ' +
              'containing triples matching the pattern ' + patternString + '."@en');
    addTriple(fragment, dcterms + 'source', dataset);
    addTriple(fragment, voID + 'triples', '"' + results.total + '"^^<' + xsd + 'integer>');

    // Add subject metadata
    if (subject) {
      addTriple(fragment, dcterms + 'subject', subject);
      if (predicate || object)
        addTriple(subject, rdfs + 'seeAlso', formTemplate.expand({ subject: subjectURI }));
      addTriple(subject, rdfs + 'seeAlso', formTemplate.expand({ object: subjectURI }));
    }

    // Add predicate metadata
    if (predicate) {
      if (!subject && !object) {
        addTriple(dataset, voID + 'propertyPartition', fragment);
        addTriple(fragment, voID + 'property', predicate);
      }
      if (subject || object)
        addTriple(predicate, rdfs + 'seeAlso', formTemplate.expand({ predicate: predicateURI }));
      addTriple(predicate, rdfs + 'seeAlso', formTemplate.expand({ subject: predicateURI }));
      addTriple(predicate, rdfs + 'seeAlso', formTemplate.expand({ object: predicateURI }));
    }

    // Add object metadata
    if (object) {
      addTriple(fragment, dcterms + 'subject', object);
      if (!subject && predicate === rdf + 'type') {
        addTriple(dataset, voID + 'classPartition', fragment);
        addTriple(fragment, voID + 'class', object);
      }
      if (!objectIsLiteral) {
        if (subject || predicate)
          addTriple(object, rdfs + 'seeAlso', formTemplate.expand({ object: objectURI }));
        addTriple(object, rdfs + 'seeAlso', formTemplate.expand({ subject: objectURI }));
      }
    }

    // Write all result data
    results.triples.forEach(writer.addTriple.bind(writer));
    writer.end();
  },
};

module.exports = TurtleFragmentWriter;