/jsonld.php

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<?php
/**
 * PHP implementation of the JSON-LD API.
 * Version: 0.4.8-dev
 *
 * @author Dave Longley
 *
 * BSD 3-Clause License
 * Copyright (c) 2011-2014 Digital Bazaar, Inc.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 * Redistributions of source code must retain the above copyright notice,
 * this list of conditions and the following disclaimer.
 *
 * Redistributions in binary form must reproduce the above copyright
 * notice, this list of conditions and the following disclaimer in the
 * documentation and/or other materials provided with the distribution.
 *
 * Neither the name of the Digital Bazaar, Inc. nor the names of its
 * contributors may be used to endorse or promote products derived from
 * this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
 * IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
 * PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED
 * TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

/**
 * Performs JSON-LD compaction.
 *
 * @param mixed $input the JSON-LD object to compact.
 * @param mixed $ctx the context to compact with.
 * @param assoc [$options] options to use:
 *          [base] the base IRI to use.
 *          [graph] true to always output a top-level graph (default: false).
 *          [documentLoader(url)] the document loader.
 *
 * @return mixed the compacted JSON-LD output.
 */
function jsonld_compact($input, $ctx, $options=array()) {
  $p = new JsonLdProcessor();
  return $p->compact($input, $ctx, $options);
}

/**
 * Performs JSON-LD expansion.
 *
 * @param mixed $input the JSON-LD object to expand.
 * @param assoc[$options] the options to use:
 *          [base] the base IRI to use.
 *          [documentLoader(url)] the document loader.
 *
 * @return array the expanded JSON-LD output.
 */
function jsonld_expand($input, $options=array()) {
  $p = new JsonLdProcessor();
  return $p->expand($input, $options);
}

/**
 * Performs JSON-LD flattening.
 *
 * @param mixed $input the JSON-LD to flatten.
 * @param mixed $ctx the context to use to compact the flattened output, or
 *          null.
 * @param [options] the options to use:
 *          [base] the base IRI to use.
 *          [documentLoader(url)] the document loader.
 *
 * @return mixed the flattened JSON-LD output.
 */
function jsonld_flatten($input, $ctx, $options=array()) {
  $p = new JsonLdProcessor();
  return $p->flatten($input, $ctx, $options);
}

/**
 * Performs JSON-LD framing.
 *
 * @param mixed $input the JSON-LD object to frame.
 * @param stdClass $frame the JSON-LD frame to use.
 * @param assoc [$options] the framing options.
 *          [base] the base IRI to use.
 *          [embed] default @embed flag (default: true).
 *          [explicit] default @explicit flag (default: false).
 *          [requireAll] default @requireAll flag (default: true).
 *          [omitDefault] default @omitDefault flag (default: false).
 *          [documentLoader(url)] the document loader.
 *
 * @return stdClass the framed JSON-LD output.
 */
function jsonld_frame($input, $frame, $options=array()) {
  $p = new JsonLdProcessor();
  return $p->frame($input, $frame, $options);
}

/**
 * **Experimental**
 *
 * Links a JSON-LD document's nodes in memory.
 *
 * @param mixed $input the JSON-LD document to link.
 * @param mixed $ctx the JSON-LD context to apply or null.
 * @param assoc [$options] the options to use:
 *          [base] the base IRI to use.
 *          [expandContext] a context to expand with.
 *          [documentLoader(url)] the document loader.
 *
 * @return the linked JSON-LD output.
 */
function jsonld_link($input, $ctx, $options) {
  // API matches running frame with a wildcard frame and embed: '@link'
  // get arguments
  $frame = new stdClass();
  if($ctx) {
    $frame->{'@context'} = $ctx;
  }
  $frame->{'@embed'} = '@link';
  return jsonld_frame($input, $frame, $options);
};

/**
 * Performs RDF dataset normalization on the given input. The input is
 * JSON-LD unless the 'inputFormat' option is used. The output is an RDF
 * dataset unless the 'format' option is used.
 *
 * @param mixed $input the JSON-LD object to normalize.
 * @param assoc [$options] the options to use:
 *          [base] the base IRI to use.
 *          [intputFormat] the format if input is not JSON-LD:
 *            'application/nquads' for N-Quads.
 *          [format] the format if output is a string:
 *            'application/nquads' for N-Quads.
 *          [documentLoader(url)] the document loader.
 *
 * @return mixed the normalized output.
 */
function jsonld_normalize($input, $options=array()) {
  $p = new JsonLdProcessor();
  return $p->normalize($input, $options);
}

/**
 * Converts an RDF dataset to JSON-LD.
 *
 * @param mixed $input a serialized string of RDF in a format specified
 *          by the format option or an RDF dataset to convert.
 * @param assoc [$options] the options to use:
 *          [format] the format if input not an array:
 *            'application/nquads' for N-Quads (default).
 *          [useRdfType] true to use rdf:type, false to use @type
 *            (default: false).
 *          [useNativeTypes] true to convert XSD types into native types
 *            (boolean, integer, double), false not to (default: false).
 *
 * @return array the JSON-LD output.
 */
function jsonld_from_rdf($input, $options=array()) {
  $p = new JsonLdProcessor();
  return $p->fromRDF($input, $options);
}

/**
 * Outputs the RDF dataset found in the given JSON-LD object.
 *
 * @param mixed $input the JSON-LD object.
 * @param assoc [$options] the options to use:
 *          [base] the base IRI to use.
 *          [format] the format to use to output a string:
 *            'application/nquads' for N-Quads.
 *          [produceGeneralizedRdf] true to output generalized RDF, false
 *            to produce only standard RDF (default: false).
 *          [documentLoader(url)] the document loader.
 *
 * @return mixed the resulting RDF dataset (or a serialization of it).
 */
function jsonld_to_rdf($input, $options=array()) {
  $p = new JsonLdProcessor();
  return $p->toRDF($input, $options);
}

/**
 * JSON-encodes (with unescaped slashes) the given stdClass or array.
 *
 * @param mixed $input the native PHP stdClass or array which will be
 *          converted to JSON by json_encode().
 * @param int $options the options to use.
 *          [JSON_PRETTY_PRINT] pretty print.
 * @param int $depth the maximum depth to use.
 *
 * @return the encoded JSON data.
 */
function jsonld_encode($input, $options=0, $depth=512) {
  // newer PHP has a flag to avoid escaped '/'
  if(defined('JSON_UNESCAPED_SLASHES')) {
     return json_encode($input, JSON_UNESCAPED_SLASHES | $options, $depth);
  }
  // use a simple string replacement of '\/' to '/'.
  return str_replace('\\/', '/', json_encode($input, $options, $depth));
}

/**
 * Decodes a serialized JSON-LD object.
 *
 * @param string $input the JSON-LD input.
 *
 * @return mixed the resolved JSON-LD object, null on error.
 */
function jsonld_decode($input) {
  return json_decode($input);
}

/**
 * Parses a link header. The results will be key'd by the value of "rel".
 *
 * Link: <http://json-ld.org/contexts/person.jsonld>; rel="http://www.w3.org/ns/json-ld#context"; type="application/ld+json"
 *
 * Parses as: {
 *   'http://www.w3.org/ns/json-ld#context': {
 *     target: http://json-ld.org/contexts/person.jsonld,
 *     type: 'application/ld+json'
 *   }
 * }
 *
 * If there is more than one "rel" with the same IRI, then entries in the
 * resulting map for that "rel" will be arrays of objects, otherwise they will
 * be single objects.
 *
 * @param string $header the link header to parse.
 *
 * @return assoc the parsed result.
 */
function jsonld_parse_link_header($header) {
  $rval = array();
  // split on unbracketed/unquoted commas
  if(!preg_match_all(
    '/(?:<[^>]*?>|"[^"]*?"|[^,])+/', $header, $entries, PREG_SET_ORDER)) {
    return $rval;
  }
  $r_link_header = '/\s*<([^>]*?)>\s*(?:;\s*(.*))?/';
  foreach($entries as $entry) {
    if(!preg_match($r_link_header, $entry[0], $match)) {
      continue;
    }
    $result = (object)array('target' => $match[1]);
    $params = $match[2];
    $r_params = '/(.*?)=(?:(?:"([^"]*?)")|([^"]*?))\s*(?:(?:;\s*)|$)/';
    preg_match_all($r_params, $params, $matches, PREG_SET_ORDER);
    foreach($matches as $match) {
      $result->{$match[1]} = $match[2] ?: $match[3];
    }
    $rel = property_exists($result, 'rel') ? $result->rel : '';
    if(!isset($rval[$rel])) {
      $rval[$rel] = $result;
    } else if(is_array($rval[$rel])) {
      $rval[$rel][] = $result;
    } else {
      $rval[$rel] = array($rval[$rel], $result);
    }
  }
  return $rval;
}

/**
 * Relabels all blank nodes in the given JSON-LD input.
 *
 * @param mixed input the JSON-LD input.
 */
function jsonld_relabel_blank_nodes($input) {
  $p = new JsonLdProcessor();
  return $p->_labelBlankNodes(new UniqueNamer('_:b'), $input);
}

/** JSON-LD shared in-memory cache. */
global $jsonld_cache;
$jsonld_cache = new stdClass();

/** The default active context cache. */
$jsonld_cache->activeCtx = new ActiveContextCache();

/** Stores the default JSON-LD document loader. */
global $jsonld_default_load_document;
$jsonld_default_load_document = 'jsonld_default_document_loader';

/**
 * Sets the default JSON-LD document loader.
 *
 * @param callable load_document(url) the document loader.
 */
function jsonld_set_document_loader($load_document) {
  global $jsonld_default_load_document;
  $jsonld_default_load_document = $load_document;
}

/**
 * Retrieves JSON-LD at the given URL.
 *
 * @param string $url the URL to retrieve.
 *
 * @return the JSON-LD.
 */
function jsonld_get_url($url) {
  global $jsonld_default_load_document;
  if($jsonld_default_load_document !== null) {
    $document_loader = $jsonld_default_load_document;
  } else {
    $document_loader = 'jsonld_default_document_loader';
  }

  $remote_doc = call_user_func($document_loader, $url);
  if($remote_doc) {
    return $remote_doc->document;
  }
  return null;
}

/**
 * The default implementation to retrieve JSON-LD at the given URL.
 *
 * @param string $url the URL to to retrieve.
 *
 * @return stdClass the RemoteDocument object.
 */
function jsonld_default_document_loader($url) {
  $doc = (object)array(
    'contextUrl' => null, 'document' => null, 'documentUrl' => $url);
  $redirects = array();

  $opts = array(
    'http' => array(
      'method' => 'GET',
      'header' =>
        "Accept: application/ld+json\r\n"),
    /* Note: Use jsonld_default_secure_document_loader for security. */
    'ssl' => array(
      'verify_peer' => false,
      'allow_self_signed' => true)
  );

  $context = stream_context_create($opts);
  $content_type = null;
  stream_context_set_params($context, array('notification' =>
    function($notification_code, $severity, $message) use (
      &$redirects, &$content_type) {
      switch($notification_code) {
      case STREAM_NOTIFY_REDIRECTED:
        $redirects[] = $message;
        break;
      case STREAM_NOTIFY_MIME_TYPE_IS:
        $content_type = $message;
        break;
      };
    }));
  $result = @file_get_contents($url, false, $context);
  if($result === false) {
    throw new JsonLdException(
      'Could not retrieve a JSON-LD document from the URL: ' . $url,
      'jsonld.LoadDocumentError', 'loading document failed');
  }
  $link_header = array();
  foreach($http_response_header as $header) {
    if(strpos($header, 'link') === 0) {
      $value = explode(': ', $header);
      if(count($value) > 1) {
        $link_header[] = $value[1];
      }
    }
  }
  $link_header = jsonld_parse_link_header(join(',', $link_header));
  if(isset($link_header['http://www.w3.org/ns/json-ld#context'])) {
    $link_header = $link_header['http://www.w3.org/ns/json-ld#context'];
  } else {
    $link_header = null;
  }
  if($link_header && $content_type !== 'application/ld+json') {
    // only 1 related link header permitted
    if(is_array($link_header)) {
      throw new JsonLdException(
        'URL could not be dereferenced, it has more than one ' .
        'associated HTTP Link Header.', 'jsonld.LoadDocumentError',
        'multiple context link headers', array('url' => $url));
    }
    $doc->{'contextUrl'} = $link_header->target;
  }

  // update document url based on redirects
  $redirs = count($redirects);
  if($redirs > 0) {
    $url = $redirects[$redirs - 1];
  }
  $doc->document = $result;
  $doc->documentUrl = $url;
  return $doc;
}

/**
 * The default implementation to retrieve JSON-LD at the given secure URL.
 *
 * @param string $url the secure URL to to retrieve.
 *
 * @return stdClass the RemoteDocument object.
 */
function jsonld_default_secure_document_loader($url) {
  if(strpos($url, 'https') !== 0) {
    throw new JsonLdException(
      "Could not GET url: '$url'; 'https' is required.",
      'jsonld.LoadDocumentError', 'loading document failed');
  }

  $doc = (object)array(
    'contextUrl' => null, 'document' => null, 'documentUrl' => $url);
  $redirects = array();

  // default JSON-LD https GET implementation
  $opts = array(
    'http' => array(
      'method' => 'GET',
      'header' =>
        "Accept: application/ld+json\r\n"),
    'ssl' => array(
      'verify_peer' => true,
      'allow_self_signed' => false,
      'cafile' => '/etc/ssl/certs/ca-certificates.crt'));
  $context = stream_context_create($opts);
  $content_type = null;
  stream_context_set_params($context, array('notification' =>
    function($notification_code, $severity, $message) use (
      &$redirects, &$content_type) {
      switch($notification_code) {
      case STREAM_NOTIFY_REDIRECTED:
        $redirects[] = $message;
        break;
      case STREAM_NOTIFY_MIME_TYPE_IS:
        $content_type = $message;
        break;
      };
    }));
  $result = @file_get_contents($url, false, $context);
  if($result === false) {
    throw new JsonLdException(
      'Could not retrieve a JSON-LD document from the URL: ' + $url,
      'jsonld.LoadDocumentError', 'loading document failed');
  }
  $link_header = array();
  foreach($http_response_header as $header) {
    if(strpos($header, 'link') === 0) {
      $value = explode(': ', $header);
      if(count($value) > 1) {
        $link_header[] = $value[1];
      }
    }
  }
  $link_header = jsonld_parse_link_header(join(',', $link_header));
  if(isset($link_header['http://www.w3.org/ns/json-ld#context'])) {
    $link_header = $link_header['http://www.w3.org/ns/json-ld#context'];
  } else {
    $link_header = null;
  }
  if($link_header && $content_type !== 'application/ld+json') {
    // only 1 related link header permitted
    if(is_array($link_header)) {
      throw new JsonLdException(
        'URL could not be dereferenced, it has more than one ' .
        'associated HTTP Link Header.', 'jsonld.LoadDocumentError',
        'multiple context link headers', array('url' => $url));
    }
    $doc->{'contextUrl'} = $link_header->target;
  }

  // update document url based on redirects
  foreach($redirects as $redirect) {
    if(strpos($redirect, 'https') !== 0) {
      throw new JsonLdException(
        "Could not GET redirected url: '$redirect'; 'https' is required.",
        'jsonld.LoadDocumentError', 'loading document failed');
    }
    $url = $redirect;
  }
  $doc->document = $result;
  $doc->documentUrl = $url;
  return $doc;
}

/** Registered global RDF dataset parsers hashed by content-type. */
global $jsonld_rdf_parsers;
$jsonld_rdf_parsers = new stdClass();

/**
 * Registers a global RDF dataset parser by content-type, for use with
 * jsonld_from_rdf. Global parsers will be used by JsonLdProcessors that do
 * not register their own parsers.
 *
 * @param string $content_type the content-type for the parser.
 * @param callable $parser(input) the parser function (takes a string as
 *          a parameter and returns an RDF dataset).
 */
function jsonld_register_rdf_parser($content_type, $parser) {
  global $jsonld_rdf_parsers;
  $jsonld_rdf_parsers->{$content_type} = $parser;
}

/**
 * Unregisters a global RDF dataset parser by content-type.
 *
 * @param string $content_type the content-type for the parser.
 */
function jsonld_unregister_rdf_parser($content_type) {
  global $jsonld_rdf_parsers;
  if(property_exists($jsonld_rdf_parsers, $content_type)) {
    unset($jsonld_rdf_parsers->{$content_type});
  }
}

/**
 * Parses a URL into its component parts.
 *
 * @param string $url the URL to parse.
 *
 * @return assoc the parsed URL.
 */
function jsonld_parse_url($url) {
  if($url === null) {
    $url = '';
  }

  $keys = array(
    'href', 'protocol', 'scheme', '?authority', 'authority',
    '?auth', 'auth', 'user', 'pass', 'host', '?port', 'port', 'path',
    '?query', 'query', '?fragment', 'fragment');
  $regex = "/^(([^:\/?#]+):)?(\/\/(((([^:@]*)(?::([^:@]*))?)?@)?([^:\/?#]*)(:(\d*))?))?([^?#]*)(\?([^#]*))?(#(.*))?/";
  preg_match($regex, $url, $match);

  $rval = array();
  $flags = array();
  $len = count($keys);
  for($i = 0; $i < $len; ++$i) {
    $key = $keys[$i];
    if(strpos($key, '?') === 0) {
      $flags[substr($key, 1)] = !empty($match[$i]);
    } else if(!isset($match[$i])) {
      $rval[$key] = null;
    } else {
      $rval[$key] = $match[$i];
    }
  }

  if(!$flags['authority']) {
    $rval['authority'] = null;
  }
  if(!$flags['auth']) {
    $rval['auth'] = $rval['user'] = $rval['pass'] = null;
  }
  if(!$flags['port']) {
    $rval['port'] = null;
  }
  if(!$flags['query']) {
    $rval['query'] = null;
  }
  if(!$flags['fragment']) {
    $rval['fragment'] = null;
  }

  $rval['normalizedPath'] = jsonld_remove_dot_segments(
    $rval['path'], !!$rval['authority']);

  return $rval;
}

/**
 * Removes dot segments from a URL path.
 *
 * @param string $path the path to remove dot segments from.
 * @param bool $has_authority true if the URL has an authority, false if not.
 */
function jsonld_remove_dot_segments($path, $has_authority) {
  $rval = '';

  if(strpos($path, '/') === 0) {
    $rval = '/';
  }

  // RFC 3986 5.2.4 (reworked)
  $input = explode('/', $path);
  $output = array();
  while(count($input) > 0) {
    if($input[0] === '.' || ($input[0] === '' && count($input) > 1)) {
      array_shift($input);
      continue;
    }
    if($input[0] === '..') {
      array_shift($input);
      if($has_authority ||
        (count($output) > 0 && $output[count($output) - 1] !== '..')) {
        array_pop($output);
      } else {
        // leading relative URL '..'
        $output[] = '..';
      }
      continue;
    }
    $output[] = array_shift($input);
  }

  return $rval . implode('/', $output);
}

/**
 * Prepends a base IRI to the given relative IRI.
 *
 * @param mixed $base a string or the parsed base IRI.
 * @param string $iri the relative IRI.
 *
 * @return string the absolute IRI.
 */
function jsonld_prepend_base($base, $iri) {
  // skip IRI processing
  if($base === null) {
    return $iri;
  }

  // already an absolute IRI
  if(strpos($iri, ':') !== false) {
    return $iri;
  }

  // parse base if it is a string
  if(is_string($base)) {
    $base = jsonld_parse_url($base);
  }

  // parse given IRI
  $rel = jsonld_parse_url($iri);

  // per RFC3986 5.2.2
  $transform = array('protocol' => $base['protocol']);

  if($rel['authority'] !== null) {
    $transform['authority'] = $rel['authority'];
    $transform['path'] = $rel['path'];
    $transform['query'] = $rel['query'];
  } else {
    $transform['authority'] = $base['authority'];

    if($rel['path'] === '') {
      $transform['path'] = $base['path'];
      if($rel['query'] !== null) {
        $transform['query'] = $rel['query'];
      } else {
        $transform['query'] = $base['query'];
      }
    } else {
      if(strpos($rel['path'], '/') === 0) {
        // IRI represents an absolute path
        $transform['path'] = $rel['path'];
      } else {
        // merge paths
        $path = $base['path'];

        // append relative path to the end of the last directory from base
        if($rel['path'] !== '') {
          $idx = strrpos($path, '/');
          $idx = ($idx === false) ? 0 : $idx + 1;
          $path = substr($path, 0, $idx);
          if(strlen($path) > 0 && substr($path, -1) !== '/') {
            $path .= '/';
          }
          $path .= $rel['path'];
        }

        $transform['path'] = $path;
      }
      $transform['query'] = $rel['query'];
    }
  }

  // remove slashes and dots in path
  $transform['path'] = jsonld_remove_dot_segments(
    $transform['path'], !!$transform['authority']);

  // construct URL
  $rval = $transform['protocol'];
  if($transform['authority'] !== null) {
    $rval .= '//' . $transform['authority'];
  }
  $rval .= $transform['path'];
  if($transform['query'] !== null) {
    $rval .= '?' . $transform['query'];
  }
  if($rel['fragment'] !== null) {
    $rval .= '#' . $rel['fragment'];
  }

  // handle empty base
  if($rval === '') {
    $rval = './';
  }

  return $rval;
}

/**
 * Removes a base IRI from the given absolute IRI.
 *
 * @param mixed $base the base IRI.
 * @param string $iri the absolute IRI.
 *
 * @return string the relative IRI if relative to base, otherwise the absolute
 *           IRI.
 */
function jsonld_remove_base($base, $iri) {
  // skip IRI processing
  if($base === null) {
    return $iri;
  }

  if(is_string($base)) {
    $base = jsonld_parse_url($base);
  }

  // establish base root
  $root = '';
  if($base['href'] !== '') {
    $root .= "{$base['protocol']}//{$base['authority']}";
  } else if(strpos($iri, '//') === false) {
    // support network-path reference with empty base
    $root .= '//';
  }

  // IRI not relative to base
  if($root === '' || strpos($iri, $root) !== 0) {
    return $iri;
  }

  // remove root from IRI
  $rel = jsonld_parse_url(substr($iri, strlen($root)));

  // remove path segments that match (do not remove last segment unless there
  // is a hash or query)
  $base_segments = explode('/', $base['normalizedPath']);
  $iri_segments = explode('/', $rel['normalizedPath']);
  $last = ($rel['query'] || $rel['fragment']) ? 0 : 1;
  while(count($base_segments) > 0 && count($iri_segments) > $last) {
    if($base_segments[0] !== $iri_segments[0]) {
      break;
    }
    array_shift($base_segments);
    array_shift($iri_segments);
  }

  // use '../' for each non-matching base segment
  $rval = '';
  if(count($base_segments) > 0) {
    // don't count the last segment (if it ends with '/' last path doesn't
    // count and if it doesn't end with '/' it isn't a path)
    array_pop($base_segments);
    foreach($base_segments as $segment) {
      $rval .= '../';
    }
  }

  // prepend remaining segments
  $rval .= implode('/', $iri_segments);

  // add query and hash
  if($rel['query'] !== null) {
    $rval .= "?{$rel['query']}";
  }
  if($rel['fragment'] !== null) {
    $rval .= "#{$rel['fragment']}";
  }

  if($rval === '') {
    $rval = './';
  }

  return $rval;
}


/**
 * A JSON-LD processor.
 */
class JsonLdProcessor {
  /** XSD constants */
  const XSD_BOOLEAN = 'http://www.w3.org/2001/XMLSchema#boolean';
  const XSD_DOUBLE = 'http://www.w3.org/2001/XMLSchema#double';
  const XSD_INTEGER = 'http://www.w3.org/2001/XMLSchema#integer';
  const XSD_STRING = 'http://www.w3.org/2001/XMLSchema#string';

  /** RDF constants */
  const RDF_LIST = 'http://www.w3.org/1999/02/22-rdf-syntax-ns#List';
  const RDF_FIRST = 'http://www.w3.org/1999/02/22-rdf-syntax-ns#first';
  const RDF_REST = 'http://www.w3.org/1999/02/22-rdf-syntax-ns#rest';
  const RDF_NIL = 'http://www.w3.org/1999/02/22-rdf-syntax-ns#nil';
  const RDF_TYPE = 'http://www.w3.org/1999/02/22-rdf-syntax-ns#type';
  const RDF_LANGSTRING =
    'http://www.w3.org/1999/02/22-rdf-syntax-ns#langString';

  /** Restraints */
  const MAX_CONTEXT_URLS = 10;

  /** Processor-specific RDF dataset parsers. */
  protected $rdfParsers = null;

  /**
   * Constructs a JSON-LD processor.
   */
  public function __construct() {}

  /**
   * Performs JSON-LD compaction.
   *
   * @param mixed $input the JSON-LD object to compact.
   * @param mixed $ctx the context to compact with.
   * @param assoc $options the compaction options.
   *          [base] the base IRI to use.
   *          [compactArrays] true to compact arrays to single values when
   *            appropriate, false not to (default: true).
   *          [graph] true to always output a top-level graph (default: false).
   *          [skipExpansion] true to assume the input is expanded and skip
   *            expansion, false not to, defaults to false.
   *          [activeCtx] true to also return the active context used.
   *          [documentLoader(url)] the document loader.
   *
   * @return mixed the compacted JSON-LD output.
   */
  public function compact($input, $ctx, $options) {
    global $jsonld_default_load_document;

    if($ctx === null) {
      throw new JsonLdException(
        'The compaction context must not be null.',
        'jsonld.CompactError', 'invalid local context');
    }

    // nothing to compact
    if($input === null) {
      return null;
    }

    self::setdefaults($options, array(
      'base' => is_string($input) ? $input : '',
      'compactArrays' => true,
      'graph' => false,
      'skipExpansion' => false,
      'activeCtx' => false,
      'documentLoader' => $jsonld_default_load_document,
      'link' => false));
    if($options['link']) {
      // force skip expansion when linking, "link" is not part of the
      // public API, it should only be called from framing
      $options['skipExpansion'] = true;
    }

    if($options['skipExpansion'] === true) {
      $expanded = $input;
    } else {
      // expand input
      try {
        $expanded = $this->expand($input, $options);
      } catch(JsonLdException $e) {
        throw new JsonLdException(
          'Could not expand input before compaction.',
          'jsonld.CompactError', null, null, $e);
      }
    }

    // process context
    $active_ctx = $this->_getInitialContext($options);
    try {
      $active_ctx = $this->processContext($active_ctx, $ctx, $options);
    } catch(JsonLdException $e) {
      throw new JsonLdException(
        'Could not process context before compaction.',
        'jsonld.CompactError', null, null, $e);
    }

    // do compaction
    $compacted = $this->_compact($active_ctx, null, $expanded, $options);

    if($options['compactArrays'] &&
      !$options['graph'] && is_array($compacted)) {
      if(count($compacted) === 1) {
        // simplify to a single item
        $compacted = $compacted[0];
      } else if(count($compacted) === 0) {
        // simplify to an empty object
        $compacted = new stdClass();
      }
    } else if($options['graph']) {
      // always use array if graph option is on
      $compacted = self::arrayify($compacted);
    }

    // follow @context key
    if(is_object($ctx) && property_exists($ctx, '@context')) {
      $ctx = $ctx->{'@context'};
    }

    // build output context
    $ctx = self::copy($ctx);
    $ctx = self::arrayify($ctx);

    // remove empty contexts
    $tmp = $ctx;
    $ctx = array();
    foreach($tmp as $v) {
      if(!is_object($v) || count(array_keys((array)$v)) > 0) {
        $ctx[] = $v;
      }
    }

    // remove array if only one context
    $ctx_length = count($ctx);
    $has_context = ($ctx_length > 0);
    if($ctx_length === 1) {
      $ctx = $ctx[0];
    }

    // add context and/or @graph
    if(is_array($compacted)) {
      // use '@graph' keyword
      $kwgraph = $this->_compactIri($active_ctx, '@graph');
      $graph = $compacted;
      $compacted = new stdClass();
      if($has_context) {
        $compacted->{'@context'} = $ctx;
      }
      $compacted->{$kwgraph} = $graph;
    } else if(is_object($compacted) && $has_context) {
      // reorder keys so @context is first
      $graph = $compacted;
      $compacted = new stdClass();
      $compacted->{'@context'} = $ctx;
      foreach($graph as $k => $v) {
        $compacted->{$k} = $v;
      }
    }

    if($options['activeCtx']) {
      return array('compacted' => $compacted, 'activeCtx' => $active_ctx);
    }

    return $compacted;
  }

  /**
   * Performs JSON-LD expansion.
   *
   * @param mixed $input the JSON-LD object to expand.
   * @param assoc $options the options to use:
   *          [base] the base IRI to use.
   *          [expandContext] a context to expand with.
   *          [keepFreeFloatingNodes] true to keep free-floating nodes,
   *            false not to, defaults to false.
   *          [documentLoader(url)] the document loader.
   *
   * @return array the expanded JSON-LD output.
   */
  public function expand($input, $options) {
    global $jsonld_default_load_document;
    self::setdefaults($options, array(
      'keepFreeFloatingNodes' => false,
      'documentLoader' => $jsonld_default_load_document));

    // if input is a string, attempt to dereference remote document
    if(is_string($input)) {
      $remote_doc = call_user_func($options['documentLoader'], $input);
    } else {
      $remote_doc = (object)array(
        'contextUrl' => null,
        'documentUrl' => null,
        'document' => $input);
    }

    try {
      if($remote_doc->document === null) {
        throw new JsonLdException(
          'No remote document found at the given URL.',
          'jsonld.NullRemoteDocument');
      }
      if(is_string($remote_doc->document)) {
        $remote_doc->document = self::_parse_json($remote_doc->document);
      }
    } catch(Exception $e) {
      throw new JsonLdException(
        'Could not retrieve a JSON-LD document from the URL.',
        'jsonld.LoadDocumentError', 'loading document failed',
        array('remoteDoc' => $remote_doc), $e);
    }

    // set default base
    self::setdefault($options, 'base', $remote_doc->documentUrl ?: '');

    // build meta-object and retrieve all @context urls
    $input = (object)array(
      'document' => self::copy($remote_doc->document),
      'remoteContext' => (object)array(
        '@context' => $remote_doc->contextUrl));
    if(isset($options['expandContext'])) {
      $expand_context = self::copy($options['expandContext']);
      if(is_object($expand_context) &&
        property_exists($expand_context, '@context')) {
        $input->expandContext = $expand_context;
      } else {
        $input->expandContext = (object)array('@context' => $expand_context);
      }
    }

    // retrieve all @context URLs in the input
    try {
      $this->_retrieveContextUrls(
        $input, new stdClass(), $options['documentLoader'], $options['base']);
    } catch(Exception $e) {
      throw new JsonLdException(
        'Could not perform JSON-LD expansion.',
        'jsonld.ExpandError', null, null, $e);
    }

    $active_ctx = $this->_getInitialContext($options);
    $document = $input->document;
    $remote_context = $input->remoteContext->{'@context'};

    // process optional expandContext
    if(property_exists($input, 'expandContext')) {
      $active_ctx = self::_processContext(
        $active_ctx, $input->expandContext, $options);
    }

    // process remote context from HTTP Link Header
    if($remote_context) {
      $active_ctx = self::_processContext(
        $active_ctx, $remote_context, $options);
    }

    // do expansion
    $expanded = $this->_expand($active_ctx, null, $document, $options, false);

    // optimize away @graph with no other properties
    if(is_object($expanded) && property_exists($expanded, '@graph') &&
      count(array_keys((array)$expanded)) === 1) {
      $expanded = $expanded->{'@graph'};
    } else if($expanded === null) {
      $expanded = array();
    }
    // normalize to an array
    return self::arrayify($expanded);
  }

  /**
   * Performs JSON-LD flattening.
   *
   * @param mixed $input the JSON-LD to flatten.
   * @param ctx the context to use to compact the flattened output, or null.
   * @param assoc $options the options to use:
   *          [base] the base IRI to use.
   *          [expandContext] a context to expand with.
   *          [documentLoader(url)] the document loader.
   *
   * @return array the flattened output.
   */
  public function flatten($input, $ctx, $options) {
    global $jsonld_default_load_document;
    self::setdefaults($options, array(
      'base' => is_string($input) ? $input : '',
      'documentLoader' => $jsonld_default_load_document));

    try {
      // expand input
      $expanded = $this->expand($input, $options);
    } catch(Exception $e) {
      throw new JsonLdException(
        'Could not expand input before flattening.',
        'jsonld.FlattenError', null, null, $e);
    }

    // do flattening
    $flattened = $this->_flatten($expanded);

    if($ctx === null) {
      return $flattened;
    }

    // compact result (force @graph option to true, skip expansion)
    $options['graph'] = true;
    $options['skipExpansion'] = true;
    try {
      $compacted = $this->compact($flattened, $ctx, $options);
    } catch(Exception $e) {
      throw new JsonLdException(
        'Could not compact flattened output.',
        'jsonld.FlattenError', null, null, $e);
    }

    return $compacted;
  }

  /**
   * Performs JSON-LD framing.
   *
   * @param mixed $input the JSON-LD object to frame.
   * @param stdClass $frame the JSON-LD frame to use.
   * @param $options the framing options.
   *          [base] the base IRI to use.
   *          [expandContext] a context to expand with.
   *          [embed] default @embed flag: '@last', '@always', '@never', '@link'
   *            (default: '@last').
   *          [explicit] default @explicit flag (default: false).
   *          [requireAll] default @requireAll flag (default: true).
   *          [omitDefault] default @omitDefault flag (default: false).
   *          [documentLoader(url)] the document loader.
   *
   * @return stdClass the framed JSON-LD output.
   */
  public function frame($input, $frame, $options) {
    global $jsonld_default_load_document;
    self::setdefaults($options, array(
      'base' => is_string($input) ? $input : '',
      'compactArrays' => true,
      'embed' => '@last',
      'explicit' => false,
      'requireAll' => true,
      'omitDefault' => false,
      'documentLoader' => $jsonld_default_load_document));

    // if frame is a string, attempt to dereference remote document
    if(is_string($frame)) {
      $remote_frame = call_user_func($options['documentLoader'], $frame);
    } else {
      $remote_frame = (object)array(
        'contextUrl' => null,
        'documentUrl' => null,
        'document' => $frame);
    }

    try {
      if($remote_frame->document === null) {
        throw new JsonLdException(
          'No remote document found at the given URL.',
          'jsonld.NullRemoteDocument');
      }
      if(is_string($remote_frame->document)) {
        $remote_frame->document = self::_parse_json($remote_frame->document);
      }
    } catch(Exception $e) {
      throw new JsonLdException(
        'Could not retrieve a JSON-LD document from the URL.',
        'jsonld.LoadDocumentError', 'loading document failed',
        array('remoteDoc' => $remote_frame), $e);
    }

    // preserve frame context
    $frame = $remote_frame->document;
    if($frame !== null) {
      $ctx = (property_exists($frame, '@context') ?
        $frame->{'@context'} : new stdClass());
      if($remote_frame->contextUrl !== null) {
        if($ctx !== null) {
          $ctx = $remote_frame->contextUrl;
        } else {
          $ctx = self::arrayify($ctx);
          $ctx[] = $remote_frame->contextUrl;
        }
        $frame->{'@context'} = $ctx;
      }
    }

    try {
      // expand input
      $expanded = $this->expand($input, $options);
    } catch(Exception $e) {
      throw new JsonLdException(
        'Could not expand input before framing.',
        'jsonld.FrameError', null, null, $e);
    }

    try {
      // expand frame
      $opts = $options;
      $opts['keepFreeFloatingNodes'] = true;
      $expanded_frame = $this->expand($frame, $opts);
    } catch(Exception $e) {
      throw new JsonLdException(
        'Could not expand frame before framing.',
        'jsonld.FrameError', null, null, $e);
    }

    // do framing
    $framed = $this->_frame($expanded, $expanded_frame, $options);

    try {
      // compact result (force @graph option to true, skip expansion, check
      // for linked embeds)
      $options['graph'] = true;
      $options['skipExpansion'] = true;
      $options['link'] = new ArrayObject();
      $options['activeCtx'] = true;
      $result = $this->compact($framed, $ctx, $options);
    } catch(Exception $e) {
      throw new JsonLdException(
        'Could not compact framed output.',
        'jsonld.FrameError', null, null, $e);
    }

    $compacted = $result['compacted'];
    $active_ctx = $result['activeCtx'];

    // get graph alias
    $graph = $this->_compactIri($active_ctx, '@graph');
    // remove @preserve from results
    $options['link'] = new ArrayObject();
    $compacted->{$graph} = $this->_removePreserve(
      $active_ctx, $compacted->{$graph}, $options);
    return $compacted;
  }

  /**
   * Performs JSON-LD normalization.
   *
   * @param mixed $input the JSON-LD object to normalize.
   * @param assoc $options the options to use:
   *          [base] the base IRI to use.
   *          [expandContext] a context to expand with.
   *          [inputFormat] the format if input is not JSON-LD:
   *            'application/nquads' for N-Quads.
   *          [format] the format if output is a string:
   *            'application/nquads' for N-Quads.
   *          [documentLoader(url)] the document loader.
   *
   * @return mixed the normalized output.
   */
  public function normalize($input, $options) {
    global $jsonld_default_load_document;
    self::setdefaults($options, array(
      'base' => is_string($input) ? $input : '',
      'documentLoader' => $jsonld_default_load_document));

    if(isset($options['inputFormat'])) {
      if($options['inputFormat'] != 'application/nquads') {
         throw new JsonLdException(
           'Unknown normalization input format.', 'jsonld.NormalizeError');
      }
      $dataset = $this->parseNQuads($input);
    } else {
      try {
        // convert to RDF dataset then do normalization
        $opts = $options;
        if(isset($opts['format'])) {
          unset($opts['format']);
        }
        $opts['produceGeneralizedRdf'] = false;
        $dataset = $this->toRDF($input, $opts);
      } catch(Exception $e) {
        throw new JsonLdException(
          'Could not convert input to RDF dataset before normalization.',
          'jsonld.NormalizeError', null, null, $e);
      }
    }

    // do normalization
    return $this->_normalize($dataset, $options);
  }

  /**
   * Converts an RDF dataset to JSON-LD.
   *
   * @param mixed $dataset a serialized string of RDF in a format specified
   *          by the format option or an RDF dataset to convert.
   * @param assoc $options the options to use:
   *          [format] the format if input is a string:
   *            'application/nquads' for N-Quads (default).
   *          [useRdfType] true to use rdf:type, false to use @type
   *            (default: false).
   *          [useNativeTypes] true to convert XSD types into native types
   *            (boolean, integer, double), false not to (default: false).
   *
   * @return array the JSON-LD output.
   */
  public function fromRDF($dataset, $options) {
    global $jsonld_rdf_parsers;

    self::setdefaults($options, array(
      'useRdfType' => false,
      'useNativeTypes' => false));

    if(!isset($options['format']) && is_string($dataset)) {
      // set default format to nquads
      $options['format'] = 'application/nquads';
    }

    // handle special format
    if(isset($options['format']) && $options['format']) {
      // supported formats (processor-specific and global)
      if(($this->rdfParsers !== null &&
        !property_exists($this->rdfParsers, $options['format'])) ||
        $this->rdfParsers === null &&
        !property_exists($jsonld_rdf_parsers, $options['format'])) {
        throw new JsonLdException(
          'Unknown input format.',
          'jsonld.UnknownFormat', null, array('format' => $options['format']));
      }
      if($this->rdfParsers !== null) {
        $callable = $this->rdfParsers->{$options['format']};
      } else {
        $callable = $jsonld_rdf_parsers->{$options['format']};
      }
      $dataset = call_user_func($callable, $dataset);
    }

    // convert from RDF
    return $this->_fromRDF($dataset, $options);
  }

  /**
   * Outputs the RDF dataset found in the given JSON-LD object.
   *
   * @param mixed $input the JSON-LD object.
   * @param assoc $options the options to use:
   *          [base] the base IRI to use.
   *          [expandContext] a context to expand with.
   *          [format] the format to use to output a string:
   *            'application/nquads' for N-Quads.
   *          [produceGeneralizedRdf] true to output generalized RDF, false
   *            to produce only standard RDF (default: false).
   *          [documentLoader(url)] the document loader.
   *
   * @return mixed the resulting RDF dataset (or a serialization of it).
   */
  public function toRDF($input, $options) {
    global $jsonld_default_load_document;
    self::setdefaults($options, array(
      'base' => is_string($input) ? $input : '',
      'produceGeneralizedRdf' => false,
      'documentLoader' => $jsonld_default_load_document));

    try {
      // expand input
      $expanded = $this->expand($input, $options);
    } catch(JsonLdException $e) {
      throw new JsonLdException(
        'Could not expand input before serialization to RDF.',
        'jsonld.RdfError', null, null, $e);
    }

    // create node map for default graph (and any named graphs)
    $namer = new UniqueNamer('_:b');
    $node_map = (object)array('@default' => new stdClass());
    $this->_createNodeMap($expanded, $node_map, '@default', $namer);

    // output RDF dataset
    $dataset = new stdClass();
    $graph_names = array_keys((array)$node_map);
    sort($graph_names);
    foreach($graph_names as $graph_name) {
      $graph = $node_map->{$graph_name};
      // skip relative IRIs
      if($graph_name === '@default' || self::_isAbsoluteIri($graph_name)) {
        $dataset->{$graph_name} = $this->_graphToRDF($graph, $namer, $options);
      }
    }

    $rval = $dataset;

    // convert to output format
    if(isset($options['format']) && $options['format']) {
      // supported formats
      if($options['format'] === 'application/nquads') {
        $rval = self::toNQuads($dataset);
      } else {
        throw new JsonLdException(
          'Unknown output format.', 'jsonld.UnknownFormat',
          null, array('format' => $options['format']));
      }
    }

    return $rval;
  }

  /**
   * Processes a local context, resolving any URLs as necessary, and returns a
   * new active context in its callback.
   *
   * @param stdClass $active_ctx the current active context.
   * @param mixed $local_ctx the local context to process.
   * @param assoc $options the options to use:
   *          [documentLoader(url)] the document loader.
   *
   * @return stdClass the new active context.
   */
  public function processContext($active_ctx, $local_ctx, $options) {
    global $jsonld_default_load_document;
    self::setdefaults($options, array(
      'base' => '',
      'documentLoader' => $jsonld_default_load_document));

    // return initial context early for null context
    if($local_ctx === null) {
      return $this->_getInitialContext($options);
    }

    // retrieve URLs in local_ctx
    $local_ctx = self::copy($local_ctx);
    if(is_string($local_ctx) or (
      is_object($local_ctx) && !property_exists($local_ctx, '@context'))) {
      $local_ctx = (object)array('@context' => $local_ctx);
    }
    try {
      $this->_retrieveContextUrls(
        $local_ctx, new stdClass(),
        $options['documentLoader'], $options['base']);
    } catch(Exception $e) {
      throw new JsonLdException(
        'Could not process JSON-LD context.',
        'jsonld.ContextError', null, null, $e);
    }

    // process context
    return $this->_processContext($active_ctx, $local_ctx, $options);
  }

  /**
   * Returns true if the given subject has the given property.
   *
   * @param stdClass $subject the subject to check.
   * @param string $property the property to look for.
   *
   * @return bool true if the subject has the given property, false if not.
   */
  public static function hasProperty($subject, $property) {
    $rval = false;
    if(property_exists($subject, $property)) {
      $value = $subject->{$property};
      $rval = (!is_array($value) || count($value) > 0);
    }
    return $rval;
  }

  /**
   * Determines if the given value is a property of the given subject.
   *
   * @param stdClass $subject the subject to check.
   * @param string $property the property to check.
   * @param mixed $value the value to check.
   *
   * @return bool true if the value exists, false if not.
   */
  public static function hasValue($subject, $property, $value) {
    $rval = false;
    if(self::hasProperty($subject, $property)) {
      $val = $subject->{$property};
      $is_list = self::_isList($val);
      if(is_array($val) || $is_list) {
        if($is_list) {
          $val = $val->{'@list'};
        }
        foreach($val as $v) {
          if(self::compareValues($value, $v)) {
            $rval = true;
            break;
          }
        }
      } else if(!is_array($value)) {
        // avoid matching the set of values with an array value parameter
        $rval = self::compareValues($value, $val);
      }
    }
    return $rval;
  }

  /**
   * Adds a value to a subject. If the value is an array, all values in the
   * array will be added.
   *
   * Note: If the value is a subject that already exists as a property of the
   * given subject, this method makes no attempt to deeply merge properties.
   * Instead, the value will not be added.
   *
   * @param stdClass $subject the subject to add the value to.
   * @param string $property the property that relates the value to the subject.
   * @param mixed $value the value to add.
   * @param assoc [$options] the options to use:
   *          [propertyIsArray] true if the property is always an array, false
   *            if not (default: false).
   *          [allowDuplicate] true to allow duplicates, false not to (uses a
   *            simple shallow comparison of subject ID or value)
   *            (default: true).
   */
  public static function addValue(
    $subject, $property, $value, $options=array()) {
    self::setdefaults($options, array(
      'allowDuplicate' => true,
      'propertyIsArray' => false));

    if(is_array($value)) {
      if(count($value) === 0 && $options['propertyIsArray'] &&
        !property_exists($subject, $property)) {
        $subject->{$property} = array();
      }
      foreach($value as $v) {
        self::addValue($subject, $property, $v, $options);
      }
    } else if(property_exists($subject, $property)) {
      // check if subject already has value if duplicates not allowed
      $has_value = (!$options['allowDuplicate'] &&
        self::hasValue($subject, $property, $value));

      // make property an array if value not present or always an array
      if(!is_array($subject->{$property}) &&
        (!$has_value || $options['propertyIsArray'])) {
        $subject->{$property} = array($subject->{$property});
      }

      // add new value
      if(!$has_value) {
        $subject->{$property}[] = $value;
      }
    } else {
      // add new value as set or single value
      $subject->{$property} = ($options['propertyIsArray'] ?
        array($value) : $value);
    }
  }

  /**
   * Gets all of the values for a subject's property as an array.
   *
   * @param stdClass $subject the subject.
   * @param string $property the property.
   *
   * @return array all of the values for a subject's property as an array.
   */
  public static function getValues($subject, $property) {
    $rval = (property_exists($subject, $property) ?
      $subject->{$property} : array());
    return self::arrayify($rval);
  }

  /**
   * Removes a property from a subject.
   *
   * @param stdClass $subject the subject.
   * @param string $property the property.
   */
  public static function removeProperty($subject, $property) {
    unset($subject->{$property});
  }

  /**
   * Removes a value from a subject.
   *
   * @param stdClass $subject the subject.
   * @param string $property the property that relates the value to the subject.
   * @param mixed $value the value to remove.
   * @param assoc [$options] the options to use:
   *          [propertyIsArray] true if the property is always an array,
   *          false if not (default: false).
   */
  public static function removeValue(
    $subject, $property, $value, $options=array()) {
    self::setdefaults($options, array(
      'propertyIsArray' => false));

    // filter out value
    $filter = function($e) use ($value) {
      return !sel…