Add optimization for finding the best CURIE.

- Build a map for searching for a matching IRI when
  computing the inverse context. Each letter of an
  IRI can be used to key into the map to find the
  best set of partial matches (which can be used
  to create CURIEs).
- This approach is a faster alternative to trying each
  possible term in the active context as a possible
  CURIE, linearly, one at a time.

jsonld.php

@@ -4701,32 +4701,41 @@ class JsonLdProcessor {
     }
 
     // no term or @vocab match, check for possible CURIEs
-    $iri_len = strlen($iri);
     $choice = null;
-    foreach($active_ctx->mappings as $term => $definition) {
-      // skip null definitions and terms with colons, they can't be prefixes
-      if($definition === null || $definition->_term_has_colon) {
-        continue;
-      }
-      // skip entries with @ids that are not partial matches
-      if(!($iri_len > $definition->_id_length &&
-        strpos($iri, $definition->{'@id'}) === 0)) {
-        continue;
+    $idx = 0;
+    $partial_matches = array();
+    $iri_map = $active_ctx->fast_curie_map;
+    // check for partial matches of against `iri`, which means look until
+    // iri.length - 1, not full length
+    $max_partial_length = strlen($iri) - 1;
+    for(; $idx < $max_partial_length && isset($iri_map[$iri[$idx]]); ++$idx) {
+      $iri_map = $iri_map[$iri[$idx]];
+      if(isset($iri_map[''])) {
+        $entry = $iri_map[''][0];
+        $entry->iri_length = $idx + 1;
+        $partial_matches[] = $entry;
       }
+    }
+    // check partial matches in reverse order to prefer longest ones first
+    $partial_matches = array_reverse($partial_matches);
+    foreach($partial_matches as $entry) {
+      $terms = $entry->terms;
+      foreach($terms as $term) {
+        // a CURIE is usable if:
+        // 1. it has no mapping, OR
+        // 2. value is null, which means we're not compacting an @value, AND
+        //   the mapping matches the IRI
+        $curie = $term . ':' . substr($iri, $entry->iri_length);
+        $is_usable_curie = (!property_exists($active_ctx->mappings, $curie) ||
+          ($value === null &&
+          $active_ctx->mappings->{$curie}->{'@id'} === $iri));
 
-      // a CURIE is usable if:
-      // 1. it has no mapping, OR
-      // 2. value is null, which means we're not compacting an @value, AND
-      //   the mapping matches the IRI)
-      $curie = $term . ':' . substr($iri, $definition->_id_length);
-      $is_usable_curie = (!property_exists($active_ctx->mappings, $curie) ||
-        ($value === null && $active_ctx->mappings->{$curie}->{'@id'} === $iri));
-
-      // select curie if it is shorter or the same length but lexicographically
-      // less than the current choice
-      if($is_usable_curie && ($choice === null ||
-        self::_compareShortestLeast($curie, $choice) < 0)) {
-        $choice = $curie;
+        // select curie if it is shorter or the same length but
+        // lexicographically less than the current choice
+        if($is_usable_curie && ($choice === null ||
+          self::_compareShortestLeast($curie, $choice) < 0)) {
+          $choice = $curie;
+        }
       }
     }
 
@@ -5363,6 +5372,10 @@ class JsonLdProcessor {
 
     $inverse = $active_ctx->inverse = new stdClass();
 
+    // variables for building fast CURIE map
+    $fast_curie_map = $active_ctx->fast_curie_map = new ArrayObject();
+    $iris_to_terms = array();
+
     // handle default language
     $default_language = '@none';
     if(property_exists($active_ctx, '@language')) {
@@ -5391,9 +5404,25 @@ class JsonLdProcessor {
       $iris = $mapping->{'@id'};
       $iris = self::arrayify($iris);
       foreach($iris as $iri) {
+        $is_keyword = self::_isKeyword($iri);
+
         // initialize container map
         if(!property_exists($inverse, $iri)) {
           $inverse->{$iri} = new stdClass();
+          if(!$is_keyword && !$mapping->_term_has_colon) {
+            // init IRI to term map and fast CURIE map
+            $iris_to_terms[$iri] = new ArrayObject();
+            $iris_to_terms[$iri][] = $term;
+            $fast_curie_entry = (object)array(
+              'iri' => $iri, 'terms' => $iris_to_terms[$iri]);
+            if(!array_key_exists($iri[0], (array)$fast_curie_map)) {
+              $fast_curie_map[$iri[0]] = new ArrayObject();
+            }
+            $fast_curie_map[$iri[0]][] = $fast_curie_entry;
+          }
+        } else if(!$is_keyword && !$mapping->_term_has_colon) {
+          // add IRI to term match
+          $iris_to_terms[$iri][] = $term;
         }
         $container_map = $inverse->{$iri};
 
@@ -5437,9 +5466,47 @@ class JsonLdProcessor {
       }
     }
 
+    // build fast CURIE map
+    foreach($fast_curie_map as $key => $value) {
+      $this->_buildIriMap($fast_curie_map, $key, 1);
+    }
+
     return $inverse;
   }
 
+  /**
+   * Runs a recursive algorithm to build a lookup map for quickly finding
+   * potential CURIEs.
+   *
+   * @param ArrayObject $iri_map the map to build.
+   * @param string $key the current key in the map to work on.
+   * @param int $idx the index into the IRI to compare.
+   */
+  function _buildIriMap($iri_map, $key, $idx) {
+    $entries = $iri_map[$key];
+    $next = $iri_map[$key] = new ArrayObject();
+
+    foreach($entries as $entry) {
+      $iri = $entry->iri;
+      if($idx >= strlen($iri)) {
+        $letter = '';
+      } else {
+        $letter = $iri[$idx];
+      }
+      if(!isset($next[$letter])) {
+        $next[$letter] = new ArrayObject();
+      }
+      $next[$letter][] = $entry;
+    }
+
+    foreach($next as $key => $value) {
+      if($key === '') {
+        continue;
+      }
+      $this->_buildIriMap($next, $key, $idx + 1);
+    }
+  }
+
   /**
    * Adds the term for the given entry if not already added.
    *