Fix division by zero in WeightedRandomStreamRanker

lib/Tumblr/StreamBuilder/StreamRankers/WeightedRandomStreamRanker.php

@@ -46,21 +46,44 @@ protected function rank_inner(array $stream_elements, StreamTracer $tracer = nul
 
         // weighted sampling array, $H: ['sampling score' => $stream_element]
         $H = [];
-        $max_rand = mt_getrandmax();
         /** @var StreamElement $element */
         foreach ($valid_elements as $element) {
-            /** @var RecommendationLeafStreamElementTrait $original_element */
-            $original_element = $element->get_original_element();
             // calculate sampling score
-            $r = pow(mt_rand() / $max_rand, (1 / $original_element->get_score()));
-            $H[strval($r)] = $element;
+            $r = $this->get_element_random_score($element);
+
+            // store the element in $H, using $r as key.
+            $key = strval($r);
+            if (array_key_exists($key, $H)) {
+                // We don't want to replace an element that was previously added to $H.
+                // so we append the element id, if the key already exists.
+                $key = sprintf('%s_%s', $key, $element->get_element_id());
+            }
+            $H[$key] = $element;
         }
+
         // sort by key in descending order
         krsort($H);
         $ranked_elements = array_values($H);
         return array_merge($ranked_elements, $not_valid_elements);
     }
 
+    /**
+     * @param StreamElement $element Stream element to rank randomly
+     * @return float|int|object
+     */
+    protected function get_element_random_score(StreamElement $element)
+    {
+        /** @var RecommendationLeafStreamElementTrait $original_element */
+        $original_element = $element->get_original_element();
+        $max_rand = mt_getrandmax();
+        $score = $original_element->get_score();
+        if ($score == 0.0) {
+            $score = 0.001;
+        }
+        // calculate sampling score
+        return pow(mt_rand() / $max_rand, (1 / $score));
+    }
+
     /**
      * Selects stream elements that have a score
      * @param StreamElement[] $stream_elements Stream elements

tests/unit/Tumblr/StreamBuilder/StreamRankers/WeightedRandomStreamRankerTest.php

@@ -105,6 +105,7 @@ public function test_ranker_with_stream_elements(): void
             1234 => 1.0,
             2345 => 2.0,
             3456 => 3.0,
+            4567 => 0.0,
         ];
         $stream_elements = $this->build_blog_stream_elements($bid2score);
         mt_srand(0);