Better distribution for set get-random-element operations.

src/dict.c

@@ -739,6 +739,30 @@ unsigned int dictGetSomeKeys(dict *d, dictEntry **des, unsigned int count) {
     return stored;
 }
 
+/* This is like dictGetRandomKey() from the POV of the API, but will do more
+ * work to ensure a better distribution of the returned element.
+ *
+ * This function improves the distribution because the dictGetRandomKey()
+ * problem is that it selects a random bucket, then it selects a random
+ * element from the chain in the bucket. However elements being in different
+ * chain lengths will have different probabilities of being reported. With
+ * this function instead what we do is to consider a "linear" range of the table
+ * that may be constituted of N buckets with chains of different lengths
+ * appearing one after the other. Then we report a random element in the range.
+ * In this way we smooth away the problem of different chain lenghts. */
+#define GETFAIR_NUM_ENTRIES 20
+dictEntry *dictGetFairRandomKey(dict *d) {
+    dictEntry *entries[GETFAIR_NUM_ENTRIES];
+    unsigned int count = dictGetSomeKeys(d,entries,GETFAIR_NUM_ENTRIES);
+    /* Note that dictGetSomeKeys() may return zero elements in an unlucky
+     * run() even if there are actually elements inside the hash table. So
+     * when we get zero, we call the true dictGetRandomKey() that will always
+     * yeld the element if the hash table has at least one. */
+    if (count == 0) return dictGetRandomKey(d);
+    unsigned int idx = rand() % count;
+    return entries[idx];
+}
+
 /* Function to reverse bits. Algorithm from:
  * http://graphics.stanford.edu/~seander/bithacks.html#ReverseParallel */
 static unsigned long rev(unsigned long v) {

src/dict.h

@@ -166,6 +166,7 @@ dictIterator *dictGetSafeIterator(dict *d);
 dictEntry *dictNext(dictIterator *iter);
 void dictReleaseIterator(dictIterator *iter);
 dictEntry *dictGetRandomKey(dict *d);
+dictEntry *dictGetFairRandomKey(dict *d);
 unsigned int dictGetSomeKeys(dict *d, dictEntry **des, unsigned int count);
 void dictGetStats(char *buf, size_t bufsize, dict *d);
 uint64_t dictGenHashFunction(const void *key, int len);

src/t_set.c

@@ -207,7 +207,7 @@ sds setTypeNextObject(setTypeIterator *si) {
  * used field with values which are easy to trap if misused. */
 int setTypeRandomElement(robj *setobj, sds *sdsele, int64_t *llele) {
     if (setobj->encoding == OBJ_ENCODING_HT) {
-        dictEntry *de = dictGetRandomKey(setobj->ptr);
+        dictEntry *de = dictGetFairRandomKey(setobj->ptr);
         *sdsele = dictGetKey(de);
         *llele = -123456789; /* Not needed. Defensive. */
     } else if (setobj->encoding == OBJ_ENCODING_INTSET) {