Add bms_get_singleton_member(), and use it where appropriate.

This patch adds a function that replaces a bms_membership() test followed
by a bms_singleton_member() call, performing both the test and the
extraction of a singleton set's member in one scan of the bitmapset.
The performance advantage over the old way is probably minimal in current
usage, but it seems worthwhile on notational grounds anyway.

David Rowley

src/backend/nodes/bitmapset.c

@@ -524,6 +524,50 @@ bms_singleton_member(const Bitmapset *a)
 	return result;
 }
 
+/*
+ * bms_get_singleton_member
+ *
+ * Test whether the given set is a singleton.
+ * If so, set *member to the value of its sole member, and return TRUE.
+ * If not, return FALSE, without changing *member.
+ *
+ * This is more convenient and faster than calling bms_membership() and then
+ * bms_singleton_member(), if we don't care about distinguishing empty sets
+ * from multiple-member sets.
+ */
+bool
+bms_get_singleton_member(const Bitmapset *a, int *member)
+{
+	int			result = -1;
+	int			nwords;
+	int			wordnum;
+
+	if (a == NULL)
+		return false;
+	nwords = a->nwords;
+	for (wordnum = 0; wordnum < nwords; wordnum++)
+	{
+		bitmapword	w = a->words[wordnum];
+
+		if (w != 0)
+		{
+			if (result >= 0 || HAS_MULTIPLE_ONES(w))
+				return false;
+			result = wordnum * BITS_PER_BITMAPWORD;
+			while ((w & 255) == 0)
+			{
+				w >>= 8;
+				result += 8;
+			}
+			result += rightmost_one_pos[w & 255];
+		}
+	}
+	if (result < 0)
+		return false;
+	*member = result;
+	return true;
+}
+
 /*
  * bms_num_members - count members of set
  */