Add SparseByteSet::size() method

Summary:
Useful, when `SparseByteSet` used as fast pre check for heavyweight
operation.

Reviewed By: marksantaniello

Differential Revision: D60285173

fbshipit-source-id: 8deac984b0f00096448af51fdfee283d0bb13ca2

folly/container/SparseByteSet.h

@@ -106,6 +106,13 @@ class SparseByteSet {
    */
   inline void clear() { size_ = 0; }
 
+  /***
+   *  size()
+   *
+   *  O(1), non-amortized.
+   */
+  inline uint16_t size() { return size_; }
+
  private:
   uint16_t size_; // can't use uint8_t because it would overflow if all
                   // possible values were inserted.

folly/container/test/SparseByteSetTest.cpp

@@ -103,3 +103,15 @@ TEST_F(SparseByteSetTest, remove_nop) {
   bool r = s.remove(12);
   EXPECT_FALSE(r);
 }
+
+TEST_F(SparseByteSetTest, size) {
+  EXPECT_EQ(s.size(), 0);
+
+  s.add(1);
+  s.add(2);
+  s.add(3);
+  EXPECT_EQ(s.size(), 3);
+
+  s.remove(1);
+  EXPECT_EQ(s.size(), 2);
+}