Implement extractFirstSet

Summary: This can be useful to iterate bitmasks that are ORs of power-of-2 enum flags.

Reviewed By: yfeldblum

Differential Revision: D15245333

fbshipit-source-id: 661e0bdbe4468b693e37fda12e513202a3b3ed17

folly/lang/Bits.h

@@ -26,6 +26,10 @@
  *    1-based.  0 = no bits are set (x == 0)
  *    for x != 0, findLastSet(x) == 1 + floor(log2(x))
  *
+ * extractFirstSet(x)  [constexpr]
+ *    extract first (least significant) bit set in a value of an integral
+ *    type, 0 = no bits are set (x == 0)
+ *
  * nextPowTwo(x)  [constexpr]
  *    Finds the next power of two >= x.
  *
@@ -146,6 +150,18 @@ inline constexpr unsigned int findLastSet(T const v) {
   // clang-format on
 }
 
+/// extractFirstSet
+///
+/// Return a value where all the bits but the least significant are cleared.
+template <typename T>
+inline constexpr T extractFirstSet(T const v) {
+  static_assert(std::is_integral<T>::value, "non-integral type");
+  static_assert(std::is_unsigned<T>::value, "signed type");
+  static_assert(!std::is_same<T, bool>::value, "bool type");
+
+  return v & -v;
+}
+
 /// popcount
 ///
 /// Returns the number of bits which are set.

folly/lang/test/BitsTest.cpp

@@ -65,6 +65,17 @@ void testFLS() {
   }
 }
 
+template <class UINT>
+void testEFS() {
+  EXPECT_EQ(0, extractFirstSet(static_cast<UINT>(0)));
+  size_t bits = std::numeric_limits<UINT>::digits;
+  for (size_t i = 0; i < bits; i++) {
+    UINT lsb = static_cast<UINT>(1) << i;
+    UINT v = (static_cast<UINT>(1) << (bits - 1)) | lsb;
+    EXPECT_EQ(lsb, extractFirstSet(v));
+  }
+}
+
 } // namespace
 
 TEST(Bits, FindFirstSet) {
@@ -95,6 +106,14 @@ TEST(Bits, FindLastSet) {
   testFLS<unsigned long long>();
 }
 
+TEST(Bits, ExtractFirstSet) {
+  testEFS<unsigned char>();
+  testEFS<unsigned short>();
+  testEFS<unsigned int>();
+  testEFS<unsigned long>();
+  testEFS<unsigned long long>();
+}
+
 TEST(Bits, nextPowTwoClz) {
   EXPECT_EQ(1, nextPowTwo(0u));
   EXPECT_EQ(1, nextPowTwo(1u));