Add count_leading_ones function

Add the following function in `src/util.hpp`:

    template<typename N> inline unsigned int count_leading_ones(N n);

This function returns the quantity of leading `1` bits in `n`.

src/tests/test_util.cpp

@@ -127,6 +127,19 @@ BOOST_AUTO_TEST_CASE( test_count_leading_zeros )
 		== bit_width<char>() - 6 );
 }
 
+BOOST_AUTO_TEST_CASE( test_count_leading_ones )
+{
+	BOOST_CHECK( count_leading_ones(0) == 0 );
+	BOOST_CHECK( count_leading_ones(1) == 0 );
+	BOOST_CHECK( count_leading_ones((boost::uint8_t) 0xFF) == 8 );
+	BOOST_CHECK( count_leading_ones((boost::uint16_t) 0xFFFF) == 16 );
+	BOOST_CHECK( count_leading_ones((boost::uint32_t) 0xFFFFFFFF) == 32 );
+	BOOST_CHECK( count_leading_ones((boost::uint64_t) 0xFFFFFFFFFFFFFFFF)
+		== 64 );
+	BOOST_CHECK( count_leading_ones((boost::uint8_t) 0xF8) == 5 );
+	BOOST_CHECK( count_leading_ones((boost::uint16_t) 54321) == 2 );
+}
+
 /* vim: set ts=4 sw=4: */
 
 BOOST_AUTO_TEST_SUITE_END()

src/util.hpp

@@ -360,6 +360,8 @@ inline unsigned int count_leading_zeros_impl(N n, std::size_t w) {
  *
  * @returns the quantity of leading `0` bits in `n`, if `N` satisfies the
  * above constraints.
+ *
+ * @see count_leading_ones()
  */
 template<typename N>
 inline unsigned int count_leading_zeros(N n) {
@@ -390,6 +392,30 @@ inline unsigned int count_leading_zeros(N n) {
 	// having an undefined result.
 }
 
+/**
+ * Returns the quantity of leading `1` bits in `n` — i.e., the quantity of
+ * bits in `n`, minus the 1-based bit index of the most significant `0` bit in
+ * `n`, or minus 0 if `n` contains no `0` bits.
+ *
+ * @tparam N The type of `n`. This should be a fundamental integer type that
+ *  (a) is not wider than `unsigned long long int`, and
+ *  (b) is no greater than `INT_MAX` bits in width;
+ * if `N` does not satisfy these constraints, the return value is undefined.
+ *
+ * @param n An integer upon which to operate.
+ *
+ * @returns the quantity of leading `1` bits in `n`, if `N` satisfies the
+ * above constraints.
+ *
+ * @see count_leading_zeros()
+ */
+template<typename N>
+inline unsigned int count_leading_ones(N n) {
+	// Explicitly specify the type for which to instantiate
+	// `count_leading_zeros` in case `~n` is of a different type.
+	return count_leading_zeros<N>(~n);
+}
+
 #ifdef __GNUC__
 #define LIKELY(a)    __builtin_expect((a),1) // Tells GCC to optimize code so that if is likely to happen
 #define UNLIKELY(a)  __builtin_expect((a),0) // Tells GCC to optimize code so that if is unlikely to happen