From 23fddb0e93dae8a241fc8234710016c00a4bec14 Mon Sep 17 00:00:00 2001
From: 8573 <8573dd@gmail.com>
Date: Wed, 19 Mar 2014 05:42:30 +0000
Subject: [PATCH] Add `count_ones` function
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Add the following function in `src/util.hpp`:

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

This function returns the quantity of `1` bits in `n` — i.e., `n`’s
population count.
---
 src/tests/test_util.cpp | 15 +++++++++++++++
 src/util.hpp            | 27 +++++++++++++++++++++++++++
 2 files changed, 42 insertions(+)

diff --git a/src/tests/test_util.cpp b/src/tests/test_util.cpp
index 08bffe75aba..e0a54100049 100644
--- a/src/tests/test_util.cpp
+++ b/src/tests/test_util.cpp
@@ -89,6 +89,21 @@ BOOST_AUTO_TEST_CASE( test_bit_width )
 	BOOST_CHECK( bit_width((boost::uint64_t) 0) == 64 );
 }
 
+BOOST_AUTO_TEST_CASE( test_count_ones )
+{
+	BOOST_CHECK( count_ones(0) == 0 );
+	BOOST_CHECK( count_ones(1) == 1 );
+	BOOST_CHECK( count_ones(2) == 1 );
+	BOOST_CHECK( count_ones(3) == 2 );
+	BOOST_CHECK( count_ones(4) == 1 );
+	BOOST_CHECK( count_ones(5) == 2 );
+	BOOST_CHECK( count_ones(6) == 2 );
+	BOOST_CHECK( count_ones(7) == 3 );
+	BOOST_CHECK( count_ones(8) == 1 );
+	BOOST_CHECK( count_ones(9) == 2 );
+	BOOST_CHECK( count_ones(12345) == 6 );
+}
+
 /* vim: set ts=4 sw=4: */
 
 BOOST_AUTO_TEST_SUITE_END()
diff --git a/src/util.hpp b/src/util.hpp
index 4bc75cdada4..edf39fc30c7 100644
--- a/src/util.hpp
+++ b/src/util.hpp
@@ -237,6 +237,33 @@ inline std::size_t bit_width(const T& x) {
 	return sizeof(x) * std::numeric_limits<unsigned char>::digits;
 }
 
+/**
+ * Returns the quantity of `1` bits in `n` — i.e., `n`’s population count.
+ *
+ * Algorithm adapted from:
+ * <https://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetKernighan>
+ *
+ * This algorithm was chosen for relative simplicity, not for speed.
+ *
+ * @tparam N The type of `n`. This should be a fundamental integer type no
+ * greater than `UINT_MAX` bits in width; if it is not, the return value is
+ * undefined.
+ *
+ * @param n An integer upon which to operate.
+ *
+ * @returns the quantity of `1` bits in `n`, if `N` is a fundamental integer
+ * type.
+ */
+template<typename N>
+inline unsigned int count_ones(N n) {
+	unsigned int r = 0;
+	while (n) {
+		n &= n-1;
+		++r;
+	}
+	return r;
+}
+
 #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