From 409c809cd8d83fd7461b0b818f820539ce23bf8b Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@gmail.com>
Date: Tue, 29 Dec 2020 01:06:22 +0200
Subject: [PATCH] Add popcount

---
 include/boost/core/bit.hpp   | 38 +++++++++++++++++++++++++++--
 test/Jamfile.v2              |  1 +
 test/bit_popcount_test.cpp   | 46 ++++++++++++++++++++++++++++++++++++
 test/has_single_bit_test.cpp |  2 +-
 4 files changed, 84 insertions(+), 3 deletions(-)
 create mode 100644 test/bit_popcount_test.cpp

diff --git a/include/boost/core/bit.hpp b/include/boost/core/bit.hpp
index 6258591..de6aada 100644
--- a/include/boost/core/bit.hpp
+++ b/include/boost/core/bit.hpp
@@ -100,7 +100,6 @@ BOOST_CORE_BIT_CONSTEXPR int countl_zero( T x ) BOOST_NOEXCEPT
     }
 }
 
-
 template<class T>
 BOOST_CORE_BIT_CONSTEXPR int countl_one( T x ) BOOST_NOEXCEPT
 {
@@ -164,8 +163,43 @@ BOOST_CORE_BIT_CONSTEXPR int countr_one( T x ) BOOST_NOEXCEPT
     return boost::core::countr_zero( static_cast<T>( ~x ) );
 }
 
+namespace detail
+{
+
+BOOST_CXX14_CONSTEXPR inline int popcount_impl( boost::uint32_t x ) BOOST_NOEXCEPT
+{
+    x = x - ( ( x >> 1 ) & 0x55555555 );
+    x = ( x & 0x33333333 ) + ( ( x >> 2 ) & 0x33333333 );
+    x = ( x + ( x >> 4 ) ) & 0x0F0F0F0F;
+
+    return static_cast<unsigned>( ( x * 0x01010101 ) >> 24 );
+}
+
+BOOST_CXX14_CONSTEXPR inline int popcount_impl( boost::uint64_t x ) BOOST_NOEXCEPT
+{
+    x = x - ( ( x >> 1 ) & 0x5555555555555555 );
+    x = ( x & 0x3333333333333333 ) + ( ( x >> 2 ) & 0x3333333333333333 );
+    x = ( x + ( x >> 4 ) ) & 0x0F0F0F0F0F0F0F0F;
+
+    return static_cast<unsigned>( ( x * 0x0101010101010101 ) >> 56 );
+}
+
+} // namespace detail
+
 template<class T>
-BOOST_CORE_BIT_CONSTEXPR int popcount( T x ) BOOST_NOEXCEPT;
+BOOST_CORE_BIT_CONSTEXPR int popcount( T x ) BOOST_NOEXCEPT
+{
+    BOOST_STATIC_ASSERT( sizeof(T) <= sizeof(boost::uint64_t) );
+
+    if( sizeof(T) <= sizeof(boost::uint32_t) )
+    {
+        return boost::core::detail::popcount_impl( static_cast<boost::uint32_t>( x ) );
+    }
+    else
+    {
+        return boost::core::detail::popcount_impl( static_cast<boost::uint64_t>( x ) );
+    }
+}
 
 // rotating
 
diff --git a/test/Jamfile.v2 b/test/Jamfile.v2
index 926e175..e2ec8ac 100644
--- a/test/Jamfile.v2
+++ b/test/Jamfile.v2
@@ -230,6 +230,7 @@ run bit_width_test.cpp ;
 run has_single_bit_test.cpp ;
 run bit_floor_test.cpp ;
 run bit_ceil_test.cpp ;
+run bit_popcount_test.cpp ;
 
 use-project /boost/core/swap : ./swap ;
 build-project ./swap ;
diff --git a/test/bit_popcount_test.cpp b/test/bit_popcount_test.cpp
new file mode 100644
index 0000000..d5cc678
--- /dev/null
+++ b/test/bit_popcount_test.cpp
@@ -0,0 +1,46 @@
+// Test for boost/core/bit.hpp (bit_ceil)
+//
+// Copyright 2020 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/core/bit.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <boost/core/detail/splitmix64.hpp>
+#include <boost/cstdint.hpp>
+#include <limits>
+#include <iostream>
+
+template<class T> void test_popcount( T x )
+{
+    int k = 0;
+    for( T y = x; y; y &= y - 1, ++k );
+
+    BOOST_TEST_EQ( boost::core::popcount( x ), k ) || ( std::cerr << "x: " << +x << std::endl );
+}
+
+int main()
+{
+    {
+        test_popcount( static_cast<unsigned char>( 0 ) );
+        test_popcount( static_cast<unsigned short>( 0 ) );
+        test_popcount( static_cast<unsigned int>( 0 ) );
+        test_popcount( static_cast<unsigned long>( 0 ) );
+        test_popcount( static_cast<unsigned long long>( 0 ) );
+    }
+
+    boost::detail::splitmix64 rng;
+
+    for( int i = 0; i < 1000; ++i )
+    {
+        boost::uint64_t x = rng();
+
+        test_popcount( static_cast<unsigned char>( x ) );
+        test_popcount( static_cast<unsigned short>( x ) );
+        test_popcount( static_cast<unsigned int>( x ) );
+        test_popcount( static_cast<unsigned long>( x ) );
+        test_popcount( static_cast<unsigned long long>( x ) );
+    }
+
+    return boost::report_errors();
+}
diff --git a/test/has_single_bit_test.cpp b/test/has_single_bit_test.cpp
index 317d769..0be8fcd 100644
--- a/test/has_single_bit_test.cpp
+++ b/test/has_single_bit_test.cpp
@@ -12,7 +12,7 @@
 
 template<class T> void test_single_bit( T x )
 {
-    // BOOST_TEST_EQ( boost::core::has_single_bit( x ), boost::core::popcount( x ) == 1 );
+    BOOST_TEST_EQ( boost::core::has_single_bit( x ), boost::core::popcount( x ) == 1 );
 }
 
 int main()