Add ones_count and zeros_count bitwise utilities.

Author: evoskuil

include/bitcoin/system/impl/math/bits.ipp

@@ -73,6 +73,18 @@ constexpr size_t right_ones(Value value) NOEXCEPT
     return to_unsigned(std::countr_one<Value>(value));
 }
 
+template <typename Value, if_unsigned_integer<Value>>
+constexpr size_t ones_count(Value value) NOEXCEPT
+{
+    return to_unsigned(std::popcount<Value>(value));
+}
+
+template <typename Value, if_unsigned_integer<Value>>
+constexpr size_t zeros_count(Value value) NOEXCEPT
+{
+    return to_unsigned(std::popcount<Value>(bit_not(value)));
+}
+
 // Bitwise logical operations.
 // ----------------------------------------------------------------------------
 

include/bitcoin/system/math/bits.hpp

@@ -51,6 +51,12 @@ constexpr size_t left_ones(Value value) NOEXCEPT;
 template <typename Value, if_unsigned_integer<Value> = true>
 constexpr size_t right_ones(Value value) NOEXCEPT;
 
+// Because std::popcount (C++20) function returns int.
+template <typename Value, if_unsigned_integer<Value> = true>
+constexpr size_t ones_count(Value value) NOEXCEPT;
+template <typename Value, if_unsigned_integer<Value> = true>
+constexpr size_t zeros_count(Value value) NOEXCEPT;
+
 /// Bitwise logical operations.
 /// ---------------------------------------------------------------------------
 

test/math/bits.cpp

@@ -123,6 +123,11 @@ static_assert(right_zeros(uint8_t{ 0b11111111 }) == 0);
 static_assert(right_zeros(uint8_t{ 0b00011100 }) == 2);
 static_assert(right_zeros(uint8_t{ 0b00011101 }) == 0);
 
+static_assert(zeros_count(uint8_t{ 0b00000000 }) == 8);
+static_assert(zeros_count(uint8_t{ 0b11111111 }) == 0);
+static_assert(zeros_count(uint8_t{ 0b01111111 }) == 1);
+static_assert(zeros_count(uint8_t{ 0b11100011 }) == 3);
+
 // ones
 
 static_assert(left_ones(uint8_t{ 0b00000000 }) == 0);
@@ -135,6 +140,11 @@ static_assert(right_ones(uint8_t{ 0b11111111 }) == 8);
 static_assert(right_ones(uint8_t{ 0b11111110 }) == 0);
 static_assert(right_ones(uint8_t{ 0b11100011 }) == 2);
 
+static_assert(ones_count(uint8_t{ 0b00000000 }) == 0);
+static_assert(ones_count(uint8_t{ 0b11111111 }) == 8);
+static_assert(ones_count(uint8_t{ 0b11110000 }) == 4);
+static_assert(ones_count(uint8_t{ 0b00011110 }) == 4);
+
 // ones_complement (NOT)
 // inverts all bits (~n, !bool)
 static_assert(bit_not(-4) == 3);