ClickHouse

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#include <base/bit_cast.h>
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#include <Functions/FunctionFactory.h>
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#include <Functions/FunctionUnaryArithmetic.h>
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#include <bit>
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namespace DB
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{
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template <typename A>
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struct BitCountImpl
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{
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    using ResultType = std::conditional_t<(sizeof(A) * 8 >= 256), UInt16, UInt8>;
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    static constexpr bool allow_string_or_fixed_string = true;
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    static inline ResultType apply(A a)
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    {
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        /// We count bits in the value representation in memory. For example, we support floats.
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        /// We need to avoid sign-extension when converting signed numbers to larger type. So, uint8_t(-1) has 8 bits.
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        if constexpr (is_big_int_v<A>)
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        {
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            ResultType res = 0;
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            for (auto item : a.items)
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                res += std::popcount(item);
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            return res;
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        }
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        if constexpr (std::is_same_v<A, UInt64> || std::is_same_v<A, Int64>)
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            return std::popcount(static_cast<UInt64>(a));
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        if constexpr (std::is_same_v<A, UInt32> || std::is_same_v<A, Int32> || std::is_unsigned_v<A>)
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            return std::popcount(static_cast<UInt32>(a));
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        if constexpr (std::is_same_v<A, Int16>)
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            return std::popcount(static_cast<UInt16>(a));
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        if constexpr (std::is_same_v<A, Int8>)
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            return std::popcount(static_cast<uint8_t>(a));
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        else
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            return std::popcount(bit_cast<uint64_t>(a));
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    }
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#if USE_EMBEDDED_COMPILER
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    static constexpr bool compilable = false;
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#endif
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};
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struct NameBitCount { static constexpr auto name = "bitCount"; };
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using FunctionBitCount = FunctionUnaryArithmetic<BitCountImpl, NameBitCount, false /* is injective */>;
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/// The function has no ranges of monotonicity.
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template <> struct FunctionUnaryArithmeticMonotonicity<NameBitCount>
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{
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    static bool has() { return false; }
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    static IFunction::Monotonicity get(const Field &, const Field &)
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    {
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        return {};
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    }
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};
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REGISTER_FUNCTION(BitCount)
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{
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    factory.registerFunction<FunctionBitCount>();
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}
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}
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