#include <DataTypes/DataTypeString.h>
#include <Functions/FunctionFactory.h>
#include <Functions/FunctionStringToString.h>
#include <IO/WriteBufferFromVector.h>
#include <IO/WriteHelpers.h>
#include <Poco/UTF8Encoding.h>

#include <string_view>

#ifdef __SSE2__
#    include <emmintrin.h>
#endif

namespace DB
{
namespace ErrorCodes
{
    extern const int ILLEGAL_COLUMN;
}

extern const UInt8 length_of_utf8_sequence[256];

namespace
{

struct ToValidUTF8Impl
{
    static void toValidUTF8One(const char * begin, const char * end, WriteBuffer & write_buffer)
    {
        static constexpr std::string_view replacement = "\xEF\xBF\xBD";

        const char * p = begin;
        const char * valid_start = begin;

        /// The last recorded character was `replacement`.
        bool just_put_replacement = false;

        auto put_valid = [&write_buffer, &just_put_replacement](const char * data, size_t len)
        {
            if (len == 0)
                return;
            just_put_replacement = false;
            write_buffer.write(data, len);
        };

        auto put_replacement = [&write_buffer, &just_put_replacement]()
        {
            if (just_put_replacement)
                return;
            just_put_replacement = true;
            write_buffer.write(replacement.data(), replacement.size());
        };

        while (p < end)
        {
#ifdef __SSE2__
            /// Fast skip of ASCII
            static constexpr size_t SIMD_BYTES = 16;
            const char * simd_end = p + (end - p) / SIMD_BYTES * SIMD_BYTES;

            while (p < simd_end && !_mm_movemask_epi8(_mm_loadu_si128(reinterpret_cast<const __m128i *>(p))))
                p += SIMD_BYTES;

            if (!(p < end))
                break;
#endif

            size_t len = length_of_utf8_sequence[static_cast<unsigned char>(*p)];

            if (len > 4)
            {
                /// Invalid start of sequence. Skip one byte.
                put_valid(valid_start, p - valid_start);
                put_replacement();
                ++p;
                valid_start = p;
            }
            else if (p + len > end)
            {
                /// Sequence was not fully written to this buffer.
                break;
            }
            else if (Poco::UTF8Encoding::isLegal(reinterpret_cast<const unsigned char *>(p), len))
            {
                /// Valid sequence.
                p += len;
            }
            else
            {
                /// Invalid sequence. Skip just first byte.
                put_valid(valid_start, p - valid_start);
                put_replacement();
                ++p;
                valid_start = p;
            }
        }

        put_valid(valid_start, p - valid_start);

        if (p != end)
            put_replacement();
    }

    static void vector(
        const ColumnString::Chars & data,
        const ColumnString::Offsets & offsets,
        ColumnString::Chars & res_data,
        ColumnString::Offsets & res_offsets)
    {
        const size_t offsets_size = offsets.size();
        /// It can be larger than that, but we believe it is unlikely to happen.
        res_data.resize(data.size());
        res_offsets.resize(offsets_size);

        size_t prev_offset = 0;
        WriteBufferFromVector<ColumnString::Chars> write_buffer(res_data);
        for (size_t i = 0; i < offsets_size; ++i)
        {
            const char * haystack_data = reinterpret_cast<const char *>(&data[prev_offset]);
            const size_t haystack_size = offsets[i] - prev_offset - 1;
            toValidUTF8One(haystack_data, haystack_data + haystack_size, write_buffer);
            writeChar(0, write_buffer);
            res_offsets[i] = write_buffer.count();
            prev_offset = offsets[i];
        }
        write_buffer.finalize();
    }

    [[noreturn]] static void vectorFixed(const ColumnString::Chars &, size_t, ColumnString::Chars &)
    {
        throw Exception("Column of type FixedString is not supported by toValidUTF8 function", ErrorCodes::ILLEGAL_COLUMN);
    }
};

struct NameToValidUTF8
{
    static constexpr auto name = "toValidUTF8";
};
using FunctionToValidUTF8 = FunctionStringToString<ToValidUTF8Impl, NameToValidUTF8>;

}

void registerFunctionToValidUTF8(FunctionFactory & factory)
{
    factory.registerFunction<FunctionToValidUTF8>();
}

}