#include <mutex>
#include <Common/FieldVisitors.h>
#include <DataTypes/DataTypesNumber.h>
#include <DataTypes/DataTypeString.h>
#include <DataTypes/DataTypeArray.h>
#include <Columns/ColumnString.h>
#include <Columns/ColumnArray.h>
#include <Columns/ColumnConst.h>
#include <Columns/ColumnsNumber.h>
#include <Common/Arena.h>
#include <Common/HashTable/HashMap.h>
#include <Common/typeid_cast.h>
#include <common/StringRef.h>
#include <Functions/IFunctionImpl.h>
#include <Functions/FunctionHelpers.h>
#include <Functions/FunctionFactory.h>
#include <DataTypes/getLeastSupertype.h>


namespace DB
{

namespace ErrorCodes
{
    extern const int ILLEGAL_TYPE_OF_ARGUMENT;
    extern const int BAD_ARGUMENTS;
    extern const int NUMBER_OF_ARGUMENTS_DOESNT_MATCH;
    extern const int ILLEGAL_COLUMN;
}


/** transform(x, from_array, to_array[, default]) - convert x according to an explicitly passed match.
  */

/** transform(x, [from...], [to...], default)
  * - converts the values according to the explicitly specified mapping.
  *
  * x - what to transform.
  * from - a constant array of values for the transformation.
  * to - a constant array of values into which values from `from` must be transformed.
  * default - what value to use if x is not equal to any of the values in `from`.
  * `from` and `to` - arrays of the same size.
  *
  * Types:
  * transform(T, Array(T), Array(U), U) -> U
  *
  * transform(x, [from...], [to...])
  * - if `default` is not specified, then for values of `x` for which there is no corresponding element in `from`, the unchanged value of `x` is returned.
  *
  * Types:
  * transform(T, Array(T), Array(T)) -> T
  *
  * Note: the implementation is rather cumbersome.
  */
class FunctionTransform : public IFunction
{
public:
    static constexpr auto name = "transform";
    static FunctionPtr create(const Context &) { return std::make_shared<FunctionTransform>(); }

    String getName() const override
    {
        return name;
    }

    bool isVariadic() const override { return true; }
    size_t getNumberOfArguments() const override { return 0; }
    bool useDefaultImplementationForConstants() const override { return true; }
    ColumnNumbers getArgumentsThatAreAlwaysConstant() const override { return {1, 2}; }

    DataTypePtr getReturnTypeImpl(const DataTypes & arguments) const override
    {
        const auto args_size = arguments.size();
        if (args_size != 3 && args_size != 4)
            throw Exception{"Number of arguments for function " + getName() + " doesn't match: passed " + toString(args_size) + ", should be 3 or 4",
                ErrorCodes::NUMBER_OF_ARGUMENTS_DOESNT_MATCH};

        const DataTypePtr & type_x = arguments[0];

        if (!type_x->isValueRepresentedByNumber() && !isString(type_x))
            throw Exception{"Unsupported type " + type_x->getName()
                + " of first argument of function " + getName()
                + ", must be numeric type or Date/DateTime or String", ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT};

        const DataTypeArray * type_arr_from = checkAndGetDataType<DataTypeArray>(arguments[1].get());

        if (!type_arr_from)
            throw Exception{"Second argument of function " + getName()
                + ", must be array of source values to transform from.", ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT};

        const auto type_arr_from_nested = type_arr_from->getNestedType();

        if ((type_x->isValueRepresentedByNumber() != type_arr_from_nested->isValueRepresentedByNumber())
            || (isString(type_x) != isString(type_arr_from_nested)))
        {
            throw Exception{"First argument and elements of array of second argument of function " + getName()
                + " must have compatible types: both numeric or both strings.", ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT};
        }

        const DataTypeArray * type_arr_to = checkAndGetDataType<DataTypeArray>(arguments[2].get());

        if (!type_arr_to)
            throw Exception{"Third argument of function " + getName()
                + ", must be array of destination values to transform to.", ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT};

        const DataTypePtr & type_arr_to_nested = type_arr_to->getNestedType();

        if (args_size == 3)
        {
            if ((type_x->isValueRepresentedByNumber() != type_arr_to_nested->isValueRepresentedByNumber())
                || (isString(type_x) != isString(type_arr_to_nested)))
                throw Exception{"Function " + getName()
                    + " has signature: transform(T, Array(T), Array(U), U) -> U; or transform(T, Array(T), Array(T)) -> T; where T and U are types.",
                    ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT};

            return type_x;
        }
        else
        {
            const DataTypePtr & type_default = arguments[3];

            if (!type_default->isValueRepresentedByNumber() && !isString(type_default))
                throw Exception{"Unsupported type " + type_default->getName()
                    + " of fourth argument (default value) of function " + getName()
                    + ", must be numeric type or Date/DateTime or String", ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT};

            bool default_is_string = WhichDataType(type_default).isString();
            bool nested_is_string = WhichDataType(type_arr_to_nested).isString();

            if ((type_default->isValueRepresentedByNumber() != type_arr_to_nested->isValueRepresentedByNumber())
                || (default_is_string != nested_is_string))
                throw Exception{"Function " + getName()
                    + " have signature: transform(T, Array(T), Array(U), U) -> U; or transform(T, Array(T), Array(T)) -> T; where T and U are types.",
                    ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT};

            if (type_arr_to_nested->isValueRepresentedByNumber() && type_default->isValueRepresentedByNumber())
            {
                /// We take the smallest common type for the elements of the array of values `to` and for `default`.
                return getLeastSupertype({type_arr_to_nested, type_default});
            }

            /// TODO More checks.
            return type_arr_to_nested;
        }
    }

    void executeImpl(Block & block, const ColumnNumbers & arguments, size_t result, size_t input_rows_count) override
    {
        const ColumnConst * array_from = checkAndGetColumnConst<ColumnArray>(block.getByPosition(arguments[1]).column.get());
        const ColumnConst * array_to = checkAndGetColumnConst<ColumnArray>(block.getByPosition(arguments[2]).column.get());

        if (!array_from || !array_to)
            throw Exception{"Second and third arguments of function " + getName() + " must be constant arrays.", ErrorCodes::ILLEGAL_COLUMN};

        initialize(array_from->getValue<Array>(), array_to->getValue<Array>(), block, arguments);

        const auto in = block.getByPosition(arguments.front()).column.get();

        if (isColumnConst(*in))
        {
            executeConst(block, arguments, result, input_rows_count);
            return;
        }

        const IColumn * default_column = nullptr;
        if (arguments.size() == 4)
            default_column = block.getByPosition(arguments[3]).column.get();

        auto column_result = block.getByPosition(result).type->createColumn();
        auto out = column_result.get();

        if (!executeNum<UInt8>(in, out, default_column)
            && !executeNum<UInt16>(in, out, default_column)
            && !executeNum<UInt32>(in, out, default_column)
            && !executeNum<UInt64>(in, out, default_column)
            && !executeNum<Int8>(in, out, default_column)
            && !executeNum<Int16>(in, out, default_column)
            && !executeNum<Int32>(in, out, default_column)
            && !executeNum<Int64>(in, out, default_column)
            && !executeNum<Float32>(in, out, default_column)
            && !executeNum<Float64>(in, out, default_column)
            && !executeString(in, out, default_column))
        {
            throw Exception{"Illegal column " + in->getName() + " of first argument of function " + getName(), ErrorCodes::ILLEGAL_COLUMN};
        }

        block.getByPosition(result).column = std::move(column_result);
    }

private:
    static void executeConst(Block & block, const ColumnNumbers & arguments, const size_t result, size_t input_rows_count)
    {
        /// Materialize the input column and compute the function as usual.

        Block tmp_block;
        ColumnNumbers tmp_arguments;

        tmp_block.insert(block.getByPosition(arguments[0]));
        tmp_block.getByPosition(0).column = tmp_block.getByPosition(0).column->cloneResized(input_rows_count)->convertToFullColumnIfConst();
        tmp_arguments.push_back(0);

        for (size_t i = 1; i < arguments.size(); ++i)
        {
            tmp_block.insert(block.getByPosition(arguments[i]));
            tmp_arguments.push_back(i);
        }

        auto impl = FunctionOverloadResolverAdaptor(std::make_unique<DefaultOverloadResolver>(std::make_shared<FunctionTransform>()))
                    .build(tmp_block.getColumnsWithTypeAndName());

        tmp_block.insert(block.getByPosition(result));
        size_t tmp_result = arguments.size();

        impl->execute(tmp_block, tmp_arguments, tmp_result, input_rows_count);

        block.getByPosition(result).column = tmp_block.getByPosition(tmp_result).column;
    }

    template <typename T>
    bool executeNum(const IColumn * in_untyped, IColumn * out_untyped, const IColumn * default_untyped)
    {
        if (const auto in = checkAndGetColumn<ColumnVector<T>>(in_untyped))
        {
            if (!default_untyped)
            {
                auto out = typeid_cast<ColumnVector<T> *>(out_untyped);
                if (!out)
                {
                    throw Exception{"Illegal column " + out_untyped->getName() + " of elements of array of third argument of function " + getName()
                        + ", must be " + in->getName(), ErrorCodes::ILLEGAL_COLUMN};
                }

                executeImplNumToNum<T>(in->getData(), out->getData());
            }
            else if (isColumnConst(*default_untyped))
            {
                if (!executeNumToNumWithConstDefault<T, UInt8>(in, out_untyped)
                    && !executeNumToNumWithConstDefault<T, UInt16>(in, out_untyped)
                    && !executeNumToNumWithConstDefault<T, UInt32>(in, out_untyped)
                    && !executeNumToNumWithConstDefault<T, UInt64>(in, out_untyped)
                    && !executeNumToNumWithConstDefault<T, Int8>(in, out_untyped)
                    && !executeNumToNumWithConstDefault<T, Int16>(in, out_untyped)
                    && !executeNumToNumWithConstDefault<T, Int32>(in, out_untyped)
                    && !executeNumToNumWithConstDefault<T, Int64>(in, out_untyped)
                    && !executeNumToNumWithConstDefault<T, Float32>(in, out_untyped)
                    && !executeNumToNumWithConstDefault<T, Float64>(in, out_untyped)
                    && !executeNumToStringWithConstDefault<T>(in, out_untyped))
                {
                    throw Exception{"Illegal column " + in->getName() + " of elements of array of second argument of function " + getName(),
                        ErrorCodes::ILLEGAL_COLUMN};
                }
            }
            else
            {
                if (!executeNumToNumWithNonConstDefault<T, UInt8>(in, out_untyped, default_untyped)
                    && !executeNumToNumWithNonConstDefault<T, UInt16>(in, out_untyped, default_untyped)
                    && !executeNumToNumWithNonConstDefault<T, UInt32>(in, out_untyped, default_untyped)
                    && !executeNumToNumWithNonConstDefault<T, UInt64>(in, out_untyped, default_untyped)
                    && !executeNumToNumWithNonConstDefault<T, Int8>(in, out_untyped, default_untyped)
                    && !executeNumToNumWithNonConstDefault<T, Int16>(in, out_untyped, default_untyped)
                    && !executeNumToNumWithNonConstDefault<T, Int32>(in, out_untyped, default_untyped)
                    && !executeNumToNumWithNonConstDefault<T, Int64>(in, out_untyped, default_untyped)
                    && !executeNumToNumWithNonConstDefault<T, Float32>(in, out_untyped, default_untyped)
                    && !executeNumToNumWithNonConstDefault<T, Float64>(in, out_untyped, default_untyped)
                    && !executeNumToStringWithNonConstDefault<T>(in, out_untyped, default_untyped))
                {
                    throw Exception{"Illegal column " + in->getName() + " of elements of array of second argument of function " + getName(),
                        ErrorCodes::ILLEGAL_COLUMN};
                }
            }

            return true;
        }

        return false;
    }

    bool executeString(const IColumn * in_untyped, IColumn * out_untyped, const IColumn * default_untyped)
    {
        if (const auto in = checkAndGetColumn<ColumnString>(in_untyped))
        {
            if (!default_untyped)
            {
                if (!executeStringToString(in, out_untyped))
                    throw Exception{"Illegal column " + in->getName() + " of elements of array of second argument of function " + getName(),
                        ErrorCodes::ILLEGAL_COLUMN};
            }
            else if (isColumnConst(*default_untyped))
            {
                if (!executeStringToNumWithConstDefault<UInt8>(in, out_untyped)
                    && !executeStringToNumWithConstDefault<UInt16>(in, out_untyped)
                    && !executeStringToNumWithConstDefault<UInt32>(in, out_untyped)
                    && !executeStringToNumWithConstDefault<UInt64>(in, out_untyped)
                    && !executeStringToNumWithConstDefault<Int8>(in, out_untyped)
                    && !executeStringToNumWithConstDefault<Int16>(in, out_untyped)
                    && !executeStringToNumWithConstDefault<Int32>(in, out_untyped)
                    && !executeStringToNumWithConstDefault<Int64>(in, out_untyped)
                    && !executeStringToNumWithConstDefault<Float32>(in, out_untyped)
                    && !executeStringToNumWithConstDefault<Float64>(in, out_untyped)
                    && !executeStringToStringWithConstDefault(in, out_untyped))
                {
                    throw Exception{"Illegal column " + in->getName() + " of elements of array of second argument of function " + getName(),
                        ErrorCodes::ILLEGAL_COLUMN};
                }
            }
            else
            {
                if (!executeStringToNumWithNonConstDefault<UInt8>(in, out_untyped, default_untyped)
                    && !executeStringToNumWithNonConstDefault<UInt16>(in, out_untyped, default_untyped)
                    && !executeStringToNumWithNonConstDefault<UInt32>(in, out_untyped, default_untyped)
                    && !executeStringToNumWithNonConstDefault<UInt64>(in, out_untyped, default_untyped)
                    && !executeStringToNumWithNonConstDefault<Int8>(in, out_untyped, default_untyped)
                    && !executeStringToNumWithNonConstDefault<Int16>(in, out_untyped, default_untyped)
                    && !executeStringToNumWithNonConstDefault<Int32>(in, out_untyped, default_untyped)
                    && !executeStringToNumWithNonConstDefault<Int64>(in, out_untyped, default_untyped)
                    && !executeStringToNumWithNonConstDefault<Float32>(in, out_untyped, default_untyped)
                    && !executeStringToNumWithNonConstDefault<Float64>(in, out_untyped, default_untyped)
                    && !executeStringToStringWithNonConstDefault(in, out_untyped, default_untyped))
                {
                    throw Exception{"Illegal column " + in->getName() + " of elements of array of second argument of function " + getName(),
                        ErrorCodes::ILLEGAL_COLUMN};
                }
            }

            return true;
        }

        return false;
    }

    template <typename T, typename U>
    bool executeNumToNumWithConstDefault(const ColumnVector<T> * in, IColumn * out_untyped)
    {
        auto out = typeid_cast<ColumnVector<U> *>(out_untyped);
        if (!out)
            return false;

        executeImplNumToNumWithConstDefault<T, U>(in->getData(), out->getData(), const_default_value.get<U>());
        return true;
    }

    template <typename T, typename U>
    bool executeNumToNumWithNonConstDefault(const ColumnVector<T> * in, IColumn * out_untyped, const IColumn * default_untyped)
    {
        auto out = typeid_cast<ColumnVector<U> *>(out_untyped);
        if (!out)
            return false;

        if (!executeNumToNumWithNonConstDefault2<T, U, UInt8>(in, out, default_untyped)
            && !executeNumToNumWithNonConstDefault2<T, U, UInt16>(in, out, default_untyped)
            && !executeNumToNumWithNonConstDefault2<T, U, UInt32>(in, out, default_untyped)
            && !executeNumToNumWithNonConstDefault2<T, U, UInt64>(in, out, default_untyped)
            && !executeNumToNumWithNonConstDefault2<T, U, Int8>(in, out, default_untyped)
            && !executeNumToNumWithNonConstDefault2<T, U, Int16>(in, out, default_untyped)
            && !executeNumToNumWithNonConstDefault2<T, U, Int32>(in, out, default_untyped)
            && !executeNumToNumWithNonConstDefault2<T, U, Int64>(in, out, default_untyped)
            && !executeNumToNumWithNonConstDefault2<T, U, Float32>(in, out, default_untyped)
            && !executeNumToNumWithNonConstDefault2<T, U, Float64>(in, out, default_untyped))
        {
            throw Exception(
                "Illegal column " + default_untyped->getName() + " of fourth argument of function " + getName(),
                ErrorCodes::ILLEGAL_COLUMN);
        }

        return true;
    }

    template <typename T, typename U, typename V>
    bool executeNumToNumWithNonConstDefault2(const ColumnVector<T> * in, ColumnVector<U> * out, const IColumn * default_untyped)
    {
        auto col_default = checkAndGetColumn<ColumnVector<V>>(default_untyped);
        if (!col_default)
            return false;

        executeImplNumToNumWithNonConstDefault<T, U, V>(in->getData(), out->getData(), col_default->getData());
        return true;
    }

    template <typename T>
    bool executeNumToStringWithConstDefault(const ColumnVector<T> * in, IColumn * out_untyped)
    {
        auto out = typeid_cast<ColumnString *>(out_untyped);
        if (!out)
            return false;

        const String & default_str = const_default_value.get<const String &>();
        StringRef default_string_ref{default_str.data(), default_str.size() + 1};
        executeImplNumToStringWithConstDefault<T>(in->getData(), out->getChars(), out->getOffsets(), default_string_ref);
        return true;
    }

    template <typename T>
    bool executeNumToStringWithNonConstDefault(const ColumnVector<T> * in, IColumn * out_untyped, const IColumn * default_untyped)
    {
        auto out = typeid_cast<ColumnString *>(out_untyped);
        if (!out)
            return false;

        auto default_col = checkAndGetColumn<ColumnString>(default_untyped);
        if (!default_col)
        {
            throw Exception{"Illegal column " + default_untyped->getName() + " of fourth argument of function " + getName(),
                ErrorCodes::ILLEGAL_COLUMN};
        }

        executeImplNumToStringWithNonConstDefault<T>(
            in->getData(),
            out->getChars(), out->getOffsets(),
            default_col->getChars(), default_col->getOffsets());

        return true;
    }

    template <typename U>
    bool executeStringToNumWithConstDefault(const ColumnString * in, IColumn * out_untyped)
    {
        auto out = typeid_cast<ColumnVector<U> *>(out_untyped);
        if (!out)
            return false;

        executeImplStringToNumWithConstDefault<U>(in->getChars(), in->getOffsets(), out->getData(), const_default_value.get<U>());
        return true;
    }

    template <typename U>
    bool executeStringToNumWithNonConstDefault(const ColumnString * in, IColumn * out_untyped, const IColumn * default_untyped)
    {
        auto out = typeid_cast<ColumnVector<U> *>(out_untyped);
        if (!out)
            return false;

        if (!executeStringToNumWithNonConstDefault2<U, UInt8>(in, out, default_untyped)
            && !executeStringToNumWithNonConstDefault2<U, UInt16>(in, out, default_untyped)
            && !executeStringToNumWithNonConstDefault2<U, UInt32>(in, out, default_untyped)
            && !executeStringToNumWithNonConstDefault2<U, UInt64>(in, out, default_untyped)
            && !executeStringToNumWithNonConstDefault2<U, Int8>(in, out, default_untyped)
            && !executeStringToNumWithNonConstDefault2<U, Int16>(in, out, default_untyped)
            && !executeStringToNumWithNonConstDefault2<U, Int32>(in, out, default_untyped)
            && !executeStringToNumWithNonConstDefault2<U, Int64>(in, out, default_untyped)
            && !executeStringToNumWithNonConstDefault2<U, Float32>(in, out, default_untyped)
            && !executeStringToNumWithNonConstDefault2<U, Float64>(in, out, default_untyped))
        {
            throw Exception{"Illegal column " + default_untyped->getName() + " of fourth argument of function " + getName(),
                ErrorCodes::ILLEGAL_COLUMN};
        }

        return true;
    }

    template <typename U, typename V>
    bool executeStringToNumWithNonConstDefault2(const ColumnString * in, ColumnVector<U> * out, const IColumn * default_untyped)
    {
        auto col_default = checkAndGetColumn<ColumnVector<V>>(default_untyped);
        if (!col_default)
            return false;

        executeImplStringToNumWithNonConstDefault<U, V>(in->getChars(), in->getOffsets(), out->getData(), col_default->getData());
        return true;
    }

    bool executeStringToString(const ColumnString * in, IColumn * out_untyped)
    {
        auto out = typeid_cast<ColumnString *>(out_untyped);
        if (!out)
            return false;

        executeImplStringToString(in->getChars(), in->getOffsets(), out->getChars(), out->getOffsets());
        return true;
    }

    bool executeStringToStringWithConstDefault(const ColumnString * in, IColumn * out_untyped)
    {
        auto out = typeid_cast<ColumnString *>(out_untyped);
        if (!out)
            return false;

        const String & default_str = const_default_value.get<const String &>();
        StringRef default_string_ref{default_str.data(), default_str.size() + 1};
        executeImplStringToStringWithConstDefault(in->getChars(), in->getOffsets(), out->getChars(), out->getOffsets(), default_string_ref);
        return true;
    }

    bool executeStringToStringWithNonConstDefault(const ColumnString * in, IColumn * out_untyped, const IColumn * default_untyped)
    {
        auto out = typeid_cast<ColumnString *>(out_untyped);
        if (!out)
            return false;

        auto default_col = checkAndGetColumn<ColumnString>(default_untyped);
        if (!default_col)
        {
            throw Exception{"Illegal column " + default_untyped->getName() + " of fourth argument of function " + getName(),
                ErrorCodes::ILLEGAL_COLUMN};
        }

        executeImplStringToStringWithNonConstDefault(
            in->getChars(), in->getOffsets(),
            out->getChars(), out->getOffsets(),
            default_col->getChars(), default_col->getOffsets());

        return true;
    }


    template <typename T, typename U>
    void executeImplNumToNumWithConstDefault(const PaddedPODArray<T> & src, PaddedPODArray<U> & dst, U dst_default)
    {
        const auto & table = *table_num_to_num;
        size_t size = src.size();
        dst.resize(size);
        for (size_t i = 0; i < size; ++i)
        {
            auto it = table.find(src[i]);
            if (it)
                memcpy(&dst[i], &it->getMapped(), sizeof(dst[i]));    /// little endian.
            else
                dst[i] = dst_default;
        }
    }

    template <typename T, typename U, typename V>
    void executeImplNumToNumWithNonConstDefault(const PaddedPODArray<T> & src, PaddedPODArray<U> & dst, const PaddedPODArray<V> & dst_default)
    {
        const auto & table = *table_num_to_num;
        size_t size = src.size();
        dst.resize(size);
        for (size_t i = 0; i < size; ++i)
        {
            auto it = table.find(src[i]);
            if (it)
                memcpy(&dst[i], &it->getMapped(), sizeof(dst[i]));    /// little endian.
            else
                dst[i] = dst_default[i];
        }
    }

    template <typename T>
    void executeImplNumToNum(const PaddedPODArray<T> & src, PaddedPODArray<T> & dst)
    {
        const auto & table = *table_num_to_num;
        size_t size = src.size();
        dst.resize(size);
        for (size_t i = 0; i < size; ++i)
        {
            auto it = table.find(src[i]);
            if (it)
                memcpy(&dst[i], &it->getMapped(), sizeof(dst[i]));
            else
                dst[i] = src[i];
        }
    }

    template <typename T>
    void executeImplNumToStringWithConstDefault(const PaddedPODArray<T> & src,
        ColumnString::Chars & dst_data, ColumnString::Offsets & dst_offsets, StringRef dst_default)
    {
        const auto & table = *table_num_to_string;
        size_t size = src.size();
        dst_offsets.resize(size);
        ColumnString::Offset current_dst_offset = 0;
        for (size_t i = 0; i < size; ++i)
        {
            auto it = table.find(src[i]);
            StringRef ref = it ? it->getMapped() : dst_default;
            dst_data.resize(current_dst_offset + ref.size);
            memcpy(&dst_data[current_dst_offset], ref.data, ref.size);
            current_dst_offset += ref.size;
            dst_offsets[i] = current_dst_offset;
        }
    }

    template <typename T>
    void executeImplNumToStringWithNonConstDefault(const PaddedPODArray<T> & src,
        ColumnString::Chars & dst_data, ColumnString::Offsets & dst_offsets,
        const ColumnString::Chars & dst_default_data, const ColumnString::Offsets & dst_default_offsets)
    {
        const auto & table = *table_num_to_string;
        size_t size = src.size();
        dst_offsets.resize(size);
        ColumnString::Offset current_dst_offset = 0;
        ColumnString::Offset current_dst_default_offset = 0;
        for (size_t i = 0; i < size; ++i)
        {
            auto it = table.find(src[i]);
            StringRef ref;

            if (it)
                ref = it->getMapped();
            else
            {
                ref.data = reinterpret_cast<const char *>(&dst_default_data[current_dst_default_offset]);
                ref.size = dst_default_offsets[i] - current_dst_default_offset;
            }

            dst_data.resize(current_dst_offset + ref.size);
            memcpy(&dst_data[current_dst_offset], ref.data, ref.size);
            current_dst_offset += ref.size;
            current_dst_default_offset = dst_default_offsets[i];
            dst_offsets[i] = current_dst_offset;
        }
    }

    template <typename U>
    void executeImplStringToNumWithConstDefault(
        const ColumnString::Chars & src_data, const ColumnString::Offsets & src_offsets,
        PaddedPODArray<U> & dst, U dst_default)
    {
        const auto & table = *table_string_to_num;
        size_t size = src_offsets.size();
        dst.resize(size);
        ColumnString::Offset current_src_offset = 0;
        for (size_t i = 0; i < size; ++i)
        {
            StringRef ref{&src_data[current_src_offset], src_offsets[i] - current_src_offset};
            current_src_offset = src_offsets[i];
            auto it = table.find(ref);
            if (it)
                memcpy(&dst[i], &it->getMapped(), sizeof(dst[i]));
            else
                dst[i] = dst_default;
        }
    }

    template <typename U, typename V>
    void executeImplStringToNumWithNonConstDefault(
        const ColumnString::Chars & src_data, const ColumnString::Offsets & src_offsets,
        PaddedPODArray<U> & dst, const PaddedPODArray<V> & dst_default)
    {
        const auto & table = *table_string_to_num;
        size_t size = src_offsets.size();
        dst.resize(size);
        ColumnString::Offset current_src_offset = 0;
        for (size_t i = 0; i < size; ++i)
        {
            StringRef ref{&src_data[current_src_offset], src_offsets[i] - current_src_offset};
            current_src_offset = src_offsets[i];
            auto it = table.find(ref);
            if (it)
                memcpy(&dst[i], &it->getMapped(), sizeof(dst[i]));
            else
                dst[i] = dst_default[i];
        }
    }

    template <bool with_default>
    void executeImplStringToStringWithOrWithoutConstDefault(
        const ColumnString::Chars & src_data, const ColumnString::Offsets & src_offsets,
        ColumnString::Chars & dst_data, ColumnString::Offsets & dst_offsets, StringRef dst_default)
    {
        const auto & table = *table_string_to_string;
        size_t size = src_offsets.size();
        dst_offsets.resize(size);
        ColumnString::Offset current_src_offset = 0;
        ColumnString::Offset current_dst_offset = 0;
        for (size_t i = 0; i < size; ++i)
        {
            StringRef src_ref{&src_data[current_src_offset], src_offsets[i] - current_src_offset};
            current_src_offset = src_offsets[i];

            auto it = table.find(src_ref);

            StringRef dst_ref = it ? it->getMapped() : (with_default ? dst_default : src_ref);
            dst_data.resize(current_dst_offset + dst_ref.size);
            memcpy(&dst_data[current_dst_offset], dst_ref.data, dst_ref.size);
            current_dst_offset += dst_ref.size;
            dst_offsets[i] = current_dst_offset;
        }
    }

    void executeImplStringToString(
        const ColumnString::Chars & src_data, const ColumnString::Offsets & src_offsets,
        ColumnString::Chars & dst_data, ColumnString::Offsets & dst_offsets)
    {
        executeImplStringToStringWithOrWithoutConstDefault<false>(src_data, src_offsets, dst_data, dst_offsets, {});
    }

    void executeImplStringToStringWithConstDefault(
        const ColumnString::Chars & src_data, const ColumnString::Offsets & src_offsets,
        ColumnString::Chars & dst_data, ColumnString::Offsets & dst_offsets, StringRef dst_default)
    {
        executeImplStringToStringWithOrWithoutConstDefault<true>(src_data, src_offsets, dst_data, dst_offsets, dst_default);
    }

    void executeImplStringToStringWithNonConstDefault(
        const ColumnString::Chars & src_data, const ColumnString::Offsets & src_offsets,
        ColumnString::Chars & dst_data, ColumnString::Offsets & dst_offsets,
        const ColumnString::Chars & dst_default_data, const ColumnString::Offsets & dst_default_offsets)
    {
        const auto & table = *table_string_to_string;
        size_t size = src_offsets.size();
        dst_offsets.resize(size);
        ColumnString::Offset current_src_offset = 0;
        ColumnString::Offset current_dst_offset = 0;
        ColumnString::Offset current_dst_default_offset = 0;
        for (size_t i = 0; i < size; ++i)
        {
            StringRef src_ref{&src_data[current_src_offset], src_offsets[i] - current_src_offset};
            current_src_offset = src_offsets[i];

            auto it = table.find(src_ref);
            StringRef dst_ref;

            if (it)
                dst_ref = it->getMapped();
            else
            {
                dst_ref.data = reinterpret_cast<const char *>(&dst_default_data[current_dst_default_offset]);
                dst_ref.size = dst_default_offsets[i] - current_dst_default_offset;
            }

            dst_data.resize(current_dst_offset + dst_ref.size);
            memcpy(&dst_data[current_dst_offset], dst_ref.data, dst_ref.size);
            current_dst_offset += dst_ref.size;
            current_dst_default_offset = dst_default_offsets[i];
            dst_offsets[i] = current_dst_offset;
        }
    }


    /// Different versions of the hash tables to implement the mapping.

    using NumToNum = HashMap<UInt64, UInt64, HashCRC32<UInt64>>;
    using NumToString = HashMap <UInt64, StringRef, HashCRC32<UInt64>>;     /// Everywhere StringRef's with trailing zero.
    using StringToNum = HashMap<StringRef, UInt64, StringRefHash>;
    using StringToString = HashMap<StringRef, StringRef, StringRefHash>;

    std::unique_ptr<NumToNum> table_num_to_num;
    std::unique_ptr<NumToString> table_num_to_string;
    std::unique_ptr<StringToNum> table_string_to_num;
    std::unique_ptr<StringToString> table_string_to_string;

    Arena string_pool;

    Field const_default_value;    /// Null, if not specified.

    std::atomic<bool> initialized {false};
    std::mutex mutex;

    /// Can be called from different threads. It works only on the first call.
    void initialize(const Array & from, const Array & to, Block & block, const ColumnNumbers & arguments)
    {
        if (initialized)
            return;

        const size_t size = from.size();
        if (0 == size)
            throw Exception{"Empty arrays are illegal in function " + getName(), ErrorCodes::BAD_ARGUMENTS};

        std::lock_guard lock(mutex);

        if (initialized)
            return;

        if (size != to.size())
            throw Exception{"Second and third arguments of function " + getName() + " must be arrays of same size", ErrorCodes::BAD_ARGUMENTS};

        Array converted_to;
        const Array * used_to = &to;

        /// Whether the default value is set.

        if (arguments.size() == 4)
        {
            const IColumn * default_col = block.getByPosition(arguments[3]).column.get();
            const ColumnConst * const_default_col = typeid_cast<const ColumnConst *>(default_col);

            if (const_default_col)
                const_default_value = (*const_default_col)[0];

            /// Do we need to convert the elements `to` and `default_value` to the smallest common type that is Float64?
            bool default_col_is_float =
                   checkColumn<ColumnFloat32>(default_col)
                || checkColumn<ColumnFloat64>(default_col)
                || checkColumnConst<ColumnFloat32>(default_col)
                || checkColumnConst<ColumnFloat64>(default_col);

            bool to_is_float = to[0].getType() == Field::Types::Float64;

            if (default_col_is_float && !to_is_float)
            {
                converted_to.resize(size);
                for (size_t i = 0; i < size; ++i)
                    converted_to[i] = applyVisitor(FieldVisitorConvertToNumber<Float64>(), to[i]);
                used_to = &converted_to;
            }
            else if (!default_col_is_float && to_is_float)
            {
                if (const_default_col)
                    const_default_value = applyVisitor(FieldVisitorConvertToNumber<Float64>(), const_default_value);
            }
        }

        /// Note: Doesn't check the duplicates in the `from` array.

        if (from[0].getType() != Field::Types::String && to[0].getType() != Field::Types::String)
        {
            table_num_to_num = std::make_unique<NumToNum>();
            auto & table = *table_num_to_num;
            for (size_t i = 0; i < size; ++i)
            {
                // Field may be of Float type, but for the purpose of bitwise
                // equality we can treat them as UInt64, hence the reinterpret().
                table[from[i].reinterpret<UInt64>()] = (*used_to)[i].reinterpret<UInt64>();
            }
        }
        else if (from[0].getType() != Field::Types::String && to[0].getType() == Field::Types::String)
        {
            table_num_to_string = std::make_unique<NumToString>();
            auto & table = *table_num_to_string;
            for (size_t i = 0; i < size; ++i)
            {
                const String & str_to = to[i].get<const String &>();
                StringRef ref{string_pool.insert(str_to.data(), str_to.size() + 1), str_to.size() + 1};
                table[from[i].reinterpret<UInt64>()] = ref;
            }
        }
        else if (from[0].getType() == Field::Types::String && to[0].getType() != Field::Types::String)
        {
            table_string_to_num = std::make_unique<StringToNum>();
            auto & table = *table_string_to_num;
            for (size_t i = 0; i < size; ++i)
            {
                const String & str_from = from[i].get<const String &>();
                StringRef ref{string_pool.insert(str_from.data(), str_from.size() + 1), str_from.size() + 1};
                table[ref] = (*used_to)[i].reinterpret<UInt64>();
            }
        }
        else if (from[0].getType() == Field::Types::String && to[0].getType() == Field::Types::String)
        {
            table_string_to_string = std::make_unique<StringToString>();
            auto & table = *table_string_to_string;
            for (size_t i = 0; i < size; ++i)
            {
                const String & str_from = from[i].get<const String &>();
                const String & str_to = to[i].get<const String &>();
                StringRef ref_from{string_pool.insert(str_from.data(), str_from.size() + 1), str_from.size() + 1};
                StringRef ref_to{string_pool.insert(str_to.data(), str_to.size() + 1), str_to.size() + 1};
                table[ref_from] = ref_to;
            }
        }

        initialized = true;
    }
};

void registerFunctionTransform(FunctionFactory & factory)
{
    factory.registerFunction<FunctionTransform>();
}

}