#include <DB/Functions/FunctionsConditional.h>
#include <DB/Functions/FunctionsArray.h>
#include <DB/Functions/FunctionsTransform.h>
#include <DB/Functions/FunctionFactory.h>
#include <DB/Functions/Conditional/common.h>
#include <DB/Functions/Conditional/ArgsInfo.h>
#include <DB/Functions/Conditional/CondSource.h>
#include <DB/Functions/Conditional/NumericPerformer.h>
#include <DB/Functions/Conditional/StringEvaluator.h>
#include <DB/Functions/Conditional/StringArrayEvaluator.h>
#include <DB/Functions/Conditional/CondException.h>
#include <DB/Columns/ColumnNullable.h>

namespace DB
{

namespace
{

/// Check whether at least one of the specified branches of the function multiIf
/// is either a nullable column or a null column inside a given block.
bool blockHasNullableBranches(const Block & block, const ColumnNumbers & args)
{
	auto check = [](const Block & block, size_t arg)
	{
		const auto & elem = block.unsafeGetByPosition(arg);
		return (elem.column && (elem.column->isNullable() || elem.column->isNull()));
	};

	size_t else_arg = Conditional::elseArg(args);
	for (size_t i = Conditional::firstThen(); i < else_arg; i = Conditional::nextThen(i))
	{
		if (check(block, args[i]))
			return true;
	}

	if (check(block, args[else_arg]))
		return true;

	return false;
}

bool hasNullableDataTypes(const DataTypes & args)
{
	size_t else_arg = Conditional::elseArg(args);

	for (size_t i = Conditional::firstThen(); i < else_arg; i = Conditional::nextThen(i))
	{
		if (args[i]->isNullable())
			return true;
	}

	return args[else_arg]->isNullable();
}

bool hasNullDataTypes(const DataTypes & args)
{
	size_t else_arg = Conditional::elseArg(args);

	for (size_t i = Conditional::firstThen(); i < else_arg; i = Conditional::nextThen(i))
	{
		if (args[i]->isNull())
			return true;
	}

	return args[else_arg]->isNull();
}

}

void registerFunctionsConditional(FunctionFactory & factory)
{
	factory.registerFunction<FunctionIf>();
	factory.registerFunction<FunctionMultiIf>();
	factory.registerFunction<FunctionCaseWithExpr>();
	factory.registerFunction<FunctionCaseWithoutExpr>();
}

/// Implementation of FunctionMultiIf.

FunctionPtr FunctionMultiIf::create(const Context & context)
{
	return std::make_shared<FunctionMultiIf>();
}

String FunctionMultiIf::getName() const
{
	return is_case_mode ? "CASE" : name;
}

bool FunctionMultiIf::hasSpecialSupportForNulls() const
{
	return true;
}

void FunctionMultiIf::setCaseMode()
{
	is_case_mode = true;
}

DataTypePtr FunctionMultiIf::getReturnTypeImpl(const DataTypes & args) const
{
	DataTypePtr data_type;

	try
	{
		data_type = getReturnTypeInternal(args);
	}
	catch (const Conditional::CondException & ex)
	{
		rethrowContextually(ex);
	}

	return data_type;
}

void FunctionMultiIf::executeImpl(Block & block, const ColumnNumbers & args, size_t result)
{
	auto perform_multi_if = [&](Block & block, const ColumnNumbers & args, size_t result, size_t tracker)
	{
		if (performTrivialCase(block, args, result, tracker))
			return;
		if (Conditional::NumericPerformer::perform(block, args, result, tracker))
			return;
		if (Conditional::StringEvaluator::perform(block, args, result, tracker))
			return;
		if (Conditional::StringArrayEvaluator::perform(block, args, result, tracker))
			return;

		if (is_case_mode)
			throw Exception{"Some THEN/ELSE clauses in CASE construction have "
				"illegal or incompatible types", ErrorCodes::ILLEGAL_COLUMN};
		else
			throw Exception{"One or more branch (then, else) columns of function "
				+ getName() + " have illegal or incompatible types",
				ErrorCodes::ILLEGAL_COLUMN};
	};

	try
	{
		if (!blockHasNullableBranches(block, args))
		{
			perform_multi_if(block, args, result, result);
			return;
		}

		/// The adopted approach is quite similar to how ordinary functions deal
		/// with nullable arguments. From the original block, we create a new block
		/// that contains only non-nullable types and an extra column, namely a "tracker"
		/// column that tracks the originating column of each row of the result column.
		/// This way, after having run multiIf on this new block, we can create
		/// a correct null byte map for the result column.

		size_t row_count = block.rowsInFirstColumn();

		/// From the block to be processed, deduce a block whose specified
		/// columns are not nullable. We accept null columns because they
		/// are processed independently later.
		ColumnNumbers args_to_transform;
		size_t else_arg = Conditional::elseArg(args);
		for (size_t i = Conditional::firstThen(); i < else_arg; i = Conditional::nextThen(i))
			args_to_transform.push_back(args[i]);
		args_to_transform.push_back(args[else_arg]);

		Block block_with_nested_cols = createBlockWithNestedColumns(block, args_to_transform);

		/// Append a column that tracks, for each result of multiIf, the index
		/// of the originating column.
		ColumnWithTypeAndName elem;
		elem.type = std::make_shared<DataTypeUInt16>();

		size_t tracker = block_with_nested_cols.columns();
		block_with_nested_cols.insert(elem);

		/// Really perform multiIf.
		perform_multi_if(block_with_nested_cols, args, result, tracker);

		/// Store the result.
		const ColumnWithTypeAndName & source_col = block_with_nested_cols.unsafeGetByPosition(result);
		ColumnWithTypeAndName & dest_col = block.unsafeGetByPosition(result);

		if (source_col.column->isNull())
		{
			/// Degenerate case: the result is a null column.
			dest_col.column = source_col.column;
			return;
		}

		dest_col.column = std::make_shared<ColumnNullable>(source_col.column);

		/// Setup the null byte map of the result column by using the branch tracker column values.
		ColumnPtr tracker_holder = block_with_nested_cols.unsafeGetByPosition(tracker).column;
		ColumnNullable & nullable_col = static_cast<ColumnNullable &>(*dest_col.column);

		if (auto col = typeid_cast<ColumnConstUInt16 *>(tracker_holder.get()))
		{
			auto pos = col->getData();
			const IColumn & origin = *block.unsafeGetByPosition(pos).column;

			ColumnPtr null_map;

			if (origin.isNull())
				null_map = std::make_shared<ColumnUInt8>(row_count, 1);
			else if (origin.isNullable())
			{
				const ColumnNullable & origin_nullable = static_cast<const ColumnNullable &>(origin);
				null_map = origin_nullable.getNullValuesByteMap();
			}
			else
				null_map = std::make_shared<ColumnUInt8>(row_count, 0);

			nullable_col.getNullValuesByteMap() = null_map;
		}
		else if (auto col = typeid_cast<ColumnUInt16 *>(tracker_holder.get()))
		{
			/// Remember which columns are nullable. This avoids us many costly
			/// calls to virtual functions.
			std::vector<UInt8> nullable_cols_map;
			nullable_cols_map.resize(args.size());
			for (const auto & arg : args)
			{
				const auto & col = block.unsafeGetByPosition(arg).column;
				bool is_nullable = col->isNullable();
				nullable_cols_map[arg] = is_nullable ? 1 : 0;
			}

			/// Remember which columns are null. The same remark as above applies.
			std::vector<UInt8> null_cols_map;
			null_cols_map.resize(args.size());
			for (const auto & arg : args)
			{
				const auto & col = block.unsafeGetByPosition(arg).column;
				bool is_null = col->isNull();
				null_cols_map[arg] = is_null ? 1 : 0;
			}

			auto null_map = std::make_shared<ColumnUInt8>(row_count);
			nullable_col.getNullValuesByteMap() = null_map;
			auto & null_map_data = null_map->getData();

			const auto & data = col->getData();
			for (size_t row = 0; row < row_count; ++row)
			{
				size_t pos = data[row];
				bool is_null;

				if (null_cols_map[pos] != 0)
					is_null = true;
				else if (nullable_cols_map[pos] != 0)
				{
					const IColumn & origin = *block.unsafeGetByPosition(pos).column;
					const auto & nullable_col = static_cast<const ColumnNullable &>(origin);
					is_null = nullable_col.isNullAt(row);
				}
				else
					is_null = false;

				null_map_data[row] = is_null ? 1 : 0;
			}
		}
		else
			throw Exception{"Internal error", ErrorCodes::LOGICAL_ERROR};
	}
	catch (const Conditional::CondException & ex)
	{
		rethrowContextually(ex);
	}
}

DataTypePtr FunctionMultiIf::getReturnTypeInternal(const DataTypes & args) const
{
	if (!Conditional::hasValidArgCount(args))
	{
		if (is_case_mode)
			throw Exception{"Some mandatory parameters are missing in the CASE "
				"construction", ErrorCodes::NUMBER_OF_ARGUMENTS_DOESNT_MATCH};
		else
			throw Exception{"Invalid number of arguments for function " + getName(),
				ErrorCodes::NUMBER_OF_ARGUMENTS_DOESNT_MATCH};
	}

	/// Check that conditions have valid types.
	for (size_t i = Conditional::firstCond(); i < Conditional::elseArg(args); i = Conditional::nextCond(i))
	{
		const IDataType * observed_type;
		if (args[i]->isNullable())
		{
			const DataTypeNullable & nullable_type = static_cast<const DataTypeNullable &>(*args[i]);
			observed_type = nullable_type.getNestedType().get();
		}
		else
			observed_type = args[i].get();

		if (!typeid_cast<const DataTypeUInt8 *>(observed_type) && !observed_type->isNull())
		{
			if (is_case_mode)
				throw Exception{"In CASE construction, illegal type of WHEN clause "
				+ toString(i / 2) + ". Must be UInt8", ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT};
			else
				throw Exception{"Illegal type of argument " + toString(i) + " (condition) "
					"of function " + getName() + ". Must be UInt8.",
					ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT};
		}
	}

	bool has_nullable_types = hasNullableDataTypes(args);
	bool has_null_types = hasNullDataTypes(args);

	if (Conditional::hasArithmeticBranches(args))
		return Conditional::getReturnTypeForArithmeticArgs(args);
	else if (Conditional::hasArrayBranches(args))
	{
		/// NOTE Сообщения об ошибках будут относится к типам элементов массивов, что немного некорректно.
		DataTypes new_args;
		new_args.reserve(args.size());

		auto push_branch_arg = [&args, &new_args](size_t i)
		{
			if (args[i]->isNull())
				new_args.push_back(args[i]);
			else
			{
				const IDataType * observed_type;
				if (args[i]->isNullable())
				{
					const auto & nullable_type = static_cast<const DataTypeNullable &>(*args[i]);
					observed_type = nullable_type.getNestedType().get();
				}
				else
					observed_type = args[i].get();

				const DataTypeArray * type_arr = typeid_cast<const DataTypeArray *>(observed_type);
				if (type_arr == nullptr)
					throw Exception{"Internal error", ErrorCodes::LOGICAL_ERROR};
				new_args.push_back(type_arr->getNestedType());
			}
		};

		for (size_t i = 0; i < Conditional::elseArg(args); ++i)
		{
			if (Conditional::isCond(i))
				new_args.push_back(args[i]);
			else
				push_branch_arg(i);
		}

		push_branch_arg(Conditional::elseArg(args));

		/// NOTE: in a future release, this code will be rewritten. Indeed
		/// the current approach is flawed since it cannot appropriately
		/// deal with null arguments and arrays that contain null elements.
		/// For now we assume that arrays do not contain any such elements.
		DataTypePtr elt_type = getReturnTypeImpl(new_args);
		if (elt_type->isNullable())
		{
			DataTypeNullable & nullable_type = static_cast<DataTypeNullable &>(*elt_type);
			elt_type = nullable_type.getNestedType();
		}

		DataTypePtr type = std::make_shared<DataTypeArray>(elt_type);
		if (has_nullable_types || has_null_types)
			type = std::make_shared<DataTypeNullable>(type);
		return type;
	}
	else if (!Conditional::hasIdenticalTypes(args))
	{
		if (Conditional::hasFixedStrings(args))
		{
			if (!Conditional::hasFixedStringsOfIdenticalLength(args))
			{
				if (is_case_mode)
					throw Exception{"THEN/ELSE clauses in CASE construction "
						"have FixedString type and different sizes",
						ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT};
				else
					throw Exception{"Branch (then, else) arguments of function " + getName() +
						" have FixedString type and different sizes",
						ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT};
			}

			const IDataType * data = args[Conditional::firstThen()].get();
			const auto * fixed_str = typeid_cast<const DataTypeFixedString *>(data);

			if (fixed_str == nullptr)
				throw Exception{"Internal error", ErrorCodes::LOGICAL_ERROR};

			DataTypePtr type = std::make_shared<DataTypeFixedString>(fixed_str->getN());
			if (has_nullable_types || has_null_types)
				type = std::make_shared<DataTypeNullable>(type);
			return type;
		}
		else if (Conditional::hasStrings(args))
		{
			DataTypePtr type = std::make_shared<DataTypeString>();
			if (has_nullable_types || has_null_types)
				type = std::make_shared<DataTypeNullable>(type);
			return type;
		}
		else
		{
			if (is_case_mode)
				throw Exception{"THEN/ELSE clauses in CASE construction "
					"have incompatible arguments", ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT};
			else
				throw Exception{
					"Incompatible branch (then, else) arguments for function " + getName(),
					ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT
				};
		}
	}
	else
	{
		/// Return the type of the first non-null branch.
		/// Make it nullable if there is at least one nullable branch
		/// or one null branch.

		auto get_type_to_return = [has_nullable_types, has_null_types](const DataTypePtr & arg) -> DataTypePtr
		{
			if (arg->isNullable())
				return arg;
			else if (has_nullable_types || has_null_types)
				return std::make_shared<DataTypeNullable>(arg);
			else
				return arg;
		};

		for (size_t i = Conditional::firstThen(); i < Conditional::elseArg(args); i = Conditional::nextThen(i))
		{
			if (!args[i]->isNull())
				return get_type_to_return(args[i]);
		}

		size_t i = Conditional::elseArg(args);
		if (!args[i]->isNull())
			return get_type_to_return(args[i]);

		/// All the branches are null.
		return std::make_shared<DataTypeNull>();
	}
}

/// The tracker parameter is an index to a column that tracks the originating column of each value of
/// the result column. Calling this function with result == tracker means that no such tracking is
/// required, which happens if multiIf is called with no nullable parameters.
bool FunctionMultiIf::performTrivialCase(Block & block, const ColumnNumbers & args, size_t result, size_t tracker)
{
	/// Check that all the branches have the same type. Moreover
	/// some or all these branches may be null.
	std::string first_type_name;
	DataTypePtr type;
	Field sample;

	size_t else_arg = Conditional::elseArg(args);
	for (size_t i = Conditional::firstThen(); i < else_arg; i = Conditional::nextThen(i))
	{
		if (!block.getByPosition(args[i]).type->isNull())
		{
			const auto & name = block.getByPosition(args[i]).type->getName();
			if (first_type_name.empty())
			{
				first_type_name = name;
				type = block.getByPosition(args[i]).type;
				block.getByPosition(args[i]).column->get(0, sample);
			}
			else
			{
				if (name != first_type_name)
					return false;
			}
		}
	}

	if (!block.getByPosition(args[else_arg]).type->isNull())
	{
		if (first_type_name.empty())
		{
			type = block.getByPosition(args[else_arg]).type;
			block.getByPosition(args[else_arg]).column->get(0, sample);
		}
		else
		{
			const auto & name = block.getByPosition(args[else_arg]).type->getName();
			if (name != first_type_name)
				return false;
		}
	}

	size_t row_count = block.rowsInFirstColumn();
	auto & res_col = block.getByPosition(result).column;

	if (!type)
	{
		/// Degenerate case: all the branches are null.
		res_col = DataTypeNull{}.createConstColumn(row_count, Field{});
		return true;
	}

	/// Check that all the conditions are constants.
	for (size_t i = Conditional::firstCond(); i < else_arg; i = Conditional::nextCond(i))
	{
		const IColumn * col = block.getByPosition(args[i]).column.get();
		if (!col->isConst())
			return false;
	}

	/// Initialize readers for the conditions.
	Conditional::CondSources conds;
	conds.reserve(Conditional::getCondCount(args));

	for (size_t i = Conditional::firstCond(); i < else_arg; i = Conditional::nextCond(i))
		conds.emplace_back(block, args, i);

	/// Perform multiIf.

	auto make_result = [&](size_t index)
	{
		res_col = block.getByPosition(index).column;
		if (res_col->isNull())
			res_col = type->createConstColumn(row_count, sample);
		if (tracker != result)
		{
			ColumnPtr & col = block.getByPosition(tracker).column;
			col = std::make_shared<ColumnConstUInt16>(row_count, index);
		}
	};

	size_t i = Conditional::firstCond();
	for (const auto & cond : conds)
	{
		if (cond.get(0))
		{
			make_result(args[Conditional::thenFromCond(i)]);
			return true;
		}
		i = Conditional::nextCond(i);
	}

	make_result(args[else_arg]);
	return true;
}

/// Translate a context-free error into a contextual error.
void FunctionMultiIf::rethrowContextually(const Conditional::CondException & ex) const
{
	if (is_case_mode)
	{
		/// CASE construction context.
		if (ex.getCode() == Conditional::CondErrorCodes::TYPE_DEDUCER_ILLEGAL_COLUMN_TYPE)
			throw Exception{"Illegal type of column " + ex.getMsg1() +
				" in CASE construction", ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT};
		else if (ex.getCode() == Conditional::CondErrorCodes::TYPE_DEDUCER_UPSCALING_ERROR)
			throw Exception{"THEN/ELSE clause parameters in CASE construction are not upscalable to a "
				"common type without loss of precision: " + ex.getMsg1(),
				ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT};
		else if (ex.getCode() == Conditional::CondErrorCodes::NUMERIC_PERFORMER_ILLEGAL_COLUMN)
		{
			size_t i = std::stoul(ex.getMsg1());
			if ((i % 2) == 1)
				throw Exception{"Illegal THEN clause " + toString(1 + (i - 1) / 2)
					+ " in CASE construction", ErrorCodes::ILLEGAL_COLUMN};
			else
				throw Exception{"Illegal ELSE clause in CASE construction",
					ErrorCodes::ILLEGAL_COLUMN};
		}
		else if (ex.getCode() == Conditional::CondErrorCodes::COND_SOURCE_ILLEGAL_COLUMN)
		{
			size_t i = std::stoul(ex.getMsg2());
			if ((i % 2) == 1)
				throw Exception{"Illegal column " + ex.getMsg1() + " of THEN clause "
					+ toString(1 + (i - 1) / 2) + " in CASE construction."
					"Must be ColumnUInt8 or ColumnConstUInt8", ErrorCodes::ILLEGAL_COLUMN};
			else
				throw Exception{"Illegal column " + ex.getMsg1() + " of ELSE clause "
					" in CASE construction. Must be ColumnUInt8 or ColumnConstUInt8",
					ErrorCodes::ILLEGAL_COLUMN};
		}
		else if (ex.getCode() == Conditional::CondErrorCodes::NUMERIC_EVALUATOR_ILLEGAL_ARGUMENT)
		{
			size_t i = std::stoul(ex.getMsg1());
			if ((i % 2) == 1)
				throw Exception{"Illegal type of THEN clause " + toString(1 + (i - 1) / 2)
					+ " in CASE construction", ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT};
			else
				throw Exception{"Illegal type of ELSE clause in CASE construction",
					ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT};
		}
		else if (ex.getCode() == Conditional::CondErrorCodes::ARRAY_EVALUATOR_INVALID_TYPES)
			throw Exception{"Internal logic error: one or more THEN/ELSE clauses of "
				"CASE construction have invalid types", ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT};
		else
			throw Exception{"An unexpected error has occurred in CASE construction",
				ErrorCodes::LOGICAL_ERROR};
	}
	else
	{
		/// multiIf function context.
		if (ex.getCode() == Conditional::CondErrorCodes::TYPE_DEDUCER_ILLEGAL_COLUMN_TYPE)
			throw Exception{"Illegal type of column " + ex.getMsg1() +
				" of function multiIf", ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT};
		else if (ex.getCode() == Conditional::CondErrorCodes::TYPE_DEDUCER_UPSCALING_ERROR)
			throw Exception{"Arguments of function multiIf are not upscalable to a "
				"common type without loss of precision: " + ex.getMsg1(),
				ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT};
		else if (ex.getCode() == Conditional::CondErrorCodes::NUMERIC_PERFORMER_ILLEGAL_COLUMN)
			throw Exception{"Illegal argument " + ex.getMsg1() + " of function multiIf",
				ErrorCodes::ILLEGAL_COLUMN};
		else if (ex.getCode() == Conditional::CondErrorCodes::COND_SOURCE_ILLEGAL_COLUMN)
			throw Exception{"Illegal column " + ex.getMsg1() + " of argument "
				+ ex.getMsg2() + " of function multiIf"
				"Must be ColumnUInt8 or ColumnConstUInt8.", ErrorCodes::ILLEGAL_COLUMN};
		else if (ex.getCode() == Conditional::CondErrorCodes::NUMERIC_EVALUATOR_ILLEGAL_ARGUMENT)
			throw Exception{"Illegal type of argument " + ex.getMsg1() + " of function multiIf",
				ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT};
		else if (ex.getCode() == Conditional::CondErrorCodes::ARRAY_EVALUATOR_INVALID_TYPES)
			throw Exception{"Internal logic error: one or more arguments of function "
				"multiIf have invalid types", ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT};
		else
			throw Exception{"An unexpected error has occurred while performing multiIf",
				ErrorCodes::LOGICAL_ERROR};
	}
}

/// Implementation of FunctionCaseWithExpr.

FunctionPtr FunctionCaseWithExpr::create(const Context & context_)
{
	return std::make_shared<FunctionCaseWithExpr>(context_);
}

FunctionCaseWithExpr::FunctionCaseWithExpr(const Context & context_)
	: context{context_}
{
}

String FunctionCaseWithExpr::getName() const
{
	return name;
}

DataTypePtr FunctionCaseWithExpr::getReturnTypeImpl(const DataTypes & args) const
{
	/// See the comments in executeImpl() to understand why we actually have to
	/// get the return type of a transform function.

	/// Get the return types of the arrays that we pass to the transform function.
	DataTypes src_array_types;
	DataTypes dst_array_types;

	for (size_t i = 1; i < (args.size() - 1); ++i)
	{
		if ((i % 2) != 0)
			src_array_types.push_back(args[i]);
		else
			dst_array_types.push_back(args[i]);
	}

	FunctionArray fun_array{context};
	fun_array.setCaseMode();

	DataTypePtr src_array_type = fun_array.getReturnTypeImpl(src_array_types);
	DataTypePtr dst_array_type = fun_array.getReturnTypeImpl(dst_array_types);

	/// Finally get the return type of the transform function.
	FunctionTransform fun_transform;
	fun_transform.setCaseMode();
	return fun_transform.getReturnTypeImpl({args.front(), src_array_type, dst_array_type, args.back()});
}

void FunctionCaseWithExpr::executeImpl(Block & block, const ColumnNumbers & args, size_t result)
{
	/// In the following code, we turn the construction:
	/// CASE expr WHEN val[0] THEN branch[0] ... WHEN val[N-1] then branch[N-1] ELSE branchN
	/// into the construction transform(expr, src, dest, branchN)
	/// where:
	/// src  = [val[0], val[1], ..., val[N-1]]
	/// dest = [branch[0], ..., branch[N-1]]
	/// then we perform it.

	/// Create the arrays required by the transform function.
	ColumnNumbers src_array_args;
	DataTypes src_array_types;

	ColumnNumbers dst_array_args;
	DataTypes dst_array_types;

	for (size_t i = 1; i < (args.size() - 1); ++i)
	{
		if ((i % 2) != 0)
		{
			src_array_args.push_back(args[i]);
			src_array_types.push_back(block.getByPosition(args[i]).type);
		}
		else
		{
			dst_array_args.push_back(args[i]);
			dst_array_types.push_back(block.getByPosition(args[i]).type);
		}
	}

	FunctionArray fun_array{context};
	fun_array.setCaseMode();

	DataTypePtr src_array_type = fun_array.getReturnTypeImpl(src_array_types);
	DataTypePtr dst_array_type = fun_array.getReturnTypeImpl(dst_array_types);

	Block temp_block = block;

	size_t src_array_pos = temp_block.columns();
	temp_block.insert({nullptr, src_array_type, ""});

	size_t dst_array_pos = temp_block.columns();
	temp_block.insert({nullptr, dst_array_type, ""});

	fun_array.executeImpl(temp_block, src_array_args, src_array_pos);
	fun_array.executeImpl(temp_block, dst_array_args, dst_array_pos);

	/// Execute transform.
	FunctionTransform fun_transform;
	fun_transform.setCaseMode();

	ColumnNumbers transform_args{args.front(), src_array_pos, dst_array_pos, args.back()};
	fun_transform.executeImpl(temp_block, transform_args, result);

	/// Put the result into the original block.
	block.getByPosition(result).column = std::move(temp_block.getByPosition(result).column);
}

/// Implementation of FunctionCaseWithoutExpr.

FunctionPtr FunctionCaseWithoutExpr::create(const Context & context_)
{
	return std::make_shared<FunctionCaseWithoutExpr>();
}

String FunctionCaseWithoutExpr::getName() const
{
	return name;
}

bool FunctionCaseWithoutExpr::hasSpecialSupportForNulls() const
{
	return true;
}

DataTypePtr FunctionCaseWithoutExpr::getReturnTypeImpl(const DataTypes & args) const
{
	FunctionMultiIf fun_multi_if;
	fun_multi_if.setCaseMode();
	return fun_multi_if.getReturnTypeImpl(args);
}

void FunctionCaseWithoutExpr::executeImpl(Block & block, const ColumnNumbers & args, size_t result)
{
	/// A CASE construction without any expression is a mere multiIf.
	FunctionMultiIf fun_multi_if;
	fun_multi_if.setCaseMode();
	fun_multi_if.executeImpl(block, args, result);
}

}