ClickHouse

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#include <DataTypes/DataTypeArray.h>
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#include <DataTypes/getLeastSupertype.h>
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#include <Functions/FunctionFactory.h>
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namespace DB
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{
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namespace ErrorCodes
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{
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    extern const int TOO_FEW_ARGUMENTS_FOR_FUNCTION;
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}
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namespace
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{
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/// Implements the CASE construction when it is
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/// provided an expression. Users should not call this function.
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class FunctionCaseWithExpression : public IFunction
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{
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public:
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    static constexpr auto name = "caseWithExpression";
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    static FunctionPtr create(ContextPtr context_) { return std::make_shared<FunctionCaseWithExpression>(context_); }
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    explicit FunctionCaseWithExpression(ContextPtr context_) : context(context_) {}
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    bool isVariadic() const override { return true; }
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    bool useDefaultImplementationForConstants() const override { return false; }
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    bool useDefaultImplementationForNulls() const override { return false; }
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    bool useDefaultImplementationForNothing() const override { return false; }
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    bool isSuitableForShortCircuitArgumentsExecution(const DataTypesWithConstInfo & /*arguments*/) const override { return false; }
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    size_t getNumberOfArguments() const override { return 0; }
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    String getName() const override { return name; }
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    DataTypePtr getReturnTypeImpl(const DataTypes & args) const override
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    {
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        if (args.empty())
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            throw Exception(ErrorCodes::TOO_FEW_ARGUMENTS_FOR_FUNCTION, "Function {} expects at least 1 arguments", getName());
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        /// See the comments in executeImpl() to understand why we actually have to
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        /// get the return type of a transform function.
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        /// Get the types of the arrays that we pass to the transform function.
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        DataTypes dst_array_types;
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        for (size_t i = 2; i < args.size() - 1; i += 2)
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            dst_array_types.push_back(args[i]);
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        // Type of the ELSE branch
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        dst_array_types.push_back(args.back());
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        return getLeastSupertype(dst_array_types);
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    }
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    ColumnPtr executeImpl(const ColumnsWithTypeAndName & args, const DataTypePtr & result_type, size_t input_rows_count) const override
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    {
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        if (args.empty())
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            throw Exception(ErrorCodes::TOO_FEW_ARGUMENTS_FOR_FUNCTION, "Function {} expects at least 1 argument", getName());
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        /// In the following code, we turn the construction:
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        /// CASE expr WHEN val[0] THEN branch[0] ... WHEN val[N-1] then branch[N-1] ELSE branchN
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        /// into the construction transform(expr, src, dest, branchN)
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        /// where:
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        /// src = [val[0], val[1], ..., val[N-1]]
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        /// dst = [branch[0], ..., branch[N-1]]
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        /// then we perform it.
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        /// Create the arrays required by the transform function.
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        ColumnsWithTypeAndName src_array_elems;
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        DataTypes src_array_types;
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        ColumnsWithTypeAndName dst_array_elems;
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        DataTypes dst_array_types;
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        for (size_t i = 1; i < (args.size() - 1); ++i)
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        {
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            if (i % 2)
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            {
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                src_array_elems.push_back(args[i]);
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                src_array_types.push_back(args[i].type);
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            }
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            else
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            {
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                dst_array_elems.push_back(args[i]);
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                dst_array_types.push_back(args[i].type);
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            }
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        }
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        DataTypePtr src_array_type = std::make_shared<DataTypeArray>(getLeastSupertype(src_array_types));
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        DataTypePtr dst_array_type = std::make_shared<DataTypeArray>(getLeastSupertype(dst_array_types));
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        ColumnWithTypeAndName src_array_col{nullptr, src_array_type, ""};
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        ColumnWithTypeAndName dst_array_col{nullptr, dst_array_type, ""};
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        auto fun_array = FunctionFactory::instance().get("array", context);
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        src_array_col.column = fun_array->build(src_array_elems)->execute(src_array_elems, src_array_type, input_rows_count);
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        dst_array_col.column = fun_array->build(dst_array_elems)->execute(dst_array_elems, dst_array_type, input_rows_count);
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        /// Execute transform.
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        ColumnsWithTypeAndName transform_args{args.front(), src_array_col, dst_array_col, args.back()};
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        return FunctionFactory::instance().get("transform", context)->build(transform_args)
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            ->execute(transform_args, result_type, input_rows_count);
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    }
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private:
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    ContextPtr context;
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};
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}
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REGISTER_FUNCTION(CaseWithExpression)
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{
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    factory.registerFunction<FunctionCaseWithExpression>();
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    /// These are obsolete function names.
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    factory.registerAlias("caseWithExpr", "caseWithExpression");
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}
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}
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