cpp/datastore/columns/lowcardinalityadaptor.h

#pragma once

#include "column.h"
#include "lowcardinality.h"

#include <cassert>

namespace datastore {

class OutputStream;
class CodedInputStream;

/** Adapts any ColumnType to be serialized\deserialized as LowCardinality,
 *  and to be castable to ColumnType via ColumnPtr->As<ColumnType>().
 *
 * It helps to ease migration of the old codebases, which can't afford to switch
 * to using ColumnLowCardinalityT or ColumnLowCardinality directly,
 * but still want to benefit from smaller on-wire LowCardinality bandwidth footprint.
 *
 * Not intended to be used by users directly.
 *
 * @see ClientOptions, CreateColumnByType
 */
template <typename AdaptedColumnType>
class
[[deprecated("Makes implementation of LC(X) harder and code uglier. Will be removed in next major release (3.0) ")]]
LowCardinalitySerializationAdaptor : public AdaptedColumnType
{
public:
    using AdaptedColumnType::AdaptedColumnType;

    bool LoadPrefix(InputStream* input, size_t rows) override {
        auto new_data_column = this->Slice(0, 0)->template As<AdaptedColumnType>();
        ColumnLowCardinalityT<AdaptedColumnType> low_cardinality_col(new_data_column);

        return low_cardinality_col.LoadPrefix(input, rows);
    }

    /// Loads column data from input stream.
    bool LoadBody(InputStream* input, size_t rows) override {
        auto new_data_column = this->CloneEmpty()->template As<AdaptedColumnType>();

        ColumnLowCardinalityT<AdaptedColumnType> low_cardinality_col(new_data_column);
        if (!low_cardinality_col.LoadBody(input, rows))
            return false;

        // It safe to reuse `flat_data_column` later since ColumnLowCardinalityT makes a deep copy, but still check just in case.
        assert(new_data_column->Size() == 0);

        for (size_t i = 0; i < low_cardinality_col.Size(); ++i)
            new_data_column->Append(low_cardinality_col[i]);

        this->Swap(*new_data_column);
        return true;
    }

    /// Saves column data to output stream.
    void SaveBody(OutputStream* output) override {
        ColumnLowCardinalityT<AdaptedColumnType>(this->template As<AdaptedColumnType>()).SaveBody(output);
    }
};

}