// Licensed to the Apache Software Foundation (ASF) under one

// or more contributor license agreements.  See the NOTICE file

// distributed with this work for additional information

// regarding copyright ownership.  The ASF licenses this file

// to you under the Apache License, Version 2.0 (the

// "License"); you may not use this file except in compliance

// with the License.  You may obtain a copy of the License at

//

//   http://www.apache.org/licenses/LICENSE-2.0

//

// Unless required by applicable law or agreed to in writing,

// software distributed under the License is distributed on an

// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY

// KIND, either express or implied.  See the License for the

// specific language governing permissions and limitations

// under the License.

#pragma once

#include <cstddef>

#include <cstdint>

#include <memory>

#include "common/status.h"

#include "core/block/block.h"

#include "format/generic_reader.h"

namespace doris {

/// A lightweight reader that emits row counts without reading any actual data.

/// Used as a decorator to replace the real reader when COUNT(*) push down is active.

///

/// Instead of duplicating the COUNT short-circuit logic in every format reader

/// (ORC, Parquet, etc.), FileScanner creates a CountReader after the real reader

/// is initialized and the total row count is known. The CountReader then serves

/// all subsequent get_next_block calls by simply resizing columns.

///

/// This cleanly separates the "how many rows" concern from the actual data reading,

/// eliminating duplicated COUNT blocks across format readers.

class CountReader : public GenericReader {

public:

    /// @param total_rows   Total number of rows to emit (post-filter).

    /// @param batch_size   Maximum rows per batch.

    /// @param inner_reader The original reader, kept alive for profile collection

    ///                     and lifecycle management. Ownership is transferred.

    CountReader(int64_t total_rows, size_t batch_size,

                std::unique_ptr<GenericReader> inner_reader = nullptr)

            : _remaining_rows(total_rows),

              _batch_size(batch_size),

              _inner_reader(std::move(inner_reader)) {

        set_push_down_agg_type(TPushAggOp::type::COUNT);

    }

    ~CountReader() override = default;

    Status _do_get_next_block(Block* block, size_t* read_rows, bool* eof) override {

        auto rows = std::min(_remaining_rows, static_cast<int64_t>(_batch_size));

        _remaining_rows -= rows;

        auto mutate_columns = block->mutate_columns();

        for (auto& col : mutate_columns) {

            col->resize(rows);

        }

        block->set_columns(std::move(mutate_columns));

        *read_rows = rows;

        *eof = (_remaining_rows == 0);

        return Status::OK();

    }

    /// CountReader counts rows by definition.

    bool count_read_rows() override { return true; }

    /// Delegate to inner reader if available, otherwise return our total.

    int64_t get_total_rows() const override {

        return _inner_reader ? _inner_reader->get_total_rows() : _initial_total_rows();

    }

    Status close() override {

        if (_inner_reader) {

            return _inner_reader->close();

        }

        return Status::OK();

    }

    /// Access the inner reader for profile collection or other lifecycle needs.

    GenericReader* inner_reader() const { return _inner_reader.get(); }

protected:

    void _collect_profile_before_close() override {

        if (_inner_reader) {

            _inner_reader->collect_profile_before_close();

        }

    }

private:

    int64_t _initial_total_rows() const { return _remaining_rows; }

    int64_t _remaining_rows;

    size_t _batch_size;

    std::unique_ptr<GenericReader> _inner_reader;

};

} // namespace doris