mongo/base/data_range.h

/**
 *    Copyright (C) 2018-present MongoDB, Inc.
 *
 *    This program is free software: you can redistribute it and/or modify
 *    it under the terms of the Server Side Public License, version 1,
 *    as published by MongoDB, Inc.
 *
 *    This program is distributed in the hope that it will be useful,
 *    but WITHOUT ANY WARRANTY; without even the implied warranty of
 *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *    Server Side Public License for more details.
 *
 *    You should have received a copy of the Server Side Public License
 *    along with this program. If not, see
 *    <http://www.mongodb.com/licensing/server-side-public-license>.
 *
 *    As a special exception, the copyright holders give permission to link the
 *    code of portions of this program with the OpenSSL library under certain
 *    conditions as described in each individual source file and distribute
 *    linked combinations including the program with the OpenSSL library. You
 *    must comply with the Server Side Public License in all respects for
 *    all of the code used other than as permitted herein. If you modify file(s)
 *    with this exception, you may extend this exception to your version of the
 *    file(s), but you are not obligated to do so. If you do not wish to do so,
 *    delete this exception statement from your version. If you delete this
 *    exception statement from all source files in the program, then also delete
 *    it in the license file.
 */

#pragma once

#include <cstring>
#include <tuple>
#include <type_traits>

#include "mongo/base/data_type.h"
#include "mongo/base/error_codes.h"
#include "mongo/base/status_with.h"
#include "mongo/platform/endian.h"
#include "mongo/util/mongoutils/str.h"

namespace mongo {

class ConstDataRange {
public:
    // begin and end should point to the first and one past last bytes in
    // the range you wish to view.
    //
    // debug_offset provides a way to indicate that the ConstDataRange is
    // located at an offset into some larger logical buffer. By setting it
    // to a non-zero value, you'll change the Status messages that are
    // returned on failure to be offset by the amount passed to this
    // constructor.
    ConstDataRange(const char* begin, const char* end, std::ptrdiff_t debug_offset = 0)
        : _begin(begin), _end(end), _debug_offset(debug_offset) {
        invariant(end >= begin);
    }

    ConstDataRange(const char* begin, std::size_t length, std::ptrdiff_t debug_offset = 0)
        : ConstDataRange(begin, begin + length, debug_offset) {}

    const char* data() const {
        return _begin;
    }

    size_t length() const {
        return _end - _begin;
    }

    bool empty() const {
        return length() == 0;
    }

    template <typename T>
    Status read(T* t, size_t offset = 0) const {
        if (offset > length()) {
            return makeOffsetStatus(offset);
        }

        return DataType::load(
            t, _begin + offset, length() - offset, nullptr, offset + _debug_offset);
    }

    template <typename T>
    StatusWith<T> read(std::size_t offset = 0) const {
        T t(DataType::defaultConstruct<T>());
        Status s = read(&t, offset);

        if (s.isOK()) {
            return StatusWith<T>(std::move(t));
        } else {
            return StatusWith<T>(std::move(s));
        }
    }

    friend bool operator==(const ConstDataRange& lhs, const ConstDataRange& rhs) {
        return std::tie(lhs._begin, lhs._end) == std::tie(rhs._begin, rhs._end);
    }

    friend bool operator!=(const ConstDataRange& lhs, const ConstDataRange& rhs) {
        return !(lhs == rhs);
    }


protected:
    const char* _begin;
    const char* _end;
    std::ptrdiff_t _debug_offset;

    Status makeOffsetStatus(size_t offset) const;
};

class DataRange : public ConstDataRange {
public:
    typedef char* bytes_type;

    DataRange(bytes_type begin, bytes_type end, std::ptrdiff_t debug_offset = 0)
        : ConstDataRange(begin, end, debug_offset) {}

    DataRange(bytes_type begin, std::size_t length, std::ptrdiff_t debug_offset = 0)
        : ConstDataRange(begin, length, debug_offset) {}

    template <typename T>
    Status write(const T& value, std::size_t offset = 0) {
        if (offset > length()) {
            return makeOffsetStatus(offset);
        }

        return DataType::store(value,
                               const_cast<char*>(_begin + offset),
                               length() - offset,
                               nullptr,
                               offset + _debug_offset);
    }
};

struct DataRangeTypeHelper {
    static Status makeStoreStatus(size_t t_length, size_t length, std::ptrdiff_t debug_offset);
};

// Enable for classes derived from ConstDataRange
template <typename T>
struct DataType::Handler<T,
                         typename std::enable_if<std::is_base_of<ConstDataRange, T>::value>::type> {
    static Status load(
        T* t, const char* ptr, size_t length, size_t* advanced, std::ptrdiff_t debug_offset) {
        if (t) {
            // Assuming you know what you're doing at the read above this
            // is fine.  Either you're reading into a readable buffer, so
            // ptr started off non-const, or the const_cast will feed back
            // to const char* taking Const variants.  So it'll get tossed
            // out again.
            *t = T(const_cast<char*>(ptr), const_cast<char*>(ptr) + length);
        }

        if (advanced) {
            *advanced = length;
        }

        return Status::OK();
    }

    static Status store(
        const T& t, char* ptr, size_t length, size_t* advanced, std::ptrdiff_t debug_offset) {
        if (t.length() > length) {
            return DataRangeTypeHelper::makeStoreStatus(t.length(), length, debug_offset);
        }

        if (ptr) {
            std::memcpy(ptr, t.data(), t.length());
        }

        if (advanced) {
            *advanced = t.length();
        }

        return Status::OK();
    }

    static T defaultConstruct() {
        return T(nullptr, nullptr);
    }
};

}  // namespace mongo