overflow_controlled_integer_accumulator.hpp

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/*
 * RESTinio
 */

/*!
 * @file
 * @brief Helper for parsing integer values.
 *
 * @since v.0.6.2
 */

#pragma once

#include <restinio/compiler_features.hpp>

#include <type_traits>
#include <limits>

namespace restinio
{

namespace impl
{

//
// check_positive_extremum
//
struct check_positive_extremum {};

//
// check_negative_extremum
//
struct check_negative_extremum {};

namespace overflow_controlled_integer_accumulator_details
{

template< typename T, typename Storage_Type >
[[nodiscard]]
typename std::enable_if< std::is_signed<T>::value, bool >::type
is_greater_than_maximum( Storage_Type v, Storage_Type maximum )
{
    return v > maximum;
}

// If T is unsigned type then this comparison has no sense.
template< typename T, typename Storage_Type >
[[nodiscard]]
typename std::enable_if< !std::is_signed<T>::value, bool >::type
is_greater_than_maximum( Storage_Type, Storage_Type )
{
    return false;
}

//
// extremum_value
//
template< typename T, typename Ext >
struct extremum_value;

template< typename T >
struct extremum_value< T, check_positive_extremum >
{
    using storage_type = std::make_unsigned_t<T>;

    static constexpr storage_type value = static_cast<storage_type>(
            std::numeric_limits<T>::max() );
};

template< typename T >
struct extremum_value< T, check_negative_extremum >
{
    static_assert( std::is_signed<T>::value,
            "extremum_value<T, check_negative_extremum> is defined only "
            "for signed numeric types" );

    using storage_type = std::make_unsigned_t<T>;

    static constexpr storage_type value = static_cast<storage_type>(
            std::numeric_limits<T>::min() );

    static_assert(
            value == (static_cast<storage_type>(std::numeric_limits<T>::max()) + 1u),
            "The integer representation is expected to be two's complement" );
};

} /* namespace overflow_controlled_integer_accumulator_details */

//
// overflow_controlled_integer_accumulator_t
//
/*!
 * @brief Helper class for accumulating integer value during parsing
 * it from string (with check for overflow).
 *
 * Usage example:
 * @code
    int parse_int(string_view_t str) {
        overflow_controlled_integer_accumulator_t<int> acc;
        for(const auto ch : str) {
            if(!is_digit(ch))
                throw not_a_digit(ch);
            acc.next_digit(ch);
            if(acc.overflow_detected())
                throw too_big_value();
        }
        return acc.value();
    }
 * @endcode
 *
 * @note
 * Since v.0.6.6 it can be used for parcing not only decinal numbers,
 * but also numbers with base of @a Multiplier.
 *
 * @note
 * Since v.0.6.6 it can be used for parsing digits of negative decimal
 * numbers. In that case it should be used like that:
 * @code
 * char ch = next_char();
 * if('-' == ch) {
 *  // Digits of negative number should be extracted.
 *  overflow_controlled_integer_accumulator_t<
 *          int,
 *          check_negative_extremum > acc;
 *  ...
 * }
 * @endcode
 *
 * @since v.0.6.2
 */
template<
    typename T,
    int Multiplier,
    typename Extremum_Type = check_positive_extremum >
class overflow_controlled_integer_accumulator_t
{
    using extremum_value =
            overflow_controlled_integer_accumulator_details::extremum_value<
                    T,
                    Extremum_Type >;

    //! Type to be used for holding intermediate value.
    using storage_type = typename extremum_value::storage_type;

    //! The current value of the accumulator.
    storage_type m_current{};
    //! Overflow detection flag.
    bool m_overflow_detected{ false };

public :
    //! Try to add another digit to the accumulator.
    /*!
     * Value of the accumulator will be changed only if there is no overflow.
     */
    void
    next_digit( T digit ) noexcept
    {
        using namespace overflow_controlled_integer_accumulator_details;

        constexpr storage_type multiplier{
                static_cast<storage_type>(Multiplier)
        };

        const storage_type updated_value = m_current * multiplier +
                static_cast<storage_type>(digit);

        if( updated_value < m_current ||
                is_greater_than_maximum<T>( updated_value, extremum_value::value ) )
            m_overflow_detected = true;
        else
            m_current = updated_value;
    }

    //! Is overflow detected during previous call to next_digit?
    bool
    overflow_detected() const noexcept
    {
        return m_overflow_detected;
    }

    //! Get the current accumulator value.
    T
    value() const noexcept
    {
        return static_cast<T>(m_current);
    }
};

} /* namespace restinio */

} /* namespace restinio */