zlib.hpp

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/*
    restinio
*/

/*!
    Transformator of data streams using zlib.

    @since v.0.4.4
*/

#pragma once

#include <string>
#include <cstring>

#include <fmt/format.h>

#include <zlib.h>

#include <restinio/exception.hpp>
#include <restinio/string_view.hpp>
#include <restinio/message_builders.hpp>
#include <restinio/request_handler.hpp>

namespace restinio
{

namespace transforms
{

namespace zlib
{

//! Default reserve buffer size for zlib transformator.
//! @since v.0.4.4
constexpr std::size_t default_output_reserve_buffer_size = 256 * 1024;

/** @name Default values for zlib tuning parameters.
 * @brief Constants are defined with values provided by zlib.
 *
 * @since v.0.4.4
*/
///@{
constexpr int default_window_bits = MAX_WBITS;
constexpr int default_mem_level = MAX_MEM_LEVEL;
constexpr int default_strategy = Z_DEFAULT_STRATEGY;
///@}

//
// params_t
//

//! Parameters of performing data transformation with zlib.
/*
    @since v.0.4.4

    \note There is a special case for compression format: format_t::identity.
    If this format is set that zlib transformator is transparently copies
    input to output and so all other params are ignored.
*/
class params_t
{
    public:
        //! Types of transformation.
        enum class operation_t
        {
            //! Compress the input data.
            compress,
            //! Decompress input data.
            decompress
        };

        //! Formats of compressed data.
        enum class format_t
        {
            //! zlib format.
            deflate,
            //! gzip format
            gzip,
            //! Identity. With semantics descrobed here: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Accept-Encoding
            /*
                Means that no compression will be used and no header/trailer will be applied.
            */
            identity
        };

        //! Init constructor.
        /*!
            It's better to use special functions to cunstruct
            initial params_t, see:
            deflate_compress(), deflate_decompress(),
            gzip_compress(), gzip_decompress(),
        */
        params_t(
            //! Operation: compress or decompress.
            operation_t op,
            //! Foramt: deflate or gzip.
            format_t f,
            //! Compression level.
            int l = -1 )
            :   m_operation{ op }
            ,   m_format{ f }
        {
            level( l );
        }

        //! Default constructor for identiry transformator
        params_t()
            :   m_operation{ operation_t::compress }
            ,   m_format{ format_t::identity }
        {
            level( -1 );
        }

        //! Get operation.
        operation_t operation() const { return m_operation; }

        //! Get format.
        format_t format() const { return m_format; }

        //! Get compression level.
        /*!
            \note Makes sense only for compression operation.
        */
        int level() const { return m_level; }

        //! Set compression level.
        /*!
            Must be an integer value in the range of -1 to 9.

            \note Makes sense only for compression operation.
        */
        params_t &
        level( int level_value ) &
        {
            if( level_value < -1 || level_value > 9 )
            {
                throw exception_t{
                    fmt::format(
                        "invalid compression level: {}, must be "
                        "an integer value in the range of -1 to 9",
                        level_value ) };
            }

            m_level = level_value;

            return reference_to_self();
        }

        //! Set compression level.
        params_t &&
        level( int level_value ) &&
        {
            return std::move( this->level( level_value ) );
        }

        //! Get window_bits.
        int window_bits() const { return m_window_bits; }

        //! Set window_bits.
        /*!
            Must be an integer value in the range of 8 to MAX_WBITS.

            \note For descompress operation it is better to use default
            value of windows beats. See https://zlib.net/manual.html
            inflateInit2() description for details.
        */
        params_t &
        window_bits( int window_bits_value ) &
        {
            // From https://zlib.net/manual.html:
            // For the current implementation of deflate(),
            // a windowBits value of 8 (a window size of 256 bytes)
            // is not supported. As a result, a request for 8 will result in 9
            // (a 512-byte window). In that case, providing 8 to inflateInit2()
            // will result in an error when the zlib header with 9 is
            // checked against the initialization of inflate().
            // The remedy is to not use 8 with deflateInit2()
            // with this initialization, or at least in that case use 9
            // with inflateInit2().
            // ...
            // windowBits can also be zero to request that inflate
            // use the window size in the zlib header of the compressed
            // stream.

            if( ( window_bits_value < 8 || window_bits_value > MAX_WBITS ) &&
                ( 0 != window_bits_value || operation_t::decompress != operation() ) )
            {
                throw exception_t{
                    fmt::format(
                        "invalid window_bits: {}, must be "
                        "an integer value in the range of 8 to {} or "
                        "0 for decompress operation",
                        window_bits_value,
                        MAX_WBITS ) };
            }

            if( 8 == window_bits_value )
                window_bits_value = 9;

            m_window_bits = window_bits_value;

            return reference_to_self();
        }

        //! Set window_bits.
        params_t &&
        window_bits( int window_bits_value ) &&
        {
            return std::move( this->window_bits( window_bits_value ) );
        }

        //! Get compression mem_level.
        /*!
            \note Makes sense only for compression operation.
        */
        int mem_level() const { return m_mem_level; }

        //! Set compression mem_level.
        /*!
            Must be an integer value in the range of 1 to 9.
            1 stands for minimum memory usage and
            9 stands for maximum memory usage.
            The amount of memory that can be used
            affects the quality of compression.

            \note Makes sense only for compression operation.
        */
        params_t &
        mem_level( int mem_level_value ) &
        {
            if( mem_level_value < 1 || mem_level_value > MAX_MEM_LEVEL )
            {
                throw exception_t{
                    fmt::format(
                        "invalid compression mem_level: {}, must be "
                        "an integer value in the range of 1 to {}",
                        mem_level_value,
                        MAX_MEM_LEVEL ) };
            }

            m_mem_level = mem_level_value;

            return reference_to_self();
        }

        //! Set compression mem_level.
        params_t &&
        mem_level( int mem_level_value ) &&
        {
            return std::move( this->mem_level( mem_level_value ) );
        }

        //! Get compression strategy.
        /*!
            \note Makes sense only for compression operation.
        */
        int strategy() const { return m_strategy; }

        //! Set compression strategy.
        /*!
            Must be an integer value defined by one of
            zlib constants: Z_FILTERED, Z_HUFFMAN_ONLY,
            Z_RLE, Z_DEFAULT_STRATEGY.

            \note Makes sense only for compression operation.
        */
        params_t &
        strategy( int strategy_value ) &
        {
            if( Z_DEFAULT_STRATEGY != strategy_value &&
                Z_FILTERED != strategy_value &&
                Z_HUFFMAN_ONLY != strategy_value &&
                Z_RLE != strategy_value )
            {
                throw exception_t{
                    fmt::format(
                        "invalid compression strategy: {}, must be "
                        "one of: "
                        "Z_DEFAULT_STRATEGY({}), ",
                        "Z_FILTERED({}), ",
                        "Z_HUFFMAN_ONLY({}), ",
                        "Z_RLE({})",
                        strategy_value,
                        Z_DEFAULT_STRATEGY,
                        Z_FILTERED,
                        Z_HUFFMAN_ONLY,
                        Z_RLE ) };
            }

            m_strategy = strategy_value;

            return reference_to_self();
        }

        //! Set compression strategy.
        params_t &&
        strategy( int strategy_value ) &&
        {
            return std::move( this->strategy( strategy_value ) );
        }

        //! Get the size initially reserved for buffer.
        /*!
            When using zlib transformator the outout buffer is used.
            The initial size of such buffer must be defined.
            zlib_t instance will use this parameter
            as the initial size of out buffer and as an increment size
            if out buffer must be enlarged.
        */
        std::size_t reserve_buffer_size() const { return m_reserve_buffer_size; }

        //! Set the size initially reserved for buffer.
        params_t &
        reserve_buffer_size( std::size_t size ) &
        {
            if( size < 10UL )
            {
                throw exception_t{ "too small reserve buffer size" };
            }

            m_reserve_buffer_size = size;

            return reference_to_self();
        }

        //! Set the size initially reserved for buffer.
        params_t &&
        reserve_buffer_size( std::size_t size ) &&
        {
            return std::move( this->reserve_buffer_size( size ) );
        }

    private:
        //! Get the reference to self.
        params_t & reference_to_self() { return *this; }

        //! Transformation type.
        operation_t m_operation;

        //! Format to be used for compressed data.
        format_t m_format;

        //! Compression level.
        /*!
            An integer value in the range of -1 to 9.
        */
        int m_level;

        int m_window_bits{ default_window_bits };
        int m_mem_level{ default_mem_level };
        int m_strategy{ default_strategy };

        //! Size initially reserved for buffer.
        std::size_t m_reserve_buffer_size{ default_output_reserve_buffer_size };
};

/** @name Create parameters for zlib transformators.
 * @brief A set of function helping to create params_t objects
 * ommiting some verbose deteils.
 *
 * Instead of writing something like this:
 * \code
 * params_t params{
 *     restinio::transforms::zlib::params_t::operation_t::compress,
 *     restinio::transforms::zlib::params_t::format_t::gzip,
 *     -1 };
 * \endcode
 * It is better to write the following:
 * \code
 * params_t params = restinio::transforms::zlib::make_gzip_compress_params();
 * \endcode
 *
 * @since v.0.4.4
*/
///@{
inline params_t
make_deflate_compress_params( int compression_level = -1 )
{
    return params_t{
            params_t::operation_t::compress,
            params_t::format_t::deflate,
            compression_level };
}

inline params_t
make_deflate_decompress_params()
{
    return params_t{
            params_t::operation_t::decompress,
            params_t::format_t::deflate };
}

inline params_t
make_gzip_compress_params( int compression_level = -1 )
{
    return params_t{
            params_t::operation_t::compress,
            params_t::format_t::gzip,
            compression_level };
}

inline params_t
make_gzip_decompress_params()
{
    return params_t{
            params_t::operation_t::decompress,
            params_t::format_t::gzip };
}

inline params_t
make_identity_params()
{
    return params_t{};
}
///@}

//
// zlib_t
//

//! Zlib transformator.
/*!
    Uses zlib (https://zlib.net) for gzip/deflate compresion/decompression.
    All higher level functionality that minimizes boilerplate code and makes
    compression and decompression logic less verbose is based on using zlib_t.

    Simple usage:
    \code
        namespace rtz = restinio::transforms::zlib;
        rtz::zlib_t z{ rtz::make_gzip_compress_params( compression_level ).level( 9 ) };
        z.write( input_data );
        z.complete();

        auto gziped_data = z.giveaway_output();
    \endcode

    Advanced usage:
    \code
    namespace rtz = restinio::transforms::zlib;
    rtz::zlib_t z{ rtz::make_gzip_compress_params( compression_level ).level( 9 ) };

    std::size_t processed_data = 0;
    for( const auto d : data_pieces )
    {
        z.write( d );

        // Track how much data is already processed:
        processed_data += d.size();

        if( processed_data > 1024 * 1024 )
        {
            // If more than 1Mb  is processed do flush.
            z.flush();
        }

        if( z.output_size() > 100 * 1024 )
        {
            // If more than 100Kb of data is ready, then append it to something.
            append_output( z.giveaway_output() );
        }
    }

    // Complete the stream and append remeining putput data.
    z.complete();
    append_output( z.giveaway_output() );
    \endcode


    @since v.0.4.4
*/
class zlib_t
{
    public:
        zlib_t( const params_t & transformation_params )
            :   m_params{ transformation_params }
        {
            if( !is_identity() )
            {
                // Setting allocator stuff before initializing
                // TODO: allocation can be done with user defined allocator.
                m_zlib_stream.zalloc = Z_NULL;
                m_zlib_stream.zfree = Z_NULL;
                m_zlib_stream.opaque = Z_NULL;

                // Track initialization result.
                int init_result;

                // Compression.
                auto current_window_bits = m_params.window_bits();

                if( params_t::format_t::gzip == m_params.format() )
                {
                    current_window_bits += 16;
                }

                if( params_t::operation_t::compress == m_params.operation() )
                {
                    // zlib format.
                    init_result =
                        deflateInit2(
                            &m_zlib_stream,
                            m_params.level(),
                            Z_DEFLATED,
                            current_window_bits,
                            m_params.mem_level(),
                            m_params.strategy() );
                }
                else
                {
                    init_result =
                        inflateInit2(
                            &m_zlib_stream,
                            current_window_bits );
                }

                if( Z_OK != init_result )
                {
                    throw exception_t{
                        fmt::format(
                            "Failed to initialize zlib stream: {}, {}",
                            init_result,
                            get_error_msg() ) };
                }

                m_zlib_stream_initialized = true;

                // Reserve initial buffer.
                inc_buffer();
            }
            // else => Nothing to initialize and to reserve.
        }

        zlib_t( const zlib_t & ) = delete;
        zlib_t( zlib_t && ) = delete;
        zlib_t & operator = ( const zlib_t & ) = delete;
        zlib_t & operator = ( zlib_t && ) = delete;

        ~zlib_t()
        {
            if( m_zlib_stream_initialized )
            {
                if( params_t::operation_t::compress == m_params.operation() )
                {
                    deflateEnd( &m_zlib_stream );
                }
                else
                {
                    inflateEnd( &m_zlib_stream );
                }
            }
        }

        //! Get parameters of current transformation.
        const params_t & params() const { return m_params; }

        //! Append input data.
        /*!
            Pushes a given data to zlib transform.
            \a input is the data to be compressed or decompressed.
        */
        void
        write( string_view_t input )
        {
            ensure_operation_in_not_completed();

            if( is_identity() )
            {
                m_out_buffer.append( input.data(), input.size() );
                m_write_pos = m_out_buffer.size();
            }
            else
            {
                if( std::numeric_limits< decltype( m_zlib_stream.avail_in ) >::max() < input.size() )
                {
                    throw exception_t{
                        fmt::format(
                            "input data is too large: {} (max possible: {}), "
                            "try to break large data into pieces",
                            input.size(),
                            std::numeric_limits< decltype( m_zlib_stream.avail_in ) >::max() ) };
                }

                if( 0 < input.size() )
                {
                    m_zlib_stream.next_in =
                        reinterpret_cast< Bytef* >( const_cast< char* >( input.data() ) );

                    m_zlib_stream.avail_in = static_cast< uInt >( input.size() );

                    if( params_t::operation_t::compress == m_params.operation() )
                    {
                        write_compress_impl( Z_NO_FLUSH );
                    }
                    else
                    {
                        write_decompress_impl( Z_NO_FLUSH );
                    }
                }
            }
        }

        //! Flush the zlib stream.
        /*!
            Flushes underlying zlib stream.
            All pending output is flushed to the output buffer.
        */
        void
        flush()
        {
            ensure_operation_in_not_completed();

            if( !is_identity() )
            {
                m_zlib_stream.next_in = nullptr;
                m_zlib_stream.avail_in = static_cast< uInt >( 0 );

                if( params_t::operation_t::compress == m_params.operation() )
                {
                    write_compress_impl( Z_SYNC_FLUSH );
                }
                else
                {
                    write_decompress_impl( Z_SYNC_FLUSH );
                }
            }
        }

        //! Complete the stream.
        void
        complete()
        {
            ensure_operation_in_not_completed();

            if( !is_identity() )
            {
                m_zlib_stream.next_in = nullptr;
                m_zlib_stream.avail_in = static_cast< uInt >( 0 );

                if( params_t::operation_t::compress == m_params.operation() )
                {
                    write_compress_impl( Z_FINISH );
                }
                else
                {
                    write_decompress_impl( Z_FINISH );
                }
            }

            m_operation_is_complete = true;
        }

        //! Get current accumulated output data
        /*!
            On this request a current accumulated output data is reterned.
            Move semantics is applied. Once current output is fetched
            zlib_t object resets its internal out buffer.

            In the following code:
            \code
            restinio::transformator::zlib_t z{ restinio::transformator::gzip_compress() };

            z.write( A );
            consume_out( z.giveaway_output() ); // (1)

            z.write( B );
            z.write( C );
            consume_out( z.giveaway_output() ); // (2)

            \endcode
            At the point (2) `consume_out()` function receives
            a string that is not an appended version of a string
            received in point (1).
        */
        std::string
        giveaway_output()
        {
            std::string result;
            const auto data_size = m_write_pos;
            std::swap( result, m_out_buffer );
            m_write_pos = 0;
            result.resize( data_size ); // Shrink output data.
            return result;
        }

        //! Get current output size.
        auto output_size() const { return m_write_pos; }

        //! Is operation complete?
        bool is_completed() const { return m_operation_is_complete; }

    private:
        bool is_identity() const
        {
            return params_t::format_t::identity == m_params.format();
        }

        //! Get zlib error message if it exists.
        const char *
        get_error_msg() const
        {
            const char * err_msg = "<no zlib error description>";
            if( m_zlib_stream.msg )
                err_msg = m_zlib_stream.msg;

            return err_msg;
        }

        //! Checks completion flag and throws if operation is is already completed.
        void
        ensure_operation_in_not_completed() const
        {
            if( is_completed() )
                throw exception_t{ "zlib operation is already completed" };
        }

        //! Increment internal buffer for receiving output.
        void
        inc_buffer()
        {
            m_out_buffer.resize(
                m_out_buffer.size() + m_params.reserve_buffer_size() );
        }

        //! Prepare out buffer for receiving data.
        auto
        prepare_out_buffer()
        {
            m_zlib_stream.next_out =
                reinterpret_cast< Bytef* >(
                    const_cast< char* >( m_out_buffer.data() + m_write_pos ) );

            const auto provided_out_buffer_size =
                m_out_buffer.size() - m_write_pos;
            m_zlib_stream.avail_out =
                static_cast<uInt>( provided_out_buffer_size );

            return provided_out_buffer_size;
        }

        //! Handle incoming data for compression operation.
        /*
            Data and its size must be already in
            `m_zlib_stream.next_in`, `m_zlib_stream.avail_in`.
        */
        void
        write_compress_impl( int flush )
        {
            while( true )
            {
                const auto provided_out_buffer_size = prepare_out_buffer();

                int operation_result = deflate( &m_zlib_stream, flush );

                if( !( Z_OK == operation_result ||
                        Z_BUF_ERROR == operation_result ||
                        ( Z_STREAM_END == operation_result && Z_FINISH == flush ) ) )
                {
                    const char * err_msg = "<no error desc>";
                    if( m_zlib_stream.msg )
                        err_msg = m_zlib_stream.msg;

                    throw exception_t{
                        fmt::format(
                            "unexpected result of deflate() (zlib): {}, {}",
                            operation_result,
                            err_msg ) };
                }

                m_write_pos += provided_out_buffer_size - m_zlib_stream.avail_out;

                if( 0 == m_zlib_stream.avail_out && Z_STREAM_END != operation_result )
                {
                    // Looks like not all the output was obtained.
                    // There is a minor chance that it just happened to
                    // occupy exactly the same space that was available,
                    // in that case it would go for a one redundant call to deflate.
                    inc_buffer();
                    continue;
                }

                if( 0 == m_zlib_stream.avail_in )
                {
                    // All the input was consumed.
                    break;
                }
            }
        }

        //! Handle incoming data for decompression operation.
        /*
            Data and its size must be already in
            `m_zlib_stream.next_in`, `m_zlib_stream.avail_in`.
        */
        void
        write_decompress_impl( int flush )
        {
            while( true )
            {
                const auto provided_out_buffer_size = prepare_out_buffer();

                int operation_result = inflate( &m_zlib_stream, flush );
                if( !( Z_OK == operation_result ||
                        Z_BUF_ERROR == operation_result ||
                        Z_STREAM_END == operation_result ) )
                {
                    throw exception_t{
                        fmt::format(
                            "unexpected result of inflate() (zlib): {}, {}",
                            operation_result,
                            get_error_msg() ) };
                }

                m_write_pos += provided_out_buffer_size - m_zlib_stream.avail_out;

                if( 0 == m_zlib_stream.avail_out && Z_STREAM_END != operation_result )
                {
                    // Looks like not all the output was obtained.
                    // There is a minor chance that it just happened to
                    // occupy exactly the same space that was available,
                    // in that case it would go for a one redundant call to deflate.
                    inc_buffer();
                    continue;
                }

                if( 0 == m_zlib_stream.avail_in )
                {
                    // All the input was consumed.
                    break;
                }
            }
        }

        //! Parameters for zlib.
        const params_t m_params;

        //! Flag: was m_zlib_stream initialized properly.
        /*!
            If deflateInit2()/inflateInit2() was completed successfully
            it is needed to call deflateEnd()/inflateEnd().
        */
        bool m_zlib_stream_initialized{ false };

        //! zlib stream.
        z_stream m_zlib_stream;

        //! Output buffer.
        std::string m_out_buffer;
        //! Next write pos in out buffer.
        std::size_t m_write_pos{ 0 };

        bool m_operation_is_complete{ false };
};

/** @name Helper functions for doing zlib transformation with less boilerplate.
 * @brief A set of handy functions helping to perform zlib transform in one line.
 *
 * Instead of writing something like this:
 * \code
 * restinio::transforms::zlib::zlib_t z{ restinio::transforms::zlib::gzip_compress() };
 * z.write( data );
 * z.complete();
 * body = z.giveaway_output();
 * \endcode
 * It is possible to write the following:
 * \code
 * body = restinio::transformators::zlib::gzip_compress( data );
 * \endcode
 *
 * @since v.0.4.4
*/
///@{

//! Do a specified zlib transformation.
inline std::string
transform( string_view_t input, const params_t & params )
{
    zlib_t z{ params };
    z.write( input );
    z.complete();

    return z.giveaway_output();
}

inline std::string
deflate_compress( string_view_t input, int compression_level = -1 )
{
    return transform( input, make_deflate_compress_params( compression_level ) );
}

inline std::string
deflate_decompress( string_view_t input )
{
    return transform( input, make_deflate_decompress_params() );
}

inline std::string
gzip_compress( string_view_t input, int compression_level = -1 )
{
    return transform( input, make_gzip_compress_params( compression_level ) );
}

inline std::string
gzip_decompress( string_view_t input )
{
    return transform( input, make_gzip_decompress_params() );
}
///@}

//
// body_appender_t
//

template < typename Response_Output_Strategy >
class body_appender_t
{
    body_appender_t() = delete;
};

namespace impl
{

//! Check if operation is a copression, and throw if it is not.
inline void ensure_is_compression_operation( params_t::operation_t op )
{
    if( params_t::operation_t::compress != op )
    {
        throw exception_t{ "operation is not copress" };
    }
}

//! Check if a pointer to zlib transformator is valid.
inline void ensure_valid_transforator( zlib_t * ztransformator )
{
    if( nullptr == ztransformator )
    {
        throw exception_t{ "invalid body appender" };
    }
}

//! Get token for copression format.
inline std::string content_encoding_token( params_t::format_t f )
{
    std::string result{ "identity" };

    if( params_t::format_t::deflate == f )
    {
        result.assign( "deflate" );
    }
    if( params_t::format_t::gzip == f )
    {
        result.assign( "gzip" );
    }

    return result;
}

} /* namespace impl */

//
// body_appender_base_t
//

//! Base class for body appenders.
template < typename Response_Output_Strategy, typename Descendant >
class body_appender_base_t
{
    public:
        using resp_t = response_builder_t< Response_Output_Strategy >;

        body_appender_base_t( const params_t & params, resp_t & resp )
            :   m_ztransformator{ std::make_unique< zlib_t >( params ) }
            ,   m_resp{ resp }
        {
            impl::ensure_is_compression_operation(
                m_ztransformator->params().operation() );

            m_resp.append_header(
                restinio::http_field::content_encoding,
                impl::content_encoding_token(
                    m_ztransformator->params().format() ) );
        }

        body_appender_base_t( const body_appender_base_t & ) = delete;
        body_appender_base_t & operator = ( const body_appender_base_t & ) = delete;
        body_appender_base_t & operator = ( body_appender_base_t && ) = delete;

        body_appender_base_t( body_appender_base_t && ba )
            :   m_ztransformator{ std::move( ba.m_ztransformator ) }
            ,   m_resp{ ba.m_resp }
        {}

        virtual ~body_appender_base_t() {}

    protected:
        std::unique_ptr< zlib_t > m_ztransformator;
        resp_t & m_resp;
};

//! Base class for body appenders with restinio or user controlled output.
template < typename X_Controlled_Output, typename Descendant >
class x_controlled_output_body_appender_base_t
    :   public body_appender_base_t< X_Controlled_Output, Descendant >
{
    public:
        using base_type_t = body_appender_base_t< X_Controlled_Output, Descendant >;

        using base_type_t::base_type_t;

        //! Append a piece of data to response.
        Descendant &
        append( string_view_t input )
        {
            impl::ensure_valid_transforator( this->m_ztransformator.get() );
            this->m_ztransformator->write( input );
            return static_cast< Descendant & >( *this );
        }

        //! Complete zlib transformation operation.
        void
        complete()
        {
            impl::ensure_valid_transforator( this->m_ztransformator.get() );

            this->m_ztransformator->complete();

            this->m_resp.append_body( this->m_ztransformator->giveaway_output() );
        }
};

/** @name Body appender.
 * @brief Helper class for setting the body of response_builder_t<restinio_controlled_output_t>.
 *
 * Sample usage:
 * \code
 * namespace rtz = restinio::transforms::zlib;
 * auto resp = req->create_response();
 *
 * resp.append_header( restinio::http_field::server, "RESTinio" )
 *   .append_header_date_field()
 *   .append_header( restinio::http_field::content_type, "text/plain; charset=utf-8" );
 *
 * auto ba = rtz::gzip_body_appender( resp );
 * ba.append( some_data );
 * // ...
 * ba.append( some_more_data );
 * ba.complete();
 * resp.done();
 * \endcode
 *
 * @since v.0.4.4
*/
template <>
class body_appender_t< restinio_controlled_output_t >
    :   public x_controlled_output_body_appender_base_t<
                    restinio_controlled_output_t,
                    body_appender_t< restinio_controlled_output_t > >
{
    public:
        using base_type_t =
            x_controlled_output_body_appender_base_t<
                restinio_controlled_output_t,
                body_appender_t< restinio_controlled_output_t > >;

        //! Get the size of transformed body.
        auto
        size() const
        {
            impl::ensure_valid_transforator( m_ztransformator.get() );

            return m_ztransformator->output_size();
        }

        using base_type_t::base_type_t;
};

/** @name Body appender.
 * @brief Helper class for setting the body of response_builder_t<user_controlled_output_t>.
 *
 * Sample usage:
 * \code
 * namespace rtz = restinio::transforms::zlib;
 * auto resp = req->create_response<user_controlled_output_t>();
 *
 * resp.append_header( restinio::http_field::server, "RESTinio" )
 *   .append_header_date_field()
 *   .append_header( restinio::http_field::content_type, "text/plain; charset=utf-8" );
 *
 * auto ba = rtz::gzip_body_appender( resp );
 * ba.append( some_data );
 * ba.flush();
 * // ...
 * ba.append( some_more_data );
 * ba.complete();
 * resp.done();
 * \endcode
 *
 * @since v.0.4.4
*/
template <>
class body_appender_t< user_controlled_output_t > final
    :   public x_controlled_output_body_appender_base_t<
                    user_controlled_output_t,
                    body_appender_t< user_controlled_output_t > >
{
    public:
        using base_type_t =
            x_controlled_output_body_appender_base_t<
                user_controlled_output_t,
                body_appender_t< user_controlled_output_t > >;

        using base_type_t::base_type_t;

        auto &
        flush()
        {
            impl::ensure_valid_transforator( m_ztransformator.get() );
            m_ztransformator->flush();
            m_resp
                .append_body( m_ztransformator->giveaway_output() )
                .flush();

            return *this;
        }
};


/** @name Body appender.
 * @brief Helper class for setting the body of response_builder_t<chunked_output_t>.
 *
 * Sample usage:
 * \code
 * namespace rtz = restinio::transforms::zlib;
 * auto resp = req->create_response<chunked_output_t>();
 *
 * resp.append_header( restinio::http_field::server, "RESTinio" )
 *   .append_header_date_field()
 *   .append_header( restinio::http_field::content_type, "text/plain; charset=utf-8" );
 *
 * auto ba = rtz::gzip_body_appender( resp );
 * ba.append( some_data );
 * ba.append( some_more_data );
 * ba.make_chunk(); // Flush copressed data and creates a chunk with it.
 * ba.flush(); // Send currently prepared chunks to client
 * // ...
 * // Copress the data and creates a chunk with it.
 * ba.make_chunk( even_more_data );
 * ba.flush(); // Send currently prepared chunks to client
 * // ...
 * ba.append( yet_even_more_data );
 * ba.append( last_data );
 * ba.complete(); // Creates last chunk, but doesn't send it to client.
 * ba.flush(); // Send chunk created by complete() call
 * // ...
 * resp.done();
 * \endcode
 *
 * @since v.0.4.4
*/
template <>
class body_appender_t< chunked_output_t > final
    :   public body_appender_base_t<
                    chunked_output_t,
                    body_appender_t< chunked_output_t > >
{
    public:
        using base_type_t =
            body_appender_base_t<
                chunked_output_t,
                body_appender_t< chunked_output_t > >;

        using base_type_t::base_type_t;

        //! Append data to be compressed.
        /*!
            Function only adds data to anderlying zlib stream
            and it doesn't affect target response right on here.
        */
        auto &
        append( string_view_t input )
        {
            impl::ensure_valid_transforator( m_ztransformator.get() );

            m_ztransformator->write( input );
            return *this;
        }

        //! Append data to be compressed and adds current zlib transformator output
        //! as a new chunk.
        /*!
            Adds data and flushes zlib transformator.
            Then ready compressed data is taken and used as a new chunk
            of target response.
        */
        auto &
        make_chunk( string_view_t input = string_view_t{} )
        {
            append( input ); // m_ztransformator is checked here.

            m_ztransformator->flush(); // Flush already compressed data.

            // Create a chunk with current output.
            m_resp.append_chunk( m_ztransformator->giveaway_output() );

            return *this;
        }

        //! Flushes currently available compressed data with possibly creating new chunk
        //! and then flushes target response.
        void
        flush()
        {
            impl::ensure_valid_transforator( m_ztransformator.get() );

            if( !m_ztransformator->is_completed() )
            {
                m_ztransformator->flush();
                m_resp.append_chunk( m_ztransformator->giveaway_output() );
            }

            m_resp.flush();
        }

        //! Complete zlib transformation operation.
        void
        complete()
        {
            impl::ensure_valid_transforator( m_ztransformator.get() );
            m_ztransformator->complete();
            m_resp.append_chunk( m_ztransformator->giveaway_output() );
        }
};

//! Create body appender with given zlib transformation parameters.
//! @since v.0.4.4
template < typename Response_Output_Strategy >
body_appender_t< Response_Output_Strategy >
body_appender(
    response_builder_t< Response_Output_Strategy > & resp,
    const params_t & params )
{
    return body_appender_t< Response_Output_Strategy >{ params, resp };
}

//! Create body appender with deflate transformation and a given compression level.
//! @since v.0.4.4
template < typename Response_Output_Strategy >
body_appender_t< Response_Output_Strategy >
deflate_body_appender(
    response_builder_t< Response_Output_Strategy > & resp,
    int compression_level = -1 )
{
    return body_appender( resp, make_deflate_compress_params( compression_level ) );
}

//! Create body appender with gzip transformation and a given compression level.
//! @since v.0.4.4
template < typename Response_Output_Strategy >
inline body_appender_t< Response_Output_Strategy >
gzip_body_appender(
    response_builder_t< Response_Output_Strategy > & resp,
    int compression_level = -1 )
{
    return body_appender( resp, make_gzip_compress_params( compression_level ) );
}

//! Create body appender with gzip transformation and a given compression level.
//! @since v.0.4.4
template < typename Response_Output_Strategy >
inline body_appender_t< Response_Output_Strategy >
identity_body_appender(
    response_builder_t< Response_Output_Strategy > & resp,
    int = -1 )
{
    return body_appender( resp, make_identity_params() );
}

//! Call a handler over a request body.
/*!
 * If body is encoded with either 'deflate' or 'gzip'
 * then it is decompressed and the handler is called.
 *
 * If body is encoded with 'identity' (or not specified)
 * the handler is called with original body.
 *
 * In other cases an exception is thrown.
 *
 * \tparam Handler a function object capable to handle std::string as an argument.
 *
 * Sample usage:
 * \code
 * namespace rtz = restinio::transforms::zlib;
 * auto decompressed_echo_handler( restinio::request_handle_t req )
 * {
 *   return
 *     rtz::handle_body(
 *       *req,
 *       [&]( auto body ){
 *         return
 *           req->create_response()
 *             .append_header( restinio::http_field::server, "RESTinio" )
 *             .append_header_date_field()
 *             .append_header(
 *               restinio::http_field::content_type,
 *               "text/plain; charset=utf-8" );
 *             .set_body( std::move( body ) )
 *             .done();
 *       } );
 * }
 * \endcode
 * @since v.0.4.4
*/
template < typename Handler >
decltype(auto)
handle_body( const request_t & req, Handler handler )
{
    const auto content_encoding =
        req.header().get_field_or( restinio::http_field::content_encoding, "identity" );

    if( caseless_cmp( content_encoding, "deflate" ) )
    {
        return handler( deflate_decompress( req.body() ) );
    }
    else if( caseless_cmp( content_encoding, "gzip" ) )
    {
        return handler( gzip_decompress( req.body() ) );
    }
    else if( !caseless_cmp( content_encoding, "identity" ) )
    {
        throw exception_t{
            fmt::format( "content-encoding '{}' not supported", content_encoding ) };
    }

    return handler( req.body() );
}

} /* namespace zlib */

} /* namespace transforms */

} /* namespace restinio */