try_parse_field.hpp

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/*
 * RESTinio
 */

/*!
 * @file
 * @brief Helper functions for parsing values of HTTP-fields.
 *
 * @since v.0.6.8
 */

#pragma once

#include <restinio/helpers/easy_parser.hpp>

#include <restinio/http_headers.hpp>
#include <restinio/request_handler.hpp>

#include <iostream>
#include <variant>

namespace restinio
{

namespace http_field_parsers
{

//
// field_not_found_t
//
/*!
 * @brief A special type to be returned in the case if HTTP-field
 * isn't found in a request.
 *
 * @since v.0.6.8
 */
struct field_not_found_t {};

namespace try_extract_field_details
{

//
// result_variant_t
//
/*!
 * @brief Type of a variant to be returned as the result of attempt
 * to parse HTTP-field.
 *
 * @since v.0.6.8
 */
template< typename Parsed_Field_Type >
using result_variant_t = std::variant<
        Parsed_Field_Type,
        field_not_found_t,
        restinio::easy_parser::parse_error_t >;

//
// valid_field_type
//
template< typename, typename = restinio::utils::metaprogramming::void_t<> >
struct valid_field_type : public std::false_type {};

template< typename T >
struct valid_field_type<
        T,
        restinio::utils::metaprogramming::void_t<
            std::enable_if_t<
                std::is_same<
                    expected_t< T, restinio::easy_parser::parse_error_t >,
                    decltype(T::try_parse(std::declval<string_view_t>()))
                >::value,
                bool
            >
        >
    > : public std::true_type
{};

//
// try_extract_field_value_from
//
template< typename Parsed_Field_Type >
[[nodiscard]]
result_variant_t< Parsed_Field_Type >
try_extract_field_value_from(
    std::optional< string_view_t > opt_value,
    string_view_t default_value )
{
    static_assert( valid_field_type<Parsed_Field_Type>::value,
            "Parsed_Field_Type should have static try_parse method that "
            "accepts string_view_t and returns "
            "expected_t<Parsed_Field_Type, parse_error_t>" );

    if( !opt_value && default_value.empty() )
        return { field_not_found_t{} };

    string_view_t content = opt_value ? *opt_value : default_value;

    auto parse_result = Parsed_Field_Type::try_parse( content );
    if( parse_result )
        return { std::move(*parse_result) };
    else
        return { parse_result.error() };
}

} /* namespace try_extract_field_details */

//
// try_parse_field
//
/*!
 * @brief A helper function for extraction and parsing a value of
 * HTTP-field.
 *
 * This helper is intended to be used when HTTP-field is identified
 * by its name.
 *
 * Usage example:
 * @code
 * auto on_post(const restinio::request_handle_t & req) {
 *  using namespace restinio::http_field_parsers;
 *
 *  const auto auth_field = try_parse_field< authorization_value_t >(
 *          req, "X-My-Authorization");
 *  if(auto * auth = std::get_if<authorization_value_t>(&auth_field)) {
 *      // X-My-Authorization is successfully parsed.
 *      if("basic" == auth->auth_scheme) {
 *          ... // Dealing with basic authentification.
 *      }
 *      else if("bearer" == auth->auth_scheme) {
 *          ... // Dealing with bearer authentification.
 *      }
 *      else {
 *          ...
 *      }
 *  }
 * }
 * @endcode
 *
 * @tparam Parsed_Field_Type The type of field value to be received as the
 * result of successful parse if the field is present.
 *
 * @tparam Extra_Data The type of extra-data incorporated into an instance
 * of restinio::generic_request_t. There is no need to specify that type,
 * it has to be detected automatically by the compiler.
 *
 * @since v.0.6.8
 */
template< typename Parsed_Field_Type, typename Extra_Data >
[[nodiscard]]
auto
try_parse_field(
    //! A request that should hold a HTTP-field.
    const generic_request_t< Extra_Data > & req,
    //! The name of HTTP-field to be extracted and parsed.
    string_view_t field_name,
    //! The default value to be used if HTTP-field is not found.
    //! If this value is not empty, then the variant with
    //! field_not_found_t won't be returned.
    string_view_t default_value = string_view_t{} )
{
    using namespace try_extract_field_details;

    return try_extract_field_value_from< Parsed_Field_Type >(
            req.header().opt_value_of( field_name ),
            default_value );
}

/*!
 * @brief A helper function for extraction and parsing a value of
 * HTTP-field.
 *
 * This helper is intended to be used when HTTP-field is identified
 * by its ID.
 *
 * Usage example:
 * @code
 * auto on_post(const restinio::request_handle_t & req) {
 *  using namespace restinio::http_field_parsers;
 *
 *  const auto auth_field = try_parse_field< authorization_value_t >(
 *          req, restinio::http_field::authorization);
 *  if(auto * auth = std::get_if<authorization_value_t>(&auth_field)) {
 *      // Authorization is successfully parsed.
 *      if("basic" == auth->auth_scheme) {
 *          ... // Dealing with basic authentification.
 *      }
 *      else if("bearer" == auth->auth_scheme) {
 *          ... // Dealing with bearer authentification.
 *      }
 *      else {
 *          ...
 *      }
 *  }
 * }
 * @endcode
 *
 * @tparam Parsed_Field_Type The type of field value to be received as the
 * result of successful parse if the field is present.
 *
 * @tparam Extra_Data The type of extra-data incorporated into an instance
 * of restinio::generic_request_t. There is no need to specify that type,
 * it has to be detected automatically by the compiler.
 *
 * @since v.0.6.8
 */
template< typename Parsed_Field_Type, typename Extra_Data >
[[nodiscard]]
auto
try_parse_field(
    //! A request that should hold a HTTP-field.
    const generic_request_t< Extra_Data > & req,
    //! The ID of a HTTP-field to be extracted and parsed.
    http_field_t field_id,
    //! The default value to be used if HTTP-field is not found.
    //! If this value is not empty, then the variant with
    //! field_not_found_t won't be returned.
    string_view_t default_value = string_view_t{} )
{
    using namespace try_extract_field_details;

    return try_extract_field_value_from< Parsed_Field_Type >(
            req.header().opt_value_of( field_id ),
            default_value );
}

} /* namespace http_field_parsers */

} /* namespace restinio */