json-validator/include/jvalidate/extension.h

#pragma once

#include <type_traits>

#include <jvalidate/constraint/extension_constraint.h>
#include <jvalidate/forward.h>
#include <jvalidate/status.h>

namespace jvalidate::extension {
/**
 * @brief A stub visitor type for building the ExtensionConstraint processing
 * hierarchy. Exposed for use with {@see ConstraintBase::visit}.
 */
struct VisitorBase {
  virtual ~VisitorBase() = default;
};

template <typename E>
concept Constraint = std::is_base_of_v<constraint::ExtensionConstraint::Impl, E>;

namespace detail {
/**
 * @brief A stub visitor type for proxying the ExtensionConstraint concrete
 * implementation type into a visitor.
 * Exists as a base class so that {@see jvalidate::extension::ConstraintBase}
 * can cast a VisitorBase to this.
 *
 * The call graph (including abstract -> virtual resolution) is as follows:
 * ExtensionConstraint::Impl::visit(VisitorBase const &)
 * -> ConstraintBase<E>::visit(VisitorBase const &)
 * ---> TypedVisitor<E>::visit(E const &)
 * -----> TypedVisitorImpl<E, V>::visit(E const &)
 * -------> Visitor::Impl::dispatch(E const &)
 * ---------> Visitor::visit<Adapter, ValidationVisitor>(E const &, ...)
 *
 * @tparam E The ExtensionConstraint type that this visitor applies to
 */
template <Constraint E> struct TypedVisitor : VisitorBase {
  virtual Status visit(E const & cons) const = 0;
};

/**
 * @brief The cleverness of this virtual class hierarchy.
 * Connects {@see jvalidate::extension::Visitor::Impl} to one of the extension
 * constraint types provided in its type signature.
 *
 * The call graph (including abstract -> virtual resolution) is as follows:
 * ExtensionConstraint::Impl::visit(VisitorBase const &)
 * -> ConstraintBase<E>::visit(VisitorBase const &)
 * ---> TypedVisitor<E>::visit(E const &)
 * -----> TypedVisitorImpl<E, V>::visit(E const &)
 * -------> Visitor::Impl::dispatch(E const &)
 * ---------> Visitor::visit<Adapter, ValidationVisitor>(E const &, ...)
 *
 * @tparam E The ExtensionConstraint type that this visitor applies to
 * @tparam CRTP A "Curiously Recurring Template Pattern" class (the actually
 * concrete class implementation of this Visitor).
 * Must implement a function "dispatch" which can accept at least E.
 */
template <Constraint E, typename CRTP> struct TypedVisitorImpl : TypedVisitor<E> {
  Status visit(E const & cons) const final {
    return static_cast<CRTP const *>(this)->dispatch(cons);
  }
};
}

/**
 * @brief The preferred base class of all user-defined constraints. Using the
 * "Curiously Recurring Template Pattern", it is able to unwrap RTTI (Run Time
 * Type Information) through {@see TypedVisitor}.
 *
 * The call graph (including abstract -> virtual resolution) is as follows:
 * ExtensionConstraint::Impl::visit(VisitorBase const &)
 * -> ConstraintBase<E>::visit(VisitorBase const &)
 * ---> TypedVisitor<E>::visit(E const &)
 * -----> TypedVisitorImpl<E, V>::visit(E const &)
 * -------> Visitor::Impl::dispatch(E const &)
 * ---------> Visitor::visit<Adapter, ValidationVisitor>(E const &, ...)
 *
 * @tparam CRTP A "Curiously Recurring Template Pattern" class (the actually
 * concrete class implementation of this ExtensionConstraint).
 */
template <typename CRTP> struct ConstraintBase : constraint::ExtensionConstraint::Impl {
  Status visit(VisitorBase const & visitor) const final {
    return dynamic_cast<detail::TypedVisitor<CRTP> const &>(visitor).visit(
        static_cast<CRTP const &>(*this));
  }
};

/**
 * @brief The visitor class responsible for performing validation on some list
 * of ExtensionConstraints as desired by the user.
 *
 * @tparam CRTP A "Curiously Recurring Template Pattern" class (the actually
 * concrete class implementation of this Visitor).
 * This class must implement the visit function for each extension constraint
 * in the Es... type list, as described by Visitor::Impl::dispatch.
 *
 * @tparam Es... A number of ExtensionConstraint implementations. Each of these
 * must fulfill the {@see jvalidate::extension::Constraint} contract.
 */
template <typename CRTP, typename... Es> class Visitor {
private:
  template <Adapter A, typename V> class Impl : public detail::TypedVisitorImpl<Es, Impl<A, V>>... {
  public:
    Impl(Visitor const * self, A const & document, V const & visitor)
        : self_(self), document_(document), visitor_(visitor) {}

    using detail::TypedVisitorImpl<Es, Impl>::visit...;

    template <Constraint E> Status dispatch(E const & cons) const {
      // static_assert(Visitable<CRTP, E, A, V>, "Must implement all visitation functions");
      return static_cast<CRTP const *>(self_)->visit(cons, document_, visitor_);
    }

  private:
    Visitor const * self_;
    A const & document_;
    V const & visitor_;
  };

public:
  template <Adapter A, typename V>
  Status operator()(constraint::ExtensionConstraint const & cons, A const & document,
                    V const & visitor) const {
    return cons.pimpl->visit(Impl<A, V>{this, document, visitor});
  }
};
}