construct/include/ircd/json/stack.h

// Matrix Construct
//
// Copyright (C) Matrix Construct Developers, Authors & Contributors
// Copyright (C) 2016-2018 Jason Volk <jason@zemos.net>
//
// Permission to use, copy, modify, and/or distribute this software for any
// purpose with or without fee is hereby granted, provided that the above
// copyright notice and this permission notice is present in all copies. The
// full license for this software is available in the LICENSE file.

#pragma once
#define HAVE_IRCD_JSON_STACK_H

namespace ircd::json
{
	struct stack;
}

/// Output stack machine for stringifying JSON as-you-go. This device allows
/// the user to create JSON without knowing the contents when it is first
/// constructed. An object or array is opened and the user can append to the
/// stack creating the members or values or recursing further. The JSON is
/// then closed automatically with exception safety. Partial JSON is written
/// to the buffer as soon as possible.
///
/// The target buffer is not required to maintain earlier output from the same
/// stack or even earlier members and values of the same object or array. The
/// buffer may be smaller than the final JSON output and reused when the user
/// chooses to flush it to some storage or socket. If the buffer becomes full
/// a flush callback is attempted to make space and continue. This can occur
/// while the output is still incomplete JSON.
///
/// The user first creates a master json::stack instance with some reasonable
/// backing buffer. A suite of classes is provided to aid with building the
/// JSON which attach to each other stackfully, and eventually lead to the
/// root. There should only be one "active path" of instances at any given
/// time, ideally following the scope of your code itself. You must force
/// instances to go out of scope to continue at the same recursion depth.
/// This way the json::stack can "follow" your code and "record" the final
/// JSON output while allowing you to free the original resources required
/// for each value.
///
struct ircd::json::stack
{
	struct array;
	struct object;
	struct member;
	using flush_callback = std::function<const_buffer (const const_buffer &)>;

	window_buffer buf;
	flush_callback flusher;

	object *co {nullptr};              ///< The root object instance.
	array *ca {nullptr};               ///< Could be union with top_object but

	template<class gen, class... attr> bool printer(gen&&, attr&&...);
	template<class gen> bool printer(gen&&);
	bool append(const window_buffer::closure &);
	bool append(const string_view &);

  public:
	bool opened() const;               ///< Current stacking in progress.
	bool closed() const;               ///< No stacking in progress.
	bool clean() const;                ///< Never opened.
	bool done() const;                 ///< Opened and closed.

	size_t remaining() const;
	const_buffer completed() const;
	void clear();

	stack(const mutable_buffer &, flush_callback = {});
	~stack() noexcept;
};

/// stack::object is constructed under the scope of either a stack::member,
/// or a stack::array, or a stack itself. Only stack::member can be
/// constructed directly under its scope.
///
/// For a stack::member parent, the named member is waiting for this value
/// after leaving the stack at ':' after the name, this object will then
/// print '{' and dtor with '}' and then return to the stack::member which
/// will then return to its parent object.
///
/// For a stack::array parent, the stack may have been left at '[' or ','
/// but either way this object will then print '{' and dtor with '}' and
/// then return to the stack::array.
///
/// For a stack itself, this object is considered the "top object" and will
/// open the stack with '{' and accept member instances under its scope
/// until closing the stack with '}' after which the stack is done()
///
struct ircd::json::stack::object
{
	stack *s {nullptr};                ///< root stack ref
	member *pm {nullptr};              ///< parent member (if value of one)
	array *pa {nullptr};               ///< parent array (if value in one)
	member *cm {nullptr};              ///< current child member
	size_t mc {0};                     ///< members witnessed (monotonic)

  public:
	object() = default;
	object(stack &s);                  ///< Object is top
	object(array &pa);                 ///< Object is value in the array
	object(member &pm);                ///< Object is value of named member
	~object() noexcept;
};

/// stack::array is constructed under the scope of either a stack::member,
/// or a stack::array, or a stack itself. stack::object and stack::array
/// can be constructed directly under its scope, but not stack::member.
///
/// The same behavior as described by stack::object documentation applies
/// here translated to arrays.
///
struct ircd::json::stack::array
{
	stack *s {nullptr};                ///< root stack ref
	member *pm {nullptr};              ///< parent member (if value of one)
	array *pa {nullptr};               ///< parent array (if value in one)
	object *co {nullptr};              ///< current child object
	array *ca {nullptr};               ///< current child array
	size_t vc {0};                     ///< values witnessed (monotonic)

	void _pre_append();
	void _post_append();

  public:
	template<class... T> void append(const json::tuple<T...> &);
	void append(const json::value &);

	array() = default;
	array(stack &s);                   ///< Array is top
	array(array &pa);                  ///< Array is value in the array
	array(member &pm);                 ///< Array is value of the named member
	~array() noexcept;
};

/// stack::member is an intermediary that is constructed under the scope of
/// a parent stack::object. It takes a name argument. It then requires one
/// object or array be constructed under its scope as its value, or a
/// json::value / already strung JSON must be appended as its value.
///
/// If the value is supplied in the constructor argument an instance of
/// this class does not have to be held (use constructor as function).
///
struct ircd::json::stack::member
{
	stack *s {nullptr};               ///< root stack ref
	object *po {nullptr};             ///< parent object
	string_view name;                 ///< member name state
	object *co {nullptr};             ///< current child object
	array *ca {nullptr};              ///< current child array

	void _pre_append();
	void _post_append();

  public:
	template<class... T> void append(const json::tuple<T...> &);
	void append(const json::value &);

	member() = default;
	member(object &po, const string_view &name);
	member(object &po, const string_view &name, const json::value &v);
	template<class... T> member(object &po, const string_view &name, const json::tuple<T...> &t);
	~member() noexcept;
};

template<class... T>
ircd::json::stack::member::member(object &po,
                                  const string_view &name,
                                  const json::tuple<T...> &t)
:member{po, name}
{
	append(t);
}

template<class... T>
void
ircd::json::stack::member::append(const json::tuple<T...> &t)
{
	_pre_append();
	const unwind post{[this]
	{
		_post_append();
	}};

	s->append([this, &t](mutable_buffer buf)
	{
		return size(stringify(buf, t));
	});
}

template<class... T>
void
ircd::json::stack::array::append(const json::tuple<T...> &t)
{
	_pre_append();
	const unwind post{[this]
	{
		_post_append();
	}};

	s->append([&t](mutable_buffer buf)
	{
		return size(stringify(buf, t));
	});
}