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construct/include/ircd/json/stack.h

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12 KiB
C++

// 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;
struct chase;
struct const_chase;
struct checkpoint;
using flush_callback = std::function<const_buffer (const const_buffer &)>;
window_buffer buf;
flush_callback flusher;
std::exception_ptr eptr;
checkpoint *cp {nullptr};
size_t hiwat; ///< autoflush watermark
size_t lowat; ///< flush(false) call min watermark
object *co {nullptr}; ///< The root object instance.
array *ca {nullptr}; ///< Could be union with top_object but
void rethrow_exception();
void append(const size_t &expect, const window_buffer::closure &) noexcept;
void append(const string_view &) noexcept;
void append(const char &) noexcept;
public:
bool opened() const; ///< Current stacking in progress.
bool closed() const; ///< No stacking in progress.
bool failed() const; ///< Exception pending in eptr.
bool clean() const; ///< Never opened.
bool done() const; ///< Opened and closed.
size_t remaining() const;
const_buffer completed() const;
size_t invalidate_checkpoints();
bool flush(const bool &force = false) noexcept;
size_t rewind(const size_t &bytes);
void clear();
stack(const mutable_buffer &,
flush_callback = {},
const size_t &hiwat = -1,
const size_t &lowat = 0);
stack(stack &&) noexcept;
stack(const stack &) = delete;
~stack() 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
bool vc {false}; ///< value witnessed
void _pre_append();
void _post_append();
public:
template<class... T> void append(const json::tuple<T...> &);
void append(const json::value &);
member(object &po, const string_view &name);
member(stack &s, const string_view &name);
member(object &po, const string_view &name, const json::value &v);
member(stack &s, const string_view &name, const json::value &);
template<class... T> member(object &po, const string_view &name, const json::tuple<T...> &t);
template<class... T> member(stack &s, const string_view &name, const json::tuple<T...> &t);
member() = default;
member(const member &) = delete;
member(member &&) noexcept;
~member() noexcept;
static const member &top(const stack &);
static member &top(stack &);
};
/// 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
{
member m; ///< optional internal member
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:
template<class... T> void append(const json::tuple<T...> &);
void append(const json::object &);
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(object &po, const string_view &name);
object(stack &s, const string_view &name);
object(object &&) noexcept;
object(const object &) = delete;
~object() noexcept;
static const object &top(const stack &);
static object &top(stack &);
};
/// 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
{
member m; ///< optional internal member
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(member &pm); ///< Array is value of the named member
array(array &pa); ///< Array is value in the array
array(object &po, const string_view &name);
array(stack &s, const string_view &name);
array(stack &s);
array(const array &) = delete;
array(array &&) noexcept;
~array() noexcept;
static const array &top(const stack &);
static array &top(stack &);
};
/// This device chases the current active path by updating its member pointers.
struct ircd::json::stack::chase
{
array *a {nullptr};
object *o {nullptr};
member *m {nullptr};
bool next();
bool prev();
chase(stack &s, const bool &prechase = false);
chase() = default;
};
/// This device chases the current active path by updating its member pointers.
struct ircd::json::stack::const_chase
{
const array *a {nullptr};
const object *o {nullptr};
const member *m {nullptr};
bool next();
bool prev();
const_chase(const stack &s, const bool &prechase = false);
const_chase() = default;
};
/// Checkpoint captures the current state of the json::stack on construction
/// and allows a restoration to that state in one of three ways:
///
/// - Calling rollback() will immediately rewind the json::stack buffer and
/// allow continuing from the check point. This should be used with care, as
/// other json::stack objects may still be pending on the stack and destruct
/// after calling rollback(), leaving an incoherent attempt to close the JSON.
///
/// - Calling decommit() will defer the rollback() until destruction time. Take
/// care again that the checkpoint was still placed on the stack to avoid the
/// rollback() pitfall.
///
/// - Destruction under an exception is equivalent to a decommit() and will
/// perform a rollback() if exception_rollback is set.
///
/// Flushes are avoided under the scope of a checkpoint, but they are still
/// forced if the json::stack buffer fills up. In this case all active
/// checkpoints are invalidated and cannot be rolled back.
///
struct ircd::json::stack::checkpoint
{
stack *s {nullptr};
checkpoint *pc {nullptr};
size_t point {0};
size_t vc {0};
bool committed {true};
bool exception_rollback {true};
public:
bool committing() const; ///< When false, destructor will rollback()
bool recommit(); ///< Sets committing() to true.
bool decommit(); ///< Sets committing() to false.
bool rollback(); ///< Performs rollback of buffer.
checkpoint(stack &s,
const bool &committed = true,
const bool &exception_rollback = true);
checkpoint(checkpoint &&) = delete;
checkpoint(const checkpoint &) = delete;
~checkpoint() noexcept;
};
template<class... T>
ircd::json::stack::member::member(stack &s,
const string_view &name,
const json::tuple<T...> &t)
:member
{
object::top(s), name, t
}
{}
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(serialized(t), [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(serialized(t), [&t](mutable_buffer buf)
{
return size(stringify(buf, t));
});
}
template<class... T>
void
ircd::json::stack::object::append(const json::tuple<T...> &t)
{
for_each(t, [this](const auto &name, const auto &_value)
{
const json::value value(_value);
if(defined(value)) json::stack::member
{
*this, name, value
};
});
}