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.. index::
   pair: ring structure; design

.. _design-ring:


Ring data structure
===================

.. mps:prefix:: design.mps.ring


Introduction
------------

:mps:tag:`source` rings are derived from the earlier use of Deques. See
design.mps.deque.


Description
-----------

.. c:type:: RingStruct *Ring

:mps:tag:`def.ring` Rings are circular doubly-linked lists of ring "nodes".
The nodes are fields of structures which are the "elements" of the
ring.

Ring node structures (:c:type:`RingStruct`) are inlined in the structures on
the ring, like this::

    typedef struct FooStruct *Foo;     /* the element type */
    typedef struct FooStruct {         /* the element structure */
      int baz, bim;
      RingStruct ring;                 /* the ring node */
      float bip, bop;
    } FooStruct;

This arrangement means that they do not need to be managed separately.
This is especially useful in avoiding re-entrancy and bootstrapping
problems in the memory manager. Rings also provide flexible insertion
and deletion because the entire ring can be found from any node.

In the MPS, rings are used to connect a "parent" structure (such as a
:c:type:`Arena`) to a number of "child" structures (such as :c:type:`Pool`), as
shown in :mps:ref:`.fig.ring`. Note the slight abuse of naming convention, in
that ``barRing`` is not called ``barRingStruct``.

:mps:tag:`fig.ring` A ring of ``Bar`` objects owned by a ``Foo`` object.

[missing figure]

:mps:tag:`fig.empty` An empty ring of ``Bar`` objects owned by a ``Foo``
object.

[missing figure]

:mps:tag:`def.singleton` A "singleton" ring is a ring containing one node,
whose previous and next nodes are itself (see :mps:ref:`.fig.single`).

:mps:tag:`fig.single` A singleton ``Bar`` object not on any ring.

[missing figure]

:mps:tag:`fig.elt` How :c:func:`RING_ELT()` gets a parent pointer from a node pointer.

[missing figure]


Init / Finish
-------------

.. c:function:: void RingInit(Ring ring)

:mps:tag:`init` Rings are initialized with the :c:func:`RingInit()` function. They
are initialized to be a singleton ring (:mps:ref:`.def.singleton`).

.. c:function:: void RingFinish(Ring ring)

:mps:tag:`finish` Rings are finished with the :c:func:`RingFinish()` function. A
ring must be a singleton ring before it can be finished (it is an
error to attempt to finish a non-singleton ring).


Iteration
---------

.. c:function:: RING_FOR(Ring node, Ring ring, Ring next)

:mps:tag:`for` A macro is used for iterating over the elements in a ring.
This macro is called :c:func:`RING_FOR()`. :c:func:`RING_FOR()` takes three arguments.
The first is an iteration variable: ``node``. The second is the
"parent" element in the ring: ``ring``. The third is a variable used
by the iterator for working state (it holds a pointer to the next
node): ``next``. All arguments must be of type :c:type:`Ring`. The ``node``
and ``next`` variables must be declared and in scope already. All
elements except for the "parent" element are iterated over. The macro
expands to a ``for`` statement. During execution of the loop, the
``node`` variable (the first argument to the macro) will be the value
of successive elements in the Ring (at the beginning of the statement
in the body of the loop).

:mps:tag:`for.error` It is an error (possibly unchecked) for the ``node``
and ``next`` variables to be modified except implicitly by using this
iterator.

:mps:tag:`for.safe` It is safe to delete the current node during the
iteration.

:mps:tag:`for.ex` An example::

    Ring node, nextNode;
    RING_FOR(node, &foo->barRing, nextNode) {
      Bar bar = RING_ELT(Bar, FooRing, node);
      frob(bar);
    }

:mps:tag:`for.ex.elt` Notice the idiomatic use of :c:func:`RING_ELT()` which is
almost universal when using :c:func:`RING_FOR()`.


Subclass
--------

.. c:function:: RING_ELT(type, field, Ring node)

:mps:tag:`elt` :c:func:`RING_ELT()` is a macro that converts a pointer to a ring
structure into a pointer to the enclosing parent structure.
:c:func:`RING_ELT()` has three arguments which are, in order: ``type``, the
type of a pointer to the enclosing structure, ``field``, the name of
the ring structure field within it, ``ring``, the ring node. The
result is a pointer to the enclosing structure.

.. note:: :c:func:`RING_ELT()` does not work for arrays of rings.


Append / Remove
---------------

.. c:function:: void RingAppend(Ring ring, Ring new)

:mps:tag:`append` :c:func:`RingAppend()` appends a singleton ring to a ring (such
that the newly added element will be last in the iteration sequence).

.. c:function:: void (RingInsert)(Ring ring, Ring new)

:mps:tag:`insert` :c:func:`RingInsert()` adds a singleton ring to a ring (such that
the newly added element will be first in the iteration sequence).

.. c:function:: void (RingRemove)(Ring old)

:mps:tag:`remove` :c:func:`RingRemove()` removes an element from a ring. The newly
removed element becomes a singleton ring. It is an error for the
element to already be a singleton.

:mps:tag:`improve.join` It would be possible to add a :c:func:`RingJoin()` operation
that joined two rings. This is not done as it is not required.


Naming
------

:mps:tag:`naming` By convention, when one structure ``Foo`` contains one
ring of ``Bar`` structures, the field in ``Foo`` is usually known as
``barRing``, and the field in ``Bar`` is known as ``fooRing``. If the
``Foo`` structure contains more than one ring of ``Bar`` structures,
then they will have names such as ``spongRing`` and ``frobRing``.


Deques
------

This section documents where rings differ significantly from deques.

:mps:tag:`head` Deques used a distinguished head structure for the head of
the ring. Rings still have a separate head structure, but it is not
distinguished by type.


Defects
-------

This section documents known defects with the current design.

:mps:tag:`app_for.misuse` It is easy to pass :c:func:`RingAppend()` and
:c:func:`RING_FOR()` the arguments in the wrong order as all the arguments
have the same type. see :mps:ref:`.head`.

:mps:tag:`check.improve` There is no method for performing a full integrity
check. This could be added.