/* Copyright 2000-2005 The Apache Software Foundation or its licensors, as
 * applicable.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/*
 * This code draws heavily from the 4.4BSD <sys/queue.h> macros
 * and Dean Gaudet's "splim/ring.h".
 * <http://www.freebsd.org/cgi/cvsweb.cgi/src/sys/sys/queue.h>
 * <http://www.arctic.org/~dean/splim/>
 *
 * We'd use Dean's code directly if we could guarantee the
 * availability of inline functions.
 */

#ifndef APR_RING_H
#define APR_RING_H

/**
 * @file apr_ring.h
 * @brief APR Rings
 */

/*
 * for offsetof()
 */
#include "apr_general.h"

/**
 * @defgroup apr_ring Ring Macro Implementations
 * @ingroup APR 
 * A ring is a kind of doubly-linked list that can be manipulated
 * without knowing where its head is.
 * @{
 */

/**
 * The Ring Element
 *
 * A ring element struct is linked to the other elements in the ring
 * through its ring entry field, e.g.
 * <pre>
 *      struct my_element_t {
 *          APR_RING_ENTRY(my_element_t) link;
 *          int foo;
 *          char *bar;
 *      };
 * </pre>
 *
 * An element struct may be put on more than one ring if it has more
 * than one APR_RING_ENTRY field. Each APR_RING_ENTRY has a corresponding
 * APR_RING_HEAD declaration.
 *
 * @warning For strict C standards compliance you should put the APR_RING_ENTRY
 * first in the element struct unless the head is always part of a larger
 * object with enough earlier fields to accommodate the offsetof() used
 * to compute the ring sentinel below. You can usually ignore this caveat.
 */
#define APR_RING_ENTRY(elem)						\
    struct {								\
	struct elem *next;						\
	struct elem *prev;						\
    }

/**
 * The Ring Head
 *
 * Each ring is managed via its head, which is a struct declared like this:
 * <pre>
 *      APR_RING_HEAD(my_ring_t, my_element_t);
 *      struct my_ring_t ring, *ringp;
 * </pre>
 *
 * This struct looks just like the element link struct so that we can
 * be sure that the typecasting games will work as expected.
 *
 * The first element in the ring is next after the head, and the last
 * element is just before the head.
 */
#define APR_RING_HEAD(head, elem)					\
    struct head {							\
	struct elem *next;						\
	struct elem *prev;						\
    }

/**
 * The Ring Sentinel
 *
 * This is the magic pointer value that occurs before the first and
 * after the last elements in the ring, computed from the address of
 * the ring's head.  The head itself isn't an element, but in order to
 * get rid of all the special cases when dealing with the ends of the
 * ring, we play typecasting games to make it look like one.
 *
 * Here is a diagram to illustrate the arrangements of the next and
 * prev pointers of each element in a single ring. Note that they point
 * to the start of each element, not to the APR_RING_ENTRY structure.
 *
 * <pre>
 *     +->+------+<-+  +->+------+<-+  +->+------+<-+
 *     |  |struct|  |  |  |struct|  |  |  |struct|  |
 *    /   | elem |   \/   | elem |   \/   | elem |  \
 * ...    |      |   /\   |      |   /\   |      |   ...
 *        +------+  |  |  +------+  |  |  +------+
 *   ...--|prev  |  |  +--|ring  |  |  +--|prev  |
 *        |  next|--+     | entry|--+     |  next|--...
 *        +------+        +------+        +------+
 *        | etc. |        | etc. |        | etc. |
 *        :      :        :      :        :      :
 * </pre>
 *
 * The APR_RING_HEAD is nothing but a bare APR_RING_ENTRY. The prev
 * and next pointers in the first and last elements don't actually
 * point to the head, they point to a phantom place called the
 * sentinel. Its value is such that last->next->next == first because
 * the offset from the sentinel to the head's next pointer is the same
 * as the offset from the start of an element to its next pointer.
 * This also works in the opposite direction.
 *
 * <pre>
 *        last                            first
 *     +->+------+<-+  +->sentinel<-+  +->+------+<-+
 *     |  |struct|  |  |            |  |  |struct|  |
 *    /   | elem |   \/              \/   | elem |  \
 * ...    |      |   /\              /\   |      |   ...
 *        +------+  |  |  +------+  |  |  +------+
 *   ...--|prev  |  |  +--|ring  |  |  +--|prev  |
 *        |  next|--+     |  head|--+     |  next|--...
 *        +------+        +------+        +------+
 *        | etc. |                        | etc. |
 *        :      :                        :      :
 * </pre>
 *
 * Note that the offset mentioned above is different for each kind of
 * ring that the element may be on, and each kind of ring has a unique
 * name for its APR_RING_ENTRY in each element, and has its own type
 * for its APR_RING_HEAD.
 *
 * Note also that if the offset is non-zero (which is required if an
 * element has more than one APR_RING_ENTRY), the unreality of the
 * sentinel may have bad implications on very perverse implementations
 * of C -- see the warning in APR_RING_ENTRY.
 *
 * @param hp   The head of the ring
 * @param elem The name of the element struct
 * @param link The name of the APR_RING_ENTRY in the element struct
 */
#define APR_RING_SENTINEL(hp, elem, link)				\
    (struct elem *)((char *)(hp) - APR_OFFSETOF(struct elem, link))

/**
 * The first element of the ring
 * @param hp   The head of the ring
 */
#define APR_RING_FIRST(hp)	(hp)->next
/**
 * The last element of the ring
 * @param hp   The head of the ring
 */
#define APR_RING_LAST(hp)	(hp)->prev
/**
 * The next element in the ring
 * @param ep   The current element
 * @param link The name of the APR_RING_ENTRY in the element struct
 */
#define APR_RING_NEXT(ep, link)	(ep)->link.next
/**
 * The previous element in the ring
 * @param ep   The current element
 * @param link The name of the APR_RING_ENTRY in the element struct
 */
#define APR_RING_PREV(ep, link)	(ep)->link.prev


/**
 * Initialize a ring
 * @param hp   The head of the ring
 * @param elem The name of the element struct
 * @param link The name of the APR_RING_ENTRY in the element struct
 */
#define APR_RING_INIT(hp, elem, link) do {				\
	APR_RING_FIRST((hp)) = APR_RING_SENTINEL((hp), elem, link);	\
	APR_RING_LAST((hp))  = APR_RING_SENTINEL((hp), elem, link);	\
    } while (0)

/**
 * Determine if a ring is empty
 * @param hp   The head of the ring
 * @param elem The name of the element struct
 * @param link The name of the APR_RING_ENTRY in the element struct
 * @return true or false
 */
#define APR_RING_EMPTY(hp, elem, link)					\
    (APR_RING_FIRST((hp)) == APR_RING_SENTINEL((hp), elem, link))

/**
 * Initialize a singleton element
 * @param ep   The element
 * @param link The name of the APR_RING_ENTRY in the element struct
 */
#define APR_RING_ELEM_INIT(ep, link) do {				\
	APR_RING_NEXT((ep), link) = (ep);				\
	APR_RING_PREV((ep), link) = (ep);				\
    } while (0)


/**
 * Splice the sequence ep1..epN into the ring before element lep
 *   (..lep.. becomes ..ep1..epN..lep..)
 * @warning This doesn't work for splicing before the first element or on
 *   empty rings... see APR_RING_SPLICE_HEAD for one that does
 * @param lep  Element in the ring to splice before
 * @param ep1  First element in the sequence to splice in
 * @param epN  Last element in the sequence to splice in
 * @param link The name of the APR_RING_ENTRY in the element struct
 */
#define APR_RING_SPLICE_BEFORE(lep, ep1, epN, link) do {		\
	APR_RING_NEXT((epN), link) = (lep);				\
	APR_RING_PREV((ep1), link) = APR_RING_PREV((lep), link);	\
	APR_RING_NEXT(APR_RING_PREV((lep), link), link) = (ep1);	\
	APR_RING_PREV((lep), link) = (epN);				\
    } while (0)

/**
 * Splice the sequence ep1..epN into the ring after element lep
 *   (..lep.. becomes ..lep..ep1..epN..)
 * @warning This doesn't work for splicing after the last element or on
 *   empty rings... see APR_RING_SPLICE_TAIL for one that does
 * @param lep  Element in the ring to splice after
 * @param ep1  First element in the sequence to splice in
 * @param epN  Last element in the sequence to splice in
 * @param link The name of the APR_RING_ENTRY in the element struct
 */
#define APR_RING_SPLICE_AFTER(lep, ep1, epN, link) do {			\
	APR_RING_PREV((ep1), link) = (lep);				\
	APR_RING_NEXT((epN), link) = APR_RING_NEXT((lep), link);	\
	APR_RING_PREV(APR_RING_NEXT((lep), link), link) = (epN);	\
	APR_RING_NEXT((lep), link) = (ep1);				\
    } while (0)

/**
 * Insert the element nep into the ring before element lep
 *   (..lep.. becomes ..nep..lep..)
 * @warning This doesn't work for inserting before the first element or on
 *   empty rings... see APR_RING_INSERT_HEAD for one that does
 * @param lep  Element in the ring to insert before
 * @param nep  Element to insert
 * @param link The name of the APR_RING_ENTRY in the element struct
 */
#define APR_RING_INSERT_BEFORE(lep, nep, link)				\
	APR_RING_SPLICE_BEFORE((lep), (nep), (nep), link)

/**
 * Insert the element nep into the ring after element lep
 *   (..lep.. becomes ..lep..nep..)
 * @warning This doesn't work for inserting after the last element or on
 *   empty rings... see APR_RING_INSERT_TAIL for one that does
 * @param lep  Element in the ring to insert after
 * @param nep  Element to insert
 * @param link The name of the APR_RING_ENTRY in the element struct
 */
#define APR_RING_INSERT_AFTER(lep, nep, link)				\
	APR_RING_SPLICE_AFTER((lep), (nep), (nep), link)