/* Copyright 2000-2004 The Apache Software Foundation
 *
 * 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)


/**
 * Splice the sequence ep1..epN into the ring before the first element
 *   (..hp.. becomes ..hp..ep1..epN..)
 * @param hp   Head of the ring
 * @param ep1  First element in the sequence to splice in
 * @param epN  Last element in the sequence to splice in
 * @param elem The name of the element struct
 * @param link The name of the APR_RING_ENTRY in the element struct
 */
#define APR_RING_SPLICE_HEAD(hp, ep1, epN, elem, link)			\
	APR_RING_SPLICE_AFTER(APR_RING_SENTINEL((hp), elem, link),	\
			     (ep1), (epN), link)

/**
 * Splice the sequence ep1..epN into the ring after the last element
 *   (..hp.. becomes ..ep1..epN..hp..)
 * @param hp   Head of the ring
 * @param ep1  First element in the sequence to splice in
 * @param epN  Last element in the sequence to splice in
 * @param elem The name of the element struct
 * @param link The name of the APR_RING_ENTRY in the element struct
 */
#define APR_RING_SPLICE_TAIL(hp, ep1, epN, elem, link)			\
	APR_RING_SPLICE_BEFORE(APR_RING_SENTINEL((hp), elem, link),	\
			     (ep1), (epN), link)

/**
 * Insert the element nep into the ring before the first element
 *   (..hp.. becomes ..hp..nep..)
 * @param hp   Head of the ring
 * @param nep  Element to insert
 * @param elem The name of the element struct
 * @param link The name of the APR_RING_ENTRY in the element struct
 */
#define APR_RING_INSERT_HEAD(hp, nep, elem, link)			\
	APR_RING_SPLICE_HEAD((hp), (nep), (nep), elem, link)

/**
 * Insert the element nep into the ring after the last element
 *   (..hp.. becomes ..nep..hp..)
 * @param hp   Head of the ring
 * @param nep  Element to insert
 * @param elem The name of the element struct
 * @param link The name of the APR_RING_ENTRY in the element struct
 */
#define APR_RING_INSERT_TAIL(hp, nep, elem, link)			\
	APR_RING_SPLICE_TAIL((hp), (nep), (nep), elem, link)

/**
 * Concatenate ring h2 onto the end of ring h1, leaving h2 empty.
 * @param h1   Head of the ring to concatenate onto
 * @param h2   Head of the ring to concatenate
 * @param elem The name of the element struct
 * @param link The name of the APR_RING_ENTRY in the element struct
 */
#define APR_RING_CONCAT(h1, h2, elem, link) do {			\
	if (!APR_RING_EMPTY((h2), elem, link)) {			\
	    APR_RING_SPLICE_BEFORE(APR_RING_SENTINEL((h1), elem, link),	\
				  APR_RING_FIRST((h2)),			\
				  APR_RING_LAST((h2)), link);		\
	    APR_RING_INIT((h2), elem, link);				\
	}								\
    } while (0)

/**
 * Prepend ring h2 onto the beginning of ring h1, leaving h2 empty.
 * @param h1   Head of the ring to prepend onto
 * @param h2   Head of the ring to prepend
 * @param elem The name of the element struct
 * @param link The name of the APR_RING_ENTRY in the element struct
 */
#define APR_RING_PREPEND(h1, h2, elem, link) do {			\
	if (!APR_RING_EMPTY((h2), elem, link)) {			\
	    APR_RING_SPLICE_AFTER(APR_RING_SENTINEL((h1), elem, link),	\
				  APR_RING_FIRST((h2)),			\
				  APR_RING_LAST((h2)), link);		\
	    APR_RING_INIT((h2), elem, link);				\
	}								\
    } while (0)

/**
 * Unsplice a sequence of elements from a ring
 * @warning The unspliced sequence is left with dangling pointers at either end
 * @param ep1  First element in the sequence to unsplice
 * @param epN  Last element in the sequence to unsplice
 * @param link The name of the APR_RING_ENTRY in the element struct
 */
#define APR_RING_UNSPLICE(ep1, epN, link) do {				\
	APR_RING_NEXT(APR_RING_PREV((ep1), link), link) =		\
		     APR_RING_NEXT((epN), link);			\
	APR_RING_PREV(APR_RING_NEXT((epN), link), link) =		\
		     APR_RING_PREV((ep1), link);			\
    } while (0)

/**
 * Remove a single element from a ring
 * @warning The unspliced element is left with dangling pointers at either end
 * @param ep   Element to remove
 * @param link The name of the APR_RING_ENTRY in the element struct
 */
#define APR_RING_REMOVE(ep, link)					\
    APR_RING_UNSPLICE((ep), (ep), link)


/**
 * Iterate through a ring
 * @param ep The current element
 * @param hp The ring to iterate over
 * @param elem The name of the element struct
 * @param link The name of the APR_RING_ENTRY in the element struct
 * @remark This is the same as either:
 * <pre>
 *	ep = APR_RING_FIRST(hp);
 * 	while (ep != APR_RING_SENTINEL(hp, elem, link)) {
 *	    ...
 * 	    ep = APR_RING_NEXT(ep, link);
 * 	}
 *   OR
 * 	for (ep = APR_RING_FIRST(hp);
 *           ep != APR_RING_SENTINEL(hp, elem, link);
 *           ep = APR_RING_NEXT(ep, link)) {
 *	    ...
 * 	}
 * </pre>
 * @warning Be aware that you cannot change the value of ep within
 * the foreach loop, nor can you destroy the ring element it points to.
 * Modifying the prev and next pointers of the element is dangerous
 * but can be done if you're careful.  If you change ep's value or
 * destroy the element it points to, then APR_RING_FOREACH
 * will have no way to find out what element to use for its next
 * iteration.  The reason for this can be seen by looking closely
 * at the equivalent loops given in the tip above.  So, for example,
 * if you are writing a loop that empties out a ring one element
 * at a time, APR_RING_FOREACH just won't work for you.  Do it
 * by hand, like so:
 * <pre>
 *      while (!APR_RING_EMPTY(hp, elem, link)) {
 *          ep = APR_RING_FIRST(hp);
 *          ...
 *          APR_RING_REMOVE(ep, link);
 *      }
 * </pre>
 * @deprecated This macro causes more headaches than it's worth.  Use
 * one of the alternatives documented here instead; the clarity gained
 * in what's really going on is well worth the extra line or two of code.
 * This macro will be removed at some point in the future.
 */
#define APR_RING_FOREACH(ep, hp, elem, link)				\
    for ((ep)  = APR_RING_FIRST((hp));					\
	 (ep) != APR_RING_SENTINEL((hp), elem, link);			\
	 (ep)  = APR_RING_NEXT((ep), link))

/**
 * Iterate through a ring backwards
 * @param ep The current element
 * @param hp The ring to iterate over
 * @param elem The name of the element struct
 * @param link The name of the APR_RING_ENTRY in the element struct
 * @see APR_RING_FOREACH
 */
#define APR_RING_FOREACH_REVERSE(ep, hp, elem, link)			\
    for ((ep)  = APR_RING_LAST((hp));					\
	 (ep) != APR_RING_SENTINEL((hp), elem, link);			\
	 (ep)  = APR_RING_PREV((ep), link))


/* Debugging tools: */

#ifdef APR_RING_DEBUG
#include <stdio.h>
#include <assert.h>

#define APR_RING_CHECK_ONE(msg, ptr)					\
	fprintf(stderr, "*** %s %p\n", msg, ptr)

#define APR_RING_CHECK(hp, elem, link, msg)				\
	APR_RING_CHECK_ELEM(APR_RING_SENTINEL(hp, elem, link), elem, link, msg)

#define APR_RING_CHECK_ELEM(ep, elem, link, msg) do {			\
	struct elem *start = (ep);					\
	struct elem *here = start;					\
	fprintf(stderr, "*** ring check start -- %s\n", msg);		\
	do {								\
	    fprintf(stderr, "\telem %p\n", here);			\
	    fprintf(stderr, "\telem->next %p\n",			\
		    APR_RING_NEXT(here, link));				\
	    fprintf(stderr, "\telem->prev %p\n",			\
		    APR_RING_PREV(here, link));				\
	    fprintf(stderr, "\telem->next->prev %p\n",			\
		    APR_RING_PREV(APR_RING_NEXT(here, link), link));	\
	    fprintf(stderr, "\telem->prev->next %p\n",			\
		    APR_RING_NEXT(APR_RING_PREV(here, link), link));	\
	    if (APR_RING_PREV(APR_RING_NEXT(here, link), link) != here) { \
		fprintf(stderr, "\t*** elem->next->prev != elem\n");	\
		break;							\
	    }								\
	    if (APR_RING_NEXT(APR_RING_PREV(here, link), link) != here) { \
		fprintf(stderr, "\t*** elem->prev->next != elem\n");	\
		break;							\
	    }								\
	    here = APR_RING_NEXT(here, link);				\
	} while (here != start);					\
	fprintf(stderr, "*** ring check end\n");			\
    } while (0)

#define APR_RING_CHECK_CONSISTENCY(hp, elem, link)			\
	APR_RING_CHECK_ELEM_CONSISTENCY(APR_RING_SENTINEL(hp, elem, link),\
					elem, link)

#define APR_RING_CHECK_ELEM_CONSISTENCY(ep, elem, link) do {		\
	struct elem *start = (ep);					\
	struct elem *here = start;					\
	do {								\
	    assert(APR_RING_PREV(APR_RING_NEXT(here, link), link) == here); \
	    assert(APR_RING_NEXT(APR_RING_PREV(here, link), link) == here); \
	    here = APR_RING_NEXT(here, link);				\
	} while (here != start);					\
    } while (0)

#else
/**
 * Print a single pointer value to STDERR
 *   (This is a no-op unless APR_RING_DEBUG is defined.)
 * @param msg Descriptive message
 * @param ptr Pointer value to print
 */
#define APR_RING_CHECK_ONE(msg, ptr)
/**
 * Dump all ring pointers to STDERR, starting with the head and looping all
 * the way around the ring back to the head.  Aborts if an inconsistency
 * is found.
 *   (This is a no-op unless APR_RING_DEBUG is defined.)
 * @param hp   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
 * @param msg  Descriptive message
 */
#define APR_RING_CHECK(hp, elem, link, msg)
/**
 * Loops around a ring and checks all the pointers for consistency.  Pops
 * an assertion if any inconsistency is found.  Same idea as APR_RING_CHECK()
 * except that it's silent if all is well.
 *   (This is a no-op unless APR_RING_DEBUG is defined.)
 * @param hp   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_CHECK_CONSISTENCY(hp, elem, link)
/**
 * Dump all ring pointers to STDERR, starting with the given element and
 * looping all the way around the ring back to that element.  Aborts if
 * an inconsistency is found.
 *   (This is a no-op unless APR_RING_DEBUG is defined.)
 * @param ep   The element
 * @param elem The name of the element struct
 * @param link The name of the APR_RING_ENTRY in the element struct
 * @param msg  Descriptive message
 */
#define APR_RING_CHECK_ELEM(ep, elem, link, msg)
/**
 * Loops around a ring, starting with the given element, and checks all
 * the pointers for consistency.  Pops an assertion if any inconsistency
 * is found.  Same idea as APR_RING_CHECK_ELEM() except that it's silent
 * if all is well.
 *   (This is a no-op unless APR_RING_DEBUG is defined.)
 * @param ep   The element
 * @param elem The name of the element struct
 * @param link The name of the APR_RING_ENTRY in the element struct
 */
#define APR_RING_CHECK_ELEM_CONSISTENCY(ep, elem, link)
#endif

/** @} */ 

#endif /* !APR_RING_H */