.\"	$OpenBSD: queue.3,v 1.42 2006/01/12 17:01:15 jmc Exp $
.\"	$NetBSD: queue.3,v 1.4 1995/07/03 00:25:36 mycroft Exp $
.\"
.\" Copyright (c) 1993 The Regents of the University of California.
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.\" modification, are permitted provided that the following conditions
.\" are met:
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.\"	@(#)queue.3	8.1 (Berkeley) 12/13/93
.\"
.Dd December 21, 2022
.Dt AG_QUEUE 3
.Os Agar 1.7
.Sh NAME
.Nm AG_Queue
.Nd "Agar singly-linked lists, doubly-linked lists, simple queues, tail queues, and circular queues"
.Sh SYNOPSIS
.Fd #define _USE_AGAR_QUEUE /* For versions without AG_ prefix */
.Fd #include <agar/core.h>
.Sh DESCRIPTION
These macros define and operate on five types of data structures:
singly-linked lists, simple queues, lists, tail queues, and circular queues.
All five structures support the following functionality:
.Pp
.Bl -enum -compact -offset indent
.It
Insertion of a new entry at the head of the list.
.It
Insertion of a new entry after any element in the list.
.It
Removal of an entry from the head of the list.
.It
Forward traversal through the list.
.El
.Pp
Singly-linked lists are the simplest of the five data structures
and support only the above functionality.
Singly-linked lists are ideal for applications with large datasets
and few or no removals, or for implementing a LIFO queue.
.Pp
Simple queues add the following functionality:
.Pp
.Bl -enum -compact -offset indent
.It
Entries can be added at the end of a list.
.El
.Pp
However:
.Pp
.Bl -enum -compact -offset indent
.It
All list insertions must specify the head of the list.
.It
Each head entry requires two pointers rather than one.
.It
Code size is about 15% greater and operations run about 20% slower
than singly-linked lists.
.El
.Pp
Simple queues are ideal for applications with large datasets and
few or no removals, or for implementing a FIFO queue.
.Pp
All doubly linked types of data structures (lists, tail queues, and circle
queues) additionally allow:
.Pp
.Bl -enum -compact -offset indent
.It
Insertion of a new entry before any element in the list.
.It
Removal of any entry in the list.
.El
.Pp
However:
.Pp
.Bl -enum -compact -offset indent
.It
Each element requires two pointers rather than one.
.It
Code size and execution time of operations (except for removal) is about
twice that of the singly-linked data-structures.
.El
.Pp
Lists are the simplest of the doubly linked data structures and support
only the above functionality over singly-linked lists.
.Pp
Tail queues add the following functionality:
.Pp
.Bl -enum -compact -offset indent
.It
Entries can be added at the end of a list.
.It
They may be traversed backwards, at a cost.
.El
.Pp
However:
.Pp
.Bl -enum -compact -offset indent
.It
All list insertions and removals must specify the head of the list.
.It
Each head entry requires two pointers rather than one.
.It
Code size is about 15% greater and operations run about 20% slower
than singly-linked lists.
.El
.Pp
Circular queues add the following functionality:
.Pp
.Bl -enum -compact -offset indent
.It
Entries can be added at the end of a list.
.It
They may be traversed backwards, from tail to head.
.El
.Pp
However:
.Pp
.Bl -enum -compact -offset indent
.It
All list insertions and removals must specify the head of the list.
.It
Each head entry requires two pointers rather than one.
.It
The termination condition for traversal is more complex.
.It
Code size is about 40% greater and operations run about 45% slower than lists.
.El
.Pp
In the macro definitions,
.Fa TYPE
is the name tag of a user defined structure that must contain a field of type
.Li AG_SLIST_ENTRY ,
.Li AG_LIST_ENTRY ,
.Li AG_SIMPLEQ_ENTRY ,
.Li AG_TAILQ_ENTRY ,
or
.Li AG_CIRCLEQ_ENTRY ,
named
.Fa NAME .
The argument
.Fa HEADNAME
is the name tag of a user defined structure that must be declared
using the macros
.Fn AG_SLIST_HEAD ,
.Fn AG_LIST_HEAD ,
.Fn AG_SIMPLEQ_HEAD ,
.Fn AG_TAILQ_HEAD ,
or
.Fn AG_CIRCLEQ_HEAD .
See the examples below for further explanation of how these macros are used.
.Sh SINGLY-LINKED LISTS
.nr nS 1
.Fn AG_SLIST_ENTRY "TYPE"
.Pp
.Fn AG_SLIST_HEAD "HEADNAME" "TYPE"
.Pp
.Fn AG_SLIST_HEAD_ "TYPE"
.Pp
.Fn AG_SLIST_HEAD_INITIALIZER "AG_SLIST_HEAD head"
.Pp
.Ft "struct TYPE *"
.Fn AG_SLIST_FIRST "AG_SLIST_HEAD *head"
.Pp
.Ft "struct TYPE *"
.Fn AG_SLIST_NEXT "struct TYPE *listelm" "AG_SLIST_ENTRY NAME"
.Pp
.Ft "struct TYPE *"
.Fn AG_SLIST_END "AG_SLIST_HEAD *head"
.Pp
.Ft "bool"
.Fn AG_SLIST_EMPTY "AG_SLIST_HEAD *head"
.Pp
.Fn AG_SLIST_FOREACH "VARNAME" "AG_SLIST_HEAD *head" "AG_SLIST_ENTRY NAME"
.Pp
.Fn AG_SLIST_FOREACH_PREVPTR "VARNAME" "VARNAMEP" "AG_SLIST_HEAD *head" "AG_SLIST_ENTRY NAME"
.Pp
.Ft void
.Fn AG_SLIST_INIT "AG_SLIST_HEAD *head"
.Pp
.Ft void
.Fn AG_SLIST_INSERT_AFTER "struct TYPE *listelm" "struct TYPE *elm" "AG_SLIST_ENTRY NAME"
.Pp
.Ft void
.Fn AG_SLIST_INSERT_HEAD "AG_SLIST_HEAD *head" "struct TYPE *elm" "AG_SLIST_ENTRY NAME"
.Pp
.Ft void
.Fn AG_SLIST_REMOVE_HEAD "AG_SLIST_HEAD *head" "AG_SLIST_ENTRY NAME"
.Pp
.Ft void
.Fn AG_SLIST_REMOVE_NEXT "AG_SLIST_HEAD *head" "struct TYPE *elm" "AG_SLIST_ENTRY NAME"
.Pp
.Ft void
.Fn AG_SLIST_REMOVE "AG_SLIST_HEAD *head" "struct TYPE *elm" "TYPE" "AG_SLIST_ENTRY NAME"
.Pp
.nr nS 0
A singly-linked list is headed by a structure defined by the
.Fn AG_SLIST_HEAD
macro.
This structure contains a single pointer to the first element on the list.
The elements are singly linked for minimum space and pointer manipulation
overhead at the expense of O(n) removal for arbitrary elements.
New elements can be added to the list after an existing element or
at the head of the list.
A
.Fa AG_SLIST_HEAD
structure is declared as follows:
.Bd -literal -offset indent
.\" SYNTAX(c)
AG_SLIST_HEAD(HEADNAME, TYPE) head;
AG_SLIST_HEAD_(TYPE) head;	/* If HEADNAME is not needed */
.Ed
.Pp
where
.Fa HEADNAME
is the name of the structure to be defined, and struct
.Fa TYPE
is the type of the elements to be linked into the list.
A pointer to the head of the list can later be declared as:
.Bd -literal -offset indent
.\" SYNTAX(c)
struct HEADNAME *headp;
.Ed
.Pp
(The names
.Li head
and
.Li headp
are user selectable.)
.Pp
The
.Fn AG_SLIST_ENTRY
macro declares a structure that connects the elements in the list.
.Pp
The
.Fn AG_SLIST_INIT
macro initializes the list referenced by
.Fa head .
.Pp
The list can also be initialized statically by using the
.Fn AG_SLIST_HEAD_INITIALIZER
macro like this:
.Bd -literal -offset indent
.\" SYNTAX(c)
AG_SLIST_HEAD(HEADNAME, TYPE) head = AG_SLIST_HEAD_INITIALIZER(head);
.Ed
.Pp
The
.Fn AG_SLIST_INSERT_HEAD
macro inserts the new element
.Fa elm
at the head of the list.
.Pp
The
.Fn AG_SLIST_INSERT_AFTER
macro inserts the new element
.Fa elm
after the element
.Fa listelm .
.Pp
The
.Fn AG_SLIST_REMOVE_HEAD
macro removes the first element of the list pointed by
.Fa head .
.Pp
The
.Fn AG_SLIST_REMOVE_NEXT
macro removes the list element immediately following
.Fa elm .
.Pp
The
.Fn AG_SLIST_REMOVE
macro removes the element
.Fa elm
of the list pointed by
.Fa head .
.Pp
The
.Fn AG_SLIST_FIRST
and
.Fn AG_SLIST_NEXT
macros can be used to traverse the list:
.Bd -literal -offset indent
.\" SYNTAX(c)
for (np = AG_SLIST_FIRST(&head);
     np != NULL;
     np = AG_SLIST_NEXT(np, NAME))
.Ed
.Pp
Or, for simplicity, one can use the
.Fn AG_SLIST_FOREACH
macro:
.Bd -literal -offset indent
.\" SYNTAX(c)
AG_SLIST_FOREACH(np, head, NAME) { /* ... */ }
.Ed
.Pp
The
.Fn AG_SLIST_FOREACH_PREVPTR
macro is similar to
.Fn AG_SLIST_FOREACH
except that it stores a pointer to the previous element in
.Fa VARNAMEP .
This provides access to the previous element while traversing the list,
as one would have with a doubly-linked list.
.Pp
The
.Fn AG_SLIST_EMPTY
macro should be used to check whether a simple list is empty.
.Sh SINGLY-LINKED LIST EXAMPLE
.Bd -literal
.\" SYNTAX(c)
AG_SLIST_HEAD(listhead, entry) head;
struct entry {
	/* ... */
	AG_SLIST_ENTRY(entry) entries;	/* Simple list. */
	/* ... */
} *n1, *n2, *np;

AG_SLIST_INIT(&head);			/* Initialize simple list. */

n1 = malloc(sizeof(struct entry));	/* Insert at the head. */
AG_SLIST_INSERT_HEAD(&head, n1, entries);

n2 = malloc(sizeof(struct entry));	/* Insert after. */
AG_SLIST_INSERT_AFTER(n1, n2, entries);

AG_SLIST_FOREACH(np, &head, entries) {	/* Forward traversal. */
	/* np-> ... */
}

while (!AG_SLIST_EMPTY(&head))		/* Delete. */
	AG_SLIST_REMOVE_HEAD(&head, entries);
.Ed
.Sh LISTS
.nr nS 1
.Fn AG_LIST_ENTRY "TYPE"
.Pp
.Fn AG_LIST_HEAD "HEADNAME" "TYPE"
.Pp
.Fn AG_LIST_HEAD_ "TYPE"
.Pp
.Fn AG_LIST_HEAD_INITIALIZER "AG_LIST_HEAD head"
.Pp
.Ft "struct TYPE *"
.Fn AG_LIST_FIRST "AG_LIST_HEAD *head"
.Pp
.Ft "struct TYPE *"
.Fn AG_LIST_NEXT "struct TYPE *listelm" "AG_LIST_ENTRY NAME"
.Pp
.Ft "struct TYPE *"
.Fn AG_LIST_END "AG_LIST_HEAD *head"
.Pp
.Ft "bool"
.Fn AG_LIST_EMPTY "AG_LIST_HEAD *head"
.Pp
.Fn AG_LIST_FOREACH "VARNAME" "AG_LIST_HEAD *head" "AG_LIST_ENTRY NAME"
.Pp
.Ft void
.Fn AG_LIST_INIT "AG_LIST_HEAD *head"
.Pp
.Ft void
.Fn AG_LIST_INSERT_AFTER "struct TYPE *listelm" "struct TYPE *elm" "AG_LIST_ENTRY NAME"
.Pp
.Ft void
.Fn AG_LIST_INSERT_BEFORE "struct TYPE *listelm" "struct TYPE *elm" "AG_LIST_ENTRY NAME"
.Pp
.Ft void
.Fn AG_LIST_INSERT_HEAD "AG_LIST_HEAD *head" "struct TYPE *elm" "AG_LIST_ENTRY NAME"
.Pp
.Ft void
.Fn AG_LIST_REMOVE "struct TYPE *elm" "AG_LIST_ENTRY NAME"
.Pp
.Ft void
.Fn AG_LIST_REPLACE "struct TYPE *elm" "struct TYPE *elm2" "AG_LIST_ENTRY NAME"
.Pp
.nr nS 0
A list is headed by a structure defined by the
.Fn AG_LIST_HEAD
macro.
This structure contains a single pointer to the first element on the list.
The elements are doubly linked so that an arbitrary element can be
removed without traversing the list.
New elements can be added to the list after an existing element,
before an existing element, or at the head of the list.
A
.Fa AG_LIST_HEAD
structure is declared as follows:
.Bd -literal -offset indent
.\" SYNTAX(c)
AG_LIST_HEAD(HEADNAME, TYPE) head;
AG_LIST_HEAD_(TYPE) head;	/* If HEADNAME is not needed */
.Ed
.Pp
where
.Fa HEADNAME
is the name of the structure to be defined, and struct
.Fa TYPE
is the type of the elements to be linked into the list.
A pointer to the head of the list can later be declared as:
.Bd -literal -offset indent
.\" SYNTAX(c)
struct HEADNAME *headp;
.Ed
.Pp
(The names
.Li head
and
.Li headp
are user selectable.)
.Pp
The
.Fn AG_LIST_ENTRY
macro declares a structure that connects the elements in the list.
.Pp
The
.Fn AG_LIST_INIT
macro initializes the list referenced by
.Fa head .
.Pp
The list can also be initialized statically by using the
.Fn AG_LIST_HEAD_INITIALIZER
macro like this:
.Bd -literal -offset indent
.\" SYNTAX(c)
AG_LIST_HEAD(HEADNAME, TYPE) head = AG_LIST_HEAD_INITIALIZER(head);
.Ed
.Pp
The
.Fn AG_LIST_INSERT_HEAD
macro inserts the new element
.Fa elm
at the head of the list.
.Pp
The
.Fn AG_LIST_INSERT_AFTER
macro inserts the new element
.Fa elm
after the element
.Fa listelm .
.Pp
The
.Fn AG_LIST_INSERT_BEFORE
macro inserts the new element
.Fa elm
before the element
.Fa listelm .
.Pp
The
.Fn AG_LIST_REMOVE
macro removes the element
.Fa elm
from the list.
.Pp
The
.Fn AG_LIST_REPLACE
macro replaces the list element
.Fa elm
with the new element
.Fa elm2 .
.Pp
The
.Fn AG_LIST_FIRST
and
.Fn AG_LIST_NEXT
macros can be used to traverse the list:
.Bd -literal -offset indent
.\" SYNTAX(c)
for (np = AG_LIST_FIRST(&head);
     np != NULL;
     np = AG_LIST_NEXT(np, NAME))
.Ed
.Pp
Or, for simplicity, one can use the
.Fn AG_LIST_FOREACH
macro:
.Bd -literal -offset indent
.\" SYNTAX(c)
AG_LIST_FOREACH(np, head, NAME) { /* ... */ }
.Ed
.Pp
The
.Fn AG_LIST_EMPTY
macro should be used to check whether a list is empty.
.Sh LIST EXAMPLE
.Bd -literal
.\" SYNTAX(c)
AG_LIST_HEAD(listhead, entry) head;
struct entry {
	/* ... */
	AG_LIST_ENTRY(entry) entries;	/* List. */
	/* ... */
} *n1, *n2, *np;

AG_LIST_INIT(&head);			/* Initialize list. */

n1 = malloc(sizeof(struct entry));	/* Insert at the head. */
AG_LIST_INSERT_HEAD(&head, n1, entries);

n2 = malloc(sizeof(struct entry));	/* Insert after. */
AG_LIST_INSERT_AFTER(n1, n2, entries);

n2 = malloc(sizeof(struct entry));	/* Insert before. */
AG_LIST_INSERT_BEFORE(n1, n2, entries);
					/* Forward traversal. */
AG_LIST_FOREACH(np, &head, entries)
	/* np-> ... */

while (!AG_LIST_EMPTY(&head))		/* Delete. */
	AG_LIST_REMOVE(AG_LIST_FIRST(&head), entries);
.Ed
.Sh SIMPLE QUEUES
.nr nS 1
.Fn AG_SIMPLEQ_ENTRY "TYPE"
.Pp
.Fn AG_SIMPLEQ_HEAD "HEADNAME" "TYPE"
.Pp
.Fn AG_SIMPLEQ_HEAD_ "TYPE"
.Pp
.Fn AG_SIMPLEQ_HEAD_INITIALIZER "AG_SIMPLEQ_HEAD head"
.Pp
.Ft "struct TYPE *"
.Fn AG_SIMPLEQ_FIRST "AG_SIMPLEQ_HEAD *head"
.Pp
.Ft "struct TYPE *"
.Fn AG_SIMPLEQ_NEXT "struct TYPE *listelm" "AG_SIMPLEQ_ENTRY NAME"
.Pp
.Ft "struct TYPE *"
.Fn AG_SIMPLEQ_END "AG_SIMPLEQ_HEAD *head"
.Pp
.Ft void
.Fn AG_SIMPLEQ_INIT "AG_SIMPLEQ_HEAD *head"
.Pp
.Ft void
.Fn AG_SIMPLEQ_INSERT_HEAD "AG_SIMPLEQ_HEAD *head" "struct TYPE *elm" "AG_SIMPLEQ_ENTRY NAME"
.Pp
.Ft void
.Fn AG_SIMPLEQ_INSERT_TAIL "AG_SIMPLEQ_HEAD *head" "struct TYPE *elm" "AG_SIMPLEQ_ENTRY NAME"
.Pp
.Ft void
.Fn AG_SIMPLEQ_INSERT_AFTER "AG_SIMPLEQ_HEAD *head" "struct TYPE *listelm" "struct TYPE *elm" "AG_SIMPLEQ_ENTRY NAME"
.Pp
.Ft void
.Fn AG_SIMPLEQ_REMOVE_HEAD "AG_SIMPLEQ_HEAD *head" "AG_SIMPLEQ_ENTRY NAME"
.Pp
.nr nS 0
A simple queue is headed by a structure defined by the
.Fn AG_SIMPLEQ_HEAD
macro.
This structure contains a pair of pointers, one to the first element in the
simple queue and the other to the last element in the simple queue.
The elements are singly linked.
New elements can be added to the queue after an existing element,
at the head of the queue or at the tail of the queue.
A
.Fa AG_SIMPLEQ_HEAD
structure is declared as follows:
.Bd -literal -offset indent
.\" SYNTAX(c)
AG_SIMPLEQ_HEAD(HEADNAME, TYPE) head;
AG_SIMPLEQ_HEAD_(TYPE) head;	/* If HEADNAME is not needed */
.Ed
.Pp
where
.Fa HEADNAME
is the name of the structure to be defined, and struct
.Fa TYPE
is the type of the elements to be linked into the queue.
A pointer to the head of the queue can later be declared as:
.Bd -literal -offset indent
.\" SYNTAX(c)
struct HEADNAME *headp;
.Ed
.Pp
(The names
.Li head
and
.Li headp
are user selectable.)
.Pp
The
.Fn AG_SIMPLEQ_ENTRY
macro declares a structure that connects the elements in
the queue.
.Pp
The
.Fn AG_SIMPLEQ_INIT
macro initializes the queue referenced by
.Fa head .
.Pp
The queue can also be initialized statically by using the
.Fn AG_SIMPLEQ_HEAD_INITIALIZER
macro like this:
.Bd -literal -offset indent
.\" SYNTAX(c)
AG_SIMPLEQ_HEAD(HEADNAME, TYPE) head =
    AG_SIMPLEQ_HEAD_INITIALIZER(head);
.Ed
.Pp
The
.Fn AG_SIMPLEQ_INSERT_HEAD
macro inserts the new element
.Fa elm
at the head of the queue.
.Pp
The
.Fn AG_SIMPLEQ_INSERT_TAIL
macro inserts the new element
.Fa elm
at the end of the queue.
.Pp
The
.Fn AG_SIMPLEQ_INSERT_AFTER
macro inserts the new element
.Fa elm
after the element
.Fa listelm .
.Pp
The
.Fn AG_SIMPLEQ_REMOVE_HEAD
macro removes the first element
from the queue.
.Pp
The
.Fn AG_SIMPLEQ_FIRST
and
.Fn AG_SIMPLEQ_NEXT
macros can be used to traverse the queue.
The
.Fn AG_SIMPLEQ_FOREACH
is used for queue traversal:
.Bd -literal -offset indent
.\" SYNTAX(c)
AG_SIMPLEQ_FOREACH(np, head, NAME) { /* ... */ }
.Ed
.Pp
The
.Fn AG_SIMPLEQ_EMPTY
macro should be used to check whether a list is empty.
.Sh SIMPLE QUEUE EXAMPLE
.Bd -literal
.\" SYNTAX(c)
AG_SIMPLEQ_HEAD(listhead, entry) head = AG_SIMPLEQ_HEAD_INITIALIZER(head);
struct entry {
	/* ... */
	AG_SIMPLEQ_ENTRY(entry) entries;	/* Simple queue. */
	/* ... */
} *n1, *n2, *np;

n1 = malloc(sizeof(struct entry));	/* Insert at the head. */
AG_SIMPLEQ_INSERT_HEAD(&head, n1, entries);

n2 = malloc(sizeof(struct entry));	/* Insert after. */
AG_SIMPLEQ_INSERT_AFTER(&head, n1, n2, entries);

n2 = malloc(sizeof(struct entry));	/* Insert at the tail. */
AG_SIMPLEQ_INSERT_TAIL(&head, n2, entries);
					/* Forward traversal. */
AG_SIMPLEQ_FOREACH(np, &head, entries) {
	/* np-> ... */
}
					/* Delete. */
while (!AG_SIMPLEQ_EMPTY(&head))
	AG_SIMPLEQ_REMOVE_HEAD(&head, entries);
.Ed
.Sh TAIL QUEUES
.nr nS 1
.Fn AG_TAILQ_ENTRY "TYPE"
.Pp
.Fn AG_TAILQ_HEAD "HEADNAME" "TYPE"
.Pp
.Fn AG_TAILQ_HEAD_ "TYPE"
.Pp
.Fn AG_TAILQ_HEAD_INITIALIZER "AG_TAILQ_HEAD head"
.Pp
.Ft "struct TYPE *"
.Fn AG_TAILQ_FIRST "AG_TAILQ_HEAD *head"
.Pp
.Ft "struct TYPE *"
.Fn AG_TAILQ_NEXT "struct TYPE *listelm" "AG_TAILQ_ENTRY NAME"
.Pp
.Ft "struct TYPE *"
.Fn AG_TAILQ_END "AG_TAILQ_HEAD *head"
.Pp
.Ft "struct TYPE *"
.Fn AG_TAILQ_LAST "AG_TAILQ_HEAD *head" "HEADNAME NAME"
.Pp
.Fn AG_TAILQ_PREV "struct TYPE *listelm" "HEADNAME NAME" "AG_TAILQ_ENTRY NAME"
.Pp
.Ft "bool"
.Fn AG_TAILQ_EMPTY "AG_TAILQ_HEAD *head"
.Pp
.Fn AG_TAILQ_FOREACH "VARNAME" "AG_TAILQ_HEAD *head" "AG_TAILQ_ENTRY NAME"
.Pp
.Fn AG_TAILQ_FOREACH_REVERSE "VARNAME" "AG_TAILQ_HEAD *head" "HEADNAME" "AG_TAILQ_ENTRY NAME"
.Pp
.Ft void
.Fn AG_TAILQ_INIT "AG_TAILQ_HEAD *head"
.Pp
.Ft void
.Fn AG_TAILQ_INSERT_AFTER "AG_TAILQ_HEAD *head" "struct TYPE *listelm" "struct TYPE *elm" "AG_TAILQ_ENTRY NAME"
.Pp
.Ft void
.Fn AG_TAILQ_INSERT_BEFORE "struct TYPE *listelm" "struct TYPE *elm" "AG_TAILQ_ENTRY NAME"
.Pp
.Ft void
.Fn AG_TAILQ_INSERT_HEAD "AG_TAILQ_HEAD *head" "struct TYPE *elm" "AG_TAILQ_ENTRY NAME"
.Pp
.Ft void
.Fn AG_TAILQ_INSERT_TAIL "AG_TAILQ_HEAD *head" "struct TYPE *elm" "AG_TAILQ_ENTRY NAME"
.Pp
.Ft void
.Fn AG_TAILQ_REMOVE "AG_TAILQ_HEAD *head" "struct TYPE *elm" "AG_TAILQ_ENTRY NAME"
.Pp
.nr nS 0
A tail queue is headed by a structure defined by the
.Fn AG_TAILQ_HEAD
macro.
This structure contains a pair of pointers,
one to the first element in the tail queue and the other to
the last element in the tail queue.
The elements are doubly linked so that an arbitrary element can be
removed without traversing the tail queue.
New elements can be added to the queue after an existing element,
before an existing element, at the head of the queue, or at the end
of the queue.
A
.Fa AG_TAILQ_HEAD
structure is declared as follows:
.Bd -literal -offset indent
.\" SYNTAX(c)
AG_TAILQ_HEAD(HEADNAME, TYPE) head;
AG_TAILQ_HEAD_(TYPE) head;	/* If HEADNAME is not needed */
.Ed
.Pp
where
.Fa HEADNAME
is the name of the structure to be defined, and struct
.Fa TYPE
is the type of the elements to be linked into the tail queue.
A pointer to the head of the tail queue can later be declared as:
.Bd -literal -offset indent
.\" SYNTAX(c)
struct HEADNAME *headp;
.Ed
.Pp
(The names
.Li head
and
.Li headp
are user selectable.)
.Pp
The
.Fn AG_TAILQ_ENTRY
macro declares a structure that connects the elements in
the tail queue.
.Pp
The
.Fn AG_TAILQ_INIT
macro initializes the tail queue referenced by
.Fa head .
.Pp
The tail queue can also be initialized statically by using the
.Fn AG_TAILQ_HEAD_INITIALIZER
macro.
.Pp
The
.Fn AG_TAILQ_INSERT_HEAD
macro inserts the new element
.Fa elm
at the head of the tail queue.
.Pp
The
.Fn AG_TAILQ_INSERT_TAIL
macro inserts the new element
.Fa elm
at the end of the tail queue.
.Pp
The
.Fn AG_TAILQ_INSERT_AFTER
macro inserts the new element
.Fa elm
after the element
.Fa listelm .
.Pp
The
.Fn AG_TAILQ_INSERT_BEFORE
macro inserts the new element
.Fa elm
before the element
.Fa listelm .
.Pp
The
.Fn AG_TAILQ_REMOVE
macro removes the element
.Fa elm
from the tail queue.
.Pp
.Fn AG_TAILQ_FOREACH
and
.Fn AG_TAILQ_FOREACH_REVERSE
are used for traversing a tail queue.
.Fn AG_TAILQ_FOREACH
starts at the first element and proceeds towards the last.
.Fn AG_TAILQ_FOREACH_REVERSE
starts at the last element and proceeds towards the first.
.Bd -literal -offset indent
.\" SYNTAX(c)
AG_TAILQ_FOREACH(np, &head, NAME) { /* ... */ }
AG_TAILQ_FOREACH_REVERSE(np, &head, HEADNAME, NAME) { /* ... */ }
.Ed
.Pp
The
.Fn AG_TAILQ_FIRST ,
.Fn AG_TAILQ_NEXT ,
.Fn AG_TAILQ_LAST
and
.Fn AG_TAILQ_PREV
macros can be used to manually traverse a tail queue or an arbitrary part of
one.
.Pp
The
.Fn AG_TAILQ_EMPTY
macro should be used to check whether a tail queue is empty.
.Sh TAIL QUEUE EXAMPLE
.Bd -literal
.\" SYNTAX(c)
AG_TAILQ_HEAD(tailhead, entry) head;
struct entry {
	/* ... */
	AG_TAILQ_ENTRY(entry) entries;	/* Tail queue. */
	/* ... */
} *n1, *n2, *np;

AG_TAILQ_INIT(&head);			/* Initialize queue. */

n1 = malloc(sizeof(struct entry));	/* Insert at the head. */
AG_TAILQ_INSERT_HEAD(&head, n1, entries);

n1 = malloc(sizeof(struct entry));	/* Insert at the tail. */
AG_TAILQ_INSERT_TAIL(&head, n1, entries);

n2 = malloc(sizeof(struct entry));	/* Insert after. */
AG_TAILQ_INSERT_AFTER(&head, n1, n2, entries);

n2 = malloc(sizeof(struct entry));	/* Insert before. */
AG_TAILQ_INSERT_BEFORE(n1, n2, entries);
					/* Forward traversal. */
AG_TAILQ_FOREACH(np, &head, entries) {
	/* np-> ... */
}
					/* Manual forward traversal. */
for (np = n2; np != NULL; np = AG_TAILQ_NEXT(np, entries)) {
	/* np-> ... */
}
					/* Delete. */
while (np = AG_TAILQ_FIRST(&head))
	AG_TAILQ_REMOVE(&head, np, entries);
.Ed
.Sh CIRCULAR QUEUES
.nr nS 1
.Fn AG_CIRCLEQ_ENTRY "TYPE"
.Pp
.Fn AG_CIRCLEQ_HEAD "HEADNAME" "TYPE"
.Pp
.Fn AG_CIRCLEQ_HEAD_ "TYPE"
.Pp
.Fn AG_CIRCLEQ_HEAD_INITIALIZER "AG_CIRCLEQ_HEAD head"
.Pp
.Ft "struct TYPE *"
.Fn AG_CIRCLEQ_FIRST "AG_CIRCLEQ_HEAD *head"
.Pp
.Ft "struct TYPE *"
.Fn AG_CIRCLEQ_LAST "AG_CIRCLEQ_HEAD *head"
.Pp
.Ft "struct TYPE *"
.Fn AG_CIRCLEQ_END "AG_CIRCLEQ_HEAD *head"
.Pp
.Ft "struct TYPE *"
.Fn AG_CIRCLEQ_NEXT "struct TYPE *listelm" "AG_CIRCLEQ_ENTRY NAME"
.Pp
.Ft "struct TYPE *"
.Fn AG_CIRCLEQ_PREV "struct TYPE *listelm" "AG_CIRCLEQ_ENTRY NAME"
.Pp
.Ft "bool"
.Fn AG_CIRCLEQ_EMPTY "AG_CIRCLEQ_HEAD *head"
.Pp
.Fn AG_CIRCLEQ_FOREACH "VARNAME" "AG_CIRCLEQ_HEAD *head" "AG_CIRCLEQ_ENTRY NAME"
.Pp
.Fn AG_CIRCLEQ_FOREACH_REVERSE "VARNAME" "AG_CIRCLEQ_HEAD *head" "AG_CIRCLEQ_ENTRY NAME"
.Pp
.Ft void
.Fn AG_CIRCLEQ_INIT "AG_CIRCLEQ_HEAD *head"
.Pp
.Ft void
.Fn AG_CIRCLEQ_INSERT_AFTER "AG_CIRCLEQ_HEAD *head" "struct TYPE *listelm" "struct TYPE *elm" "AG_CIRCLEQ_ENTRY NAME"
.Pp
.Ft void
.Fn AG_CIRCLEQ_INSERT_BEFORE "AG_CIRCLEQ_HEAD *head" "struct TYPE *listelm" "struct TYPE *elm" "AG_CIRCLEQ_ENTRY NAME"
.Pp
.Ft void
.Fn AG_CIRCLEQ_INSERT_HEAD "AG_CIRCLEQ_HEAD *head" "struct TYPE *elm" "AG_CIRCLEQ_ENTRY NAME"
.Pp
.Ft void
.Fn AG_CIRCLEQ_INSERT_TAIL "AG_CIRCLEQ_HEAD *head" "struct TYPE *elm" "AG_CIRCLEQ_ENTRY NAME"
.Pp
.Ft void
.Fn AG_CIRCLEQ_REMOVE "AG_CIRCLEQ_HEAD *head" "struct TYPE *elm" "AG_CIRCLEQ_ENTRY NAME"
.Pp
.nr nS 0
A circular queue is headed by a structure defined by the
.Fn AG_CIRCLEQ_HEAD
macro.
This structure contains a pair of pointers,
one to the first element in the circular queue and the other to the
last element in the circular queue.
The elements are doubly linked so that an arbitrary element can be
removed without traversing the queue.
New elements can be added to the queue after an existing element,
before an existing element, at the head of the queue, or at the end
of the queue.
A
.Fa AG_CIRCLEQ_HEAD
structure is declared as follows:
.Bd -literal -offset indent
.\" SYNTAX(c)
AG_CIRCLEQ_HEAD(HEADNAME, TYPE) head;
AG_CIRCLEQ_HEAD_(TYPE) head;	/* If HEADNAME is not needed */
.Ed
.Pp
where
.Fa HEADNAME
is the name of the structure to be defined, and struct
.Fa TYPE
is the type of the elements to be linked into the circular queue.
A pointer to the head of the circular queue can later be declared as:
.Bd -literal -offset indent
.\" SYNTAX(c)
struct HEADNAME *headp;
.Ed
.Pp
(The names
.Li head
and
.Li headp
are user selectable.)
.Pp
The
.Fn AG_CIRCLEQ_ENTRY
macro declares a structure that connects the elements in the circular queue.
.Pp
The
.Fn AG_CIRCLEQ_INIT
macro initializes the circular queue referenced by
.Fa head .
.Pp
The circular queue can also be initialized statically by using the
.Fn AG_CIRCLEQ_HEAD_INITIALIZER
macro.
.Pp
The
.Fn AG_CIRCLEQ_INSERT_HEAD
macro inserts the new element
.Fa elm
at the head of the circular queue.
.Pp
The
.Fn AG_CIRCLEQ_INSERT_TAIL
macro inserts the new element
.Fa elm
at the end of the circular queue.
.Pp
The
.Fn AG_CIRCLEQ_INSERT_AFTER
macro inserts the new element
.Fa elm
after the element
.Fa listelm .
.Pp
The
.Fn AG_CIRCLEQ_INSERT_BEFORE
macro inserts the new element
.Fa elm
before the element
.Fa listelm .
.Pp
The
.Fn AG_CIRCLEQ_REMOVE
macro removes the element
.Fa elm
from the circular queue.
.Pp
The
.Fn AG_CIRCLEQ_FIRST ,
.Fn AG_CIRCLEQ_LAST ,
.Fn AG_CIRCLEQ_END ,
.Fn AG_CIRCLEQ_NEXT
and
.Fn AG_CIRCLEQ_PREV
macros can be used to traverse a circular queue.
The
.Fn AG_CIRCLEQ_FOREACH
is used for circular queue forward traversal:
.Bd -literal -offset indent
.\" SYNTAX(c)
AG_CIRCLEQ_FOREACH(np, head, NAME) { /* ... */ }
.Ed
.Pp
The
.Fn AG_CIRCLEQ_FOREACH_REVERSE
macro acts like
.Fn AG_CIRCLEQ_FOREACH
but traverses the circular queue backwards.
.Pp
The
.Fn AG_CIRCLEQ_EMPTY
macro should be used to check whether a circular queue is empty.
.Sh CIRCULAR QUEUE EXAMPLE
.Bd -literal
.\" SYNTAX(c)
AG_CIRCLEQ_HEAD(circleq, entry) head;
struct entry {
	/* ... */
	AG_CIRCLEQ_ENTRY(entry) entries;	/* Circular queue. */
	/* ... */
} *n1, *n2, *np;

AG_CIRCLEQ_INIT(&head);			/* Initialize circular queue. */

n1 = malloc(sizeof(struct entry));	/* Insert at the head. */
AG_CIRCLEQ_INSERT_HEAD(&head, n1, entries);

n1 = malloc(sizeof(struct entry));	/* Insert at the tail. */
AG_CIRCLEQ_INSERT_TAIL(&head, n1, entries);

n2 = malloc(sizeof(struct entry));	/* Insert after. */
AG_CIRCLEQ_INSERT_AFTER(&head, n1, n2, entries);

n2 = malloc(sizeof(struct entry));	/* Insert before. */
AG_CIRCLEQ_INSERT_BEFORE(&head, n1, n2, entries);
					/* Forward traversal. */
AG_CIRCLEQ_FOREACH(np, &head, entries) {
	/* np-> ... */
}
					/* Reverse traversal. */
AG_CIRCLEQ_FOREACH_REVERSE(np, &head, entries) {
	/* np-> ... */
}
					/* Delete. */
while (!AG_CIRCLEQ_EMPTY(&head))
	AG_CIRCLEQ_REMOVE(&head, AG_CIRCLEQ_FIRST(&head), entries);
.Ed
.Sh NOTES
It is an error to assume the next and previous fields are preserved
after an element has been removed from a list or queue.
Using any macro (except the various forms of insertion) on an element
removed from a list or queue is incorrect.
An example of erroneous usage is removing the same element twice.
.Pp
The
.Fn AG_SLIST_END ,
.Fn AG_LIST_END ,
.Fn AG_SIMPLEQ_END
and
.Fn AG_TAILQ_END
macros are provided for symmetry with
.Fn AG_CIRCLEQ_END .
They expand to
.Dv NULL
and don't serve any useful purpose.
.Pp
Trying to free a list in the following way is a common error:
.Bd -literal -offset indent
.\" SYNTAX(c)
AG_LIST_FOREACH(var, head, entry) {
	free(var);
}
free(head);
.Ed
.Pp
Since
.Va var
is free'd, the
.Fn FOREACH
macro refers to a pointer that may have been reallocated already.
Proper code needs a second variable.
.Bd -literal -offset indent
.\" SYNTAX(c)
for (var = AG_LIST_FIRST(head);
     var != AG_LIST_END(head);
     var = nxt) {
	nxt = AG_LIST_NEXT(var, entry);
	free(var);
}
AG_LIST_INIT(head);	/* to put the list back in order */
.Ed
.Pp
A similar situation occurs when the current element is deleted
from the list.
Correct code saves a pointer to the next element in the list before
removing the element:
.Bd -literal -offset indent
.\" SYNTAX(c)
for (var = AG_LIST_FIRST(head);
     var != AG_LIST_END(head);
     var = nxt) {
	nxt = AG_LIST_NEXT(var, entry);
	if (some_condition) {
		AG_LIST_REMOVE(var, entry);
		some_function(var);
	}
}
.Ed
.Sh HISTORY
The
.Nm
macros first appeared in Agar 1.0 and are based on the
.Bx 4.4
queue macros in
.Pa sys/queue.h .