.\" $Id: heap.mdoc,v 1.1.2.1.10.1 2004/03/09 08:33:43 marka Exp $
.\"
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.\" Copyright (c) 1997,1999 by Internet Software Consortium.
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.Dd January 1, 1997
.\"Os OPERATING_SYSTEM [version/release]
.Os BSD 4
.Dt HEAP @SYSCALL_EXT@
.Sh NAME
.Nm heap_new ,
.Nm heap_free ,
.Nm heap_insert ,
.Nm heap_delete ,
.Nm heap_increased ,
.Nm heap_decreased ,
.Nm heap_element ,
.Nm heap_for_each 
.Nd heap implementation of priority queues
.Sh SYNOPSIS
.Fd #include \&"heap.h\&"
.Ft heap_context    
.Fn heap_new "heap_higher_priority_func higher_priority" \
"heap_index_func index" "int array_size_increment"
.Ft int
.Fn heap_free "heap_context ctx"
.Ft int
.Fn heap_insert "heap_context ctx" "void *elt"
.Ft int
.Fn heap_delete "heap_context ctx" "int i"
.Ft int
.Fn heap_increased "heap_context ctx" "int i"
.Ft int
.Fn heap_decreased "heap_context ctx" "int i"
.Ft void *
.Fn heap_element "heap_context ctx" "int i"
.Ft int 
.Fn heap_for_each "heap_context ctx" "heap_for_each_func action" "void *uap"
.Sh DESCRIPTION
These functions implement heap\-based priority queues.  The user defines a
priority scheme, and provides a function for comparison of the priority
of heap elements
(see the description of the
.Ft heap_higher_priority_func
function pointer, below).
.Pp
Each of the functions depends upon the
.Ft heap_context
type, which is a pointer to a
.Ft struct heap_context 
.Pq see Pa heap.h No for more information .
.Pp
The
.Pa heap.h
header file also defines the following set of function
function pointers:
.Bd -literal -offset indent
typedef int (*heap_higher_priority_func)(void *, void *);
typedef void (*heap_index_func)(void *, int);
typedef void (*heap_for_each_func)(void *, void *);
.Ed
.Pp
These are pointers to user-defined functions.
The 
.Ft heap_higher_priority_func
type is a pointer to a function which compares two
different heap (queue) elements and returns an
.Ft int
which answers the question, "Does the first queue element 
have a higher priority than the second?"  In other words, 
a function pointer of this type 
.Em must 
return a number greater than zero
if the element indicated by the first argument is of a higher priority than 
that indicated by the second element, and zero otherwise.  
.Pp
The other two function pointers are documented in the descriptions
of 
.Fn heap_new
.Pq Va heap_index_func
and
.Fn heap_for_each
.Pq Va heap_for_each_func ,
below.
.Pp
The function
.Fn heap_new
initializes a 
.Ft struct heap_context
and returns a pointer to it.  The
.Fa higher_priority
function pointer 
.Em must 
be 
.No non\- Ns Dv NULL .
As explained above, this refers to a 
function supplied by the user which compares the priority of two different
queue or heap elements; see above for more information. 
The second argument, 
.Fa index ,
is a pointer to a user-defined function whose arguments are
a heap element and its index in the heap.
.Fa Index 
is intended to provide the user a means of knowing the internal index
of an element in the heap while maintaining the opacity of the implementation;
since the user has to know the actual indexes of heap elements in order to use,
e.g., 
.Fn heap_delete
or
.Fn heap_element ,
the user 
.Fa index
function could store the index in the heap element, itself.  If 
.Fa index
is 
.No non\- Ns Dv NULL ,
then it is called 
.Em whenever 
the index of an element changes, allowing the user to stay up\-to\-date
with index changes.
The last argument, 
.Fa array_size_increment
will be used, as its name suggests, by
.Xr malloc 3
or
.Xr realloc 3
to increment the array which implements the heap; if zero, a default value 
will be used.
.Pp
The
.Fn heap_free
function frees the given
.Ft heap_context
argument 
.Pq Fa ctx ,
which also frees the entire
.Nm heap ,
if it is
.No non\- Ns Dv NULL .
The argument
.Fa ctx
should be
.No non\- Ns Dv NULL .
.Pp
The 
.Fn heap_insert
function is used to insert the new heap element
.Fa elt
into the appropriate place (priority\-wise) in the
.Ft heap
indicated by 
.Fa ctx
(a pointer to a
.Ft heap_context ) .
If 
.No non\- Ns Dv NULL ,
the user-defined
.Ft higher_priority
function pointer associated with the indicated 
.Nm heap
is used to determine that
.Dq appropriate place ;
the highest\-priority elements are at the front of the queue (top of
the heap).
(See the description of 
.Fn heap_new , 
above, for more information.)
.Pp
The function
.Fn heap_delete
is used to delete the 
.Fa i\- Ns th
element of the queue (heap), and fixing up the queue (heap) from that
element onward via the priority as determined by the user function
pointed to by
.Ft higher_priority 
function pointer
(see description of
.Fn heap_new ,
above).
.Pp
.Fn heap_increased
.Pp
.Fn heap_decreased
.Pp
The 
.Fn heap_element
function returns the
.Fa i\- Ns th
element of the queue/heap indicated by
.Fa ctx ,
if possible.
.Pp
The
.Fn heap_for_each
function provides a mechanism for the user to increment through the entire
queue (heap) and perform some
.Fa action 
upon each of the queue elements.  This
.Fa action 
is pointer to a user\-defined function with two arguments, the first of
which should be interpreted by the user's function as a heap element.  The 
second value passed to the user function is just the
.Fa uap
argument to 
.Fn heap_for_each ;
this allows the user to specify additional arguments, if necessary, to
the function pointed to by 
.Fa action .
.\" The following requests should be uncommented and
.\" used where appropriate.  This next request is
.\" for sections 2 and 3 function return values only.
.Sh RETURN VALUES
.Bl -tag -width "heap_decreased()"
.It Fn heap_new
.Dv NULL
if unable to 
.Xr malloc 3
a 
.Ft struct heap_context
or if the
.Fa higher_priority
function pointer is 
.Dv NULL ;
otherwise, a valid
.Ft heap_context 
.Ns .
.It Fn heap_free
-1 if 
.Fa ctx
is 
.Dv NULL 
(with 
.Va errno
set to
.Dv EINVAL ) ;
otherwise, 0.
.It Fn heap_insert
-1 
if either
.Fa ctx
or 
.Fa elt
is 
.Dv NULL ,
or if an attempt to 
.Xr malloc 3
or 
.Xr realloc 3
the heap array fails (with
.Va errno
set to 
.Dv EINVAL
or 
.Dv ENOMEM ,
respectively).
Otherwise, 0.
.It Fn heap_delete
-1 if 
.Fa ctx
is 
.Dv NULL
or 
.Fa i 
is out\-of\-range (with
.Va errno
set to
.Dv EINVAL ) ;
0 otherwise.
.It Fn heap_increased
As for
.Fn heap_delete .
.It Fn heap_decreased
As for
.Fn heap_delete .
.It Fn heap_element
NULL if 
.Fa ctx 
is 
.Dv NULL
or
.Fa i
out\-of-bounds (with
.Va errno
set to
.Dv EINVAL ) ;
otherwise, a pointer to the
.Fa i\- Ns th
queue element.
.It Fn heap_for_each
-1 if either
.Fa ctx
or 
.Fa action
is 
.Dv NULL
(with 
.Va errno
set to
.Dv EINVAL ) ;
0 otherwise.
.El
.\" This next request is for sections 1, 6, 7 & 8 only
.\" .Sh ENVIRONMENT
.Sh FILES
.Bl -tag -width "heap.h000"
.It Pa heap.h
 heap library header file
.El
.\" .Sh EXAMPLES
.\" This next request is for sections 1, 6, 7 & 8 only
.\"     (command return values (to shell) and
.\"    fprintf/stderr type diagnostics)
.Sh DIAGNOSTICS
Please refer to
.Sx RETURN VALUES .
.\" The next request is for sections 2 and 3 error
.\" and signal handling only.
.Sh ERRORS
The variable
.Va errno
is set by
.Fn heap_free , 
.Fn heap_insert , 
.Fn heap_delete , 
.Fn heap_increased , 
and
.Fn heap_decreased 
under the conditions of invalid input
.Pq Dv EINVAL
or lack of memory
.Pq Dv ENOMEM ;
please refer to
.Sx RETURN VALUES .
.Sh SEE ALSO
.Xr malloc 3 ,
.Xr realloc 3 .
.Rs
.%A Cormen
.%A Leiserson
.%A Rivest
.%B Introduction to Algorithms
.%Q "MIT Press / McGraw Hill"
.%D 1990
.%O ISBN 0\-262\-03141\-8
.%P chapter 7
.Re
.Rs
.%A Sedgewick
.%B Algorithms, 2nd ed'n
.%Q Addison\-Wesley
.%D 1988
.%O ISBN 0\-201\-06673\-4
.%P chapter 11
.Re
.\" .Sh STANDARDS
.\" .Sh HISTORY
.Sh AUTHORS
The
.Nm heap
library was implemented by Bob Halley (halley@vix.com) of Vixie Enterprises, 
Inc., for the Internet Software consortium, and was adapted from
the two books listed in the 
.Sx SEE ALSO
section, above.
.\" .Sh BUGS