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.\" Author: Archie Cobbs <archie@freebsd.org>
.\"
.\" $Id: ghash.3,v 1.1.1.1 2012/02/21 23:25:53 misho Exp $
.\"
.Dd April 22, 2002
.Dt GHASH 3
.Os
.Sh NAME
.Nm ghash
.Nd generic hash table library
.Sh LIBRARY
PDEL Library (libpdel, \-lpdel)
.Sh SYNOPSIS
.In sys/types.h
.In pdel/util/ghash.h
.Ft "struct ghash *"
.Fn ghash_create "void *arg" "u_int isize" "u_int maxload" "const char *mtype" "ghash_hash_t *hash" "ghash_equal_t *equal" "ghash_add_t *add" "ghash_del_t *del"
.Ft void
.Fn ghash_destroy "struct ghash **gp"
.Ft void
.Fn ghash_arg "struct ghash *g"
.Ft "void *"
.Fn ghash_get "struct ghash *g" "const void *item"
.Ft int
.Fn ghash_put "struct ghash *g" "const void *item"
.Ft int
.Fn ghash_remove "struct ghash *g" "const void *item"
.Ft u_int
.Fn ghash_size "struct ghash *g"
.Ft int
.Fn ghash_dump "struct ghash *g" "void ***listp" "const char *mtype"
.Ft void
.Fn ghash_walk_init "struct ghash *g" "struct ghash_walk *walk"
.Ft "void *"
.Fn ghash_walk_next "struct ghash *g" "struct ghash_walk *walk"
.Ft "struct ghash_iter *"
.Fn ghash_iter_create "struct ghash *g"
.Ft void
.Fn ghash_iter_destroy "struct ghash_iter **iterp"
.Ft int
.Fn ghash_iter_has_next "struct ghash_iter *iter"
.Ft "void *"
.Fn ghash_iter_next "struct ghash_iter *iter"
.Ft int
.Fn ghash_iter_remove "struct ghash_iter *iter"
.Sh DESCRIPTION
The
.Nm ghash
functions implement a generic hash table data structure.
The hash table stores
.Em items
of type
.Ft "void *" .
The user code supplies callback routines for:
.Bl -bullet -offset 3n
.It
Computing the hash value of an item
.It
Comparing two items for equality
.It
Housekeeping associated with adding and removing an item
.El
.Pp
.Fn ghash_create
creates a new hash table.
The
.Fa arg
parameter can be retrieved at any time via
.Fn ghash_arg
and is otherwise ignored.
The initial size of the hash table (in buckets) is determined by
.Fa isize ,
or defaults to 31 if
.Fa isize
is zero.
.Fa mtype
is a typed memory type string (see
.Xr typed_mem 3 )
used when allocating memory for the hash table.
.Pp
.Fa maxload
is a maximum load value, measured in percent.
If the ratio of the number of items in the hash table to the number
of buckets grows larger than this value, the number of buckets is
increased.
For example, if
.Fa maxload
is 200, then on average there will never be more than 2 items per bucket.
If
.Fa maxload
is given as zero, the default value of 75% is used.
.Pp
The
.Fa hash ,
.Fa equal ,
.Fa add ,
and
.Fa del
parameters are pointers to user-supplied callback functions having
the following types:
.Bd -literal -offset 3n
typedef int       ghash_equal_t(struct ghash *g,
                      const void *item1, const void *item2);
typedef u_int32_t ghash_hash_t(struct ghash *g, const void *item);
typedef void      ghash_add_t(struct ghash *g, void *item);
typedef void      ghash_del_t(struct ghash *g, void *item);

.Ed
The
.Fa equal
function should return 1 if the items are equal, otherwise 0.
The
.Fa hash
function should return the item's 32-bit hash value.
Note that
.Fa equal
and
.Fa hash
must be consistent, i.e., two items that are equal must
have the same hash value.
If
.Fa equal
and
.Fa hash
are
.Dv NULL ,
the item pointers themselves are compared and hashed;
in effect, the hash table behaves like a set of pointers.
.Pp
The
.Fa add
and
.Fa del
routines, if not
.Dv NULL ,
will be called whenever an item is added to, or removed from, the hash table.
For example, if
.Fn ghash_put
causes a new item to replace an old item, there will be a call to the
.Fa del
function for the old item, followed by a call to the
.Fa add
function for the new item.
These callbacks are typically used to increase and decrease reference counts.
.Pp
.Fn ghash_destroy
destroys a hash table and free's all associated memory.
If any items remain in the hash table, the
.Fa del
callback will be invoked once for each item.
Note that this function takes a pointer to a pointer to a hash table.
Upon return, the pointer to the hash table will be set to
.Dv NULL .
If it is already equal to
.Dv NULL ,
.Fn ghash_destroy
does nothing.
.Pp
.Fn ghash_get
retrieves an item previously stored in the hash table, or else returns
.Dv NULL
if the item is not found.
.Pp
.Fn ghash_put
adds an item to the hash table, replacing any item that is equal to it
(as determined by the
.Fa equal
callback).
.Dv NULL
is an invalid item and may not be stored in a hash table.
.Pp
.Fn ghash_remove
removes an item from the hash table.
If the item does not exist, nothing happens.
.Pp
.Fn ghash_size
returns the number of items in the hash table.
.Pp
.Fn ghash_dump
generates an array of all the items in the hash table.
A pointer to the array is stored in
.Fa "*listp" .
The array is allocated with memory type
.Fa mtype .
The caller must eventually free the array, also using
.Fa mtype .
.Pp
.Fn ghash_walk_init
and
.Fn ghash_walk_next
are used to traverse all items in the hash table consecutively.
First,
.Fn ghash_walk_init
is called with a pointer to the caller-supplied
.Dv "struct ghash_walk" .
Then, each invocation of
.Fn ghash_walk_next
returns the next item in the hash table, or
.Dv NULL
if no more items remain.
.Pp
Another way to traverse all hash table elements is using a
.Dv "struct ghash_iter" ,
which acts as an iterator object.
.Fn ghash_iter_create
returns such a structure.
.Fn ghash_iter_has_next
returns non-zero if there are items remaining.
Each invocation of
.Fn ghash_iter_next
returns the next item.
.Fn ghash_iter_remove
removes item the most recently returned by
.Fn ghash_iter_next .
.Fn ghash_iter_destroy
destroys the iterator.
Note: all associated iterators must be destroyed before calling
.Fn ghash_destroy .
.Sh RETURN VALUES
.Fn ghash_put
returns 0 if the item is new, or 1 if the item replaced an existing item.
.Fn ghash_remove
returns 0 if the item was not found, or 1 if the item was found and removed.
.Fn ghash_dump
returns the number of items in the generated array.
.Fn ghash_create ,
.Fn ghash_put ,
.Fn ghash_dump ,
and
.Fn ghash_iter_create
return -1 or
.Dv NULL
to indicate an error;
.Va errno
will be set to the appropriate value.
.Sh IMPLEMENTATION NOTES
The
.Nm ghash
library is designed to gracefully handle buggy code.
For example, a reentrant call to
.Fn ghash_put
from within the
.Fa add
callback function called as a result of a previous call to
.Fn ghash_put
will return an error with
.Va errno
set to
.Er EBUSY .
Similarly, if the hash table is modified in the middle of a traversal,
.Fn ghash_walk_next
or
.Fn ghash_iter_next
will return an error.
.Sh SEE ALSO
.Xr gtree 3 ,
.Xr libpdel 3 ,
.Xr typed_mem 3
.Sh HISTORY
The PDEL library was developed at Packet Design, LLC.
.Dv "http://www.packetdesign.com/"
.Sh AUTHORS
.An Archie Cobbs Aq archie@freebsd.org