.\" $NetBSD: thmap.9,v 1.3 2020/08/28 07:03:41 riastradh Exp $
.\"
.\" Copyright (c) 2018 Mindaugas Rasiukevicius <rmind at noxt eu>
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.Dd December 11, 2018
.Dt THMAP 9
.Os
.Sh NAME
.Nm thmap
.Nd concurrent trie-hash map
.Sh SYNOPSIS
.In thmap.h
.\" -----
.Ft thmap_t *
.Fn thmap_create "uintptr_t baseptr" "const thmap_ops_t *ops" "unsigned flags"
.Ft void
.Fn thmap_destroy "thmap_t *thmap"
.Ft void *
.Fn thmap_get "thmap_t *thmap" "const void *key" "size_t len"
.Ft void *
.Fn thmap_put "thmap_t *thmap" "const void *key" "size_t len" "void *val"
.Ft void *
.Fn thmap_del "thmap_t *thmap" "const void *key" "size_t len"
.Ft void *
.Fn thmap_stage_gc "thmap_t *thmap"
.Ft void
.Fn thmap_gc "thmap_t *thmap" "void *ref"
.Ft void
.Fn thmap_setroot "thmap_t *thmap" "uintptr_t root_offset"
.Ft uintptr_t
.Fn thmap_getroot "const thmap_t *thmap"
.\" -----
.Sh DESCRIPTION
Concurrent trie-hash map \(em a general purpose associative array,
combining the elements of hashing and radix trie.
Highlights:
.Pp
.Bl -hyphen -compact
.It
Very competitive performance, with logarithmic time complexity on average.
.It
Lookups are lock-free and inserts/deletes are using fine-grained locking.
.It
Incremental growth of the data structure (no large resizing/rehashing).
.It
Optional support for use with shared memory, e.g. memory-mapped file.
.El
.Pp
Delete operations (the key/data destruction) must be synchronized with
the readers using some reclamation mechanism.
.\" -----
.Sh FUNCTIONS
.Bl -tag -width abcd
.It Fn thmap_create baseptr ops flags
Construct a new trie-hash map.
The optional
.Fa ops
parameter can
used to set the custom allocate/free operations (see the description of
.Vt thmap_ops_t
below).
In such case, the
.Fa baseptr
is the base (start) address of the address space mapping (it must be
word-aligned).
If
.Fa ops
is set to
.Dv NULL ,
then
.Xr malloc 3
and
.Xr free 3
will be used as the default operations and
.Fa baseptr
should be set to zero.
Currently, the supported
.Fa flags
are:
.Bl -tag -width THMAP_SETROOT
.It Dv THMAP_NOCOPY
The keys on insert will not be copied and the given pointers to them will
be expected to be valid and the values constant until the key is deleted;
by default, the put operation will make a copy of the key.
.It Dv THMAP_SETROOT
Indicate that the root of the map will be manually set using the
.Fn thmap_setroot
routine;
by default, the map is initialized and the root node is set on
.Fn thmap_create .
.El
.\" ---
.It Fn thmap_destroy thmap
Destroy the map, freeing the memory it uses.
.\" ---
.It Fn thmap_get thmap key len
Lookup the key (of a given length) and return the value associated with it.
Return
.Dv NULL
if the key is not found (see the
.Sx CAVEATS
section).
.\" ---
.It Fn thmap_put thmap key len val
Insert the key with an arbitrary value.
If the key is already present, return the already existing associated value
without changing it.
Otherwise, on a successful insert, return the given value.
Just compare the result against
.Fa val
to test whether the insert was successful.
.\" ---
.It Fn thmap_del thmap key len
Remove the given key.
If the key was present, return the associated value;
otherwise return
.Dv NULL .
The memory associated with the entry is not released immediately, because
in the concurrent environment (e.g., multi-threaded application) the caller
may need to ensure it is safe to do so.
It is managed using the
.Fn thmap_stage_gc
and
.Fn thmap_gc
routines.
.\" ---
.It Fn thmap_stage_gc thmap
Stage the currently pending entries (the memory not yet released after
the deletion) for reclamation (G/C).
This operation should be called
.Em before
the synchronization barrier.
.Pp
Returns a reference which must be passed to
.Fn thmap_gc .
Not calling the G/C function for the returned reference would result in
a memory leak.
.\" ---
.It Fn thmap_gc thmap ref
Reclaim (G/C) the staged entries i.e. release any memory associated
with the deleted keys.
The reference must be the value returned by the call to
.Fn thmap_stage_gc .
.Pp
This function must be called
.Em after
the synchronization barrier which guarantees that there are no active
readers referencing the staged entries.
.\" ---
.El
.Pp
If the map is created using the
.Fa THMAP_SETROOT
flag, then the following functions are applicable:
.\" ---
.Bl -tag -width abcd
.It Fn thmap_setroot thmap root_offset
Set the root node.
The address must be relative to the base address, as if allocated by the
.Fn thmap_ops_t::alloc
routine.
Return 0 on success and \-1 on failure (if already set).
.It Fn thmap_getroot thmap
Get the root node address.
The returned address will be relative to the base address.
.El
.\" ---
.Pp
Members of
.Vt thmap_ops_t
are
.Bd -literal
        uintptr_t (*alloc)(size_t len);
        void      (*free)(uintptr_t addr, size_t len);
.Ed
.\" -----
.Sh EXAMPLES
Simple case backed by
.Xr malloc 3 ,
which could be used in multi-threaded environment:
.Bd -literal
	thmap_t *kvmap;
	struct obj *obj;

	kvmap = thmap_create(0, NULL);
	assert(kvmap != NULL);
	...
	obj = obj_create();
	thmap_put(kvmap, "test", sizeof("test") - 1, obj);
	...
	obj = thmap_get(kvmap, "test", sizeof("test") - 1);
	...
	thmap_destroy(kvmap);
.Ed
.\" -----
.Sh AUTHORS
.An Mindaugas Rasiukevicius Aq Mt rmind@noxt.eu
.Sh CAVEATS
The implementation uses pointer tagging and atomic operations.
This requires the base address and the allocations to provide at least word
alignment.
.Pp
While the
.Dv NULL
values may be inserted,
.Fn thmap_get
and
.Fn thmap_del
cannot indicate whether the key was not found or a key with a
.Dv NULL
value was found.
If the caller needs to indicate an "empty" value, it can use a
special pointer value, such as
.Li (void *)(uintptr_t)0x1 .
.\" -----