=pod

=head1 NAME

ossl_ht_new, ossl_ht_free,
ossl_ht_read_lock, ossl_ht_read_unlock,
ossl_ht_write_lock, ossl_ht_write_unlock,
ossl_ht_flush, ossl_ht_insert,
ossl_ht_delete, ossl_ht_count,
ossl_ht_foreach_until, ossl_ht_filter,
ossl_ht_value_list_free, ossl_ht_get,
ossl_ht_put, HT_START_KEY_DEFN,
HT_END_KEY_DEFN, HT_DEF_KEY_FIELD_CHAR_ARRAY,
HT_DEF_KEY_FIELD_UINT8T_ARRAY, HT_DEF_KEY_FIELD,
HT_INIT_KEY, HT_KEY_RESET, HT_SET_KEY_FIELD,
HT_SET_KEY_STRING, HT_SET_KEY_BLOB,
TO_HT_KEY, FROM_HT_KEY,
IMPLEMENT_HT_VALUE_TYPE_FNS
- internal rcu locked hashtables

=head1 SYNOPSIS

 HT *ossl_ht_new(const HT_CONFIG *conf);
 void ossl_ht_free(HT *htable);
 void ossl_ht_read_lock(HT *htable);
 void ossl_ht_read_unlock(HT *htable);
 void ossl_ht_write_lock(HT *htable);
 void ossl_ht_write_unlock(HT *htable);
 int  ossl_ht_flush(HT *htable);
 int  ossl_ht_insert(HT *htable, HT_KEY *key, HT_VALUE *data, HT_VALUE **olddata);
 int ossl_ht_delete(HT *htable, HT_KEY *key);
 size_t ossl_ht_count(HT *htable);
 void ossl_ht_foreach_until(HT *htable, int (*cb)(HT_VALUE *obj, void *arg), void *arg);
 HT_VALUE_LIST *ossl_ht_filter(HT *htable, size_t max_len, int (*filter)(HT_VALUE *obj));
 void ossl_ht_value_list_free(HT_VALUE_LIST *list);
 HT_VALUE *ossl_ht_get(HT *htable, HT_KEY *key);
 void ossl_ht_put(HT_VALUE *value);
 HT_START_KEY_DEFN(keyname);
 HT_END_KEY_DEFN(keyname);
 HT_DEF_KEY_FIELD(name, type);
 HT_DEF_KEY_FIELD_CHAR_ARRAY(name, size);
 HT_DEF_KEY_FIELD_UINT8T_ARRAY(name, size);
 HT_INIT_KEY(key);
 HT_KEY_RESET(key);
 HT_SET_KEY_FIELD(key, member, value);
 HT_SET_KEY_STRING(key, member, value);
 HT_SET_KEY_BLOB(key, member, value, len);
 TO_HT_KEY(key);
 FROM_HT_KEY(key, type);
 IMPLEMENT_HT_VALUE_TYPE_FNS(vtype, name, pfx);

=head1 DESCRIPTION

This API provides a library-internal implementation of a hashtable that provides
reference counted object retrieval under the protection of an rcu lock.  API
type safety is offered via conversion macros to and from the generic B<HT_VALUE>
type.

=over 2

=item *

ossl_ht_new() returns a new B<HT> (hashtable object) used to store data
elements based on a defined key.  The call accepts an HT_CONFIG pointer which
contains configurations options for hashtable.  Current config options consist
of:
    I<ht_free_fn> The function to call to free a value, may be NULL.
    I<ht_hash_fn> The function to generate a hash value for a key, may be NULL.
    I<init_neighborhoods> The initial number of neighborhoods in the hash table.

Note that init_bucket_len may be set to zero, which will use the default initial
bucket size, which will be automatically expanded with the hash table load
average reaches 0.75.

Note that lockless_read operation implies behavioral restrictions. Specifically
only element additions are allowed, deletion operations will fail
    Hash table growth is inhibited.  init_bucket_len should be set to an
    appropriate value to prevent performance degradation
    The table owner is responsible for ensuring there are no readers during a
    freeing of the table.

Note that lockless_write operations are done at your own risk.  Lockless
    operation in a multithreaded environment will cause data corruption.  It
    is the callers responsibility in this mode of operation to provide thread
    synchronization.

=item *

ossl_ht_free() frees an allocated hash table.  Each element in the table
will have its reference count dropped, and, if said count reaches zero, the hash
tables registered free function will be called to release the element data.

=item *

ossl_ht_read_lock(), ossl_ht_read_unlock(), ossl_ht_write_lock() and
ossl_ht_write_unlock() lock the table for reading and writing/modification.
These function are not required for use in the event a table is to be used in a
lockless fashion, but if they are not used, it is the responsibility of the caller
to ensure thread synchronization.  Note that an rcu lock is used internally for these
operations, so for table modifying actions (ossl_ht_flush() and ossl_ht_delete()
the write lock must be taken and released to ensure rcu synchronization takes
place.

=item *

ossl_ht_flush() empties a hash table.  All elements will have their
reference counts decremented, and, on reaching zero, the free function will be
called to release the element data.

=item *

ossl_ht_insert() inserts an B<HT_VALUE> element into the hash table, to be
hashed using the corresponding B<HT_KEY> value.

=item *

ossl_ht_delete() deletes an entry from the hashtable indexed by the passed
B<HT_KEY> value.

=item *

ossl_ht_count() returns the number of elements within the hash table.

=item *

ossl_ht_foreach_until() iterates over all elements in the hash table, calling
the passed callback function for each. The return value of the callback
indicates if the iteration should continue or not.  Returning 1 indicates
iteration should continue, while returning 0 indicates that iteration should
terminate.

Note that the iteration is done under read lock protection, and as such
modifications to the table are disallowed in the callback function.
Modification to the value content are permitted, if the caller is able to
properly synchronize such modifications with other threads.

=item *

ossl_ht_filter() iterates over all elements of the hash table, calling
the filter callback for each element.  If the callback returns 1, the
corresponding B<HT_VALUE> is placed on a list, and its reference count incremented.
The completed list is returned to the caller as an B<HT_VALUE_LIST> object

=item *

ossl_ht_value_list_free() frees an B<HT_VALUE_LIST>.  For each element on
the list, its reference count is decremented, and after traversing the list, the
list object is freed.  Note, NULL elements are allowed on the list, but for any
element which is taken from the list by a caller, they must call
ossl_ht_put() on the B<HT_VALUE> to prevent memory leaks.

=item *

ossl_ht_get() performs a lookup of an B<HT_KEY> in the hashtable, returning
its corresponding value.

=item *

HT_START_KEY_DEFN() Begins the definition of a key type.  the keyname parameter
defines the structure name, and presets a common key header.

=item *

HT_END_KEY_DEFN() Finalizes a key definition.  the keyname parameter (which may
differ from the name passed in HT_START_KEY_DEFN(), defines the key type name.
The resulting type may be converted to an HT_KEY variable via the HT_TO_KEY()
macro, and back using the HT_FROM_KEY() macro.

=item *

HT_DEF_KEY_FIELD() Allows for the creation of data fields within a key. Note,
this macro can be used for any data type, but it is recommended that strings and
binary arrays be created with the HT_DEF_KEY_FIELD_CHAR_ARRAY() and
HT_DEF_KEY_FIELD_UINT8T_ARRAY() macros to ensure proper in-lining of key data.

=item *

HT_DEF_KEY_FIELD_CHAR_ARRAY() Creates a string field of fixed size
within a key definition. Note these items will be NULL terminated.

=item *

HT_DEF_KEY_FIELD_UINT8T_ARRAY() Creates an array of uint8_t elements within a
key.

=item *

HT_INIT_KEY() Initializes a key for use.  Can be called multiple times, but must
be called at least once before using in any hashtable method.

=item *

HT_KEY_RESET() Resets a key's data to all zeros.

=item *

HT_SET_KEY_FIELD() Sets a field in a key (as defined by HT_DEF_KEY_FIELD()) to a
given value.

=item *

HT_SET_KEY_STRING() Performs a strncpy() of a source string to the destination
key field.

=item *

HT_SET_KEY_BLOB() Performs a memcpy() of a source uint8_t buffer to a
destination key field.

=item *

TO_HT_KEY() Converts a key type as defined by HT_START_KEY_DEFN() and
HE_END_KEY_DEFN() to the generic HT_KEY type

=item *

FROM_HT_KEY() Converts an HT_KEY back to a specific key type as defined by
HT_START_KEY_DEFN() and HT_END_KEY_DEFN()

=item *

IMPLEMENT_HT_VALUE_TYPE_FNS() creates template conversion functions for
manipulating the hashtable using specific data types.  This macro accepts two
parameters, a NAME, which is used to prefix the hashtable function so that it
may be associated with a specific hash table, and TYPE which defines the type of
data the instantiated function accepts.  The list of functions instantiated by
this macro are below.

=over 2

=item *

int ossl_ht_NAME_TYPE_insert(HT* h, HT_KEY *key, <type> *value, HT_VALUE **olddata)
Inserts a value to the hash table of type TYPE into the hash table using the
provided key.  If olddata is not NULL, and a matching key already exists in the
table, the operation is a replacement, and the old data is returned in this
pointer

=item *

<TYPE> ossl_ht_NAME_TYPE_get(HT *h, HT_KEY *key, HT_VALUE **v)
Looks up an item in the hash table based on key, and returns the data it found,
if any.  v holds a pointer to the B<HT_VALUE> associated with the data.

=item *

<TYPE> *ossl_ht_NAME_TYPE_from_value(HT_VALUE *v)
Validates that the B<HT_VALUE> provided matches the TYPE specified, and returns the
value data.  If there is a type mismatch, NULL is returned

=item *

HT_VALUE *ossl_ht_NAME_TYPE_to_value(<TYPE> *data)
Converts the data pointer provided to an B<HT_VALUE> object

=item *

int ossl_ht_NAME_TYPE_type(HT_VALUE *v)
Returns true if the B<HT_VALUE> object passed in is of type <TYPE>

=back

=back

=head1 RETURN VALUES

ossl_ht_new() returns an B<HT*> struct on success and NULL on error

void ossl_ht_free(HT *htable);

ossl_ht_flush() and ossl_ht_insert() return 1 on success and 0 on error

ossl_ht_delete() returns 1 if the key was successfully deleted, and 0 if the
key was not found. 

ossl_ht_count() returns the number of elements in the hash table

ossl_ht_filter() returns an B<HT_VALUE_LIST> of all elements matching the
provided filter

ossl_ht_get() returns an B<HT_VALUE> pointer, or NULL if the element was not
found.

ossl_ht_insert() returns 1 if an element was inserted, 0 if the element is
already present, -1 on fatal errors (memory allocation or growth not allowed).

=head1 EXAMPLES

#include <stdio.h>
#include <string.h>

#include <openssl/err.h>
#include <openssl/crypto.h>
#include <internal/hashtable.h>

HT_START_KEY_DEFN(intkey)
HT_DEF_KEY_FIELD(myintkey, int)
HT_END_KEY_DEFN(INTKEY)

IMPLEMENT_HT_VALUE_TYPE_FNS(int, test, static)

static void int_free_fn(HT_VALUE *v)
{
    int *i = ossl_crypto_test_int_from_value(v);

    fprintf(stderr, "Freeing an element\n");
    OPENSSL_free(i);
}

static int test_int_hashtable(void)
{
    /*
     * our config says:
     * int_free_fn - Our free handler
     * NULL - Use default hash fn
     * 0 - use default initial bucket size
     */
    HT_CONFIG hash_conf = {
        int_free_fn,
        NULL,
        0
    };
    INTKEY key;
    HT *ht = NULL;
    HT_VALUE *v;
    int rc;
    int *newval = OPENSSL_malloc(sizeof(int));

    ht = ossl_ht_new(&hash_conf);

    if (ht == NULL)
        return 0;

    if (newval == NULL)
        goto out;

    *newval = 1;

    /* insert */
    HT_INIT_KEY(&key);
    HT_SET_KEY_FIELD(&key, myintkey, 47);
    ossl_ht_write_lock(ht);
    rc = ossl_ht_test_int_insert(ht, TO_HT_KEY(&key), newval, NULL);
    ossl_ht_write_unlock(ht);
    if (rc == 0)
        goto out;

    /* num_items */
    if (ossl_ht_count(ht) != 1)
        goto out;

    /* lookup */
    HT_RESET_KEY(&key);
    HT_SET_KEY_FIELD(&key, myintkey, 47);
    ossl_ht_read_lock(ht);
    v = ossl_ht_get(ht, TO_HT_KEY(&key);
    fprintf(stderr, "found element with key 47 holding value %d\n",
            *ossl_ht_test_int_from_value(v));
    ossl_ht_read_unlock(ht);

    rc = 1;
end:
    /* this will call the free function for our element */
    ossl_ht_free(ht);
    return rc;
}

=head1 COPYRIGHT

Copyright 2024 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file LICENSE in the source distribution or at
L<https://www.openssl.org/source/license.html>.

=cut
