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       snmp_index - Abstract Data Type for SNMP Indexing


       The  module  snmp_index implements an Abstract Data Type (ADT) for an SNMP index structure
       for SNMP tables. It is implemented as an ets table of  the  ordered_set  data-type,  which
       means  that  all  operations  are  O(log  n).  In  the  table,  the key is an ASN.1 OBJECT

       This index is used to separate the implementation of the SNMP  ordering  from  the  actual
       implementation  of  the  table.  The SNMP ordering, that is implementation of GET NEXT, is
       implemented in this module.

       For example, suppose there is an SNMP table, which is best implemented in  Erlang  as  one
       process  per  SNMP table row. Suppose further that the INDEX in the SNMP table is an OCTET
       STRING. The index structure would be created as follows:


       For each new process we create, we insert an item in an snmp_index structure:

       new_process(Name, SnmpIndex) ->
         Pid = start_process(),
         NewSnmpIndex =
           snmp_index:insert(SnmpIndex, Name, Pid),

       With this structure, we can now map an OBJECT IDENTIFIER in e.g. a GET  NEXT  request,  to
       the correct process:

       get_next_pid(Oid, SnmpIndex) ->
         {ok, {_, Pid}} = snmp_index:get_next(SnmpIndex, Oid),


       The following data types are used in the functions below:

         * index()

         * oid() = [byte()]

         * key_types = type_spec() | {type_spec(), type_spec(), ...}

         * type_spec() = fix_string | string | integer

         * key() = key_spec() | {key_spec(), key_spec(), ...}

         * key_spec() = string() | integer()

       The index() type denotes an snmp index structure.

       The oid() type is used to represent an ASN.1 OBJECT IDENTIFIER.

       The key_types() type is used when creating the index structure, and the key() type is used
       when inserting and deleting items from the structure.

       The key_types() type defines the types of the SNMP INDEX columns for  the  table.  If  the
       table has one single INDEX column, this type should be a single atom, but if the table has
       multiple INDEX columns, it should be a tuple with atoms.

       If the INDEX column is of type INTEGER, or derived from INTEGER,  the  corresponding  type
       should be integer. If it is a variable length type (e.g. OBJECT IDENTIFIER, OCTET STRING),
       the corresponding type should be string. Finally, if the type is of variable  length,  but
       with  a  fixed  size  restriction  (e.g.  IpAddress),  the  corresponding  type  should be

       For example, if the SNMP table has two INDEX columns, the first one an OCTET  STRING  with
       size  2,  and  the  second  one an OBJECT IDENTIFER, the corresponding key_types parameter
       would be {fix_string, string}.

       The key() type correlates to the key_types() type. If the key_types() is  a  single  atom,
       the  corresponding  key() is a single type as well, but if the key_types() is a tuple, key
       must be a tuple of the same size.

       In the example above, valid keys could be {"hi",  "mom"}  and  {"no",  "thanks"},  whereas
       "hi", {"hi", 42} and {"hello", "there"} would be invalid.


       All  API  functions  that  update  the  index return a NewIndex term. This is for backward
       compatibility with a previous implementation that used a B+ tree written purely in  Erlang
       for  the  index. The NewIndex return value can now be ignored. The return value is now the
       unchanged table identifier for the ets table.

       The implementation using ets tables  introduces  a  semantic  incompatibility  with  older
       implementations.  In  those  older implementations, using pure Erlang terms, the index was
       garbage collected like any other  Erlang  term  and  did  not  have  to  be  deleted  when
       discarded. An ets table is deleted only when the process creating it explicitly deletes it
       or when the creating process terminates.

       A new interface delete/1 is now added to handle the case when a process wants  to  discard
       an  index  table  (i.e.  to  build a completely new). Any application using transient snmp
       indexes has to be modified to handle this.

       As an snmp adaption usually keeps the index for the whole of the systems lifetime, this is
       rarely a problem.


       delete(Index) -> true


                 Index = NewIndex = index()
                 Key = key()

              Deletes  a  complete  index  structure  (i.e. the ets table holding the index). The
              index can no longer be referenced after this call. See the warning note above.

       delete(Index, Key) -> NewIndex


                 Index = NewIndex = index()
                 Key = key()

              Deletes a key and its value from the index structure. Returns a new structure.

       get(Index, KeyOid) -> {ok, {KeyOid, Value}} | undefined


                 Index = index()
                 KeyOid = oid()
                 Value = term()

              Gets the item with key KeyOid. Could be used from within  an  SNMP  instrumentation

       get_last(Index) -> {ok, {KeyOid, Value}} | undefined


                 Index = index()
                 KeyOid = oid()
                 Value = term()

              Gets the last item in the index structure.

       get_next(Index, KeyOid) -> {ok, {NextKeyOid, Value}} | undefined


                 Index = index()
                 KeyOid = NextKeyOid = oid()
                 Value = term()

              Gets  the  next  item in the SNMP lexicographic ordering, after KeyOid in the index
              structure. KeyOid does not have to refer to an existing item in the index.

       insert(Index, Key, Value) -> NewIndex


                 Index = NewIndex = index()
                 Key = key()
                 Value = term()

              Inserts a new key value tuple into the index structure. If an item  with  the  same
              key already exists, the new Value overwrites the old value.

       key_to_oid(Index, Key) -> KeyOid


                 Index = index()
                 Key = key()
                 KeyOid = NextKeyOid = oid()

              Converts Key to an OBJECT IDENTIFIER.

       new(KeyTypes) -> Index


                 KeyTypes = key_types()
                 Index = index()

              Creates a new snmp index structure. The key_types() type is described above.