FB_HashTableCtrl

The hash table can be used to find an individual data element quickly among a larger number of data elements. The data objects must have a unique key. The key enables the data objects to be identified unambiguously and found quickly in the table.

The function block FB_HashTableCtrl can be used to realize a simple hash table in the PLC project. The hashing with chaining (separate chaining) procedure is used.

The maximum number of data elements cannot be changed at runtime and must be specified in advance. Adding/removing/finding of data elements is controlled through action calls. The function block features the following tasks:

A_Add (adds a new data element to the table (key/value). If an element with the same key already exists, it is overwritten! )
A_GetFirst (reads the first table data element. If successful, getValue supplies the associated value.)
A_GetNext (reads the next table data element. The address: putPosPtr must point to the previous data element!)
A_Lookup (looks for a data element matching the key. If successful, getValue supplies the associated value.)
A_Remove (removes a data element matching the key.)
A_RemoveAll (removes all data elements)
A_RemoveFirst (removes the first data element)
A_reset (deletes all data elements and resets the table.)
A_GetIndexAtPosPtr (returns the array index of the data element at the address: putPosPtr. When successful, getValue returns the null-based array index. The value putValue is not used. Please note that the value getValue returns a data element index, not the data element value! )

Inputs

VAR_INPUT
    key       : DWORD := 0;
    putValue  : PVOID := 0;
    putPosPtr : POINTER TO T_HashTableEntry := 0;
END_VAR

Name	Type	Description
key	DWORD	Key (unsigned 32 bit number or pointer). This key enables a data object to be identified and found quickly in the table.
putValue	PVOID	Value/data element (input parameter, 32/64 bit, unsigned number or pointer).
putPosPtr	T_HashTableEntry	Address for data element (input parameter)

Inputs/outputs

VAR_IN_OUT
    hTable  : T_HHASHTABLE;
END_VAR

Name	Type	Description
hTable	T_HHASHTABLE	Hash table handle. The handle must be initialized once with the function: F_CreateHashTableHnd before it will be used. A corresponding instance of the handle variable must be created and initialized for each table.

Outputs

VAR_OUTPUT
    bOk        : BOOL := FALSE;
    getValue   : PVOID := 0;
    getPosPtr  : POINTER TO T_HashTableEntry := 0;
END_VAR

Name	Type	Description
bOk	BOOL	Returns TRUE if a new data element was added to/removed from or found in the table. Returns FALSE, if the searched data element was not found, the table is empty or an overflow occurred (no more free data elements).
getValue	PVOID	The value matching the key/data element (output parameter, 32/64 bit, unsigned number or pointer).
getPosPtr	T_HashTableEntry	The address for the data element (output parameter).

Example:

See: Example: Hash table (FB_HashTableCtrl).

Requirements

Development environment	Target platform	PLC libraries to be integrated (category group)
TwinCAT v3.1.0	PC or CX (x86, x64, ARM)	Tc2_Utilities (System)