EXCALIBURAdaptive Constraint-Based Agents in Artificial Environments

 [SYMBOLIC SRC] [Structures]   [Selection]   [SRC-H1]   [SRC-H2]   [SRC-H3]   [SRC-H4]   [SRC-H5]

 [ Please note: The project has been discontinued as of May 31, 2005 and is superseded by the projects of the ii Labs. There won't be further updates to these pages. ]

## SRC-H3: Moving a Precondition

This heuristic is applicable if there is at least one inconsistent Precondition Task that is not connected to the Task Constraint of SGoals. The idea is to temporally shift a Precondition Task such that its inconsistency is decreased.

### Selecting an Inconsistent Precondition Task

One of the SRC's inconsistent Precondition Tasks that are not connected to the Task Constraint of SGoals is selected with a choice probability for a Precondition Task that is proportional to the inconsistency caused by the Precondition Task. If one of the Action Tasks that belong to the chosen Precondition Task's Task Constraint has already been executed or is currently under execution, the heuristic is stopped and a negative success value is returned.

### Selecting an Interval

The new places considered for the chosen Precondition Task p are in the predecessor interval ip or successor interval is of p's interval i. The predecessor option is dropped if i is the first interval. The successor option is dropped if i is the last interval.

Each time the state-distance table provides the information that it is not possible to transform the state of ip into the state requested by p, ip is set to its predecessor interval. If ip is the first interval and a state transition is still not possible, then the predecessor option is dropped.

Each time the state-distance table provides the information that it is not possible to transform the state of is into the state requested by p, is is set to its successor interval. If is is the last interval and a state transition is still not possible, then the successor option is dropped.

If p's inconsistency is not equal to the dead-end value, p's current interval i is also an option, i.e., no change will be made if i is chosen.

From all interval options, an interval is chosen with a choice probability for an interval that is proportional to , where d is the state distance from the interval's state to the state required by p. If there are no interval options for the choice, or if i is the chosen interval, the heuristic is stopped and a negative success value is returned.