Module org.cicirello.chips_n_salsa
Class WeightedShortestProcessingTimeLateOnly
java.lang.Object
org.cicirello.search.problems.scheduling.WeightedShortestProcessingTime
org.cicirello.search.problems.scheduling.WeightedShortestProcessingTimeLateOnly
 All Implemented Interfaces:
ConstructiveHeuristic<Permutation>
This is an implementation of the weighted shortest process time heuristic. There are two
variations of the weighted shortest processing time heuristic. The basic form (implemented in
WeightedShortestProcessingTime
) is defined as: h(j) = w[j] / p[j], where w[j] is the
weight of job j, and p[j] is its processing time. However, for some scheduling cost functions,
performance is improved if the heuristic is modified as follows. If the job j is already late
(i.e., its due date d[j] < T, where T is the current time), then its heuristic value is: h(j)
= w[j] / p[j], just like in the basic version. Otherwise, if the due date hasn't passed yet, then
h(j) = 0. This implementation alters this definition slightly as: If job j is already late, then
h(j) = max( MIN_H
, w[j] / p[j]), where MIN_H
is a small nonzero value. If the
job j is not yet late, then h(j) = MIN_H
. For deterministic construction of a schedule,
this adjustment is unnecessary. However, for stochastic sampling algorithms it is important for
the heuristic to return positive values.
Field Summary

Constructor Summary
ConstructorDescriptionConstructs an WeightedShortestProcessingTimeLateOnly heuristic. 
Method Summary
Modifier and TypeMethodDescriptionfinal int
Gets the required length of complete solutions to the problem instance for which this constructive heuristic is configured.Creates an IncrementalEvaluation object corresponding to an initially empty Partial for use in incrementally constructing a solution to the problem for which this heuristic is designed.final Partial<Permutation>
createPartial
(int n) Creates an empty Partial solution, which will be incrementally transformed into a complete solution of a specified length.final Problem<Permutation>
Gets a reference to the instance of the optimization problem that is the subject of this heuristic.double
h
(Partial<Permutation> p, int element, IncrementalEvaluation<Permutation> incEval) Heuristically evaluates the possible addition of an element to the end of a Partial.

Field Details

MIN_H
public static final double MIN_HThe minimum heuristic value. If the heuristic value as calculated is lower than MIN_H, then MIN_H is used as the heuristic value. The reason is related to the primary purpose of the constructive heuristics in the library: heuristic guidance for stochastic sampling algorithms, which assume positive heuristic values (e.g., an h of 0 would be problematic). See Also:


Constructor Details

WeightedShortestProcessingTimeLateOnly
Constructs an WeightedShortestProcessingTimeLateOnly heuristic. Parameters:
problem
 The instance of a scheduling problem that is the target of the heuristic. Throws:
IllegalArgumentException
 if problem.hasDueDates() returns false.


Method Details

h
Description copied from interface:ConstructiveHeuristic
Heuristically evaluates the possible addition of an element to the end of a Partial. Higher evaluations imply that the element is a better choice for the next element to add. For example, if you evaluate two elements, x and y, with h, and h returns a higher value for y than for x, then this means that y is believed to be the better choice according to the heuristic. Implementations of this interface must ensure that h always returns a positive result. This is because stochastic sampling algorithms such as HBSS and VBSS assume that the constructive heuristic returns only positive values. Specified by:
h
in interfaceConstructiveHeuristic<Permutation>
 Overrides:
h
in classWeightedShortestProcessingTime
 Parameters:
p
 The current state of the Partialelement
 The element under consideration for adding to the PartialincEval
 An IncrementalEvaluation of p. This method assumes that incEval is of the same runtime type as the object returned byConstructiveHeuristic.createIncrementalEvaluation()
. Returns:
 The heuristic evaluation of the hypothetical addition of element to the end of p. The higher the evaluation, the more important the heuristic believes that element should be added next. The intention is to compare the value returned with the heuristic evaluations of other elements. Individual results in isolation are not necessarily meaningful.

createIncrementalEvaluation
Description copied from interface:ConstructiveHeuristic
Creates an IncrementalEvaluation object corresponding to an initially empty Partial for use in incrementally constructing a solution to the problem for which this heuristic is designed. The object returned incrementally computes any data associated with a Partial as needed by theConstructiveHeuristic.h(org.cicirello.search.ss.Partial<T>, int, org.cicirello.search.ss.IncrementalEvaluation<T>)
method. TheConstructiveHeuristic.h(org.cicirello.search.ss.Partial<T>, int, org.cicirello.search.ss.IncrementalEvaluation<T>)
method will assume that it will be given an object of the specific runtime type returned by this method. It is unsafe to pass IncrementalEvaluation objects created by one heuristic to theConstructiveHeuristic.h(org.cicirello.search.ss.Partial<T>, int, org.cicirello.search.ss.IncrementalEvaluation<T>)
method of another.The default implementation simply returns null, which is appropriate for heuristics that won't benefit from incrementally computing heuristic information.
 Returns:
 An IncrementalEvaluation for an empty Partial to be used for incrementally computing
any data required by the
ConstructiveHeuristic.h(org.cicirello.search.ss.Partial<T>, int, org.cicirello.search.ss.IncrementalEvaluation<T>)
method.

getProblem
Description copied from interface:ConstructiveHeuristic
Gets a reference to the instance of the optimization problem that is the subject of this heuristic. Specified by:
getProblem
in interfaceConstructiveHeuristic<Permutation>
 Returns:
 the instance of the optimization problem that is the subject of this heuristic.

createPartial
Description copied from interface:ConstructiveHeuristic
Creates an empty Partial solution, which will be incrementally transformed into a complete solution of a specified length. Specified by:
createPartial
in interfaceConstructiveHeuristic<Permutation>
 Parameters:
n
 the desired length of the final complete solution. Returns:
 an empty Partial solution

completeLength
public final int completeLength()Description copied from interface:ConstructiveHeuristic
Gets the required length of complete solutions to the problem instance for which this constructive heuristic is configured. Specified by:
completeLength
in interfaceConstructiveHeuristic<Permutation>
 Returns:
 length of solutions to the problem instance for which this heuristic is configured
