Module org.cicirello.chips_n_salsa

Package org.cicirello.search.problems.tsp

Class NearestCityPairHeuristic

java.lang.Object

org.cicirello.search.problems.tsp.NearestCityPairHeuristic

All Implemented Interfaces:

Splittable<ConstructiveHeuristic<Permutation>>, ConstructiveHeuristic<Permutation>

public final class NearestCityPairHeuristic
extends Object
implements ConstructiveHeuristic<Permutation>

This class implements a constructive heuristic for the TSP that prefers the first city of the nearest pair of cities. Since the stochastic sampling algorithms of the library require higher heuristic values to imply preferred choice, this heuristic is implemented as: h(j) == 1.0 / (1.0 + distance(i, j) + min_k(distance(j, k))), where h(j) is the heuristic value for city j, and i is the most recently added city.

Constructor Summary

Constructors

Constructor	Description
NearestCityPairHeuristic(BaseTSP problem)	Constructs a nearest city pair heuristic for an instance of the TSP.

Method Summary

Modifier and Type	Method	Description
final int	completeLength()	Gets the required length of complete solutions to the problem instance for which this constructive heuristic is configured.
IncrementalEvaluation<Permutation>	createIncrementalEvaluation()	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>	getProblem()	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.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Methods inherited from interface org.cicirello.search.ss.ConstructiveHeuristic

split

Constructor Details

NearestCityPairHeuristic

public NearestCityPairHeuristic(BaseTSP problem)

Constructs a nearest city pair heuristic for an instance of the TSP.

Parameters:

problem - The TSP instance to solve.

Method Details

h

public double h(Partial<Permutation> p,
 int element,
 IncrementalEvaluation<Permutation> incEval)

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 interface ConstructiveHeuristic<Permutation>

Parameters:

p - The current state of the Partial

element - The element under consideration for adding to the Partial

incEval - An IncrementalEvaluation of p. This method assumes that incEval is of the same runtime type as the object returned by ConstructiveHeuristic.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

public IncrementalEvaluation<Permutation> 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 the ConstructiveHeuristic.h(org.cicirello.search.ss.Partial<T>, int, org.cicirello.search.ss.IncrementalEvaluation<T>) method. The ConstructiveHeuristic.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 the ConstructiveHeuristic.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.

Specified by:

createIncrementalEvaluation in interface ConstructiveHeuristic<Permutation>

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

public final Problem<Permutation> 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 interface ConstructiveHeuristic<Permutation>

Returns:

the instance of the optimization problem that is the subject of this heuristic.

createPartial

public final Partial<Permutation> createPartial(int n)

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 interface ConstructiveHeuristic<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 interface ConstructiveHeuristic<Permutation>

Returns:

length of solutions to the problem instance for which this heuristic is configured