@Override
public Solution[] evolve(Solution[] parents) {
Solution result1 = parents[0].copy();
Solution result2 = parents[1].copy();
if (pprng.nextDouble() <= probability) {
for (int i = 0; i < result1.getNumberOfVariables(); i++) {
Variable variable1 = result1.getVariable(i);
Variable variable2 = result2.getVariable(i);
if (pprng.nextBoolean()
&& (variable1 instanceof RealVariable)
&& (variable2 instanceof RealVariable)) {
evolve((RealVariable) variable1, (RealVariable) variable2, distributionIndex);
}
}
}
return new Solution[] {result1, result2};
}
/**
* Evolves the specified variables using the SBX operator.
*
* @param v1 the first variable
* @param v2 the second variable
* @param distributionIndex the distribution index of this SBX operator
*/
public void evolve(RealVariable v1, RealVariable v2, double distributionIndex) {
double x0 = v1.getValue();
double x1 = v2.getValue();
double dx = Math.abs(x1 - x0);
if (dx > Settings.EPS) {
double lb = v1.getLowerBound();
double ub = v1.getUpperBound();
double bl;
double bu;
if (x0 < x1) {
bl = 1 + 2 * (x0 - lb) / dx;
bu = 1 + 2 * (ub - x1) / dx;
} else {
bl = 1 + 2 * (x1 - lb) / dx;
bu = 1 + 2 * (ub - x0) / dx;
}
// use symmetric distributions
if (bl < bu) {
bu = bl;
} else {
bl = bu;
}
double p_bl = 1 - 1 / (2 * Math.pow(bl, distributionIndex + 1));
double p_bu = 1 - 1 / (2 * Math.pow(bu, distributionIndex + 1));
double u = pprng.nextDouble();
// prevent out-of-bounds values if PRNG draws the value 1.0
if (u == 1.0) {
u = Math.nextAfter(u, -1.0);
}
double u0 = u * p_bl;
double u1 = u * p_bu;
double b0;
double b1;
if (u0 <= 0.5) {
b0 = Math.pow(2 * u0, 1 / (distributionIndex + 1));
} else {
b0 = Math.pow(0.5 / (1 - u0), 1 / (distributionIndex + 1));
}
if (u1 <= 0.5) {
b1 = Math.pow(2 * u1, 1 / (distributionIndex + 1));
} else {
b1 = Math.pow(0.5 / (1 - u1), 1 / (distributionIndex + 1));
}
if (x0 < x1) {
v1.setValue(0.5 * (x0 + x1 + b0 * (x0 - x1)));
v2.setValue(0.5 * (x0 + x1 + b1 * (x1 - x0)));
} else {
v1.setValue(0.5 * (x0 + x1 + b1 * (x0 - x1)));
v2.setValue(0.5 * (x0 + x1 + b0 * (x1 - x0)));
}
// this makes PISA's SBX compatible with other implementations
// which swap the values
if (pprng.nextBoolean()) {
double temp = v1.getValue();
v1.setValue(v2.getValue());
v2.setValue(temp);
}
// guard against out-of-bounds values
if (v1.getValue() < lb) {
v1.setValue(lb);
} else if (v1.getValue() > ub) {
v1.setValue(ub);
}
if (v2.getValue() < lb) {
v2.setValue(lb);
} else if (v2.getValue() > ub) {
v2.setValue(ub);
}
}
}