/// <summary>
/// Run an unconstrained minimization using BFGS
/// </summary>
/// <param name="x0">Starting step</param>
/// <param name="normOfFirstStep">Norm of first step</param>
/// <param name="func">Multivariate function and derivative evaluator</param>
/// <returns>The local minimum point</returns>
public virtual Vector Run(Vector x0, double normOfFirstStep, FunctionEval func)
{
dimension = x0.Count;
currentX = Vector.Copy(x0);
if (func == null)
{
throw new ArgumentException("Null function");
}
cancel = false;
// Initialize all the fields needed for optimisation
feval = new FunctionEval(func);
searchDir = Vector.Zero(dimension);
currentDeriv = Vector.Zero(dimension);
// Following vectors are the components of the
// inverse Hessian update
Vector S = Vector.Zero(dimension);
Vector Y = Vector.Zero(dimension);
Vector minusRhoS = Vector.Zero(dimension);
Matrix EyeMinusSYt = new Matrix(dimension, dimension);
Vector prevDeriv = Vector.Zero(dimension);
PositiveDefiniteMatrix H = new PositiveDefiniteMatrix(dimension, dimension);
H.SetToIdentity();
Matrix HWork = new Matrix(dimension, dimension);
// Get the derivatives
currentObj = feval(currentX, ref currentDeriv);
double prevObj = currentObj;
double rho;
double step = 0.0;
double step0 = initialStep;
double convergenceCheck = double.MaxValue;
double invDim = 1.0 / dimension;
iter = 0;
while (convergenceCheck > this.epsilon && !cancel)
{
prevDeriv.SetTo(currentDeriv);
// Compute search direction
searchDir.SetToProduct(H, currentDeriv);
// Negate
for (int i = 0; i < dimension; i++)
{
searchDir[i] = -searchDir[i];
}
if (iter == 0)
{
double sdNorm = System.Math.Sqrt(searchDir.Inner(searchDir));
double sdMult = normOfFirstStep / sdNorm;
searchDir.SetToProduct(searchDir, sdMult);
for (int i = 0; i < dimension; i++)
{
H[i, i] = sdMult;
}
}
//------------------------------------------------------
// Line search enforces strong Wolfe conditions
// so curvature condition is guaranteed
//------------------------------------------------------
step = TryLineSearch(step0, stepMax, currentDeriv.Inner(searchDir));
// Get the delta between the old and new point
S.SetToProduct(searchDir, step);
// Update the current point
currentX.SetToSum(currentX, S);
// Calculate the objective and the derivative at the new point
//double objVal = feval(currentX, ref currentDeriv);
// If the step is 0, break now, once we have re-evaluated the function and derivs.
if (step == 0.0)
{
break;
}
// Difference of derivs
Y.SetToDifference(currentDeriv, prevDeriv);
// BFGS update
rho = 1.0 / (S.Inner(Y));
if (iter == 0)
{
// Modify the Hessian to yTs/yTy times Identity
double beta = S.Inner(S) * rho;
for (int i = 0; i < dimension; i++)
{
H[i, i] = beta;
}
}
EyeMinusSYt.SetToIdentity();
EyeMinusSYt.SetToSumWithOuter(EyeMinusSYt, -rho, S, Y);
HWork.SetToProduct(EyeMinusSYt, H);
H.SetToProduct(HWork, EyeMinusSYt.Transpose());
H.SetToSumWithOuter(H, rho, S, S);
switch (convergenceCriteria)
{
case ConvergenceCriteria.Gradient:
convergenceCheck = System.Math.Sqrt(invDim * currentDeriv.Inner(currentDeriv));
break;
case ConvergenceCriteria.Objective:
convergenceCheck = System.Math.Abs(prevObj - currentObj) / System.Math.Max(1, System.Math.Max(prevObj, currentObj));
break;
}
prevObj = currentObj;
// Let anyone who's interested know that we have completed an iteration
RaiseIterationEvent(iter, currentObj, convergenceCheck);
if (++iter > maxIterations)
{
break;
}
}
return(currentX);
}
