// A Householder reflection matrix is a rank-1 update to the identity.
// P = I - b v v^T
// Unitarity requires b = 2 / |v|^2. To anihilate all but the first components of vector x,
// P x = a e_1
// we must have
// v = x +/- |x| e_1
// that is, all elements the same except the first, from which we have either added or subtracted |x|. This makes
// a = -/+ |x|
// There are two way to handle the sign. One is to simply choose the sign that avoids cancelation when calculating v_1,
// i.e. + for positive x_1 and - for negative x_1. This works fine, but makes a negative for positive x_1, which is
// weird-looking (1 0 0 gets turned into -1 0 0). An alternative is to choose the negative sign even for positive x_1,
// but to avoid cancelation write
// v_1 = x_1 - |x| = ( x_1 - |x| ) ( x_1 + |x|) / ( x_1 + |x|) = ( x_1^2 - |x|^2 ) / ( x_1 + |x| )
// = - ( x_2^2 + \cdots + x_n^2 ) / ( x_1 + |x| )
// We have now moved to the second method. Note that v is the same as x except for the first element.
public static void GenerateHouseholderReflection(double[] store, int offset, int stride, int count, out double a)
{
double x0 = store[offset];
// Compute |x| and u_0.
double xm, u0;
if (x0 < 0.0)
{
xm = Blas1.dNrm2(store, offset, stride, count);
u0 = x0 - xm;
}
else
{
// This method of computing ym and xm does incur and extra square root compared to naively computing x_2 + \cdots + x_n^2,
// but doing it this way allows us to use dNrm2's over/under-flow prevention when we have large/small elements.
double ym = Blas1.dNrm2(store, offset + stride, stride, count - 1);
xm = MoreMath.Hypot(x0, ym);
// Writing ym / (x0 + xm) * ym instead of ym * ym / (x0 + xm) prevents over/under-flow for large/small ym. Note this will
// be 0 / 0 = NaN when xm = 0.
u0 = -ym / (x0 + xm) * ym;
}
// Set result element.
a = xm;
// If |x| = 0 there is nothing to do; we could have done this a little earlier but the compiler requires us to set a before returning.
if (xm == 0.0)
{
return;
}
// Set the new value of u_0
store[offset] = u0;
// Rescale to make b = 1.
double um = Math.Sqrt(xm * Math.Abs(u0));
if (um > 0.0)
{
Blas1.dScal(1.0 / um, store, offset, stride, count);
}
}
/// <summary>
/// Computes the magnitude of the vector.
/// </summary>
/// <returns>The Euclidean norm of the vector.</returns>
public virtual double Norm()
{
return(Blas1.dNrm2(store, offset, stride, dimension));
}
