[Octave-bug-tracker] [bug #41453] norm is really slow for sparse matrices

[Jordi Gutiérrez Hermoso]

Follow-up Comment #20, bug #41453 (project octave):

> preparing a header file for eigs_base.cc. I don't know why that hasn't been
done before, and why also libinterp/dldfcn/__eigs__.cc includes the file I am
using as a .cc.

You have to understand the arrangement of Octave and why these functions are
in dldfcn. First, it's possible to compile Octave without ARPACK. Sometimes
people do this. Secondly, "dldfcn" means "dynamically loaded functions", i.e.
dynamically linked. The code in this directory gets turned into oct files
instead of being compiled into liboctave or libinterp. The idea here is to
only load the libraries into RAM with dlopen when the user requests them (this
is part of the reason why the first call to eigs is slower than later calls).
And the reason it doesn't have a header file is because it's just meant to be
a dynamically loaded oct file, somewhat "external" to Octave.

It may make sense to turn ARPACK into a core library like LAPACK, but it seems
weird to make ARPACK a hard requirement just for two Octave functions: eigs,
and svds.

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