[Gsll-devel] lu-decomp error

[Zach]

It looks like the definition of lu-decomp (linear-algebra/lu.lisp) is
incorrect.  Instead of (signum :int) it should read (signum :pointer),
right?  My code (see below) works with this change, but not without it (it
complains that the pointer isn't a fixnum).  However, signum is really just
a return value of the function, depending on how high level this should all
be, we may want to write a wrapper that allocates that memory and passes it
back as a second value.  But then we would have to worry about memory leaks
(since the GC won't free the memory, right?)  Further, this `function'
really just modifies it's arguments.  I guess the Lisp way would be to
change this into a function that returns the modified structures and mark it
as a destructive function... It is hard for me to make Lisp style and C
style coexist in my head.

How were the GSL definitions generated?  How did this :int :pointer
confusion slip through?  If the point here is to make a system where
mathematics can be done interactively, i.e. (invert-matrix #2A((1 0) (3 4)))
==> #2A(...), then a big part of this is going to be making wrapper
functions that hide the C like nuts and bolts that make GSL a bit annoying
to use in the first place.

(defun invert-matrix (mat)
  (gsl:letm ((mat2 (gsl:matrix-double mat))
             (per (gsl:permutation 2))
             (inv (gsl:matrix-double 2 2))
             (sig (cffi:foreign-alloc :int :count 1)) )
    (gsl:lu-decomp mat2 per sig)
    (gsl:lu-invert mat2 per inv)
    (cffi:foreign-free sig)
    (gsl:data inv) ))

How can I get rid of the explicit foreign-alloc/free?  Is this outside of
the scope of GSLL (seems like a border case to me).

Thanks,
Zach
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