Liam Healy

06:15 pm - Passing a complex number to a foreign function

GSL has a number of functions that take complex scalars as arguments. It defines them with either

typedef struct
  {
    double dat[2];
  }
gsl_complex;

typedef struct
  {
    float dat[2];
  }
gsl_complex_float;

I thought maybe there was a way around this. Could it be that the real and imaginary parts are simply passed sequentially as if they were two successive arguments to the function? In fact, this does work for numbers of type (complex double-float), but when I tried that on (complex single-float), it would return without error but with the result computed as if the imaginary part was 0. I thought maybe there was a padding issue, so I packed the two single-floats into a single 64-bit word that I then interpreted as a double-float. When passing this single argument to a function that expects a (complex single-float), the correct answer is computed.

For example, the BLAS function caxpy works on single-floats:

(FOREIGN-FUNCALL "gsl_blas_caxpy" 
		 :DOUBLE (PACK-COMPLEX-AS-DOUBLE ALPHA) 
		 :POINTER (MPOINTER X) :POINTER (MPOINTER Y)
		 :INT)

(FOREIGN-FUNCALL "gsl_blas_zaxpy" 
		 :DOUBLE (REALPART ALPHA) 
		 :DOUBLE (IMAGPART ALPHA) 
		 :POINTER (MPOINTER X) :POINTER (MPOINTER Y) 
		 :INT)

This has now been incorporated into GSLL. I assume it is not portable; I am on Debian amd64 but it works on x86 (32 bit) as well. It seems to be portable across CL implementations, at least it works in SBCL and CCL. So I have built in a test that tries a simple function; if it gets the wrong answer than all the functions that want a complex scalar will signal an error.

03:00 pm - GSLL new version

The new version GNU Scientific Library for Lisp (GSLL) is now available. This library has many mathematical functions used in science and engineering applications. It works on several Lisp implementations.

The most significant changes from the previous version include:

• All objects are memory managed (garbage collected), which means they can have indefinite extent like any other Lisp object. There is no need to put the object creation in a letm; in fact letm doesn't exist anymore. Use let, or defparameter, etc.
  
• All array (vector and matrix) element types that are supported by the platform, CL implementation, CFFI, and GSL, are supported by GSLL.
  
• On SBCL both Lisp and C use the same representation for array contents; they are not copied between the two sides. Thanks to Tamas Papp's foreign-friendly arrays for the inspiration.
  

There have been some function name and argument changes, so users of the previous version will need to update their code. Please use the git repository and discontinue using the old svn repository.

Feedback welcome, here or on the mailing list.

06:27 pm - GNU Scientific Library for Lisp

The GNU Scientific Library is a library of applied mathemetics commonly used in science and engineering. I have written GNU Scientific Library for Lisp (GSLL), a fairly complete interface to this library from Common Lisp. My intent is that the interface be as Lisp-natural as possible. Though this will be useful for those of us that do scientific programming in Lisp, even those who aren't Lisp programmers might use this library as a desk-calculator interface to GSL.