/*  This is an C++ version of multA.m which can be dynamically linked
    with Octave using the command

        mkoctfile multA.cc

    It performs the same functions as multA.m except 30 times faster 
    when n=10000 on my machine.  For n>10000 it is likely that a cache
    blocked algorithm would be necessary.  Note that Matlab has a 
    similar ability to dynamically link C++ code; however, the Matlab
    version of multA.m on the UNR Citrix server was already fast enough.

    Written 2005 by Eric Olson for MATH/CS 466/666
*/
   
#include<octave/oct.h>
     
DEFUN_DLD (multA, args, , 
"% multA.m -- Multiply a vector by a matrix\n\n"
"function y=multA(x)\n"
"    n=length(x);\n"
"    c=[-1.0./[n-1:-1:1].^2,5,-1.0./[1:n-1].^2];\n"
"    for i=n:-1:1\n"
"        y(i,1)=c(n-i+1:2*n-i)*x;\n"
"    end"
){
    ColumnVector x(args(0).vector_value ());
    int n=x.length();
    {
        ColumnVector y(n);
        for(int i=0;i<n;i++){
            double s=0.0;
            for(int j=0;j<n;j++){
                if(i==j) s+=5*x(j);
                else {
                    int d=abs(i-j);
                    s-=x(j)/(d*d);
                }
            }
            y(i)=s;
        }
        return octave_value(y);
    }
}