/* From: "COMPUTATIONAL PHYSICS, 2nd Ed" 
   by RH Landau, MJ Paez, and CC Bordeianu 
   Copyright Wiley-VCH, 2007.
   Electronic Materials copyright: R Landau, Oregon State Univ, 2007;
   MJ Paez, Univ Antioquia, 2007; & CC Bordeianu, Univ Bucharest, 2007
   Support by National Science Foundation
*/
// gauss.c: Points and weights for Gaussian quadrature  
// comment: this file has to reside in the same directory as integ.c  
#include <math.h>
void gauss(int npts, int job, double a, double b, 
                                           double x[], double w[]) {
	
//     npts     number of points                                 
//     job= 0  rescaling uniformly between (a,b)                  
//           1  for integral (0,b) with 50% pts inside (0, ab/(a+b)
//           2  for integral (a,inf) with 50% inside (a,b+2a)        
//     x, w     output grid points and weights.                     

  int     m, i, j; 
  double  t, t1, pp, p1, p2, p3;
  double  pi= 3.1415926535897932385E0, eps= 3.e-10;             
	                                         // eps= accuracy to adjust
  m= (npts+1)/2;
      for (i = 1; i<= m; i++) {  
        t = cos(pi*(i-0.25)/(npts+0.5));
        t1= 1;
        while((fabs(t-t1))>= eps) { 
          p1= 1.0;
          p2= 0.0;
          for (j = 1; j<= npts; j++) {
            p3= p2;
            p2= p1;
            p1= ((2*j-1)*t*p2-(j-1)*p3)/j;
         }
         pp= npts*(t*p1-p2)/(t*t-1);
         t1= t;
         t = t1 - p1/pp;
       }   
       x[i-1]= -t;
       x[npts-i]= t;
       w[i-1]   = 2.0/((1-t*t)*pp*pp);
       w[npts-i]= w[i-1];
    }
    if (job == 0)   {
      for (i = 0; i<npts ; i++)  {
            x[i]= x[i]*(b-a)/2.0+(b+a)/2.0;
            w[i]= w[i]*(b-a)/2.0;
      }
    }
      if (job == 1)  {
        for (i = 0; i<npts; i++)  {
          x[i]= a*b*(1+x[i]) / (b+a-(b-a)*x[i]);
          w[i]= w[i]*2*a*b*b /((b+a-(b-a)*x[i])*(b+a-(b-a)*x[i]));
       }
    }
      if (job == 2)   {
        for (i = 0; i<npts; i++)  {
          x[i]= (b*x[i]+b+a+a) / (1-x[i]);
          w[i]=  w[i]*2*(a+b)  /((1-x[i])*(1-x[i]));
         }
      }
}