/*  qrbdi.c    CCMATH mathematics library source code.
 *
 *  Copyright (C)  2000   Daniel A. Atkinson    All rights reserved.
 *  This code may be redistributed under the terms of the GNU library
 *  public license (LGPL). ( See the lgpl.license file for details.)
 * ------------------------------------------------------------------------
 */
#include <math.h>
int qrbdi(double *dm,double *em,int m)
{ int i,j,k,n;
  double u,x,y,a,b,c,s,t;
  for(j=1,t=fabs(dm[0]); j<m ;++j)
    if((s=fabs(dm[j])+fabs(em[j-1]))>t) t=s;
  t*=1.e-15; n=100*m;
  for(j=0; m>1 && j<n ;++j){
    for(k=m-1; k>0 ;--k){
      if(fabs(em[k-1])<t) break;
      if(fabs(dm[k-1])<t){
        for(i=k,s=1.,c=0.; i<m ;++i){
          a=s*em[i-1]; b=dm[i]; em[i-1]*=c;
          dm[i]=u=sqrt(a*a+b*b); s= -a/u; c=b/u;
         }
        break;
       }
     }
    y=dm[k]; x=dm[m-1]; u=em[m-2];
    a=(y+x)*(y-x)-u*u; s=y*em[k]; b=s+s;
    u=sqrt(a*a+b*b);
	if(u>0.){
      c=sqrt((u+a)/(u+u));
	  if(c!=0.) s/=(c*u); else s=1.;
	  for(i=k; i<m-1 ;++i){
        b=em[i];
        if(i>k){
          a=s*em[i]; b*=c;
          em[i-1]=u=sqrt(x*x+a*a);
          c=x/u; s=a/u;
         }
        a=c*y+s*b; b=c*b-s*y;
        s*=dm[i+1]; dm[i]=u=sqrt(a*a+s*s);
        y=c*dm[i+1]; c=a/u; s/=u;
        x=c*b+s*y; y=c*y-s*b;
	   }
     }
    em[m-2]=x; dm[m-1]=y;
    if(fabs(x)<t) --m;
    if(m==k+1) --m;
   }
  return j;
}