/* chi2.c 
          cc -o chi2 chi2.c -lm
          cl -O2 chi2.c

   This program calculates the chi-square significance values for given 
   degrees of freedom and the tail probability (type I error rate) for 
   given observed chi-square statistic and degree of freedom.

   Ziheng Yang,  October 1993.
*/

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

double QuantileChi2 (double prob, double v);

#define QuantileGamma(prob,alpha,beta) QuantileChi2(prob,2.0*(alpha))/(2.0*(beta))
#define CDFGamma(x,alpha,beta) IncompleteGamma((beta)*(x),alpha,LnGammaFunction(alpha))
#define CDFChi2(x,v) CDFGamma(x,(v)/2.0,0.5)

double QuantileNormal (double prob);
double CDFNormal (double x);
double LnGammaFunction (double alpha);
double IncompleteGamma (double x, double alpha, double ln_gamma_alpha);

int main(int argc, char*argv[])
{
   int i,j, ndf=200, nprob=8, df;
   double chi2, prob[]={.005, .025, .1, .5, .90, .95, .99, .999};

   if (argc!=2 && argc!=3) {
      printf ("\n\nChi-square critical values\n");
      for (i=0; i<ndf; i++) {
         if (i%15==0) {
            printf ("\n\t\t\t\tSignificance level\n");
            printf ("\n DF ");  
            for (j=0; j<nprob; j++) printf ("%9.4f", 1-prob[j]);
            printf ("\n");  
         }
         printf ("\n%3d ", i+1);
         for (j=0; j<nprob; j++)
            printf ("%9.4f", QuantileChi2(prob[j],(double)(i+1)));
         if (i%5==4) printf ("\n");
         if (i%15==14) {
            printf ("\nENTER for more, (q+ENTER) for quit... ");
            if (getchar()=='q') break;
         }
      }
   }
   else if(argc==2) {
      for (; ; ) {
         printf ("\nd.f. & Chi^2 value (Ctrl-c to break)? ");
         scanf ("%d%lf", &df, &chi2);
         if(df<1 || chi2<0) break;
         prob[0] = 1-CDFChi2(chi2,df);
         printf ("\ndf = %2d  prob = %.9f = %.3e\n", df, prob[0], prob[0]);
      }
   }
   else if(argc==3) {
      df = atoi(argv[1]);
      chi2 = atof(argv[2]);
      if(df<1 || chi2<0) {
         printf("df = %2d  ch2 = %.4f invalid", df, chi2);
         exit(-1);
      }
      prob[0] = 1 - CDFChi2(chi2, df);
      printf ("\ndf = %2d  prob = %.9f = %.3e\n", df, prob[0], prob[0]);
   }
   printf ("\n");
   return (0);
}

double QuantileNormal (double prob)
{
/* returns z so that Prob{x<z}=prob where x ~ N(0,1) and (1e-12)<prob<1-(1e-12)
   returns (-9999) if in error
   Odeh RE & Evans JO (1974) The percentage points of the normal distribution.
   Applied Statistics 22: 96-97 (AS70)
*/
   double a0=-.322232431088, a1=-1, a2=-.342242088547, a3=-.0204231210245;
   double a4=-.453642210148e-4, b0=.0993484626060, b1=.588581570495;
   double b2=.531103462366, b3=.103537752850, b4=.0038560700634;
   double y, z=0, p=prob, p1;

   p1 = (p<0.5 ? p : 1-p);
   if (p1<1e-20) return (-9999);

   y = sqrt (log(1/(p1*p1)));   
   z = y + ((((y*a4+a3)*y+a2)*y+a1)*y+a0) / ((((y*b4+b3)*y+b2)*y+b1)*y+b0);
   return (p<0.5 ? -z : z);
}

double CDFNormal (double x)
{
/*  Hill ID  (1973)  The normal integral.  Applied Statistics, 22:424-427.
    Algorithm AS 66.
    adapted by Z. Yang, March 1994.  Hill's routine is quite bad, and I 
    haven't consulted 
      Adams AG  (1969)  Algorithm 39.  Areas under the normal curve.
      Computer J. 12: 197-198.
*/
    int invers=0;
    double p, limit=10, y=x*x/2;

    if (x<0) {  invers=1;  x*=-1; }
    if (x>limit)  return (invers?0:1);
    if (x<1.28)  
       p = .5 - x * (    .398942280444 - .399903438504 * y
                   /(y + 5.75885480458 - 29.8213557808
		   /(y + 2.62433121679 + 48.6959930692
		   /(y + 5.92885724438))));
    else 
       p = 0.398942280385 * exp(-y) /
           (x - 3.8052e-8 + 1.00000615302 /
           (x + 3.98064794e-4 + 1.98615381364 /
           (x - 0.151679116635 + 5.29330324926 /
           (x + 4.8385912808 - 15.1508972451 /
           (x + 0.742380924027 + 30.789933034 /
           (x + 3.99019417011))))));

    return  invers ? p : 1-p;
}

double LnGammaFunction (double alpha)
{
/* returns ln(gamma(alpha)) for alpha>0, accurate to 10 decimal places.  
   Stirling's formula is used for the central polynomial part of the procedure.
   Pike MC & Hill ID (1966) Algorithm 291: Logarithm of the gamma function.
   Communications of the Association for Computing Machinery, 9:684
*/
   double x=alpha, f=0, z;

   if (x<7) {
       f=1;  z=x-1;
       while (++z<7)  f*=z;
       x=z;   f=-log(f);
   }
   z = 1/(x*x);
   return  f + (x-0.5)*log(x) - x + .918938533204673 
          + (((-.000595238095238*z+.000793650793651)*z-.002777777777778)*z
               +.083333333333333)/x;  
}

double IncompleteGamma (double x, double alpha, double ln_gamma_alpha)
{
/* returns the incomplete gamma ratio I(x,alpha) where x is the upper 
           limit of the integration and alpha is the shape parameter.
   returns (-1) if in error
   ln_gamma_alpha = ln(Gamma(alpha)), is almost redundant.
   (1) series expansion     if (alpha>x || x<=1)
   (2) continued fraction   otherwise
   RATNEST FORTRAN by
   Bhattacharjee GP (1970) The incomplete gamma integral.  Applied Statistics,
   19: 285-287 (AS32)
*/
   int i;
   double p=alpha, g=ln_gamma_alpha;
   double accurate=1e-8, overflow=1e30;
   double factor, gin=0, rn=0, a=0,b=0,an=0,dif=0, term=0, pn[6];

   if (x==0) return (0);
   if (x<0 || p<=0) return (-1);

   factor=exp(p*log(x)-x-g);   
   if (x>1 && x>=p) goto l30;
   /* (1) series expansion */
   gin=1;  term=1;  rn=p;
 l20:
   rn++;
   term*=x/rn;   gin+=term;

   if (term > accurate) goto l20;
   gin*=factor/p;
   goto l50;
 l30:
   /* (2) continued fraction */
   a=1-p;   b=a+x+1;  term=0;
   pn[0]=1;  pn[1]=x;  pn[2]=x+1;  pn[3]=x*b;
   gin=pn[2]/pn[3];
 l32:
   a++;  b+=2;  term++;   an=a*term;
   for (i=0; i<2; i++) pn[i+4]=b*pn[i+2]-an*pn[i];
   if (pn[5] == 0) goto l35;
   rn=pn[4]/pn[5];   dif=fabs(gin-rn);
   if (dif>accurate) goto l34;
   if (dif<=accurate*rn) goto l42;
 l34:
   gin=rn;
 l35:
   for (i=0; i<4; i++) pn[i]=pn[i+2];
   if (fabs(pn[4]) < overflow) goto l32;
   for (i=0; i<4; i++) pn[i]/=overflow;
   goto l32;
 l42:
   gin=1-factor*gin;

 l50:
   return (gin);
}


double QuantileChi2 (double prob, double v)
{
/* returns z so that Prob{x<z}=prob where x is Chi2 distributed with df=v
   returns -1 if in error.   0.000002<prob<0.999998
   RATNEST FORTRAN by
       Best DJ & Roberts DE (1975) The percentage Quantiles of the 
       Chi2 distribution.  Applied Statistics 24: 385-388.  (AS91)
   Converted into C by Ziheng Yang, Oct. 1993.
*/
   double e=.5e-6, aa=.6931471805, p=prob, g;
   double xx, c, ch, a=0,q=0,p1=0,p2=0,t=0,x=0,b=0,s1,s2,s3,s4,s5,s6;

   if (p<.000002 || p>.999998 || v<=0) return (-1);

   g = LnGammaFunction (v/2);
   xx=v/2;   c=xx-1;
   if (v >= -1.24*log(p)) goto l1;

   ch=pow((p*xx*exp(g+xx*aa)), 1/xx);
   if (ch-e<0) return (ch);
   goto l4;
l1:
   if (v>.32) goto l3;
   ch=0.4;   a=log(1-p);
l2:
   q=ch;  p1=1+ch*(4.67+ch);  p2=ch*(6.73+ch*(6.66+ch));
   t=-0.5+(4.67+2*ch)/p1 - (6.73+ch*(13.32+3*ch))/p2;
   ch-=(1-exp(a+g+.5*ch+c*aa)*p2/p1)/t;
   if (fabs(q/ch-1)-.01 <= 0) goto l4;
   else                       goto l2;
  
l3: 
   x = QuantileNormal (p);
   p1=0.222222/v;   ch=v*pow((x*sqrt(p1)+1-p1), 3.0);
   if (ch>2.2*v+6)  ch=-2*(log(1-p)-c*log(.5*ch)+g);
l4:
   q=ch;   p1=.5*ch;
   if ((t=IncompleteGamma (p1, xx, g))<0) {
      printf ("\nerr IncompleteGamma");
      return (-1);
   }
   p2=p-t;
   t=p2*exp(xx*aa+g+p1-c*log(ch));   
   b=t/ch;  a=0.5*t-b*c;

   s1=(210+a*(140+a*(105+a*(84+a*(70+60*a))))) / 420;
   s2=(420+a*(735+a*(966+a*(1141+1278*a))))/2520;
   s3=(210+a*(462+a*(707+932*a)))/2520;
   s4=(252+a*(672+1182*a)+c*(294+a*(889+1740*a)))/5040;
   s5=(84+264*a+c*(175+606*a))/2520;
   s6=(120+c*(346+127*c))/5040;
   ch+=t*(1+0.5*t*s1-b*c*(s1-b*(s2-b*(s3-b*(s4-b*(s5-b*s6))))));
   if (fabs(q/ch-1) > e) goto l4;

   return (ch);
}
