/*
 SUBROUTINE SCALE(FMN, FMX, N, VALMIN, STEP, VALMAX, IFAULT)

 ALGORITHM AS 96  APPL. STATIST. (1976) VOL.25, NO.1

 Given extreme values FMN, FMX, and the need for a scale with N
 marks, calculates value for the lowest scale mark (VALMIN) and
 step length (STEP) and highest scale mark (VALMAX).
*/

#include <math.h>

int scale(float fmn, float fmx, int n, float *valmin, float *step, float *valmax) {

 float unit[12]={0.0, 1.0, 1.2, 1.6, 2.0, 2.5, 3.0, 4.0, 5.0, 6.0, 8.0, 10.0};
 int   i, j, ifault, nunit=11;;
 float tol=5.0e-6;
 float bias=1.0e-4;
 float fmax, fmin, rn, x, s, range;

 fmax=fmx;
 fmin=fmn;
 ifault=1;

 /* Test for valid parameter values */
 if ((fmax < fmin) || (n <= 1)) return(ifault);
 ifault=0;
 rn=(float)(n - 1.0);
 x=fabs(fmax);
 if (x == 0.0) x=1.0;
 if ((fmax-fmin)/x <= tol) {
   /* All values effectively equal */
   if (fmax < 0.0) fmax=0.0;
   else if (fmax == 0.0) fmax=1.0;
   else fmin=0.0;
 }
 *step=(fmax-fmin)/rn;
 s=*step;

 /* Find power of 10 */
 while (s<1.0) s*=10.0;
 while (s>=10.0) s/=10.0;

 /* Calculate STEP */
 x=s-bias;
 i=1;
 while ((i<=nunit) && (x>unit[i])) i++;
 *step=(*step)*unit[i]/s;
 range=(*step)*rn;

 /* Make first estimate of VALMIN */
 x=0.5 * (1.0 +(fmax+fmin-range)/(*step));
 j=(int)(x-bias);
 if (x<0.0) j--;
 *valmin=(*step)*(float)j;
 /* Test if VALMIN could be zero */
 if ((fmin >= 0.0) && (range >= fmax)) *valmin=0.0;
 *valmax=(*valmin)+range;

 /* Test if VALMAX could be zero */
 if ((fmax > 0.0) || (range < (-1)*fmin)) return(ifault);
 *valmax=0.0;
 *valmin=(-1)*range;
 return(ifault);
}