lib/math/tanh.c

   1 /*
   2  * (c) copyright 1988 by the Vrije Universiteit, Amsterdam, The Netherlands.
   3  * See the copyright notice in the ACK home directory, in the file "Copyright".
   4  *
   5  * Author: Ceriel J.H. Jacobs
   6  */
   7 /* $Header$ */
   8
   9 #include        <float.h>
  10 #include        <math.h>
  11 #include        <errno.h>
  12 #include        "localmath.h"
  13
  14 double
  15 tanh(double x)
  16 {
  17         /*      Algorithm and coefficients from:
  18                         "Software manual for the elementary functions"
  19                         by W.J. Cody and W. Waite, Prentice-Hall, 1980
  20         */
  21
  22         static double p[] = {
  23                 -0.16134119023996228053e+4,
  24                 -0.99225929672236083313e+2,
  25                 -0.96437492777225469787e+0
  26         };
  27         static double q[] = {
  28                  0.48402357071988688686e+4,
  29                  0.22337720718962312926e+4,
  30                  0.11274474380534949335e+3,
  31                  1.0
  32         };
  33         int     negative = x < 0;
  34
  35         if (__IsNan(x)) {
  36                 errno = EDOM;
  37                 return x;
  38         }
  39         if (negative) x = -x;
  40
  41         if (x >= 0.5*M_LN_MAX_D) {
  42                 x = 1.0;
  43         }
  44 #define LN3D2   0.54930614433405484570e+0       /* ln(3)/2 */
  45         else if (x > LN3D2) {
  46                 x = 0.5 - 1.0/(exp(x+x)+1.0);
  47                 x += x;
  48         }
  49         else {
  50                 /* ??? avoid underflow ??? */
  51                 double g = x*x;
  52                 x += x * g * POLYNOM2(g, p)/POLYNOM3(g, q);
  53         }
  54         return negative ? -x : x;
  55 }