newlib/libm/mathfp/sf_tanh.c

   1
   2 /* @(#)z_tanhf.c 1.0 98/08/13 */
   3 /*****************************************************************
   4  * The following routines are coded directly from the algorithms
   5  * and coefficients given in "Software Manual for the Elementary
   6  * Functions" by William J. Cody, Jr. and William Waite, Prentice
   7  * Hall, 1980.
   8  *****************************************************************/
   9 /******************************************************************
  10  * Hyperbolic Tangent
  11  *
  12  * Input:
  13  *   x - floating point value
  14  *
  15  * Output:
  16  *   hyperbolic tangent of x
  17  *
  18  * Description:
  19  *   This routine calculates hyperbolic tangent.
  20  *
  21  *****************************************************************/
  22
  23 #include <float.h>
  24 #include "fdlibm.h"
  25 #include "zmath.h"
  26
  27 static const float LN3_OVER2 = 0.5493061443;
  28 static const float p[] = { -0.2059432032,
  29                            -0.0009577527 };
  30 static const float q[] = {  0.6178299136,
  31                             0.25 };
  32
  33 float
  34 tanhf (float x)
  35 {
  36   float f, res, g, P, Q, R;
  37
  38   f = fabsf (x);
  39
  40   /* Check if the input is too big. */
  41   if (f > BIGX)
  42     res = 1.0;
  43
  44   else if (f > LN3_OVER2)
  45     res = 1.0 - 2.0 / (exp (2 * f) + 1.0);
  46
  47   /* Check if the input is too small. */
  48   else if (f < z_rooteps_f)
  49     res = f;
  50
  51   /* Calculate the Taylor series. */
  52   else
  53     {
  54       g = f * f;
  55
  56       P = p[1] * g + p[0];
  57       Q = (g + q[1]) * g + q[0];
  58       R = g * (P / Q);
  59
  60       res = f + f * R;
  61     }
  62
  63   if (x < 0.0)
  64     res = -res;
  65
  66   return (res);
  67 }
  68
  69 #ifdef _DOUBLE_IS_32BITS
  70
  71 double tanh (double x)
  72 {
  73   return (double) tanhf ((float) x);
  74 }
  75
  76 #endif _DOUBLE_IS_32BITS