Return to tanh.c CVS log

Up to [MW Coherent from dump] / coherent / b / lib / libm

Annotation of coherent/b/lib/libm/tanh.c, revision 1.1.1.1

1.1       root        1: /*
                      2:  * libm/tanh.c
                      3:  * Hyperbolic tangent.
                      4:  */
                      5: 
                      6: #include <math.h>
                      7: 
                      8: #if    EMU87
                      9: #include "emumath.h"
                     10: #endif
                     11: 
                     12: /*
                     13:  * To get a correct result for very small |x|,
                     14:  * the code below just returns x for |x| < THRESHOLD.
                     15:  * We can derive a theoretical value for THRESHOLD from the series:
                     16:  *     tanh(x)  = x - x^3/3 + 2*x^5/17 - ...
                     17:  * so if |x| < sqrt(3 * DBL_EPSILON) then x^3/3 < x * DBL_EPSILON
                     18:  * and the low-order terms must be insignificant.
                     19:  * The threshold value below, arrived at empirically, is somewhat larger;
                     20:  * it is for IEEE fp, the DECVAX value must be slightly different but...
                     21:  */
                     22: #define        THRESHOLD       7.000900090031528e-06
                     23: 
                     24: double
                     25: tanh(x) double x;
                     26: {
                     27:        double r;
                     28:        register int s;
                     29: 
                     30:        if (fabs(x) < THRESHOLD)
                     31:                return x;
                     32:        /*
                     33:         * Hart (6.2.28) gives an alternative formula for |x| < 1,
                     34:         * unused here; should it be used?
                     35:         */
                     36:        if (s = (x < 0.0))
                     37:                x = -x;
                     38:        r = exp(-2.0*x);
                     39:        r = (1.0-r) / (1.0+r);
                     40:        return s ? -r : r;
                     41: }
                     42: 
                     43: /* end of tanh.c */

This archive runs on limited infrastructure. Preserving old code on modern bandwidth. Automated agents are requested to crawl responsibly.