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

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