source: sasview/src/sas/models/c_extension/cephes/psi.c @ c95a1a5

/*                                                      psi.c
 *
 *      Psi (digamma) function
 *
 *
 * SYNOPSIS:
 *
 * double x, y, psi();
 *
 * y = psi( x );
 *
 *
 * DESCRIPTION:
 *
 *              d      -
 *   psi(x)  =  -- ln | (x)
 *              dx
 *
 * is the logarithmic derivative of the gamma function.
 * For integer x,
 *                   n-1
 *                    -
 * psi(n) = -EUL  +   >  1/k.
 *                    -
 *                   k=1
 *
 * This formula is used for 0 < n <= 10.  If x is negative, it
 * is transformed to a positive argument by the reflection
 * formula  psi(1-x) = psi(x) + pi cot(pi x).
 * For general positive x, the argument is made greater than 10
 * using the recurrence  psi(x+1) = psi(x) + 1/x.
 * Then the following asymptotic expansion is applied:
 *
 *                           inf.   B
 *                            -      2k
 * psi(x) = log(x) - 1/2x -   >   -------
 *                            -        2k
 *                           k=1   2k x
 *
 * where the B2k are Bernoulli numbers.
 *
 * ACCURACY:
 *    Relative error (except absolute when |psi| < 1):
 * arithmetic   domain     # trials      peak         rms
 *    DEC       0,30         2500       1.7e-16     2.0e-17
 *    IEEE      0,30        30000       1.3e-15     1.4e-16
 *    IEEE      -30,0       40000       1.5e-15     2.2e-16
 *
 * ERROR MESSAGES:
 *     message         condition      value returned
 * psi singularity    x integer <=0      MAXNUM
 */

/*
Cephes Math Library Release 2.8:  June, 2000
Copyright 1984, 1987, 1992, 2000 by Stephen L. Moshier
*/

#include "mconf.h"

#ifdef UNK
static double A[] = {
 8.33333333333333333333E-2,
-2.10927960927960927961E-2,
 7.57575757575757575758E-3,
-4.16666666666666666667E-3,
 3.96825396825396825397E-3,
-8.33333333333333333333E-3,
 8.33333333333333333333E-2
};
#endif

#ifdef DEC
static unsigned short A[] = {
0037252,0125252,0125252,0125253,
0136654,0145314,0126312,0146255,
0036370,0037017,0101740,0174076,
0136210,0104210,0104210,0104211,
0036202,0004040,0101010,0020202,
0136410,0104210,0104210,0104211,
0037252,0125252,0125252,0125253
};
#endif

#ifdef IBMPC
static unsigned short A[] = {
0x5555,0x5555,0x5555,0x3fb5,
0x5996,0x9599,0x9959,0xbf95,
0x1f08,0xf07c,0x07c1,0x3f7f,
0x1111,0x1111,0x1111,0xbf71,
0x0410,0x1041,0x4104,0x3f70,
0x1111,0x1111,0x1111,0xbf81,
0x5555,0x5555,0x5555,0x3fb5
};
#endif

#ifdef MIEEE
static unsigned short A[] = {
0x3fb5,0x5555,0x5555,0x5555,
0xbf95,0x9959,0x9599,0x5996,
0x3f7f,0x07c1,0xf07c,0x1f08,
0xbf71,0x1111,0x1111,0x1111,
0x3f70,0x4104,0x1041,0x0410,
0xbf81,0x1111,0x1111,0x1111,
0x3fb5,0x5555,0x5555,0x5555
};
#endif

#define EUL 0.57721566490153286061

#ifdef ANSIPROT
extern double floor ( double );
extern double log ( double );
extern double tan ( double );
extern double polevl ( double, void *, int );
#else
double floor(), log(), tan(), polevl();
#endif
extern double PI, MAXNUM;


double psi(x)
double x;
{
double p, q, nz, s, w, y, z;
int i, n, negative;

negative = 0;
nz = 0.0;

if( x <= 0.0 )
        {
        negative = 1;
        q = x;
        p = floor(q);
        if( p == q )
                {
                mtherr( "psi", SING );
                return( MAXNUM );
                }
/* Remove the zeros of tan(PI x)
 * by subtracting the nearest integer from x
 */
        nz = q - p;
        if( nz != 0.5 )
                {
                if( nz > 0.5 )
                        {
                        p += 1.0;
                        nz = q - p;
                        }
                nz = PI/tan(PI*nz);
                }
        else
                {
                nz = 0.0;
                }
        x = 1.0 - x;
        }

/* check for positive integer up to 10 */
if( (x <= 10.0) && (x == floor(x)) )
        {
        y = 0.0;
        n = x;
        for( i=1; i<n; i++ )
                {
                w = i;
                y += 1.0/w;
                }
        y -= EUL;
        goto done;
        }

s = x;
w = 0.0;
while( s < 10.0 )
        {
        w += 1.0/s;
        s += 1.0;
        }

if( s < 1.0e17 )
        {
        z = 1.0/(s * s);
        y = z * polevl( z, A, 6 );
        }
else
        y = 0.0;

y = log(s)  -  (0.5/s)  -  y  -  w;

done:

if( negative )
        {
        y -= nz;
        }

return(y);
}