# include <math.h>
# include <stdio.h>
# include <stdlib.h>
# include <time.h>

# include "asa103.h"

/******************************************************************************/

void psi_values ( int *n_data, double *x, double *fx )

/******************************************************************************/
/*
  Purpose:

    psi_values() returns some values of the Psi or Digamma function.

  Discussion:

    In Mathematica, the function can be evaluated by:

      PolyGamma[x]

    or

      Polygamma[0,x]

    PSI(X) = d ln ( Gamma ( X ) ) / d X = Gamma'(X) / Gamma(X)

    PSI(1) = - Euler's constant.

    PSI(X+1) = PSI(X) + 1 / X.

  Licensing:

    This code is distributed under the MIT license. 

  Modified:

    17 August 2004

  Author:

    John Burkardt

  Reference:

    Milton Abramowitz, Irene Stegun,
    Handbook of Mathematical Functions,
    National Bureau of Standards, 1964,
    ISBN: 0-486-61272-4,
    LC: QA47.A34.

    Stephen Wolfram,
    The Mathematica Book,
    Fourth Edition,
    Cambridge University Press, 1999,
    ISBN: 0-521-64314-7,
    LC: QA76.95.W65.

  Parameters:

    Input/output, int *N_DATA.  The user sets N_DATA to 0 before the
    first call.  On each call, the routine increments N_DATA by 1, and
    returns the corresponding data; when there is no more data, the
    output value of N_DATA will be 0 again.

    Output, double *X, the argument of the function.

    Output, double *FX, the value of the function.
*/
{
# define N_MAX 11

  double fx_vec[N_MAX] = { 
     -0.5772156649015329E+00,  
     -0.4237549404110768E+00,  
     -0.2890398965921883E+00,  
     -0.1691908888667997E+00,  
     -0.6138454458511615E-01,  
      0.3648997397857652E-01,  
      0.1260474527734763E+00,  
      0.2085478748734940E+00,  
      0.2849914332938615E+00,  
      0.3561841611640597E+00,  
      0.4227843350984671E+00 };

  double x_vec[N_MAX] = { 
     1.0E+00,  
     1.1E+00,  
     1.2E+00,  
     1.3E+00,  
     1.4E+00,  
     1.5E+00,  
     1.6E+00,  
     1.7E+00,  
     1.8E+00,  
     1.9E+00,  
     2.0E+00 };

  if ( *n_data < 0 )
  {
    *n_data = 0;
  }

  *n_data = *n_data + 1;

  if ( N_MAX < *n_data )
  {
    *n_data = 0;
    *x = 0.0;
    *fx = 0.0;
  }
  else
  {
    *x = x_vec[*n_data-1];
    *fx = fx_vec[*n_data-1];
  }

  return;
# undef N_MAX
}