GEANT321/fluka/betarn.F

   1 *
   2 * $Id$
   3 *
   4 * $Log$
   5 * Revision 1.1.1.1  1995/10/24 10:19:54  cernlib
   6 * Geant
   7 *
   8 *
   9 #include "geant321/pilot.h"
  10 *CMZ :  3.21/02 29/03/94  15.41.41  by  S.Giani
  11 *-- Author :
  12 *$ CREATE BETARN.FOR
  13 *COPY BETARN
  14 *
  15 *=== betarn ===========================================================*
  16 *
  17       FUNCTION BETARN(GAM,ETA)
  18
  19 #include "geant321/dblprc.inc"
  20 #include "geant321/dimpar.inc"
  21 #include "geant321/iounit.inc"
  22 *
  23 *----------------------------------------------------------------------*
  24 *                                                                      *
  25 *     New version:                                                     *
  26 *     Created on 20 february 1991  by    Alfredo Ferrari & Paola Sala  *
  27 *                                                   Infn - Milan       *
  28 *                                                                      *
  29 *     Last change on 20-feb-91     by    Alfredo Ferrari               *
  30 *                                                                      *
  31 *     Sampling from beta distribution in [0,1) :                       *
  32 *                                                                      *
  33 *     P(X) = X**(GAM-1.D0)*(1.D0-X)**(ETA-1)*GAMM(ETA+GAM)             *
  34 *          / (GAMM(GAM*GAMM(ETA))                                      *
  35 *                                                                      *
  36 *----------------------------------------------------------------------*
  37 *
  38       REAL RNDM(2)
  39 *
  40       GAMI  = 1.D+00 / GAM
  41       ETAM1 = ETA - 1.D+00
  42       NTAM1 = NINT (ETA - 1.D+00)
  43 *  +-------------------------------------------------------------------*
  44 *  |
  45       IF ( ETAM1 - NTAM1 .NE. 0.D+00 ) THEN
  46 *  |  +----------------------------------------------------------------*
  47 *  |  |  First sample from X**(gam-1) and then reject according to
  48 *  |  |  (1-X)**(eta-1)
  49   100    CONTINUE
  50             CALL GRNDM(RNDM,2)
  51             BETARN = RNDM (1)**GAMI
  52             REJE   = ( 1.D+00 - BETARN )**ETAM1
  53          IF ( RNDM (2) .GE. REJE ) GO TO 100
  54 *  |  |
  55 *  |  +----------------------------------------------------------------*
  56 *  |
  57 *  +-------------------------------------------------------------------*
  58 *  |
  59       ELSE
  60 *  |  +----------------------------------------------------------------*
  61 *  |  |  First sample from X**(gam-1) and then reject according to
  62 *  |  |  (1-X)**(eta-1)
  63   200    CONTINUE
  64             CALL GRNDM(RNDM,2)
  65             BETARN = RNDM (1)**GAMI
  66             REJE   = ( 1.D+00 - BETARN )**NTAM1
  67          IF ( RNDM (2) .GE. REJE ) GO TO 200
  68 *  |  |
  69 *  |  +----------------------------------------------------------------*
  70       END IF
  71 *  |
  72 *  +-------------------------------------------------------------------*
  73       RETURN
  74 *=== End of function betarn ===========================================*
  75       END