!> @file random_function_mod.f90
!--------------------------------------------------------------------------------------------------!
! This file is part of the PALM model system.
!
! PALM is free software: you can redistribute it and/or modify it under the terms of the GNU General
! Public License as published by the Free Software Foundation, either version 3 of the License, or
! (at your option) any later version.
!
! PALM is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the
! implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
! Public License for more details.
!
! You should have received a copy of the GNU General Public License along with PALM. If not, see
! <http://www.gnu.org/licenses/>.
!
! Copyright 1997-2021 Leibniz Universitaet Hannover
!--------------------------------------------------------------------------------------------------!
!
!
! Description:
! ------------
!> Random number generator, produces numbers equally distributed in interval [0,1].
!> This routine is taken from the "numerical recipes".
!--------------------------------------------------------------------------------------------------!
 MODULE random_function_mod


    USE kinds

    IMPLICIT NONE

    PRIVATE

    PUBLIC random_function, random_function_ini

    INTEGER(iwp), PUBLIC, SAVE ::  random_iv(32)  !<
    INTEGER(iwp), PUBLIC, SAVE ::  random_iy      !<

    INTERFACE random_function_ini
       MODULE PROCEDURE random_function_ini
    END INTERFACE random_function_ini

    INTERFACE random_function
       MODULE PROCEDURE random_function
    END INTERFACE random_function

 CONTAINS

!--------------------------------------------------------------------------------------------------!
! Description:
! ------------
!> @todo Missing subroutine description.
!--------------------------------------------------------------------------------------------------!
 SUBROUTINE random_function_ini

    IMPLICIT NONE

    random_iv = 0
    random_iy = 0

 END SUBROUTINE random_function_ini

!--------------------------------------------------------------------------------------------------!
! Description:
! ------------
!> @todo Missing function description.
!--------------------------------------------------------------------------------------------------!
 FUNCTION random_function( idum )


    IMPLICIT NONE

    INTEGER(iwp) ::  ia               !<
    INTEGER(iwp) ::  idum             !<
    INTEGER(iwp) ::  im               !<
    INTEGER(iwp) ::  iq               !<
    INTEGER(iwp) ::  ir               !<
    INTEGER(iwp) ::  ndiv             !<
    INTEGER(iwp) ::  ntab             !<

    INTEGER(iwp) ::  j                !<
    INTEGER(iwp) ::  k                !<

    REAL(wp)     ::  am               !<
    REAL(wp)     ::  eps              !<
    REAL(wp)     ::  random_function  !<
    REAL(wp)     ::  rnmx             !<

    PARAMETER ( ia = 16807, im = 2147483647, am = 1.0_wp / im, iq = 127773, ir = 2836, ntab = 32,  &
                ndiv = 1 + INT( REAL( im - 1 ) / ntab ), eps = 1.2e-7_wp, rnmx = 1.0_wp - eps )

    IF ( idum <= 0  .OR.  random_iy == 0 )  THEN
       idum = MAX( -idum, 1 )
       DO  j = ntab + 8, 1, -1
          k    = idum / iq
          idum = ia * ( idum - k * iq ) - ir * k
          IF ( idum < 0 )  idum = idum + im
          IF ( j <= ntab )  random_iv(j) = idum
       ENDDO
       random_iy = random_iv(1)
    ENDIF

    k    = idum / iq
    idum = ia * ( idum - k * iq ) - ir * k
    IF ( idum < 0 )  idum = idum + im
    j            = 1 + random_iy / ndiv
    random_iy    = random_iv(j)
    random_iv(j) = idum
    random_function  = MIN( am * random_iy , rnmx )

 END FUNCTION random_function

 END MODULE random_function_mod