!***********************************************************************************************************************************
!
!                                              R A N D O M _ N U M B E R _ N O R M A L
!
!  Program:      RANDOM_NUMBER_NORMAL
!
!  Programmer:   David G. Simpson
!                NASA Goddard Space Flight Center
!                Greenbelt, Maryland  20771
!
!  Date:         February 18, 2016
!
!  Language:     Fortran-90
!
!  Version:      1.00a
!
!  Description:  This subroutine returns (to its argument) a normally-distributed
!                random number between 0 and 1, with mean 0 and standard deviation 1.
!
!  Notes:        1. Based on Algorithm given in Knuth, "The Art of Computer Programming,"
!                   Vol. 2 (Seminumerical Algoriths), Chapter 3.
!                2. Before any calls to this routine, Fortran subroutine RANDOM_SEED
!                   should be called in the main program.
!
!***********************************************************************************************************************************

      SUBROUTINE RANDOM_NUMBER_NORMAL (Z1)

      IMPLICIT NONE

      DOUBLE PRECISION, INTENT(OUT) :: Z1

      DOUBLE PRECISION :: U1, U2, V1, V2, S

  100 CALL RANDOM_NUMBER(U1)
      CALL RANDOM_NUMBER(U2)

      V1 = 2.0D0*U1 - 1.0D0
      V2 = 2.0D0*U2 - 1.0D0
      S = V1**2 + V2**2

      IF (S .GE. 1.0D0) GO TO 100

      Z1 = V1*SQRT(-2.0D0*LOG(S)/S)
!      Z2 = V2*SQRT(-2.0D0*LOG(S)/S)     ! Z2 is available as a second normal deviate, but is not used

      RETURN

      END SUBROUTINE RANDOM_NUMBER_NORMAL