C       Written by Simon Lacoste-Julien 02/7/29
C       This is a test to implement Euler function with call-back
C       mechanism in Fortran, to extend Python...
C
C EULER(N, fun, x, t, dt, newx)
C   - N is size of array input (x)
C   - fun is the ODE function - it's really an array of function; but this will
C     be taken care of in the python code: it will be called as follows
C        fun(N,i,x,t, result) where i will be the index of which function to evaluate (i starts at 0 in Python)
C        x will be vector
C        t will be scalar
C        result will hold result
C   - x is state vector
C   - t is time (double)
C   - dt is time step (double)
C
        SUBROUTINE EULER(N, fun, x, t, dt, newx)
            EXTERNAL fun
            INTEGER N
            DOUBLE PRECISION x(N), t, dt, newx(N)
            DOUBLE PRECISION res
            
            DO 10 i = 1, N
               CALL fun(N,i-1,x,t, res)
               newx(i) = x(i) + dt * res
 10         CONTINUE
            
        END SUBROUTINE EULER