MODULE linear_md
IMPLICIT NONE
  CONTAINS
  SUBROUTINE linear(maskin,maskout,datain,dataout,      &
                    xo,xn,io,in)
  IMPLICIT NONE

! Doing linear interpolation .....
! Author: Biju Thomas  on 2007/11/27 ....
! Land Values are excluded from interpolation ....

    INTEGER, INTENT(IN)                       :: io,in 
    REAL,    INTENT(IN)                       :: maskin,maskout
    REAL, DIMENSION(io), INTENT(IN)           :: xo(io), datain(io)
    REAL, DIMENSION(in), INTENT(IN)           :: xn(in)
    REAL, DIMENSION(in), INTENT(OUT)          :: dataout(in)
    INTEGER                                   :: i,i1,i2,ik
    REAL                                      :: x,x1,x2,dx,sum
    REAL, DIMENSION(2)                        :: flag
!
      IF ( xo(1) > xo(io)) THEN
       PRINT*, 'Input Xs are not acending oreder'
       print*,xo(1),xo(io)
       RETURN
      END IF

    DO  i=1,in
        IF ( xn(i) > xo(io) ) THEN
          dataout(i) = datain(io)
          CYCLE
        END IF
        IF ( xn(i) < xo(1) ) THEN
          dataout(i) = datain(1)
          CYCLE
        END IF

!
       x=xn(i)
       i1=1
       i2=1 
       x1=xo(i1)
       x2=xo(i2)
       DO ik=1,io
        IF( xo(ik) <= x ) THEN
          i1 = ik
          x1=xo(ik)
        END IF
       END DO
       DO ik=io,1,-1
        IF( xo(ik) >= x ) THEN
          i2 = ik
          x2=xo(ik)
        END IF
       END DO
       

      IF (datain(i1) == maskin) THEN
           flag(1)=0.
      ELSE
           flag(1)=1.
      END IF
      IF(datain(i2) == maskin) THEN
           flag(2)=0.
      ELSE
           flag(2)=1.
      END IF

      IF (x2 /= x1) THEN
        sum=flag(1) + flag(2)
        IF (sum .eq. 0.) THEN
          dataout(i)=maskout
        ELSE
         dx = flag(1)*(x2-x) + flag(2)*(x-x1)

!--->> y = [ y1*(x2-x) + y2*(x-x1) ] / [x2-x1]

         dataout(i) = flag(1)*datain(i1)*(x2-x)/dx       &
          +           flag(2)*datain(i2)*(x-x1)/dx         
        END IF
      ELSE
        sum=flag(1)
        IF (sum == 0.) THEN
          dataout(i)=maskout
        ELSE
          dataout(i) = datain(i1)
        END IF
      END IF
      
    END DO
  END SUBROUTINE linear
END MODULE linear_md