module principal_cshift_m

  IMPLICIT NONE

CONTAINS

  SUBROUTINE principal_cshift(is2, xlon, xprimm)

    ! Add or subtract 2 pi so that xlon is near [-pi, pi], then cshift
    ! so that xlon is in ascending order. Make the same cshift on
    ! xprimm.

    USE lmdz_physical_constants, ONLY: twopi
    USE serre_mod, ONLY: clon

    INCLUDE "dimensions.h"
    ! for iim

    INTEGER, INTENT(IN):: is2
    REAL, INTENT(INOUT):: xlon(:), xprimm(:) ! (iim + 1)

    !-----------------------------------------------------

    IF (is2 /= 0) THEN
       IF (clon <= 0.) THEN
          IF (is2 /= 1) THEN
             xlon(:is2 - 1) = xlon(:is2 - 1) + twopi
             xlon(:iim) = cshift(xlon(:iim), shift = is2 - 1)
             xprimm(:iim) = cshift(xprimm(:iim), shift = is2 - 1)
          END IF
       else
          xlon(is2 + 1:iim) = xlon(is2 + 1:iim) - twopi
          xlon(:iim) = cshift(xlon(:iim), shift = is2)
          xprimm(:iim) = cshift(xprimm(:iim), shift = is2)
       end IF
    end if

    xlon(iim + 1) = xlon(1) + twopi
    xprimm(iim + 1) = xprimm(1)

  END SUBROUTINE  principal_cshift

END MODULE principal_cshift_m