! $Id: regr3_lint_m.F90 3065 2017-11-10 13:25:09Z nfevrier $
module regr3_lint_m

  ! Author: Lionel GUEZ

  implicit none

  interface regr3_lint
     ! Each procedure regrids by linear interpolation.
     ! The regridding operation is done on the third dimension of the
     ! input array.
     ! The difference betwwen the procedures is the rank of the first argument.
     module procedure regr33_lint, regr34_lint
  end interface

  private
  public regr3_lint

contains

  function regr33_lint(vs, xs, xt) result(vt)

    ! "vs" has rank 3.

    use assert_eq_m, only: assert_eq
    use interpolation, only: hunt

    real, intent(in):: vs(:, :, :)
    ! (values of the function at source points "xs")

    real, intent(in):: xs(:)
    ! (abscissas of points in source grid, in strictly monotonic order)

    real, intent(in):: xt(:)
    ! (abscissas of points in target grid)

    real vt(size(vs, 1), size(vs, 2), size(xt))
    ! (values of the function on the target grid)

    ! Variables local to the procedure:
    integer is, it, ns
    integer is_b ! "is" bound between 1 and "ns - 1"

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

    ns = assert_eq(size(vs, 3), size(xs), "regr33_lint ns")

    is = -1 ! go immediately to bisection on first call to "hunt"

    do it = 1, size(xt)
       call hunt(xs, xt(it), is)
       is_b = min(max(is, 1), ns - 1)
       vt(:, :, it) = ((xs(is_b+1) - xt(it)) * vs(:, :, is_b) &
            + (xt(it) - xs(is_b)) * vs(:, :, is_b+1)) / (xs(is_b+1) - xs(is_b))
    end do

  end function regr33_lint

  !*********************************************************

  function regr34_lint(vs, xs, xt) result(vt)

    ! "vs" has rank 4.

    use assert_eq_m, only: assert_eq
    use interpolation, only: hunt

    real, intent(in):: vs(:, :, :, :)
    ! (values of the function at source points "xs")

    real, intent(in):: xs(:)
    ! (abscissas of points in source grid, in strictly monotonic order)

    real, intent(in):: xt(:)
    ! (abscissas of points in target grid)

    real vt(size(vs, 1), size(vs, 2), size(xt), size(vs, 4))
    ! (values of the function on the target grid)

    ! Variables local to the procedure:
    integer is, it, ns
    integer is_b ! "is" bound between 1 and "ns - 1"

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

    ns = assert_eq(size(vs, 3), size(xs), "regr34_lint ns")

    is = -1 ! go immediately to bisection on first call to "hunt"

    do it = 1, size(xt)
       call hunt(xs, xt(it), is)
       is_b = min(max(is, 1), ns - 1)
       vt(:, :, it, :) = ((xs(is_b+1) - xt(it)) * vs(:, :, is_b, :) &
            + (xt(it) - xs(is_b)) * vs(:, :, is_b+1, :)) &
            / (xs(is_b+1) - xs(is_b))
    end do

  end function regr34_lint

end module regr3_lint_m