subroutine perpar(gamma0, factor, ier)
!----- GPL ---------------------------------------------------------------------
!                                                                               
!  Copyright (C)  Stichting Deltares, 2011-2019.                                
!                                                                               
!  This program is free software: you can redistribute it and/or modify         
!  it under the terms of the GNU General Public License as published by         
!  the Free Software Foundation version 3.                                      
!                                                                               
!  This program is distributed in the hope that it will be useful,              
!  but WITHOUT ANY WARRANTY; without even the implied warranty of               
!  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the                
!  GNU General Public License for more details.                                 
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!  You should have received a copy of the GNU General Public License            
!  along with this program.  If not, see <http://www.gnu.org/licenses/>.        
!                                                                               
!  contact: delft3d.support@deltares.nl                                         
!  Stichting Deltares                                                           
!  P.O. Box 177                                                                 
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!  All indications and logos of, and references to, "Delft3D" and "Deltares"    
!  are registered trademarks of Stichting Deltares, and remain the property of  
!  Stichting Deltares. All rights reserved.                                     
!                                                                               
!-------------------------------------------------------------------------------
!  $Id$
!  $HeadURL$
!!--description-----------------------------------------------------------------
!
! This routine as used in Delft3D-WAVE is based on subroutine PERPAR in 
! Delft3D-MOR, without the HISWA stuff.
!
! This routine determines HISWA period parameters given a value 
! of the peak enhancement parameter gamma0 by interpolation from
! a table; see reference:
!
! M.W. Dingemans
! 'Verification of numerical wave propagation models with
! laboratory measurements'; HISWA verification in the
! directional wave basin;
! H228 Part 1B; Nov. 1987; Appendix G, p. 208
!
! VARIABLE   TYPE  I/O   DESCRIPTION
!
! gamma0     REAL   I    JONSWAP peak enhancement factor;
!                        Default=3.3; range 1-20
! FACTOR     REAL   O    Factor peak period/mean period
!                        (Tp/PERIOD)
!
!!--pseudo code and references--------------------------------------------------
! NONE
!!--declarations----------------------------------------------------------------
    implicit none
!
! Local parameters
!
    integer, parameter :: n = 14
!
! Global variables
!
    integer, intent(out) :: ier
    real   , intent(out) :: factor
    real   , intent(in)  :: gamma0
!
! Local variables
!
    integer            :: i
    real               :: fac1
    real               :: fac2
    real, dimension(n) :: gamlst
    real, dimension(n) :: nu01
    real, dimension(n) :: num10
    !
    data gamlst/1.00, 2.00, 3.00, 3.30, 3.64, 4.00, 5.00, 6.00, 7.00, &
       &        8.00, 9.00, 10.00, 15.00, 20.00/
    data num10/1.16656, 1.13181, 1.11166, 1.10706, 1.10239, 1.09798,  &
       &       1.08791, 1.08011, 1.07384, 1.06868, 1.06434, 1.06063,  &
       &       1.04788, 1.04029/
    data nu01 /1.29572, 1.24024, 1.20646, 1.19857, 1.19050, 1.18282,  &
       &       1.16503, 1.15103, 1.13963, 1.13015, 1.12210, 1.11516,  &
       &       1.09098, 1.07631/
!
!! executable statements -----------------------------------------------
!
    ier    = 0
    factor = 0.0
    if (gamma0 < gamlst(1)) then
       ier = -1
       return
    elseif (gamma0 > gamlst(n)) then
       ier = -2
       return
    else
       do i = 1, n - 1
          if (gamma0 <= gamlst(i + 1)) then
             fac1   = (gamlst(i + 1) - gamma0)/  &
                    & (gamlst(i + 1) - gamlst(i))
             fac2   = 1.0 - fac1
             factor = fac1* nu01(i) + fac2* nu01(i + 1)
             exit
          endif
       enddo
    endif
end subroutine perpar