!
!  SPDX-License-Identifier: GPL-3.0-or-later
!  Copyright (C) 2019-2022, respective authors of MCFM.
!

      subroutine ubarspinor0(q,i,f)
      implicit none
      include 'types.f'
      include 'cplx.h'
c-----subroutine for massless ubar spinor
c     Weyl representation

      include 'swapxz.f'
      integer:: i
      complex(dp):: p(4),q(4),f(4),fc,czip
      real(dp)::  px,py,phase
      parameter(czip=(0._dp,0._dp))
      logical,save::first
      data first/.true./

      if (first) then
      write(6,*) 'ubarspinor0:swapxz=',swapxz
      first=.false.
      endif

c     Translate from MCFM notation
      p(1)=q(4)
      p(2)=q(3)
      p(3)=-q(2)
      p(4)=q(1)
      if (swapxz) then
c performing the swap (x<->z),(y->-y)
      p(1)=q(4)
      p(2)=q(3)
      p(3)=-q(2)
      p(4)=q(1)
      else
      p(1)=q(4)
      p(2)=q(1)
      p(3)=q(2)
      p(4)=q(3)
      endif

      px=+real(p(2))
      py=+real(p(3))
c      pz=+real(p(4))

      fc=sqrt(p(1)+p(4))
      if (real(p(1)+p(4)) > 0) then
        phase=1._dp
      else
        phase=-1._dp
      endif



      if (abs(fc)>1.e-8_dp) then
      if (i == 1) then
      f(1)=czip
      f(2)=czip
      f(3)=+fc
      f(4)=cplx2(px,-py)/fc
      elseif (i == -1) then
      f(1)=cplx2(px,py)/fc
      f(2)=-fc
      f(3)=czip
      f(4)=czip
      endif

      else
      if (i == 1) then
        f(1)=czip
        f(2)=czip
        f(3)=czip
        f(4)=-phase*sqrt(2._dp*p(1))
      elseif (i == -1) then
        f(1)=phase*sqrt(2._dp*p(1))
        f(2)=czip
        f(3)=czip
        f(4)=czip
      endif
      endif
      return
      end