; twod1.pro
; Based on TWOD1.FR
; Revisions:
;	2001 Nov 5 - IDL version

;***file twod1.fr
;***dick french
;***latest revision 6//81
;
;---this fortran 5 routine is used in program golc.  it computes the
;   integrand used for determination of the bending angle for
;   two-dimensional waves using geometric optics.  see rgf mit#3 p.131+.
;   this version is for constant fractional amplitude gravity waves.
;
;
function twod1,ha,i,j,dxhat,hhat,eps,wlpsh, $
	psi,rhat,shphw,idiag
;
	twopi=!dpi*2.d0
;
	zkhat=twopi*wlpsh
	zlhat=twopi/shphw
	xj=double(j)*dxhat
	rdif=ha+xj*xj/(2.d0*rhat) - double(i-1)/hhat
	arg=zkhat*rdif + zlhat*xj + psi
	ca=cos(arg)
	sa=sin(arg)
	ex=exp(-rdif)
	znu=ex*(1.d0 + eps*sa)
	fac=eps*ex*ca
	dndr=-znu + fac*zkhat
	dndx=fac*zlhat
	twod1=dndr-xj*dndx
	if(idiag eq 0) then return,twod1
	print, "j=",j," znu=",znu
	print, "zkhat=",zkhat," zlhat=",zlhat
	print, "arg=",arg," ex=",ex
	print, "dndr=",dndr," dndx=",dndx
	print, "xj=",xj," rdif=",rdif
	print, "twod1=",twod1
	print
	return,twod1
	end