WSJT-X/igray.f

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FUNCTION igray(n,is)
INTEGER igray,is,n
INTEGER idiv,ish
if (is.ge.0) then
igray=ieor(n,n/2)
else
ish=-1
igray=n
1 continue
idiv=ishft(igray,ish)
igray=ieor(igray,idiv)
if(idiv.le.1.or.ish.eq.-16)return
ish=ish+ish
goto 1
endif
return
END
C (C) Copr. 1986-92 Numerical Recipes Software *(t9,12.