WSJT-X/lib/slope.f90

subroutine slope(y,npts,xpk)

! Remove best-fit slope from data in y(i).  When fitting the straight line,
! ignore the peak around xpk +/- 2.

  real y(npts)

  sumw=0.
  sumx=0.
  sumy=0.
  sumx2=0.
  sumxy=0.
  sumy2=0.
  do i=1,npts
     if(abs(i-xpk).gt.2.0) then
        sumw=sumw + 1.0
        x=i
        sumx=sumx + x
        sumy=sumy + y(i)
        sumx2=sumx2 + x*x
        sumxy=sumxy + x*y(i)
        sumy2=sumy2 + y(i)**2
     endif
  enddo

  delta=sumw*sumx2 - sumx**2
  a=(sumx2*sumy - sumx*sumxy) / delta
  b=(sumw*sumxy - sumx*sumy) / delta

  sq=0.
  do i=1,npts
     y(i)=y(i)-(a + b*i)
     if(abs(i-xpk).gt.2.0) sq=sq + y(i)**2
  enddo
  rms=sqrt(sq/(sumw-2.0))
  y=y/rms

  return
end subroutine slope