FUNCTION QQSM, X,YIN,SIGMA

;!+
;!
;!  Program:
;!      QQSM
;!
;!  Purpose:
;!      Apply a box ca Gauusianr smoothing function to
;!      function YIN(X),
;!      where the FWHM of the box is 2.354*SIGMA. Output is YOUT.
;!      Data assumed to be equally distributed in X.
;!
;!  Accuracy:
;!      Preserves integrated equivalent widths of lines in smoothed spectra
;!      to 0.04 %
;!
;!  Language:
;!     IDL - adapted from Fortran 77
;!
;!  Authors:
;!     C.S.Jeffery (CSJ) (Armagh Observatory)
;!
;!  History:
;!     03-FEB-2000: CSJ
;!        original version based on qsm function in DIPSO
;!     07-OCT-2003: CSJ
;!        IDL version (includes truncation to keep local mean valid near ends of data)       
;!     08-OCT-2003: CSJ
;!        Adapted from QSMOOTH -- much much faster !!!
;!     09-OCT-2003: CSJ
;!        Converted to a function
;!-


SIGMA2 = 2.0*SIGMA*SIGMA
THRSIG = 4.242640687*SIGMA

NP = N_ELEMENTS(X)
YOUT = FLTARR(NP)

DX = (X(NP-1)-X(0)) / (NP-1)
NG = FIX ( THRSIG / DX )

XWIN = (FINDGEN(2L*NG+1L)*DX) - THRSIG
GWIN = EXP ( -0.5 * (XWIN/SIGMA)^2 ) /  SQRT (2*!PI) 

FOR I = 0L, NP-1 DO BEGIN

  KL = MAX ( [0L,   I - NG ] )
  KU = MIN ( [NP-1L, MAX( [I + NG,0L] ) ] )  
  JL = MAX ( [0L,   NG - (I-0L)    ] )
  JU = MIN ( [2L*NG,NG + (NP-1L-I) ] ) 

  YOUT(I) = TOTAL ( GWIN(JL:JU) * YIN(KL:KU) ) / TOTAL ( GWIN(JL:JU) )

ENDFOR

  RETURN,YOUT

END