FUNCTION fwhm_g1dfit, xdata, ydata, sky, param

; Description: This function is designed for use by the function "fwhm_g2dfit.pro" and it fits a 
;              one-dimensional Gaussian function to input data. The model that is fitted is: 
;
;              G(x) = A + B*exp(-0.5*(x/C)) 
;
;              The parameter A corresponds to the sky background (ADU).
;              The parameter B corresponds to the Gaussian peak value (ADU).
;              The parameter C is the sigma of the Gaussian (pix).
;                The function returns a value of "1" if the fit was successful and a value of "0" 
;              otherwise. The parameter "param" is used to record the fitted parameters A, B and C 
;              as a three-element vector.
;                This function assumes that all values of the independent variable "xdata" are
;              positive and represent the distance of each data point from the centre of the
;              Gaussian function at x = 0. It is assumed that the parameter B is positive, since
;              this function is modelling a positive brightness profile when called from
;              "fwhm_g2dfit.pro". If the fitted value of B is negative, then the function reports 
;              that the fit has failed.
;
; Input Parameters:
;
;   xdata - FLOAT/DOUBLE VECTOR - A one-dimensional vector containing the set of data values for 
;                                 the independent variable.
;   ydata - FLOAT/DOUBLE VECTOR - A one-dimensional vector containing the set of data values for 
;                                 the dependent variable.
;   sky - FLOAT/DOUBLE - An initial and reliable estimate of the parameter A.
;
; Output Parameters:
;
;   param - DOUBLE VECTOR - This parameter is returned as a three-element vector of DOUBLES with 
;                           the fitted values of A, B and C in order. 
;
; Return value:
;
;   The function returns a status value as an INTEGER. If the status value is "0", then the fit 
;   has failed. If the status value is "1", then the fit was successful.
;
; Author: Dan Bramich (dan.bramich@hotmail.co.uk)
;
; History:
;
;   06/05/2008 - Module created (dmb)

;Check that "xdata" and "ydata" are vectors of the correct number type

;Check that "xdata" and "ydata" have the same number of elements, and that they have at least 
;three elements

;Check that "sky" is a number of the correct type

;Compute an initial estimate of the sigma of the gaussian to be fitted

;Fit a one-dimensional Gaussian to the input data

;If the fit was unsuccessful, then return the status of the fit

;Force the sigma of the fitted gaussian to be positive

;If the height of the fitted Gaussian is positive and the sigma of the fitted Gaussian is non-zero, 
;then record that the fit was successful

;Return the status of the fit