source: sasview/calculator/slit_length_calculator.py @ afeb37a

"""
This module is a small tool to allow user to quickly
determine the slit length value of data.
"""

class SlitlengthCalculator(object):
    """
    compute slit length from SAXSess beam profile (1st col. Q , 2nd col. I ,
    and 3rd col. dI.: don't need the 3rd)
    """
    def __init__(self):
        
        # x data
        self.x = None
        # y data
        self.y = None
        #default slit length
        self.slit_length = 0.0
        
        # The unit is unknown from SAXSess profile: 
        # It seems 1/nm but it could be not fixed, 
        # so users should be notified to determine the unit by themselves.
        self.slit_length_unit = "unknown"
    
    def set_data(self, x=None, y=None):
        """
         Receive two vector x, y and prepare the slit calculator for 
         computation.
        
        :param x: array
        :param y: array
        """
        self.x = x
        self.y = y
        
    def calculate_slit_length(self):
        """
        Calculate slit length.
        
        :return: the slit length calculated value.
        """
        # None data do nothing
        if self.y == None or self.x == None:
            return
        # set local variable
        y = self.y
        x = self.x

        # find max y
        max_y = y.max()
        
        # initial values 
        y_sum = 0.0
        y_max = 0.0
        ind = 0.0
        
        # sum 10 or more y values until getting max_y, 
        while (True):
            if ind >= 10 and y_max == max_y:
                break
            y_sum = y_sum + y[ind]
            if y[ind] > y_max: y_max = y[ind]
            ind += 1
     
        # find the average value/2 of the top values  
        y_half = y_sum/(2.0*ind)
        
        # defaults
        y_half_d = 0.0
        ind = 0.0
        # find indices where it crosses y = y_half.
        while (True):
            # no need to check when ind == 0
            ind += 1     
            # y value and ind just after passed the spot of the half height            
            y_half_d = y[ind]       
            if y[ind] < y_half: 
                break
        
        # y value and ind just before passed the spot of the half height
        y_half_u = y[ind-1]        
        
        # get corresponding x values
        x_half_d = x[ind]
        x_half_u = x[ind-1] 
        
        # calculate x at y = y_half using linear interpolation
        if y_half_u == y_half_d:
            x_half = (x_half_d + x_half_u)/2.0
        else:
            x_half = (x_half_u * (y_half - y_half_d) + x_half_d*(y_half_u-y_half))\
                        /(y_half_u - y_half_d)
        
        # multiply by 2 due to the beam profile is for half beam
        slit_length = 2.0 * x_half
        
        # set slit_length
        self.slit_length = slit_length
        return self.slit_length
  
    def get_slit_length_unit(self): 
        """
        :return: the slit length unit.
        """
        return self.slit_length_unit