Implement slicing support for LB

[RudolfWeeber]

i.e., lbf[4:20,:,1].velocity
or
lbf[:,:,:].velocity = np.zeros(lbf.shape))

• the getitem() method of The LB Python class should accept slices
• If slices are passed in, rather than an instance of LB Node, an instance of a new LBSlice class shuold be returned

An initial implementation of the LBSlice class should just act as proxy to single node getters. this can be done with 20-30 lines of Python. This will give the usability benefit immediately. A more efficient implementation can be done after the Walberla transition, if the performance increase is ever needed.

Steps:

• In src/python/espressomd.lb.pyx, create a new class LBSlice

  • constructor should take three slice objects for x,y,z

  • indivudal properties:

  • Using those, calcualte the x,y,z dimensions of the resulting array (i.e., how many lattice sites are requested in each direction) should be (max-min)/step, or 1 if the value for the coordinate is a single int.

    • Find out the number and type of values per lb site (either hard-code 1 float for density, 3 floats for velocity, 3 ints for index, ...) or fetch the first value (e.g., lower left corner) using a single node getter
    • Shape the result array accordingly
    • Use a nested loop to fetch all the individual values and fill the result array

• Modify the getimem() method of the lb fluid class in src/python/espresso/lb.pyx:

  If any of the arguments is an instance of slice, create an instance of LBSlice with the three arguments for x,y,z

Draft:

class LBSlice:

    def __init__(self, x,y,z, lb_fluid):
        self.x = x
        ...
        self.lb_fluid = lb_fluid
        self.dim = self.calculate_dimensions(x,y,z)

    def calculate_dimensions(x,y,z):
        # x,y,z either an instance of slcie or an int
        # Return tubple of 3 ints for the dimensions calculated from x,y,z
        return (...)

    
    def get_values(property_name, values_per_site, result_type):
        dim = self.dim
        res =  np.zeors((dim[0],dim[1],dim[22], values_per_site), dtype=result_type)
        for x in ...
           for y in ...
              for z in ...
                  res[x,y,z,:] = getattr(self.lb_fluid[x,y,z], property_name)
    def set_values(property_name, values): 
        for x in ...
           for y in ...
              for z in ...
                  setattr(self.lb_fluid[x,y,z], property_name, values[x,y,z])

    
    property velocity:
        def __get__(self):
           return self.get_vavlues("velocity", 3, float)
       def __set__(values):
           self.set_values("velocity", values)
# analogous for other properties like density

[RudolfWeeber]

Coding day: Martin and Deniz