Question

I’m having a lot of difficulty putting together what the following code is supposed to do (I understand what it’s supposed to do, I just can’t put together the pieces):

def roles=(roles)
  self.roles_mask = (roles & ROLES).map { |r| 2**ROLES.index(r) }.inject(0, :+)
end

def roles
  ROLES.reject do |r|
    ((roles_mask || 0) & 2**ROLES.index(r)).zero?
  end
end

ROLES is an array specified in the link. I’m assuming roles is also an array.

1. Why have a setter method and a regular method with the same name?
2. What array serves as the object of the map method? If roles is ['author', 'editor'] and ROLES is ['author', 'editor', 'manager'], how does the & operator create an array for map?
3. What is 2**ROLES? I figured out that this is really just 2 raised to the power of ROLES.index(r).
4. “Bitmask attributes on a Rails application” says that (roles & ROLES) is sanitizing the parameters roles against the array ROLES, but what does sanitizing it mean?
5. If r is the current value of the array (roles & ROLES), how does the index method of 2**ROLES return r? I figured out that this is returning the position of roles in the array ROLES, but I still don’t know how (roles & ROLES).map interacts/works with this.
6. How does the inject method work on the condition in the brackets for the map method?

I’d like to figure out how this bitmasking works, but I have no idea how what’s on the right side of the equation for def roles=(roles) returns an integer.

Answer 1

Basically the index call converts the given roles to integers by determining their position in the array of ROLES. 2 is then raised to the power of this index and inject simply sums the the results.

Put simply

Step 1 is to get roles that are found in ROLES by using & to se which values in the two arrays match.

Step 2, for each matching role compute 2**index. Map returns an array of these values.

Step 3, sum the array to get the final integer.

The idea is to compute a unique integer for each combination of roles.

The getter simply unwinds the process returning the combination of roles that match.