Flagpole: Simple bitmap for storing flags

Flagpole allows bundling a bunch of flags into a single integer field, by storing each flag as a bit.

notifications_via = Flagpole.new([:email, :sms, :phone_push])
notifications_via.to_h #=> { email: false, sms: false, phone_push: false }

notifications_via[:email] = true
notifications_via[:sms] = true

notifications_via.to_h #=> { email: true, sms: true, phone_push: false }
notifications_via.to_i #=> 3

Now you can just store that 3 in your database. In order to get that back, just pass it to the constructor:

notifications_via = Flagpole.new(3, [:email, :sms, :phone_push])
notifications_via.to_h #=> { email: true, sms: true, phone_push: false }

gem install flagpole

If you consider each flag as a "bit" (1 when true, 0 when false), then the above example could be represented as [1, 1, 0]. If you reverse this, you get the binary representation of the number 3 (11).

All Flagpole does is give you a nice API to treat sets of named binary settings ("flags") as an integer, by doing the marshaling for you.

Read more about bitmaps on Wikipedia.

Easy! Just add a new name at the end of the array of flag names. So, In the example above, if you later add support for twitter notifications, just do this:

notifications_via = Flagpole.new(3, [:email, :sms, :phone_push, :twitter])
notifications_via.to_h #=> { email: true, sms: true, phone_push: false, twitter: false }

As you see, the values are maintained.

If you wanted the new flag to default to true, you just need to increment all the values. You do this by using #value_of:

flags = Flagpole([:email, :sms, :phone_push, :twitter])
flags.value_of(:twitter) #=> 8

Now all you have to do is add 8 to all your stored values, and voila, everyone has twitter notifications enabled.

In order to remove a flag, you will need to modify the integer values from your storage. Assuming the example from above, let's say you no longer wish to support SMS notifications.

In order to find out the value to substract, you can use #value_of:

flags = Flagpole([:email, :sms, :phone_push, :twitter])
flags.value_of(:sms) #=> 2

Now you need to substract 2 from all the values that have the SMS flag set. In order to do this, you need to do a bitwise and between the value and 2. Those that are non-zero, are the ones that have it set. In SQL this would be:

UPDATE users
SET notification_settings = notification_settings - 2
WHERE notification_settings & 2 <> 0

This project is shared under the MIT license. See the attached LICENSE file for details.

Photo credit: Cesarexpo via Wikimedia Commons.