Permutations, Combinations and URL Shortening - Creating random tokens
/I have a model, let's call it Post. I want the public URL for a post record to be similar to Instagram's public URLs: https://www.instagram.com/p/BC3HEDGiJrp.
Instead of using post#id as the identifier in the URL, I'll use a token. The token doesn't need to be difficult to guess or secure, but it needs to be unique and URL safe.
Instead of using the often suggested SecureRandom let's follow blog.codinghorror.com's approach on URL Shortening. Towards the end of the article, he asks: "Each new URL gets a unique three character combination until no more are left. How many URLs would that take?"
And it just takes a bit of math to figure it out! Permutations to the rescue. Or wait, is it combinations? Let's have a quick refresher:
If the order doesn't matter, it is a Combination.
If the order does matter it is a Permutation.
For post tokens, order does matter so we will use permutations. Luckily Ruby's Array class defines a few methods that are helpful for this:
array#permutationarray#repeated_permutation- allows for character repeatsarray#repeated_combination- allows for character repeats
It is to our advantage that we can generate the set of possible tokens ahead of time. Now, when a new Post is created, we can assign it an unused token. This solves our problem of making sure each token is unique.
Here's the final code:
