Supabase Edge Functions T-Shirt Contest #2

Mix.install([
  {:req, "~> 0.3.2"}
])

Summary

Supabase made annother announcement on Twitter

It's time for another #SupaFunctionsChallenge! This one is a little harder! Can you solve it?
To enter the draw, make a GET request with your email address, your Twitter handle, your t-shirt size, and the correct answer 👇
https://obuldanrptloktxcffvn.functions.supabase.co/get-tshirt-competition?email=tshirt@supabase.io&twitter=supabase&size=2XL&answer=42

Analysis

1. Go to the url to get the error response.

             {
               "error": "Sorry, that's wrong, please try again! HINT: https://github.com/supabase/supabase/blob/master/examples/edge-functions/supabase/functions/get-tshirt-competition/index.ts"
             }
   

2. Inspecting the source [https://github.com/supabase/supabase/blob/master/examples/edge-functions/supabase/functions/get-tshirt-competition/index.ts], we find the answer check is on line #49.

             if (answer !== countEmailSegments(email!))
               throw new Error(
                 `Sorry, that's wrong, please try again! HINT: https://github.com/supabase/supabase/blob/master/examples/edge-functions/supabase/functions/get-tshirt-competition/index.ts`
               )
   

3. Translate the countEmailSegments() function to Elixir

             function countEmailSegments(email: string): string {
               const [localPart, domain] = email.split('@')
               const [hostname, ...countryCodes] = domain.split('.')
               return `${localPart.length}${hostname.length}${countryCodes.reduce((a, cc) => a + cc.length, '')}`
             }
   

4. We can use the same TypeScript playground from our previous contest entry to brute force an answer at [https://www.typescriptlang.org/play]. This will help us check our work.
5. We should be able to get a usable answer from the playground but this seems somewhat straightforward to translate. Let's try to go through it line by line.
6. The first line of countEmailSegments is const [localPart, domain] = email.split('@'). This uses destructuring to String.split() the email by the @ into 2 parts, the localPart before the @ and the domain, after the @. Let's try this in the next cell.
7. First we need to create the secret for our EMAIL_ADDRESS.

email = System.fetch_env!("LB_EMAIL_ADDRESS")
email |> String.split("@")

1. We can see that splits into a list of ["user", "domain.com"]. Using good ol' pattern matching, we can also destructure into the local_part and domain variables. We show the output as a tuple to show we captured the same information in a different data structure.

email = System.fetch_env!("LB_EMAIL_ADDRESS")
[local_part, domain] = email |> String.split("@")
{local_part, domain}

1. Now we handle the next line const [hostname, ...countryCodes] = domain.split('.'). This also uses destructuring to split the domain variable on the . with the hostname as a single variable and countryCodes as an array.

2. This again translates quite well to pattern matching as [head | tail] where tail is the rest of our list. Let's see this in the next code example.

email = System.fetch_env!("LB_EMAIL_ADDRESS")
[local_part, domain] = email |> String.split("@")
[host_name, country_codes] = domain |> String.split(".")
{host_name, country_codes}

1. For my address, this returns the host_name as my domain name and com as the country_codes. In this case it's a single string (I'm a little unclear when this becomes an array personally, maybe for subdomains?).
2. The last line is particularly fun as I don't use reduce as often as I should if I'm being honest. I take slower approaches that build similar accumulators because my brain works with them a little easier. The code is return ${localPart.length}${hostname.length}${countryCodes.reduce((a, cc) => a + cc.length, '')} and I'm missing the string interpolation to make this work with markdown's backticks.
3. The first parts aren't too difficult, ${localPart.length} gets the length of the localPart string, ${hostname.length} gets the length of the hostname string and ${countryCodes.reduce((a, cc) => a + cc.length, '') reduces the array by concatenating the length of each element in the array.
4. To bring this all together, with an email like jeremy@some.example.com, localPart is jeremy, hostname is some, and countryCodes is ["example", "com"]. 15. For a single country code of comorus, we'll likely just fudge this and get the length of just that since I'm honestly too lazy to turn the Elixir country_codesvariable into a list where I believe the syntax...countryCodesin javascript **always pushes the result into an array**. 16. In fact, I'll just use the playground to verify this shortly. Turns out this is correct as thecountryCodesusing my email is["com"]. We can always fudge this by just adding our expected string as a new list or if you're reading along you probably have the right pattern to match this expectancy. 17. From this knowledge we should be able to work out a complete solution. I'll start with handling just the single country_codein Elixir and figure out how to require this to be a list. 1. Man, I can't believe I fudged the[head | tail]syntax as[head, tail]even after specifying it above! 18. For my email address, we should see the response body"Thanks for playing! 6@10.3 has been added to the draw \o/"`. This correctly parses the string we gave it into the individual lengths for each part. Neat.

Solution

url = "https://obuldanrptloktxcffvn.functions.supabase.co/get-tshirt-competition"

countEmailSegments = fn email ->
  [local_part, domain] = email |> String.split("@")
  [host_name | country_codes] = domain |> String.split(".")

  country_codes_length =
    Enum.reduce(country_codes, "", fn code, accumulator ->
      code_length = String.length(code) |> to_string()
      accumulator <> code_length
    end)

  "#{String.length(local_part)}#{String.length(host_name)}#{country_codes_length}"
end

email = System.fetch_env!("LB_EMAIL_ADDRESS")
answer = countEmailSegments.(email)

parameters = %{
  "email" => email,
  "twitter" => System.fetch_env!("LB_TWITTER_HANDLE"),
  "size" => System.fetch_env!("LB_TSHIRT_SIZE"),
  "answer" => answer
}

Req.get!(url, params: parameters)