Stripe Interview

# Dependencies
Mix.install([
  {:heap, "~> 3.0"},
  {:libgraph, "~> 0.16.0"},
  # visualizations for Livebooks
  {:kino, "~> 0.11.0"},
  # http request library
  {:req, "~> 0.4.4"},
  # Repository data mapper
  {:ecto, "~> 3.10"},
  # GraphQL library
  {:absinthe, "~> 1.7"},
  {:jason, "~> 1.4"}
])

# Test Framework
ExUnit.start(autorun: false)

Requirements

Assignment:

> Given a list of Users and a list of Features, implement a function get_user_features that will return the feature id’s for a user. Note that locations property and abTest property may be absent

Questions / Clarifications

*

Approach

1. for a user’s location l, get the set of features whos location array includes l, or where the features location is empty/absent
2. for a user’s id, if id is even, get the set of features which include property abTest true, or abTest is absent.
3. return union of those two sets.

Examples

defmodule Assignment.Fixtures do
  use ExUnit.CaseTemplate

  setup _context do
    users = ~s(
      [
          { "id": 0, "name": "eva",    "location": "US" },
          { "id": 1, "name": "tess",   "location": "US" },
          { "id": 2, "name": "rahool", "location": "CA" },
          { "id": 3, "name": "amanda", "location": "CA" }
      ])
    features = ~s(
      [
        {
          "id": "annual_sale",
          "locations": ["US"],
          "abTest": true
        },
        {
            "id": "enhanced_comments",
            "abTest": true
        },
        {
            "id": "canada_promotion",
            "locations": ["CA"]
        }
      ])

    %{
      users: Jason.decode!(users, keys: :atoms),
      features: Jason.decode!(features, keys: :atoms)
    }
  end
end

Implementation

defmodule Assignment do
  def get_user_features(user, features) when not is_nil(user) and not is_nil(features) do
    features
    |> Enum.reduce([], fn feature, matches ->
      [include_feature?(user.location, feature) | matches]
    end)
    |> Enum.reject(&is_nil/1)
  end

  defp include_feature?(location, %{id: id, locations: locations}) do
    if Enum.member?(locations, location), do: id
  end

  defp include_feature?(_, %{id: id, abTest: true}), do: id
end

Testing

defmodule Test.Assignment do
  import Assignment
  use ExUnit.Case, async: true
  use Assignment.Fixtures

  describe "Assignment.get_user_features/2" do
    test "returns feature ids for a user", %{users: users, features: features} do
      eva = Enum.at(users, 0)
      assert get_user_features(eva, features) == ["enhanced_comments", "annual_sale"]
    end

    test "nil throws an error" do
      assert_raise FunctionClauseError,
                   "no function clause matching in Assignment.get_user_features/2",
                   fn ->
                     get_user_features(nil, nil)
                   end
    end
  end
end

ExUnit.run()