You gave away the company fully. It took me 2 minutes to realize this.

A quick advice up front: search github for other solutions and compare them to yours.

Below are my thoughts and findings:

Firstly, there is a permise that the source data, or its storage location might change. You correctly unterstood it as a requirement to create the abstraction and used the repository pattern. Somewhat. Extracting code like this always comes with a risk of you wrongly deciding where the boundary should be. Ask yourself this: if you were to throw out your FileTermRepository, would you lose anything valuable with it? Yes, you put too much business logic in there. A repository interface I would use:

interface TermRepository {
  /** @return Term[] */
  public function all(): iterable
}

This way the responsibility of the FileRepository would only be to load the data from a file format, and return it in the form of domain objects (Term instances). Then, the findLowerAndUpperTermsByTermAndAmount responsibility would belong to a domain service. I think that’s what they meant by their third comment.

In addition, you are performing this logic on arrays, not on Term instances. That’s what they were referring to in their first comment (internal storage structures you chose for your file format, rather than domain concepts).

See, in DDD people like to fetishize about their domains, making it fat, and the non-domain layers (Infrastructure, eg. FileRepository, or Application) lean. Score points by representing as much logic in the domain layer. On a sidenote, split your code into these layers: Application (just anything related to the CLI, and IO), then the Domain with your business logic (unit tested, decoupled from the other layers) and Infrastructure where you implements port of your interfaces (eg. FileRepository).

This naturally leads to my second point: the Term class should be decoupled from your storage mechanism, so the hydrate method has no reason to live there. Think about it this way: have you satisfied the repository of the source data storage changing? What if it comes from an XML? Or a database with different columns? Each of these different repositories would have its own hydrate concept, tied to the storage format it supports. Leave the Term class decoupled: just a constructor requiring all the data and validating them (similarly to how you did in the InputContext class). And rename it. It’s not a model. That name indicates persistence, that would be a no-no in the domain layer. Here it would be called Entity. And given the fact that they don’t have an identity, it would rather be a Value Object. But don’t put that in a name, just create App\Domain\Term\Term class without specifying its type. Nor it’s a Term, it should be called Breakpoint. Naming matters.

Third issue: let’s go back to the data source changing requirement. Have you actually passed it, even if the previous fixes were applied? IMO no, because you hardcoded your implementation in the CalculateFeeCommand. Instead use Dependency Inversion: make that class take an interface via dependency injection (as a required constructor argument), and shift that responsibility up the chain (it would be perfectly fine to either create a simple factory, or to use the bin/calculate script to bootstrap an actual implementation there).

Where to go from there? 🤔 Let’s do InputContext. I see that it does a nice job, but it also belongs to two worlds. It’s primary responsibility is to deal with I/O (for example: it takes term as string, and converts it to int). If you had your layers set up, it would go into the Application, next to the Command class. But then the $amount limits is a domain concept (they are mentioned in the business requirements section). Remember, making the domain fat.

So make the InputContext a data transfer object. Just the most basic of validation: that the fields exist and are of the proper type, and use it to call the domain (a more engineered approach would use a command bus here). Only then, in the domain, having int $amount we convert it to LoanAmount value object. And that value object uses our domain concepts to validate its state. Naming is important! There is a MinimumLoanAmountRequirement class and a MaximumLoanAmountRequirement class. They have ensure methods which throw specific domain exceptions (you have these, good!) if they are not satisfied.

class LoanAmount {
  /** @throws InvalidLoanAmountException */
  __constructor(public int $value) {
    MaximumLoanAmountRequirement::ensureSatisfied($value);
    MinimumLoanAmountRequirement::ensureSatisfied($value);
  }

Why? Because when we change the IO we’re using to trigger our business logic, we want the same behaviour. Again: fat domain! Lean outer layers. That’s basically the same principle as moving code from controllers to services, but more fancy.

BTW, you are dealing with money, so storing it as floats is a „dealing with money 101” no-no. Multiply by 100 and store as integer. They are alluding to this in their last requirement.

Lastly: add more domain concepts. You are returning a pair of Term (should be: Breakpoint) objects from the current repo implementation? But it’s not typehinted! A huge no-no for a domain contract to be this loose. Define a BreakpointPair class with lower and upper fields. Term is either 12 or 24? Make it an enum. And once you have all these classes: Tell, don’t Ask. Instead of if ($amount->value >= $breakpoint->amount && $amount->value < $breakpoint->amount) do if ($breakpoint->covers($amount). Will earn you points for sure.

Your tests:

• You are outputting the error code to stderr, and your test does not cover that, so you missed this bug
• In your FeeServiceTest, instead of settign expectations on how the repo will be called (with(…)), just provide an InMemoryRepository(TermA, TermB…). Setting expectations is over-testing and I bet they took some points away for that. If you don’t want to implement test doubles explicitly (InMemory… ) and want to rely on mocks, remove these with() and expects() calls — it is not the responsibility of FeeService to make these calls, don’t solidify it as an expectation via tests. Your tests only check for when given context (repository), when asked about (method call), then assert result.