Repository pattern in PHP - Time & Space Complexity

Time Complexity: Repository pattern

O(n)

Understanding Time Complexity

When using the Repository pattern, it's important to understand how the time it takes to get data changes as the data grows.

We want to know how the number of operations grows when fetching or saving many items.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

class UserRepository {
    private array $users = [];

    public function addUser(array $user): void {
        $this->users[] = $user;
    }

    public function findUserByEmail(string $email): ?array {
        foreach ($this->users as $user) {
            if ($user['email'] === $email) {
                return $user;
            }
        }
        return null;
    }
}
    

This code stores users in an array and searches for a user by email using a loop.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping through the users array to find a matching email.
• How many times: Up to once for each user in the array until a match is found or all checked.

How Execution Grows With Input

As the number of users grows, the search takes longer because it may check more users.

Input Size (n)	Approx. Operations
10	Up to 10 checks
100	Up to 100 checks
1000	Up to 1000 checks

Pattern observation: The number of checks grows roughly in direct proportion to the number of users.

Final Time Complexity

Time Complexity: O(n)

This means the time to find a user grows linearly with the number of users stored.

Common Mistake

[X] Wrong: "Searching in the repository is always fast and constant time."

[OK] Correct: Because the repository uses a simple array and loops through it, the search time grows as more users are added.

Interview Connect

Understanding how data retrieval time grows helps you explain design choices and shows you can think about efficiency in real projects.

Self-Check

"What if we changed the users storage from an array to a hash map keyed by email? How would the time complexity change?"

Practice

1. What is the main purpose of the Repository pattern in PHP?

easy

A. To separate data access logic from business logic

B. To directly connect the database with the user interface

C. To write all code inside a single class

D. To avoid using interfaces in the code

Solution

1. Step 1: Understand the role of Repository pattern

   The Repository pattern is designed to separate how data is accessed from how the business logic works.

2. Step 2: Identify the correct purpose

   Separating data access from business logic makes code cleaner and easier to maintain.

3. Final Answer:

   To separate data access logic from business logic -> Option A

4. Quick Check:

   Repository pattern = separation of concerns [OK]

Hint: Remember: Repository separates data logic from business logic [OK]

Common Mistakes:

• Thinking Repository connects UI directly to database
• Believing all code should be in one class
• Ignoring the use of interfaces

2. Which of the following is the correct way to declare a Repository interface in PHP?

easy

A. UserRepository interface { public function findUser(int $id); }

B. class UserRepository { public function findUser(int $id); }

C. interface UserRepository { function findUser; }

D. interface UserRepository { public function findUser(int $id); }

Solution

1. Step 1: Recall PHP interface syntax

   Interfaces use the keyword interface followed by method declarations with parentheses and types.

2. Step 2: Check each option

   interface UserRepository { public function findUser(int $id); } correctly declares an interface with a method having a typed parameter. class UserRepository { public function findUser(int $id); } uses class instead of interface. interface UserRepository { function findUser; } misses parentheses. UserRepository interface { public function findUser(int $id); } has wrong syntax order.

3. Final Answer:

   interface UserRepository { public function findUser(int $id); } -> Option D

4. Quick Check:

   Interface syntax = interface UserRepository { public function findUser(int $id); } [OK]

Hint: Interfaces use 'interface' keyword and method signatures with parentheses [OK]

Common Mistakes:

• Using 'class' instead of 'interface'
• Omitting parentheses in method declaration
• Wrong keyword order in interface declaration

3. Given the following PHP code, what will be the output?

interface UserRepository {
    public function findUser(int $id);
}

class UserRepo implements UserRepository {
    public function findUser(int $id) {
        return "User_" . $id;
    }
}

$repo = new UserRepo();
echo $repo->findUser(5);

medium

A. 5

B. User_5

C. UserRepo

D. Error: Interface method not implemented

Solution

1. Step 1: Understand interface and class implementation

   The class UserRepo implements UserRepository and defines the method findUser that returns "User_" concatenated with the id.

2. Step 2: Trace the method call

   Calling $repo->findUser(5) returns "User_5" which is then printed.

3. Final Answer:

   User_5 -> Option B

4. Quick Check:

   Method returns 'User_' + id = User_5 [OK]

Hint: Look for string concatenation in method return [OK]

Common Mistakes:

• Expecting just the number 5 as output
• Confusing class name with output
• Thinking interface causes error if implemented correctly

4. Identify the error in this PHP Repository implementation:

interface ProductRepository {
    public function getProduct(int $id);
}

class ProductRepo implements ProductRepository {
    public function getProduct() {
        return "Product";
    }
}

medium

A. Class name should be ProductRepository, not ProductRepo

B. Interface keyword is misspelled

C. Method getProduct() missing parameter in class implementation

D. Return type is missing in interface method

Solution

1. Step 1: Compare interface and class method signatures

   The interface method getProduct requires an int $id parameter, but the class method has no parameters.

2. Step 2: Understand PHP interface implementation rules

   When implementing an interface, method signatures must match exactly, including parameters.

3. Final Answer:

   Method getProduct() missing parameter in class implementation -> Option C

4. Quick Check:

   Method signatures must match [OK]

Hint: Check method parameters match interface exactly [OK]

Common Mistakes:

• Ignoring parameter mismatch
• Thinking class name must match interface name
• Assuming return type is mandatory in interface

5. You want to create a flexible Repository pattern for managing users and products. Which design approach best supports adding new data types without changing existing code?

hard

A. Create separate interfaces like UserRepository and ProductRepository, then implement them in separate classes

B. Write one big class handling all data types with many methods

C. Use global functions to access data directly

D. Store all data in arrays inside a single class without interfaces

Solution

1. Step 1: Understand flexibility in Repository pattern

   Using separate interfaces and classes for each data type allows adding new repositories without changing existing ones.

2. Step 2: Evaluate options for maintainability

   Create separate interfaces like UserRepository and ProductRepository, then implement them in separate classes supports open/closed principle: open for extension, closed for modification. Other options mix concerns or lack structure.

3. Final Answer:

   Create separate interfaces like UserRepository and ProductRepository, then implement them in separate classes -> Option A

4. Quick Check:

   Separate interfaces = flexible extension [OK]

Hint: Use separate interfaces and classes for each data type [OK]

Common Mistakes:

• Putting all logic in one big class
• Using global functions instead of classes
• Avoiding interfaces reduces flexibility