Strategy pattern in PHP - Time & Space Complexity

Time Complexity: Strategy pattern

O(n)

Understanding Time Complexity

When using the Strategy pattern, we want to understand how the program's running time changes as we use different strategies.

We ask: how does choosing and running a strategy affect the total work done?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

interface Strategy {
    public function execute(array $data): array;
}

class ConcreteStrategyA implements Strategy {
    public function execute(array $data): array {
        return array_map(fn($x) => $x * 2, $data);
    }
}

class Context {
    private Strategy $strategy;
    public function __construct(Strategy $strategy) {
        $this->strategy = $strategy;
    }
    public function run(array $data): array {
        return $this->strategy->execute($data);
    }
}

This code runs a chosen strategy on an array of data, transforming it according to the strategy's rules.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The array_map function inside the strategy loops over each element of the input array.
• How many times: It runs once for each item in the input array.

How Execution Grows With Input

As the input array gets bigger, the number of times the strategy's execute method processes elements grows directly with the size.

Input Size (n)	Approx. Operations
10	10 function calls inside array_map
100	100 function calls inside array_map
1000	1000 function calls inside array_map

Pattern observation: The work grows evenly as the input size grows; doubling input doubles work.

Final Time Complexity

Time Complexity: O(n)

This means the time to run the strategy grows in direct proportion to the number of items in the input.

Common Mistake

[X] Wrong: "The Strategy pattern adds extra loops and makes the program slower by a lot."

[OK] Correct: The pattern itself just chooses which code to run; it does not add extra loops beyond what the chosen strategy does.

Interview Connect

Understanding how design patterns affect time helps you explain your choices clearly and shows you think about both design and performance.

Self-Check

"What if the strategy used nested loops inside execute? How would the time complexity change?"

Practice

1.

What is the main purpose of the Strategy pattern in PHP?

easy

A. To allow changing the algorithm or behavior at runtime without modifying the context.

B. To create multiple instances of the same class.

C. To store data in a database efficiently.

D. To handle errors and exceptions in the code.

Solution

1. Step 1: Understand the Strategy pattern concept

   The Strategy pattern is designed to let you swap different algorithms or behaviors easily without changing the main code that uses them.

2. Step 2: Identify the correct purpose

   Among the options, only To allow changing the algorithm or behavior at runtime without modifying the context. describes this behavior: changing the algorithm at runtime without modifying the context class.

3. Final Answer:

   To allow changing the algorithm or behavior at runtime without modifying the context. -> Option A

4. Quick Check:

   Strategy pattern = change behavior easily [OK]

Hint: Strategy pattern swaps algorithms without changing main code [OK]

Common Mistakes:

• Confusing Strategy with Singleton or Factory patterns
• Thinking it manages data storage
• Assuming it handles errors

2.

Which of the following is the correct way to define a strategy interface in PHP?

?

easy

A. interface Strategy { function execute() {} }

B. class Strategy { function execute() {} }

C. interface Strategy { public function execute(); }

D. abstract class Strategy { public execute(); }

Solution

1. Step 1: Recall PHP interface syntax

   In PHP, interfaces are declared with the keyword interface, and methods inside must be declared without a body and with visibility (usually public).

2. Step 2: Check each option

   interface Strategy { public function execute(); } correctly declares an interface with a public method without a body. class Strategy { function execute() {} } uses a class, not interface. interface Strategy { function execute() {} } incorrectly provides a method body inside interface. abstract class Strategy { public execute(); } uses abstract class but misses parentheses and visibility.

3. Final Answer:

   interface Strategy { public function execute(); } -> Option C

4. Quick Check:

   Interface method = declaration only [OK]

Hint: Interface methods have no body, use 'public function name();' [OK]

Common Mistakes:

• Adding method body inside interface
• Using class instead of interface
• Omitting visibility keyword

3.

What will be the output of the following PHP code using the Strategy pattern?

interface Strategy {
    public function execute();
}

class AddStrategy implements Strategy {
    public function execute() {
        return 5 + 3;
    }
}

class MultiplyStrategy implements Strategy {
    public function execute() {
        return 5 * 3;
    }
}

class Context {
    private Strategy $strategy;
    public function __construct(Strategy $strategy) {
        $this->strategy = $strategy;
    }
    public function executeStrategy() {
        return $this->strategy->execute();
    }
}

$context = new Context(new MultiplyStrategy());
echo $context->executeStrategy();

medium

A. 8

B. 15

C. 53

D. Error: Cannot instantiate interface

Solution

1. Step 1: Understand the code flow

   The code defines two strategies: AddStrategy returns 5+3=8, MultiplyStrategy returns 5*3=15. The Context class uses the strategy passed in constructor.

2. Step 2: Identify which strategy is used

   The context is created with new MultiplyStrategy(), so calling executeStrategy() returns 15.

3. Final Answer:

   15 -> Option B

4. Quick Check:

   MultiplyStrategy returns 15 [OK]

Hint: Check which strategy object is passed to context [OK]

Common Mistakes:

• Confusing AddStrategy and MultiplyStrategy output
• Thinking interface cannot be instantiated (only concrete classes)
• Ignoring which strategy is passed

4.

Identify the error in the following PHP code implementing the Strategy pattern:

interface Strategy {
    public function execute();
}

class ConcreteStrategy implements Strategy {
    public function execute() {
        echo "Running strategy";
    }
}

class Context {
    private $strategy;
    public function __construct(Strategy $strategy) {
        $this->strategy = $strategy;
    }
    public function executeStrategy() {
        $this->strategy->execute;
    }
}

$context = new Context(new ConcreteStrategy());
$context->executeStrategy();

medium

A. Cannot instantiate interface directly

B. Strategy interface missing method body

C. Context constructor missing type hint

D. Missing parentheses when calling execute method

Solution

1. Step 1: Check method call syntax in Context

   Inside executeStrategy(), the code calls $this->strategy->execute; without parentheses, so the method is not actually called.

2. Step 2: Identify correct fix

   Method calls require parentheses even if no arguments: $this->strategy->execute();.

3. Final Answer:

   Missing parentheses when calling execute method -> Option D

4. Quick Check:

   Method call needs () [OK]

Hint: Always use () to call methods in PHP [OK]

Common Mistakes:

• Forgetting parentheses on method calls
• Confusing interface method declaration with implementation
• Ignoring type hints in constructor

5.

You want to add a new strategy SubtractStrategy to the existing PHP Strategy pattern code. Which of the following is the best way to implement it?

interface Strategy {
    public function execute(int $a, int $b): int;
}

class AddStrategy implements Strategy {
    public function execute(int $a, int $b): int {
        return $a + $b;
    }
}

// Your code here

hard

A. class SubtractStrategy implements Strategy {
public function execute(int $a, int $b): int {
return $a - $b;
}
}

B. class SubtractStrategy {
public function execute(int $a, int $b): int {
return $a - $b;
}
}

C. class SubtractStrategy implements Strategy {
public function execute(int $a, int $b) {
return $a - $b;
}
}

D. class SubtractStrategy implements Strategy {
public function execute($a, $b): int {
return $a - $b;
}
}

Solution

1. Step 1: Check interface method signature

   The interface requires execute(int $a, int $b): int with type hints and return type.

2. Step 2: Match method signature exactly in SubtractStrategy

   class SubtractStrategy implements Strategy { public function execute(int $a, int $b): int { return $a - $b; } } matches the interface exactly with parameter types and return type. class SubtractStrategy { public function execute(int $a, int $b): int { return $a - $b; } } misses implementing the interface. class SubtractStrategy implements Strategy { public function execute(int $a, int $b) { return $a - $b; } } misses return type. class SubtractStrategy implements Strategy { public function execute($a, $b): int { return $a - $b; } } misses parameter types.

3. Final Answer:

   class SubtractStrategy implements Strategy { public function execute(int $a, int $b): int { return $a - $b; } } -> Option A

4. Quick Check:

   Implement interface with exact method signature [OK]

Hint: Match interface method signature exactly when implementing [OK]

Common Mistakes:

• Omitting interface implementation
• Missing type hints or return types
• Changing method signature