Timelock pattern in Blockchain / Solidity - Time & Space Complexity

Time Complexity: Timelock pattern

O(n)

Understanding Time Complexity

When using the Timelock pattern in blockchain, it's important to know how the time to execute changes as more actions are queued.

We want to see how the number of queued transactions affects the time to process them.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

function executeTransactions() public {
  for (uint i = 0; i < queuedTransactions.length; i++) {
    if (block.timestamp >= queuedTransactions[i].executeAfter) {
      queuedTransactions[i].execute();
      removeTransaction(i);
      i--;
    }
  }
}
    

This code goes through all queued transactions and executes those whose time has come.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping through the list of queued transactions.
• How many times: Once for each transaction in the queue.

How Execution Grows With Input

As the number of queued transactions grows, the time to check and execute them grows too.

Input Size (n)	Approx. Operations
10	About 10 checks and possible executions
100	About 100 checks and possible executions
1000	About 1000 checks and possible executions

Pattern observation: The work grows directly with the number of queued transactions.

Final Time Complexity

Time Complexity: O(n)

This means the time to execute grows in a straight line as more transactions are queued.

Common Mistake

[X] Wrong: "Executing transactions takes the same time no matter how many are queued."

[OK] Correct: Because the code checks each transaction one by one, more transactions mean more work and more time.

Interview Connect

Understanding how loops over queued actions affect performance shows you can think about real blockchain contract costs and user experience.

Self-Check

"What if we stored transactions in a data structure that lets us quickly find only those ready to execute? How would the time complexity change?"

Practice

1.

What is the main purpose of the Timelock pattern in blockchain smart contracts?

easy

A. To delay certain actions until a specific time has passed

B. To speed up transaction processing

C. To encrypt user data

D. To reduce gas fees

Solution

1. Step 1: Understand the Timelock pattern concept

   The Timelock pattern is designed to delay actions in smart contracts until a set time has passed.

2. Step 2: Identify the purpose of the delay

   This delay helps protect users by preventing instant changes that could be harmful or unexpected.

3. Final Answer:

   To delay certain actions until a specific time has passed -> Option A

4. Quick Check:

   Timelock pattern = delay actions [OK]

Hint: Timelock means waiting before action happens [OK]

Common Mistakes:

• Thinking it speeds up transactions
• Confusing with encryption
• Assuming it lowers gas fees

2.

Which of the following Solidity code snippets correctly enforces a timelock using block.timestamp?

function execute() public {
  require(__________, "Too early to execute");
  // action code
}

easy

A. block.timestamp >= unlockTime

B. block.timestamp < unlockTime

C. block.number >= unlockTime

D. block.difficulty > unlockTime

Solution

1. Step 1: Understand the condition for timelock

   The action should only execute if the current time is equal or after the unlock time.

2. Step 2: Choose the correct comparison

   Using block.timestamp >= unlockTime ensures the function runs only after the unlock time.

3. Final Answer:

   block.timestamp >= unlockTime -> Option A

4. Quick Check:

   Time check uses block.timestamp >= unlockTime [OK]

Hint: Use block.timestamp and >= for timelock checks [OK]

Common Mistakes:

• Using < instead of >=
• Using block.number instead of block.timestamp
• Using unrelated block properties

3.

What will be the output of the following Solidity function call if block.timestamp is 1650000000 and unlockTime is 1650000100?

function canExecute() public view returns (bool) {
  return block.timestamp >= unlockTime;
}

medium

A. true

B. Revert with error

C. Compilation error

D. false

Solution

1. Step 1: Compare block.timestamp and unlockTime values

   Given block.timestamp = 1650000000 and unlockTime = 1650000100, block.timestamp is less than unlockTime.

2. Step 2: Evaluate the return statement

   The expression block.timestamp >= unlockTime evaluates to false.

3. Final Answer:

   false -> Option D

4. Quick Check:

   1650000000 >= 1650000100 = false [OK]

Hint: Compare timestamps carefully for true/false output [OK]

Common Mistakes:

• Assuming >= means true when timestamp is smaller
• Confusing block.timestamp with block.number
• Expecting errors instead of boolean

4.

Identify the error in this Solidity timelock function and choose the fix:

uint256 public unlockTime;

function execute() public {
  require(block.timestamp > unlockTime, "Too early");
  // perform action
}

medium

A. Use block.number instead of block.timestamp

B. Change block.timestamp > unlockTime to block.timestamp >= unlockTime

C. Remove the require statement

D. Change unlockTime to block.timestamp

Solution

1. Step 1: Analyze the require condition

   The condition block.timestamp > unlockTime disallows execution exactly at unlockTime.

2. Step 2: Adjust condition to allow execution at unlockTime

   Changing to block.timestamp >= unlockTime allows execution starting from unlockTime.

3. Final Answer:

   Change block.timestamp > unlockTime to block.timestamp >= unlockTime -> Option B

4. Quick Check:

   Use >= to include unlockTime moment [OK]

Hint: Use >= to allow execution at unlock time [OK]

Common Mistakes:

• Using > excludes unlockTime moment
• Removing require loses protection
• Using block.number causes wrong timing

5.

You want to create a timelock contract that allows an admin to schedule a withdrawal only after 1 day from scheduling. Which approach correctly implements this?

contract Timelock {
  address public admin;
  uint256 public unlockTime;

  constructor() {
    admin = msg.sender;
  }

  function scheduleWithdrawal() public {
    require(msg.sender == admin, "Not admin");
    unlockTime = block.timestamp + 86400; // 1 day
  }

  function withdraw() public {
    require(msg.sender == admin, "Not admin");
    require(block.timestamp >= unlockTime, "Too early");
    // withdrawal logic
  }
}

hard

A. Admin cannot schedule withdrawal

B. Withdrawal can happen immediately after scheduling

C. Correctly enforces 1-day delay before withdrawal

D. unlockTime is set incorrectly causing errors

Solution

1. Step 1: Check scheduling sets unlockTime correctly

   The scheduleWithdrawal function sets unlockTime to current time plus 86400 seconds (1 day).

2. Step 2: Verify withdraw enforces timelock

   The withdraw function requires current time to be at or after unlockTime, enforcing the delay.

3. Final Answer:

   Correctly enforces 1-day delay before withdrawal -> Option C

4. Quick Check:

   UnlockTime = now + 1 day, withdraw requires >= unlockTime [OK]

Hint: Add 86400 seconds and check block.timestamp >= unlockTime [OK]

Common Mistakes:

• Not adding delay in schedule function
• Using > instead of >= in withdraw
• Not restricting functions to admin