Writing test cases in Blockchain / Solidity - Time & Space Complexity

Time Complexity: Writing test cases

O(n)

Understanding Time Complexity

When writing test cases for blockchain code, it's important to know how the time to run tests grows as you add more cases.

We want to understand how the number of tests affects the total time needed to check the code.

Scenario Under Consideration

Analyze the time complexity of the following test-running code snippet.

function runTests(tests) {
  for (let i = 0; i < tests.length; i++) {
    tests[i].execute();
  }
}
    

This code runs each test case one by one from a list of tests.

Identify Repeating Operations

Look for loops or repeated actions.

• Primary operation: Calling execute() on each test case.
• How many times: Once for each test in the list.

How Execution Grows With Input

As you add more tests, the total time grows in a simple way.

Input Size (n)	Approx. Operations
10	10 calls to execute()
100	100 calls to execute()
1000	1000 calls to execute()

Pattern observation: The total work grows directly with the number of tests.

Final Time Complexity

Time Complexity: O(n)

This means if you double the number of tests, the total time to run them roughly doubles.

Common Mistake

[X] Wrong: "Running more tests doesn't affect total time much because each test is small."

[OK] Correct: Even if each test is quick, running many tests adds up, so total time grows with the number of tests.

Interview Connect

Understanding how test time grows helps you write efficient tests and shows you can think about code quality and maintenance.

Self-Check

"What if each test case itself runs a loop over data of size m? How would the time complexity change?"

Practice

1. What is the main purpose of writing test cases for blockchain smart contracts?

easy

A. To find and fix errors before deploying the contract

B. To make the contract run faster on the blockchain

C. To increase the contract's gas cost

D. To make the contract code longer

Solution

1. Step 1: Understand the role of test cases

   Test cases are written to check if the code works correctly and to catch mistakes early.

2. Step 2: Relate to blockchain deployment

   Since blockchain contracts are hard to change after deployment, testing helps avoid costly errors.

3. Final Answer:

   To find and fix errors before deploying the contract -> Option A

4. Quick Check:

   Test cases catch errors early = C [OK]

Hint: Test cases catch errors before deployment [OK]

Common Mistakes:

• Thinking tests speed up contract execution
• Believing tests increase gas cost intentionally
• Assuming tests make code longer for no reason

2. Which of the following is the correct syntax to group tests in a blockchain testing framework like Mocha?

easy

A. itGroup('Token tests', () => { /* tests */ })

B. group('Token tests', () => { /* tests */ })

C. testGroup('Token tests', () => { /* tests */ })

D. describe('Token tests', () => { /* tests */ })

Solution

1. Step 1: Recall common test grouping syntax

   In Mocha and similar frameworks, describe is used to group related tests.

2. Step 2: Identify correct function name

   Only describe is the standard function for grouping; others are incorrect or do not exist.

3. Final Answer:

   describe('Token tests', () => { /* tests */ }) -> Option D

4. Quick Check:

   Group tests with describe() = A [OK]

Hint: Use describe() to group tests clearly [OK]

Common Mistakes:

• Using group() instead of describe()
• Confusing it() with describe()
• Inventing non-existent functions like testGroup()

3. Consider this test case snippet for a smart contract function that stores a value:

it('should store the value correctly', async () => {
  await contract.store(42);
  const result = await contract.retrieve();
  assert.equal(result.toNumber(), 42);
});

What will be the output if the contract stores 42 correctly?

medium

A. Test passes with no errors

B. Test fails with assertion error

C. Syntax error in test code

D. Runtime error due to missing function

Solution

1. Step 1: Analyze the test steps

   The test calls store(42) then retrieves the value and checks if it equals 42.

2. Step 2: Understand assertion behavior

   If the contract works correctly, result.toNumber() will be 42, so assert.equal passes without error.

3. Final Answer:

   Test passes with no errors -> Option A

4. Quick Check:

   Correct value stored means test passes = A [OK]

Hint: If assert matches expected, test passes [OK]

Common Mistakes:

• Assuming test fails even if values match
• Confusing syntax errors with assertion failures
• Ignoring async/await usage in tests

4. Identify the error in this test case for transferring tokens:

it('should transfer tokens', async () => {
  await contract.transfer(receiver, 100);
  const balance = await contract.balanceOf(receiver);
  assert.equal(balance, 100);
});

medium

A. Missing await before balanceOf call

B. No error, test is correct

C. Incorrect assertion comparing balance object to number

D. transfer function called without sender address

Solution

1. Step 1: Check async calls and returns

   The balanceOf likely returns a BigNumber object, not a plain number.

2. Step 2: Verify assertion correctness

   Comparing balance directly to 100 fails because types differ; conversion to number is needed.

3. Final Answer:

   Incorrect assertion comparing balance object to number -> Option C

4. Quick Check:

   Compare values with matching types = D [OK]

Hint: Convert BigNumber to number before assert [OK]

Common Mistakes:

• Forgetting to convert BigNumber to number
• Assuming balanceOf returns a plain number
• Missing await on async calls

5. You want to write a test case that checks if a smart contract correctly rejects unauthorized users from changing the owner. Which approach is best?

hard

A. Call the changeOwner function from the owner account and expect success

B. Call the changeOwner function from an unauthorized account and expect an error

C. Call the changeOwner function without any account specified

D. Skip testing unauthorized access because it is handled by blockchain

Solution

1. Step 1: Understand test goal

   The test should verify that unauthorized users cannot change ownership.

2. Step 2: Choose test action

   Calling changeOwner from a non-owner account and expecting an error confirms access control works.

3. Step 3: Eliminate wrong options

   Testing with owner account or no account does not check unauthorized rejection; skipping test misses bugs.

4. Final Answer:

   Call the changeOwner function from an unauthorized account and expect an error -> Option B

5. Quick Check:

   Test unauthorized access by expecting failure = B [OK]

Hint: Test unauthorized calls expect errors [OK]

Common Mistakes:

• Testing only authorized calls
• Ignoring unauthorized access tests
• Assuming blockchain prevents all unauthorized actions