Packing variables for gas savings in Blockchain / Solidity - Time & Space Complexity

Time Complexity: Packing variables for gas savings

O(n)

Understanding Time Complexity

When writing smart contracts, saving gas means saving money. Packing variables helps reduce storage cost.

We want to see how packing affects the number of operations as data size grows.

Scenario Under Consideration

Analyze the time complexity of this Solidity snippet packing variables:

contract Packed {
    uint128 a;
    uint128 b;
    uint256 c;

    function setValues(uint128 _a, uint128 _b, uint256 _c) public {
        a = _a;
        b = _b;
        c = _c;
    }
}
    

This code packs two 128-bit variables into one 256-bit storage slot, then sets values.

Identify Repeating Operations

Look for repeated actions that cost gas.

• Primary operation: Writing to storage slots.
• How many times: Three assignments happen, but two fit in one slot.

How Execution Grows With Input

As you add more variables, storage writes increase.

Input Size (variables)	Approx. Storage Writes
2 (packed)	1
3 (unpacked)	3
10 (packed)	5

Pattern observation: Packing reduces the number of storage writes roughly by grouping variables.

Final Time Complexity

Time Complexity: O(n)

This means the number of storage writes grows linearly with the number of variables, but packing reduces the constant factor.

Common Mistake

[X] Wrong: "Packing variables changes the time complexity class."

[OK] Correct: Packing only reduces how many times you write, but the growth still depends on the number of variables.

Interview Connect

Understanding how packing affects gas cost shows you know how to write efficient smart contracts, a valuable skill in blockchain development.

Self-Check

"What if we added dynamic arrays instead of fixed variables? How would the time complexity change?"

Practice

1.

What is the main benefit of packing variables in blockchain smart contracts?

easy

A. It makes the code run faster on the CPU

B. It reduces gas costs by using fewer storage slots

C. It increases the contract's storage size

D. It improves the contract's security automatically

Solution

1. Step 1: Understand storage slots in blockchain

   Each storage slot costs gas. Packing variables reduces the number of slots used.

2. Step 2: Effect of packing variables

   By grouping smaller variables into one slot, gas usage decreases because fewer slots are needed.

3. Final Answer:

   It reduces gas costs by using fewer storage slots -> Option B

4. Quick Check:

   Packing = Less gas [OK]

Hint: Packing saves gas by reducing storage slots used [OK]

Common Mistakes:

• Thinking packing speeds up CPU execution
• Believing packing increases storage size
• Assuming packing improves security automatically

2.

Which of the following Solidity variable declarations correctly packs two uint8 variables into one storage slot?

contract Example {
    uint8 a;
    uint8 b;
}

easy

A. Both a and b are declared consecutively as uint8

B. a is uint8 and b is uint256

C. a is uint256 and b is uint8

D. Both a and b are declared as uint256

Solution

1. Step 1: Check variable types for packing

   Variables of the same small type declared consecutively can be packed into one slot.

2. Step 2: Analyze given declarations

   Both a and b are uint8, so they fit together in one 256-bit slot.

3. Final Answer:

   Both a and b are declared consecutively as uint8 -> Option A

4. Quick Check:

   Same small types together = packed [OK]

Hint: Use same small types consecutively to pack [OK]

Common Mistakes:

• Mixing large and small types breaks packing
• Declaring variables far apart prevents packing
• Assuming different types pack automatically

3.

Consider this Solidity contract snippet:

contract Test {
    uint128 a = 1;
    uint64 b = 2;
    uint64 c = 3;
}

How many storage slots does this contract use for these variables?

medium

A. 1 storage slot

B. 3 storage slots

C. 2 storage slots

D. 4 storage slots

Solution

1. Step 1: Calculate sizes of variables

   a is 128 bits, b and c are 64 bits each.

2. Step 2: Pack variables into 256-bit slots

   a (128 bits) starts the slot, b and c (64+64=128 bits) fit exactly in the remaining space of the same slot.

3. Final Answer:

   1 storage slot -> Option A

4. Quick Check:

   128 + 64 + 64 = 256 bits = 1 slot [OK]

Hint: Sum consecutive vars' bits: 128+64+64=256 -> 1 slot [OK]

Common Mistakes:

• Counting each variable as one slot
• Ignoring packing into remaining space after uint128
• Assuming different sizes cannot pack together

4.

Identify the error in this Solidity contract that tries to pack variables for gas savings:

contract Faulty {
    uint256 a;
    uint8 b;
    uint256 d;
    uint8 c;
}

medium

A. Variables b and c must be declared before a

B. Variables a and d should be uint8 to pack

C. All variables are packed correctly without error

D. Variables b and c cannot be packed because d breaks the sequence

Solution

1. Step 1: Understand packing order

   Packing works best when small variables are declared consecutively without large variables interrupting.

2. Step 2: Analyze variable order

   b and c are small but separated by large a and d, so they cannot be packed together.

3. Final Answer:

   Variables b and c cannot be packed because d breaks the sequence -> Option D

4. Quick Check:

   Interrupting large vars break packing [OK]

Hint: Keep small vars together without large vars interrupting [OK]

Common Mistakes:

• Assuming all small vars pack regardless of order
• Changing types unnecessarily
• Thinking order does not affect packing

5.

You want to optimize this Solidity struct for gas savings by packing variables:

struct Data {
    uint8 flag;
    uint256 id;
    uint16 count;
    uint8 status;
}

Which rearrangement packs variables best into the fewest storage slots?

hard

A. struct Data {
uint8 flag;
uint256 id;
uint16 count;
uint8 status;
}

B. struct Data {
uint16 count;
uint256 id;
uint8 flag;
uint8 status;
}

C. struct Data {
uint8 flag;
uint8 status;
uint16 count;
uint256 id;
}

D. struct Data {
uint8 flag;
uint16 count;
uint256 id;
uint8 status;
}

Solution

1. Step 1: Order variables from smallest to largest

   Place small variables (uint8, uint16) together first to pack tightly.

2. Step 2: Place largest variable last

   uint256 id is largest and should be last to avoid breaking packing of smaller vars.

3. Final Answer:

   struct Data { uint8 flag; uint8 status; uint16 count; uint256 id; } -> Option C

4. Quick Check:

   Small vars first, large last = best packing [OK]

Hint: Put small vars first, big vars last for best packing [OK]

Common Mistakes:

• Placing large vars before small ones
• Mixing variable sizes without order
• Ignoring packing benefits in structs