Variable packing in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to pack two uint128 variables into one uint256 variable.

Blockchain / Solidity

uint256 packed = (uint256(a) << [1]) | uint256(b);

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A128

B64

C256

D32

Attempts:

3 left

💡 Hint

Common Mistakes

Using 64 instead of 128 bits for shifting.

Not shifting at all.

Shifting by 256 bits which is too large.

We shift a by 128 bits to the left to pack it into the higher half of the 256-bit variable.

2fill in blank

medium

Complete the code to unpack the higher 128 bits from a packed uint256 variable.

Blockchain / Solidity

uint128 a = uint128(packed >> [1]);

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A256

B32

C128

D64

Attempts:

3 left

💡 Hint

Common Mistakes

Shifting by 64 bits instead of 128.

Not shifting before casting.

Shifting by 256 bits which clears all bits.

We shift right by 128 bits to get the higher 128 bits into the lower part, then cast to uint128.

3fill in blank

hard

Fix the error in unpacking the lower 128 bits from the packed uint256 variable.

Blockchain / Solidity

uint128 b = uint128(packed [1] 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF);

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A&

B^

C|

D>>

Attempts:

3 left

💡 Hint

Common Mistakes

Using bitwise OR which sets bits incorrectly.

Using XOR which toggles bits.

Using right shift which moves bits instead of masking.

We use bitwise AND & with a mask to extract the lower 128 bits.

4fill in blank

hard

Fill both blanks to pack three uint85 variables into one uint256 variable.

Blockchain / Solidity

uint256 packed = (uint256(x) << [1]) | (uint256(y) << [2]) | uint256(z);

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A170

B85

C128

D64

Attempts:

3 left

💡 Hint

Common Mistakes

Using 128 bits shift which is too large for 85-bit variables.

Using 64 bits shift which is too small.

Mixing up the order of shifts.

Shift x by 170 bits (85*2) and y by 85 bits to pack three 85-bit values.

5fill in blank

hard

Fill all three blanks to unpack three uint85 variables from a packed uint256 variable.

Blockchain / Solidity

uint85 x = uint85(packed >> [1]);
uint85 y = uint85((packed >> [2]) & [3]);

Drag options to blanks, or click blank then click option'

A170

B85

C0x1FFFFFFFFFFFFFFFFFF

D128

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect mask size.

Using wrong shift amounts.

Not masking after shifting.

Shift x by 170 bits, y by 85 bits, and mask with 0x1FFFFFFFFFFFFFFFFFF to extract 85 bits.