Decode the multiple variables stored in one storage slot

Question

In the following code the 3 declared variables will be packed and stored in one slot together.

When the slot 0 zero is accessed to get the value we get an encoded value, like here we get 0x000000000000000000001f0190416e8285169f15346fcf9e336b6e1443b8c30a

What should we do so that we can deocode the returned data into 3 seperate readable values?

contract Storage {
    // slot 0
    address private owner = msg.sender; // 20 bytes
    bool private isTrue = true;  // 1 byte
    uint16 private u16 = 31;  // 2 bytes
}

Answer 1

The rules on storage slot packing can be found in the Solidity documentation ("Layout of State Variables in Storage"):

Multiple, contiguous items that need less than 32 bytes are packed into a single storage slot if possible, according to the following rules:

• The first item in a storage slot is stored lower-order aligned.
• Value types use only as many bytes as are necessary to store them.
• If a value type does not fit the remaining part of a storage slot, it is stored in the next storage slot.
• Structs and array data always start a new slot and their items are packed tightly according to these rules.
• Items following struct or array data always start a new storage slot.

"Lower-order aligned" means the first variable (owner) will be the right-most value in the storage slot, and each subsequent variable is filled from right to left.

The rest of the rules should be self-explanatory.

Visualization of the storage slot:

0x 000000000000000000 001f 01 90416e8285169f15346fcf9e336b6e1443b8c30a
  └─────────┬────────┴──┬─┴┬─┴──────────────────┬─────────────────────┘
       empty padding    │  └isTrue            owner
                        └u16

To decode the data, cast the hexadecimal value from the appropriate place in the storage slot to the desired type. For example, in JavaScript:

const value = '0x000000000000000000001f0190416e8285169f15346fcf9e336b6e1443b8c30a';

const owner = `0x${value.substring(26)}`; // '0x90416e8285169f15346fcf9e336b6e1443b8c30a'
const isTrue = Boolean(Number(value.substring(24, 26))); // true
const u16 = parseInt(value.substring(20, 24), 16); // 31