advanced-evm

EVM Encoding/Decoding

abi.encode()

Concatenates given arguments and returns a bytes object. It returns the bytes with padding exactly how it would look like in the contract binary.

abi.encodePacked()

Concatenates given arguments and returns a bytes object. Difference with abi.encode: It compresses the returned bytes (no padding).

To return a string, simply wrap this in a string()

Example:

string(abi.encodePacked(stringA, stringB));

Alternative for strings: (^0.8.12)

string.concat(stringA, stringB);

abi.decode()

ABI-decodes the given bytes into the specified format.

Example:

abi.decode(BYTES_TO_DECODE, (string));

Example multi decode:

(string memory firstString, string memory secondString) = abi.decode(BYTES_TO_DECODE, (string, string));

This won't work on ABI packed encoded bytes (abi.encodePacked()), instead we can use string(abi.encodePacked(string)) to decode it.

call/staticcall

call:

call functions that changes the state of the chain.

Example:

(bool success, ) = address.call{value: ...}("")

in the brackets {} we can pass value, gasLimit, gasPrice. in the parentheses () we can send the data, in order to call a specific function.

staticcall

How at a low level we do our "pure" or "view" function calls. Read data but does not affect the state of the chain.

EVM signatures and selectors

In order to call a function using only the data field of call, we need to encode:

1. The function name
2. The func paramaters

Function signature

The function signature is a string that defines the function name and parameters

Example

transfer(address,uint256)

Function selector

The function selector is the first 4 bytes of the function signature

Example:

0xa9059cbb

To get the data needed for the call:

Step1: To get the function selector we can use:

function getSelector() public pure returns(bytes4 selector){
  selector = bytes4(keccak256(bytes("transfer(address,uint256")));
}

Step2: To Encode with the arguments:

function getDataToCallTransfer(address someAddress, uint256 someAmount) public pure returns(bytes memory) {
  return abi.encodeWithSelector(getSelector(), someAddress, someAmount);
}

Alternatively we can directly use abi.encodeWithSignature which combines step1 and step2:

function getDataToCallTransfer(address someAddress, uint256 someAmount) public pure returns(bytes memory) {
    return abi.encodeWithSignature("transfer(address,uint256)", someAddress, someAmount);
}