Lesson 7: FunWithStorage.sol Mapping Location & Data

[PCWCFA]

This question is about finding the location and data of the mappings.

The FunWithStorage.sol comment states a mapping's location can be found using the storage location of the mapping and the value of the key.

uint256 favoriteNumber; // Stored at slot 0
bool someBool; // Stored at slot 1
uint256[] myArray; /* Array Length Stored at slot 2,
  but the objects will be the keccak256(2), since 2 is the storage slot of the array */
mapping(uint256 => bool) myMap; /* An empty slot is held at slot 3
  and the elements will be stored at keccak256(h(k) . p)

  p: The storage slot (aka, 3)
  k: The key in hex
  h: Some function based on the type. For uint256, it just pads the hex
  */

   …
  
  constructor() {
    …
   myMap[0] = true; // SSTORE
  }

Given the above, then

p = 3 because myMap is the fourth state variable declared in the contract.
k = 0 because myMapp[0].

Is the below the correct implementation of the formula in the Solidity comment? Here's the ethers.utils.keccak256 documentation.

const firstMappingLocation = ethers.utils.keccak256([
    0x0000000000000000000000000000000000000000000000000000000000000000,  // key padded
    0x0000000000000000000000000000000000000000000000000000000000000003  // storage location
  ]);

const firstMappingElement = await ethers.provider.getStorageAt(
    funWithStorage.address,
    firstMappingLocation
  );

No, I have not been able to get the 0x1 representing the true stored at myMap[0]. Please advise.

[synix]

I use code below to calculate the myMap[0] in FunWithStorage contract:

const abiCoder = new ethers.utils.AbiCoder()
const secondElementLocation = ethers.utils.keccak256(
  ethers.utils.concat([
    abiCoder.encode(["uint256"], ["0"]),
    "0x0000000000000000000000000000000000000000000000000000000000000003",
  ])
)

const mapElement = await ethers.provider.getStorageAt(
  funWithStorage.address,
  secondElementLocation
)

log(`Location ${secondElementLocation}: ${mapElement}`)

[PCWCFA]

@synix Thanks! Your code works!