Retrieve the storage slot value of a (mapping => Struct) -> (mapping => uint) in Solidity.

get_inner_struct_mapping.ts

/*
architecture is like so:

Struct struct {
   ...
   mapping(address => uint) map;
   ...
}

contract X {
  ...
  mapping(address => struct) structs;
  ...
}

ie. base_slot corresponds to storage position in contract X, inner_slot corresponds to storage position in Struct struct.

Rationale: this is useful to get slots where there is no getters available for this kind of (complex) setup.
*/

export async function getInnerStructMapping(contract: string, address: string, base_slot: number, inner_slot: number): Promise<BigNumber> {
    // first get the start slot of the struct for the address parameter
    const struct_slot = ethers.utils.keccak256( ethers.utils.defaultAbiCoder.encode(['address', 'uint'],[address, base_slot]))
    
    // then, get the start slot of the 'map' mapping
    const struct_base_slot = ethers.BigNumber.from(struct_slot).add(inner_slot).toHexString()
    
    // Finally, get the slot of the address => uint mapping
    const maps_slot = ethers.utils.keccak256( ethers.utils.defaultAbiCoder.encode(['address', 'uint'],[address, struct_base_slot]))
    
    // get the storage slot value and parse as a BigNumber
    const result = ethers.BigNumber.from(await ethers.provider.getStorageAt(contract, maps_slot));
    return result;
}