Pet Shop Tutorial with exercise on callers

Adoption.sol

pragma solidity ^0.5.0;

contract Adoption {
    address[16] public adopters;
    
    // Adopting a pet
    function adopt(uint petId) public returns(uint) {
        require(petId >= 0 && petId <= 15);
        require(adopters[petId] != msg.sender, "Pet cannot be adopted by the same owners.");
        require(adopters[petId] == address(0), "Pet has owner");
        
        adopters[petId] = msg.sender;
        return petId;
    }
    
    // Retrieving adopters
    function getAdopters() public view returns(address[16] memory) {
        return adopters;
    }
}

contract AdoptionCaller {
    address[16] public adopters;
    
    function baseAdopt(address base, uint petId) public {
        Adoption oAdopt = Adoption(base);
        oAdopt.adopt(petId);
    }
    
    function callBaseAdopt(address base, uint petId) public returns(bool) {
        (bool status, bytes memory returnData) = base.call(abi.encodeWithSignature("adopt(uint256)", petId));
        return status;
    }
    
    function callBaseDelegateAdopt(address base, uint petId) public returns(bool) {
        (bool status, bytes memory returnData) = base.delegatecall(abi.encodeWithSignature("adopt(uint256)", petId));
        return status;
    }
    
    // Retrieving adopters
    function localAdopters() public view returns(address[16] memory) {
        return adopters;
    }
}

// Some other external pet caller
contract ExternalPetCaller {
    function externalDelegateAdopt(AdoptionCaller adoptCaller, address base, uint petId) public returns(bool) {
        (bool result) = adoptCaller.callBaseDelegateAdopt(base, petId);
        return result;
    }
}