Ch 2: Making the Shell

In this chapter, you’ll build the shell, the protective layer that hides the kernel. You’ll initialize the shell’s strength and implement a hit function to decrement it. Additionally, you’ll add a look() function with a requiredCracked modifier to ensure the kernel can only be viewed once the shell is fully broken. Estimated Time: ~10 minutes.

Defining the shell

The shell determines the Walnut’s resilience. It has an integer strength (shellStrength), which represents how many hits it can withstand before cracking. Let’s define the shell and initialize it in the constructor:

    uint256 shellStrength; // The strength of the Walnut's shell.

    constructor(uint256 _shellStrength, uint256 _kernel) {
        shellStrength = _shellStrength; // Set the initial shell strength.
        kernel = suint256(_kernel); // Initialize the kernel (cast to shielded).
    }

Adding the hit function

Each time the Walnut is hit, the shell strength decreases, simulating damage to the protective shell. This is crucial for revealing the kernel, as the shell must be fully broken for the kernel to be accessed:

    // Event to log hits
    event Hit(address indexed hitter, uint256 remainingShellStrength);

    // Modifier to ensure the shell is not cracked.
    modifier requireIntact() {
        require(shellStrength > 0, "SHELL_ALREADY_CRACKED");
        _;
    }

    // Function to hit the walnut shell
    function hit() public requireIntact {
        shellStrength--; // Decrease the shell strength.
        emit Hit(msg.sender, shellStrength); // Log the hit event.
    }

What’s happening here?

• The requireIntact modifier: Ensures that the function cannot be called if the Walnut’s shell is already broken (shellStrength == 0). This prevents underflow and unnecessary calls after the shell is fully cracked. We also add this modifier to the shake function:

    function shake(suint256 _numShakes) public requireIntact {
        kernel += _numShakes; // Increment the kernel value using the shielded parameter.
        emit Shake(msg.sender); // Log the shake event.
    }

• Decrementing the shell: Each call to hit decreases the shell’s strength (shellStrength) by one.

• Logging the action: The Hit event records the hitter’s address (msg.sender) and the remaining shell strength.

Example call:

Here’s how calling the hit function works in practice:

• Initial State: The shell strength is set to 5.

• First Hit: A player calls hit(). The shell strength decreases to 4.

• Subsequent Hits: Each additional hit reduces the shell strength by 1 until it reaches 0

Revealing the Kernel

Now that we have implemented the shell’s durability and the ability to break it using the hit function, we can introduce a new condition: the kernel should only be revealed once the shell is fully cracked.

Currently, there is no way to access the kernel’s value. However, now that we have a shell with a decreasing strength, we can apply a condition that restricts when the kernel can be seen. Specifically:

• The kernel should remain hidden while the shell is intact.

• The kernel can only be revealed once the shell’s strength reaches zero, i.e. when it is cracked.

To enforce this, we will create a function called look() , which will return the kernel’s value, but only if the Walnut has been fully cracked.

Here’s how we define look() with a requireCracked modifier:

    // Function to reveal the kernel if the shell is fully cracked.
    function look() public view requireCracked returns (uint256) {
        return uint256(kernel); // Reveal the kernel as a standard uint256.
    }
    
    // Modifier to ensure the shell is fully cracked before revealing the kernel.
    modifier requireCracked() {
        require(shellStrength == 0, "SHELL_INTACT"); // Ensure the shell is broken before revealing the kernel.
        _;
    }

What's happening here?

• Restricting Access with a Condition: The requireCracked modifier ensures that look() can only be called if shellStrength == 0, meaning the Walnut has been fully cracked.

• Revealing the Kernel: Once the condition is met, look() returns the unshielded value of the kernel.

• Preventing Premature Access: If look() is called before the shell is broken, the function will revert with the error "SHELL_INTACT".

Updated contract with hit, shake and look

// SPDX-License-Identifier: MIT License
pragma solidity ^0.8.13;

contract Walnut {
    uint256 shellStrength; // The strength of the Walnut's shell.
    suint256 kernel; // The hidden kernel (number inside the Walnut).

    // Events
    event Hit(address indexed hitter, uint256 remainingShellStrength); // Logs when the Walnut is hit.
    event Shake(address indexed shaker); // Logs when the Walnut is shaken.

    // Constructor to initialize the shell and kernel.
    constructor(uint256 _shellStrength, uint256 _kernel) {
        shellStrength = _shellStrength; // Set the initial shell strength.
        kernel = suint256(_kernel); // Initialize the kernel (cast to shielded).
    }

    // Function to hit the Walnut and reduce its shell strength.
    function hit() public requireIntact {
        shellStrength--; // Decrease the shell strength.
        emit Hit(msg.sender, shellStrength); // Log the hit action.
    }

    // Function to shake the Walnut and increment the kernel.
    function shake(suint256 _numShakes) public requireIntact {
        kernel += _numShakes; // Increment the kernel by the given number of shakes.
        emit Shake(msg.sender); // Log the shake action.
    }
    
    // Function to reveal the kernel if the shell is fully cracked.
    function look() public view requireCracked returns (uint256) {
        return uint256(kernel); // Reveal the kernel as a standard uint256.
    }
    
    // Modifier to ensure the shell is fully cracked before revealing the kernel.
    modifier requireCracked() {
        require(shellStrength == 0, "SHELL_INTACT"); // Ensure the shell is broken before revealing the kernel.
        _;
    }
    
    // Modifier to ensure the shell is not cracked.
    modifier requireIntact() {
        require(shellStrength > 0, "SHELL_ALREADY_CRACKED");
        _;
    }
    
}