Solidity Snippets for Frontier Workshop 2021

Incrementor.sol

// SPDX-License-Identifier: GPL-3.0-only
pragma solidity >=0.8.0;

contract Incrementor {
    uint private value;
    address private last_caller;

    function inc() public {
        value += 1;
        last_caller = msg.sender;
    }

    function add(uint delta) public {
        value += delta;
        last_caller = msg.sender;
    }

    function get_value() view public returns (uint) {
        return value;
    }
    
    function get_last_caller() view public returns (address) {
        return last_caller;
    }
}

ModExp.sol

pragma solidity >=0.8.0;
contract ModularCheck {
        // Verify simple modular exponentiation
    constructor() {
        require(modExp(3, 5, 7) == 5);
        require(modExp(5, 7, 11) == 3);
        // This one is incorrect. Uncomment it and confirm deploy fails
        require(modExp(5, 7, 11) == 2);
    }
        // Wrapper function to use the precompile.
    // Taken from https://ethereum.stackexchange.com/a/71590/9963
    function modExp(uint256 _b, uint256 _e, uint256 _m) public returns (uint256 result) {
        assembly {
            // Free memory pointer
            let pointer := mload(0x40)
                // Define length of base, exponent and modulus. 0x20 == 32 bytes
            mstore(pointer, 0x20)
            mstore(add(pointer, 0x20), 0x20)
            mstore(add(pointer, 0x40), 0x20)
                // Define variables base, exponent and modulus
            mstore(add(pointer, 0x60), _b)
            mstore(add(pointer, 0x80), _e)

            mstore(add(pointer, 0xa0), _m)
            // Store the result
            let value := mload(0xc0)
                // Call the precompiled contract 0x05 = bigModExp
            if iszero(call(not(0), 0x05, 0, pointer, 0xc0, value, 0x20)) {
                revert(0, 0)
            }
                result := mload(value)
        }
    }
}

PalletTemplatePrecompile.sol

// SPDX-License-Identifier: GPL-3.0-only
pragma solidity >=0.8.0;

/// @author The Moonbeam Team
/// @title The interface through which solidity contracts will interact with Crowdloan Rewards
/// We follow this same interface including four-byte function selectors, in the precompile that
/// wraps the pallet
interface PalletTemplate {
    
    /// @dev Store a value in the pallet's storage.
    /// @param value The new value to store. 32 bit maximum
    function do_something(uint256 value) external;

    /// @dev Retrieve the stored value
    /// @return A uint256 (with max 32-bit value) indicating the value stored in the pallet
    function get_value() external view returns (uint256);
}