openzeppelin-contracts/contracts/governance/utils/Votes.sol

// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts (last updated v4.8.0) (governance/utils/Votes.sol)
pragma solidity ^0.8.0;

import "../../utils/Context.sol";
import "../../utils/Counters.sol";
import "../../utils/Checkpoints.sol";
import "../../utils/cryptography/EIP712.sol";
import "./IVotes.sol";

/**
 * @dev This is a base abstract contract that tracks voting units, which are a measure of voting power that can be
 * transferred, and provides a system of vote delegation, where an account can delegate its voting units to a sort of
 * "representative" that will pool delegated voting units from different accounts and can then use it to vote in
 * decisions. In fact, voting units _must_ be delegated in order to count as actual votes, and an account has to
 * delegate those votes to itself if it wishes to participate in decisions and does not have a trusted representative.
 *
 * This contract is often combined with a token contract such that voting units correspond to token units. For an
 * example, see {ERC721Votes}.
 *
 * The full history of delegate votes is tracked on-chain so that governance protocols can consider votes as distributed
 * at a particular block number to protect against flash loans and double voting. The opt-in delegate system makes the
 * cost of this history tracking optional.
 *
 * When using this module the derived contract must implement {_getVotingUnits} (for example, make it return
 * {ERC721-balanceOf}), and can use {_transferVotingUnits} to track a change in the distribution of those units (in the
 * previous example, it would be included in {ERC721-_beforeTokenTransfer}).
 *
 * _Available since v4.5._
 */
abstract contract Votes is IVotes, Context, EIP712 {
    using Checkpoints for Checkpoints.History;
    using Counters for Counters.Counter;

    bytes32 private constant _DELEGATION_TYPEHASH =
        keccak256("Delegation(address delegatee,uint256 nonce,uint256 expiry)");

    mapping(address => address) private _delegation;
    mapping(address => Checkpoints.History) private _delegateCheckpoints;
    Checkpoints.History private _totalCheckpoints;

    mapping(address => Counters.Counter) private _nonces;

    /**
     * @dev Returns the current amount of votes that `account` has.
     */
    function getVotes(address account) public view virtual override returns (uint256) {
        return _delegateCheckpoints[account].latest();
    }

    /**
     * @dev Returns the amount of votes that `account` had at the end of a past block (`blockNumber`).
     *
     * Requirements:
     *
     * - `blockNumber` must have been already mined
     */
    function getPastVotes(address account, uint256 blockNumber) public view virtual override returns (uint256) {
        return _delegateCheckpoints[account].getAtProbablyRecentBlock(blockNumber);
    }

    /**
     * @dev Returns the total supply of votes available at the end of a past block (`blockNumber`).
     *
     * NOTE: This value is the sum of all available votes, which is not necessarily the sum of all delegated votes.
     * Votes that have not been delegated are still part of total supply, even though they would not participate in a
     * vote.
     *
     * Requirements:
     *
     * - `blockNumber` must have been already mined
     */
    function getPastTotalSupply(uint256 blockNumber) public view virtual override returns (uint256) {
        require(blockNumber < block.number, "Votes: block not yet mined");
        return _totalCheckpoints.getAtProbablyRecentBlock(blockNumber);
    }

    /**
     * @dev Returns the current total supply of votes.
     */
    function _getTotalSupply() internal view virtual returns (uint256) {
        return _totalCheckpoints.latest();
    }

    /**
     * @dev Returns the delegate that `account` has chosen.
     */
    function delegates(address account) public view virtual override returns (address) {
        return _delegation[account];
    }

    /**
     * @dev Delegates votes from the sender to `delegatee`.
     */
    function delegate(address delegatee) public virtual override {
        address account = _msgSender();
        _delegate(account, delegatee);
    }

    /**
     * @dev Delegates votes from signer to `delegatee`.
     */
    function delegateBySig(
        address delegatee,
        uint256 nonce,
        uint256 expiry,
        uint8 v,
        bytes32 r,
        bytes32 s
    ) public virtual override {
        require(block.timestamp <= expiry, "Votes: signature expired");
        address signer = ECDSA.recover(
            _hashTypedDataV4(keccak256(abi.encode(_DELEGATION_TYPEHASH, delegatee, nonce, expiry))),
            v,
            r,
            s
        );
        require(nonce == _useNonce(signer), "Votes: invalid nonce");
        _delegate(signer, delegatee);
    }

    /**
     * @dev Delegate all of `account`'s voting units to `delegatee`.
     *
     * Emits events {IVotes-DelegateChanged} and {IVotes-DelegateVotesChanged}.
     */
    function _delegate(address account, address delegatee) internal virtual {
        address oldDelegate = delegates(account);
        _delegation[account] = delegatee;

        emit DelegateChanged(account, oldDelegate, delegatee);
        _moveDelegateVotes(oldDelegate, delegatee, _getVotingUnits(account));
    }

    /**
     * @dev Transfers, mints, or burns voting units. To register a mint, `from` should be zero. To register a burn, `to`
     * should be zero. Total supply of voting units will be adjusted with mints and burns.
     */
    function _transferVotingUnits(address from, address to, uint256 amount) internal virtual {
        if (from == address(0)) {
            _totalCheckpoints.push(_add, amount);
        }
        if (to == address(0)) {
            _totalCheckpoints.push(_subtract, amount);
        }
        _moveDelegateVotes(delegates(from), delegates(to), amount);
    }

    /**
     * @dev Moves delegated votes from one delegate to another.
     */
    function _moveDelegateVotes(address from, address to, uint256 amount) private {
        if (from != to && amount > 0) {
            if (from != address(0)) {
                (uint256 oldValue, uint256 newValue) = _delegateCheckpoints[from].push(_subtract, amount);
                emit DelegateVotesChanged(from, oldValue, newValue);
            }
            if (to != address(0)) {
                (uint256 oldValue, uint256 newValue) = _delegateCheckpoints[to].push(_add, amount);
                emit DelegateVotesChanged(to, oldValue, newValue);
            }
        }
    }

    function _add(uint256 a, uint256 b) private pure returns (uint256) {
        return a + b;
    }

    function _subtract(uint256 a, uint256 b) private pure returns (uint256) {
        return a - b;
    }

    /**
     * @dev Consumes a nonce.
     *
     * Returns the current value and increments nonce.
     */
    function _useNonce(address owner) internal virtual returns (uint256 current) {
        Counters.Counter storage nonce = _nonces[owner];
        current = nonce.current();
        nonce.increment();
    }

    /**
     * @dev Returns an address nonce.
     */
    function nonces(address owner) public view virtual returns (uint256) {
        return _nonces[owner].current();
    }

    /**
     * @dev Returns the contract's {EIP712} domain separator.
     */
    // solhint-disable-next-line func-name-mixedcase
    function DOMAIN_SEPARATOR() external view returns (bytes32) {
        return _domainSeparatorV4();
    }

    /**
     * @dev Must return the voting units held by an account.
     */
    function _getVotingUnits(address) internal view virtual returns (uint256);
}