Merge pull request #19 from cygnusv/checkpoints

Optimize storage of balance checkpoints in T token contract

Author: cygnusv

contracts/governance/Checkpoints.sol

@@ -0,0 +1,267 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import "@openzeppelin/contracts/utils/cryptography/ECDSA.sol";
+import "@openzeppelin/contracts/utils/math/Math.sol";
+import "@openzeppelin/contracts/utils/math/SafeCast.sol";
+
+/// @title Checkpoints
+/// @dev Abstract contract to support checkpoints for Compound-like voting and
+///      delegation. This implementation supports token supply up to 2^96 - 1.
+///      This contract keeps a history (checkpoints) of each account's vote
+///      power. Vote power can be delegated either by calling the {delegate}
+///      function directly, or by providing a signature to be used with
+///      {delegateBySig}. Voting power can be publicly queried through
+///      {getVotes} and {getPastVotes}.
+///      NOTE: Extracted from OpenZeppelin ERCVotes.sol.
+abstract contract Checkpoints {
+    struct Checkpoint {
+        uint32 fromBlock;
+        uint96 votes;
+    }
+
+    mapping(address => address) internal _delegates;
+    mapping(address => uint128[]) internal _checkpoints;
+    uint128[] internal _totalSupplyCheckpoints;
+
+    /// @notice Emitted when an account changes their delegate.
+    event DelegateChanged(
+        address indexed delegator,
+        address indexed fromDelegate,
+        address indexed toDelegate
+    );
+
+    /// @notice Emitted when a balance or delegate change results in changes
+    ///         to an account's voting power.
+    event DelegateVotesChanged(
+        address indexed delegate,
+        uint256 previousBalance,
+        uint256 newBalance
+    );
+
+    function checkpoints(address account, uint32 pos)
+        public
+        view
+        virtual
+        returns (Checkpoint memory checkpoint)
+    {
+        (uint32 fromBlock, uint96 votes) = decodeCheckpoint(
+            _checkpoints[account][pos]
+        );
+        checkpoint = Checkpoint(fromBlock, votes);
+    }
+
+    /// @notice Get number of checkpoints for `account`.
+    function numCheckpoints(address account)
+        public
+        view
+        virtual
+        returns (uint32)
+    {
+        return SafeCast.toUint32(_checkpoints[account].length);
+    }
+
+    /// @notice Get the address `account` is currently delegating to.
+    function delegates(address account) public view virtual returns (address) {
+        return _delegates[account];
+    }
+
+    /// @notice Gets the current votes balance for `account`.
+    /// @param account The address to get votes balance
+    /// @return The number of current votes for `account`
+    function getVotes(address account) public view returns (uint96) {
+        uint256 pos = _checkpoints[account].length;
+        return pos == 0 ? 0 : decodeValue(_checkpoints[account][pos - 1]);
+    }
+
+    /// @notice Determine the prior number of votes for an account as of
+    ///         a block number.
+    /// @dev Block number must be a finalized block or else this function will
+    ///      revert to prevent misinformation.
+    /// @param account The address of the account to check
+    /// @param blockNumber The block number to get the vote balance at
+    /// @return The number of votes the account had as of the given block
+    function getPastVotes(address account, uint256 blockNumber)
+        public
+        view
+        returns (uint96)
+    {
+        return lookupCheckpoint(_checkpoints[account], blockNumber);
+    }
+
+    /// @notice Retrieve the `totalSupply` at the end of `blockNumber`.
+    ///         Note, this value is the sum of all balances, but it is NOT the
+    ///         sum of all the delegated votes!
+    /// @param blockNumber The block number to get the total supply at
+    /// @dev `blockNumber` must have been already mined
+    function getPastTotalSupply(uint256 blockNumber)
+        public
+        view
+        returns (uint96)
+    {
+        return lookupCheckpoint(_totalSupplyCheckpoints, blockNumber);
+    }
+
+    /// @notice Change delegation for `delegator` to `delegatee`.
+    function delegate(address delegator, address delegatee) internal virtual;
+
+    /// @notice Moves voting power from one delegate to another
+    /// @param src Address of old delegate
+    /// @param dst Address of new delegate
+    /// @param amount Voting power amount to transfer between delegates
+    function moveVotingPower(
+        address src,
+        address dst,
+        uint256 amount
+    ) internal {
+        if (src != dst && amount > 0) {
+            if (src != address(0)) {
+                (uint256 oldWeight, uint256 newWeight) = writeCheckpoint(
+                    _checkpoints[src],
+                    subtract,
+                    amount
+                );
+                emit DelegateVotesChanged(src, oldWeight, newWeight);
+            }
+
+            if (dst != address(0)) {
+                (uint256 oldWeight, uint256 newWeight) = writeCheckpoint(
+                    _checkpoints[dst],
+                    add,
+                    amount
+                );
+                emit DelegateVotesChanged(dst, oldWeight, newWeight);
+            }
+        }
+    }
+
+    /// @notice Writes a new checkpoint based on operating last stored value
+    ///         with a `delta`. Usually, said operation is the `add` or
+    ///         `subtract` functions from this contract, but more complex
+    ///         functions can be passed as parameters.
+    /// @param ckpts The checkpoints array to use
+    /// @param op The function to apply over the last value and the `delta`
+    /// @param delta Variation with respect to last stored value to be used
+    ///              for new checkpoint
+    function writeCheckpoint(
+        uint128[] storage ckpts,
+        function(uint256, uint256) view returns (uint256) op,
+        uint256 delta
+    ) internal returns (uint256 oldWeight, uint256 newWeight) {
+        uint256 pos = ckpts.length;
+        oldWeight = pos == 0 ? 0 : decodeValue(ckpts[pos - 1]);
+        newWeight = op(oldWeight, delta);
+
+        if (pos > 0) {
+            uint32 fromBlock = decodeBlockNumber(ckpts[pos - 1]);
+            if (fromBlock == block.number) {
+                ckpts[pos - 1] = encodeCheckpoint(
+                    fromBlock,
+                    SafeCast.toUint96(newWeight)
+                );
+                return (oldWeight, newWeight);
+            }
+        }
+
+        ckpts.push(
+            encodeCheckpoint(
+                SafeCast.toUint32(block.number),
+                SafeCast.toUint96(newWeight)
+            )
+        );
+    }
+
+    /// @notice Lookup a value in a list of (sorted) checkpoints.
+    /// @param ckpts The checkpoints array to use
+    /// @param blockNumber Block number when we want to get the checkpoint at
+    function lookupCheckpoint(uint128[] storage ckpts, uint256 blockNumber)
+        internal
+        view
+        returns (uint96)
+    {
+        // We run a binary search to look for the earliest checkpoint taken
+        // after `blockNumber`. During the loop, the index of the wanted
+        // checkpoint remains in the range [low-1, high). With each iteration,
+        // either `low` or `high` is moved towards the middle of the range to
+        // maintain the invariant.
+        // - If the middle checkpoint is after `blockNumber`,
+        //   we look in [low, mid)
+        // - If the middle checkpoint is before or equal to `blockNumber`,
+        //   we look in [mid+1, high)
+        // Once we reach a single value (when low == high), we've found the
+        // right checkpoint at the index high-1, if not out of bounds (in that
+        // case we're looking too far in the past and the result is 0).
+        // Note that if the latest checkpoint available is exactly for
+        // `blockNumber`, we end up with an index that is past the end of the
+        // array, so we technically don't find a checkpoint after
+        // `blockNumber`, but it works out the same.
+        require(blockNumber < block.number, "Block not yet determined");
+
+        uint256 high = ckpts.length;
+        uint256 low = 0;
+        while (low < high) {
+            uint256 mid = Math.average(low, high);
+            uint32 midBlock = decodeBlockNumber(ckpts[mid]);
+            if (midBlock > blockNumber) {
+                high = mid;
+            } else {
+                low = mid + 1;
+            }
+        }
+
+        return high == 0 ? 0 : decodeValue(ckpts[high - 1]);
+    }
+
+    /// @notice Maximum token supply. Defaults to `type(uint96).max` (2^96 - 1)
+    function maxSupply() internal view virtual returns (uint96) {
+        return type(uint96).max;
+    }
+
+    /// @notice Encodes a `blockNumber` and `value` into a single `uint128`
+    ///         checkpoint.
+    /// @dev `blockNumber` is stored in the first 32 bits, while `value` in the
+    ///      remaining 96 bits.
+    function encodeCheckpoint(uint32 blockNumber, uint96 value)
+        internal
+        pure
+        returns (uint128)
+    {
+        return (uint128(blockNumber) << 96) | uint128(value);
+    }
+
+    /// @notice Decodes a block number from a `uint128` `checkpoint`.
+    function decodeBlockNumber(uint128 checkpoint)
+        internal
+        pure
+        returns (uint32)
+    {
+        return uint32(bytes4(bytes16(checkpoint)));
+    }
+
+    /// @notice Decodes a voting value from a `uint128` `checkpoint`.
+    function decodeValue(uint128 checkpoint) internal pure returns (uint96) {
+        return uint96(checkpoint);
+    }
+
+    /// @notice Decodes a block number and voting value from a `uint128`
+    ///         `checkpoint`.
+    function decodeCheckpoint(uint128 checkpoint)
+        internal
+        pure
+        returns (uint32 blockNumber, uint96 value)
+    {
+        blockNumber = decodeBlockNumber(checkpoint);
+        value = decodeValue(checkpoint);
+    }
+
+    // slither-disable-next-line dead-code
+    function add(uint256 a, uint256 b) internal pure returns (uint256) {
+        return a + b;
+    }
+
+    // slither-disable-next-line dead-code
+    function subtract(uint256 a, uint256 b) internal pure returns (uint256) {
+        return a - b;
+    }
+}