SharesMathLib.sol

// SPDX-License-Identifier: GPL-2.0-or-later
pragma solidity ^0.8.0;

import {MathLib} from "./MathLib.sol";

/// @title SharesMathLib
/// @author Morpho Labs
/// @custom:contact security@morpho.org
/// @notice Shares management library.
/// @dev This implementation mitigates share price manipulations, using OpenZeppelin's method of virtual shares:
/// https://docs.openzeppelin.com/contracts/4.x/erc4626#inflation-attack.
library SharesMathLib {
    using MathLib for uint256;

    /// @dev The number of virtual shares has been chosen low enough to prevent overflows, and high enough to ensure
    /// high precision computations.
    /// @dev Virtual shares can never be redeemed for the assets they are entitled to, but it is assumed the share price
    /// stays low enough not to inflate these assets to a significant value.
    /// @dev Warning: The assets to which virtual borrow shares are entitled behave like unrealizable bad debt.
    uint256 internal constant VIRTUAL_SHARES = 1e6;

    /// @dev A number of virtual assets of 1 enforces a conversion rate between shares and assets when a market is
    /// empty.
    uint256 internal constant VIRTUAL_ASSETS = 1;

    /// @dev Calculates the value of `assets` quoted in shares, rounding down.
    function toSharesDown(uint256 assets, uint256 totalAssets, uint256 totalShares) internal pure returns (uint256) {
        return assets.mulDivDown(totalShares + VIRTUAL_SHARES, totalAssets + VIRTUAL_ASSETS);
    }

    /// @dev Calculates the value of `shares` quoted in assets, rounding down.
    function toAssetsDown(uint256 shares, uint256 totalAssets, uint256 totalShares) internal pure returns (uint256) {
        return shares.mulDivDown(totalAssets + VIRTUAL_ASSETS, totalShares + VIRTUAL_SHARES);
    }

    /// @dev Calculates the value of `assets` quoted in shares, rounding up.
    function toSharesUp(uint256 assets, uint256 totalAssets, uint256 totalShares) internal pure returns (uint256) {
        return assets.mulDivUp(totalShares + VIRTUAL_SHARES, totalAssets + VIRTUAL_ASSETS);
    }

    /// @dev Calculates the value of `shares` quoted in assets, rounding up.
    function toAssetsUp(uint256 shares, uint256 totalAssets, uint256 totalShares) internal pure returns (uint256) {
        return shares.mulDivUp(totalAssets + VIRTUAL_ASSETS, totalShares + VIRTUAL_SHARES);
    }
}