harb/src/Stake.sol

// SPDX-License-Identifier: GPL-3.0-or-later

pragma solidity ^0.8.13;

import "./interfaces/IStake.sol";
import "./interfaces/IHarb.sol";

contract Stake is IStake {

    uint256 internal constant MAX_STAKE = 20;  // 20% of HARB supply
    uint256 internal constant MAX_TAX = 1000;  // max 1000% tax
    uint256 internal constant TAX_RATE_BASE = 100;
    uint256 internal constant TAX_FLOOR_DURATION = 60 * 60 * 24 * 3; //this duration is the minimum basis for fee calculation, regardless of actual holding time.
    /**
     * @dev Attempted to deposit more assets than the max amount for `receiver`.
     */
    error ExceededAvailableStake(address receiver, uint256 stakeWanted, uint256 availableStake);
    error TaxTooLow(address receiver, uint64 taxRateWanted, uint64 taxRateMet, uint256 positionId);
    error SharesTooLow(address receiver, uint256 assets, uint256 sharesWanted, uint256 minStake);
    error NoPermission(address requester, address owner);


    struct StakingPosition {
        uint256 share;
        address owner;
        uint32 creationTime;
        uint32 lastTaxTime;
        uint32 taxRate;     // e.g. value of 60 = 60% tax per year
    }

    uint256 public immutable totalSupply;
    address private immutable tokenContract;
    address private immutable taxPool;
    uint256 public outstandingStake;
    uint256 private lastTokenId;
    uint256 public minStake;
    mapping (uint256 positionID => StakingPosition) public positions;


    constructor(
        address _tokenContract
    ) {
        tokenContract = _tokenContract;
        IHarb harb = IHarb(_tokenContract);
        totalSupply = 100 * 10 ** 5 * harb.decimals();
        taxPool = harb.taxPool();
    }

    function dormantSupply() public view override returns(uint256) {
        return totalSupply * (100 - MAX_STAKE) / 100;
    }

    function assetsToShares(uint256 assets) private view returns (uint256) {
        return assets * totalSupply / IERC20(_tokenContract).totalSupply();
    }

    function sharesToAssets(uint256 shares) private view returns (uint256) {
        return shares * IERC20(_tokenContract).totalSupply() / totalSupply;
    }

    function snatch(uint256 assets, address receiver, uint64 taxRate, uint256[] positions) public returns(uint256) {

        // check lower boundary
        uint256 sharesWanted = assetsToShares(assets);
        if (sharesWanted < minStake) {
            revert SharesTooLow(receiver, assets, sharesWanted, minStake);
        }

        // run through all suggested positions
        for (uint i = 0; i < positions.length; i++) {
            StakingPosition pos = positions[i];
            // check that tax lower
            if (taxRate <= pos.perSecondTaxRate) {
                revert TaxTooLow(receiver, taxRate, pos.perSecondTaxRate, i);
            }
            // dissolve position
            _payTax(pos);
            _exitPosition(pos);
        }
        
        // now try to make a new position in the free space and hope it is big enough
        uint256 availableStake = authorizedStake - outstandingStake;
        if (sharesWanted > availableStake) {
            revert ExceededAvailableStake(receiver, sharesWanted, availableStake);
        }

        // transfer
        SafeERC20.safeTransferFrom(tokenContract, _msgSender(), address(this), assets);

        // mint
        StakingPosition storage sp = c.funders[lastTokenId++];
        sp.share = shares;
        sp.owner = receiver;
        sp.lastTaxTime = now;
        sp.creationTime = now;
        sp.perSecondTaxRate = taxRate;

        outstandingStake += sharesWanted;

        return lastTokenId;
    }


    function exitPosition(uint256 positionID) public {
        StakingPosition pos = positions[positionID];
        if(pos.owner != _msgSender()) {
            NoPermission(_msgSender(), pos.owner);
        }
        // to prevent snatch-and-exit grieving attack
        if(now - pos.creationTime < 60 * 60 * 24 * 3) {

            ExitTooEarly(pos.owner, positionID, pos.creationTime);
        }
        _payTax(pos, TAX_FLOOR_DURATION);
        _exitPosition(pos);     
    }

    function payTax(uint256 positionID) public {
        StakingPosition pos = positions[positionID];
        _payTax(pos, 0);
    }


    function _payTax(StakingPosition storage pos, uint256 taxFloorDuration) private {
        // ihet = Implied Holding Expiry Timestamp
        uint256 ihet = (now - pos.creationTime < taxFloorDuration) ? pos.creationTime + taxFloorDuration : now;
        uint256 elapsedTime = ihet - pos.lastTaxTime;
        uint256 assetsBefore = sharesToAssets(pos.share);
        uint256 taxDue = assetsBefore * pos.taxRate * elapsedTime / (365 * 24 * 60 * 60) / TAX_RATE_BASE;
        if (taxDue >= assetsBefore) {
            // can not pay more tax than value of position
            taxDue = assetsBefore;
        }
        SafeERC20.safeTransfer(tokenContract, taxPool, taxDue);
        if (assetsBefore - taxDue > 0) {
            // if something left over, update storage
            sp.shares = assetsToShares(assetsBefore - taxDue);
            sp.lastTaxTime = now;
        } else {
            // if nothing left over, liquidate position
            outstandingStake -= sp.share;
            delete sp;
        }
    }

    function _exitPosition(StakingPosition storage pos) private {
        outstandingStake -= pos.share;
        address owner = pos.owner;
        uint256 assets = sharesToAssets(pos.share);
        delete pos;
        SafeERC20.safeTransfer(tokenContract, owner, assets);
    }

}