skimmer

contracts/pcv/utils/FeiSkimmer.sol

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+// SPDX-License-Identifier: GPL-3.0-or-later
+pragma solidity ^0.8.4;
+
+import "../IPCVDeposit.sol"; 
+import "../../refs/CoreRef.sol";
+
+/// @title a contract to skim excess FEI from addresses
+/// @author Fei Protocol
+contract FeiSkimmer is CoreRef {
+
+    event ThresholdUpdate(uint256 newThreshold);
+
+    /// @notice source PCV deposit to skim excess FEI from
+    IPCVDeposit public immutable source;
+
+    /// @notice the threshold of FEI above which to skim
+    uint256 public threshold;
+
+    /// @notice FEI Skimmer
+    /// @param _core Fei Core for reference
+    /// @param _source the target to skim from
+    /// @param _threshold the threshold of FEI to be maintained by source
+    constructor(
+        address _core,
+        IPCVDeposit _source,
+        uint256 _threshold
+    ) 
+        CoreRef(_core)
+    {
+        source = _source;
+        threshold = _threshold;
+        emit ThresholdUpdate(threshold);
+    }
+
+    /// @return true if FEI balance of source exceeds threshold
+    function skimEligible() external view returns (bool) {
+        return fei().balanceOf(address(source)) > threshold;
+    }
+
+    /// @notice skim FEI above the threshold from the source. Pausable. Requires skimEligible()
+    function skim()
+        external
+        whenNotPaused
+    {
+        IFei _fei = fei();
+        uint256 feiTotal = _fei.balanceOf(address(source));
+
+        require(feiTotal > threshold, "under threshold");
+
+        uint256 burnAmount = feiTotal - threshold;
+        source.withdrawERC20(address(_fei), address(this), burnAmount);
+
+        _fei.burn(burnAmount);
+    }
+
+    /// @notice set the threshold for FEI skims. Only Governor or Admin
+    /// @param newThreshold the new value above which FEI is skimmed.
+    function setThreshold(uint256 newThreshold) external onlyGovernorOrAdmin {
+        threshold = newThreshold;
+        emit ThresholdUpdate(newThreshold);
+    }
+}

test/unit/pcv/FeiSkimmer.test.ts

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+import { Core, FeiSkimmer, MockPCVDepositV2 } from '@custom-types/contracts';
+import { expectRevert, getAddresses, getCore, getImpersonatedSigner } from '@test/helpers';
+import { expect } from 'chai';
+import { Signer } from 'ethers';
+import { ethers } from 'hardhat';
+
+describe.only('FeiSkimmer', function () {
+  let minterAddress: string;
+  let userAddress: string;
+  let governorAddress: string;
+  let core: Core;
+  let skimmer: FeiSkimmer;
+  let source: MockPCVDepositV2;
+
+  const threshold = ethers.constants.WeiPerEther;
+
+  const impersonatedSigners: { [key: string]: Signer } = {};
+
+  before(async () => {
+    const addresses = await getAddresses();
+
+    // add any addresses you want to impersonate here
+    const impersonatedAddresses = [addresses.userAddress, addresses.governorAddress, addresses.minterAddress];
+
+    for (const address of impersonatedAddresses) {
+      impersonatedSigners[address] = await getImpersonatedSigner(address);
+    }
+  });
+
+  beforeEach(async function () {
+    ({ userAddress, governorAddress, minterAddress } = await getAddresses());
+    core = await getCore();
+
+    source = await (await ethers.getContractFactory('MockPCVDepositV2')).deploy(core.address, await core.fei(), 0, 0);
+
+    skimmer = await (await ethers.getContractFactory('FeiSkimmer')).deploy(core.address, source.address, threshold);
+  });
+
+  describe('Initial configuration', function () {
+    it('has source set', async function () {
+      expect(await skimmer.source()).to.be.equal(source.address);
+    });
+
+    it('has threshold set', async function () {
+      expect(await skimmer.threshold()).to.be.equal(threshold);
+    });
+
+    it('is not skim eligible', async function () {
+      expect(await skimmer.skimEligible()).to.be.false;
+    });
+  });
+
+  describe('Skim', function () {
+    it('is eligible and functional over threshold', async function () {
+      const fei = await ethers.getContractAt('IFei', await core.fei());
+
+      await fei.connect(impersonatedSigners[minterAddress]).mint(source.address, ethers.constants.WeiPerEther.mul(2));
+
+      expect(await skimmer.skimEligible()).to.be.true;
+
+      await skimmer.skim();
+
+      expect(await fei.balanceOf(source.address)).to.be.equal(threshold);
+    });
+  });
+
+  describe('Set Threshold', function () {
+    it('from governor succeeds', async function () {
+      expect(await skimmer.threshold()).to.be.equal(threshold);
+
+      await skimmer.connect(impersonatedSigners[governorAddress]).setThreshold(0);
+
+      expect(await skimmer.threshold()).to.be.equal(0);
+    });
+
+    it('not from governor succeeds', async function () {
+      await expectRevert(
+        skimmer.connect(impersonatedSigners[userAddress]).setThreshold(0),
+        'CoreRef: Caller is not a governor or contract admin'
+      );
+    });
+  });
+});