Feature/new interest model

contracts/funding/GentleInterestModel.sol

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+/*
+
+    Copyright 2019 The Hydro Protocol Foundation
+
+    Licensed under the Apache License, Version 2.0 (the "License");
+    you may not use this file except in compliance with the License.
+    You may obtain a copy of the License at
+
+        http://www.apache.org/licenses/LICENSE-2.0
+
+    Unless required by applicable law or agreed to in writing, software
+    distributed under the License is distributed on an "AS IS" BASIS,
+    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+    See the License for the specific language governing permissions and
+    limitations under the License.
+
+*/
+
+pragma solidity ^0.5.8;
+pragma experimental ABIEncoderV2;
+
+contract GentleInterestModel {
+    uint256 constant BASE = 10**18;
+
+    /**
+     * @param borrowRatio a decimal with 18 decimals
+     */
+    function polynomialInterestModel(uint256 borrowRatio) external pure returns(uint256) {
+        // 0.1r + 0.2r^16 + 0.2*r^32
+
+        // the valid range of borrowRatio is [0, 1]
+        uint256 r = borrowRatio > BASE ? BASE : borrowRatio;
+        uint256 r16 = r*r/BASE; // r^2
+        r16 = r16*r16/BASE; // r^4
+        r16 = r16*r16/BASE; // r^8
+        r16 = r16*r16/BASE; // r^16
+
+        return r / 10 + r16 / 5 + r16 * r16 / BASE / 5;
+    }
+}

contracts/helper/Eth2daiDirect.sol

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+/**
+ *Submitted for verification at Etherscan.io on 2018-03-16
+*/
+
+pragma solidity ^0.5.8;
+
+contract Eth2daiInterface {
+    // sellAllAmount(ERC20 pay_gem, uint pay_amt, ERC20 buy_gem, uint min_fill_amount)
+    function sellAllAmount(address, uint, address, uint) public returns (uint);
+}
+
+contract TokenInterface {
+    function balanceOf(address) public returns (uint);
+    function allowance(address, address) public returns (uint);
+    function approve(address, uint) public;
+    function transfer(address,uint) public returns (bool);
+    function transferFrom(address, address, uint) public returns (bool);
+    function deposit() public payable;
+    function withdraw(uint) public;
+}
+
+contract Eth2daiDirect {
+
+    Eth2daiInterface public constant eth2dai = Eth2daiInterface(0x39755357759cE0d7f32dC8dC45414CCa409AE24e);
+    TokenInterface public constant wethToken = TokenInterface(0xC02aaA39b223FE8D0A0e5C4F27eAD9083C756Cc2);
+    TokenInterface public constant daiToken = TokenInterface(0x89d24A6b4CcB1B6fAA2625fE562bDD9a23260359);
+
+    function marketBuyEth(
+        uint256 payDaiAmount,
+        uint256 minBuyEthAmount
+    )
+        public
+    {
+        daiToken.transferFrom(msg.sender, address(this), payDaiAmount);
+        uint256 fillAmount = eth2dai.sellAllAmount(address(daiToken), payDaiAmount, address(wethToken), minBuyEthAmount);
+        wethToken.withdraw(fillAmount);
+        msg.sender.transfer(fillAmount);
+    }
+
+    function marketSellEth(
+        uint256 payEthAmount,
+        uint256 minBuyDaiAmount
+    )
+        public
+        payable
+    {
+        require(msg.value == payEthAmount, "MSG_VALUE_NOT_MATCH");
+        wethToken.deposit.value(msg.value)();
+        uint256 fillAmount = eth2dai.sellAllAmount(address(wethToken), payEthAmount, address(daiToken), minBuyDaiAmount);
+        daiToken.transfer(msg.sender, fillAmount);
+    }
+
+    function() external payable {
+        require(msg.sender == address(wethToken), "CONTRACT_NOT_PAYABLE");
+    }
+}