Minterest

Minterest.sol

pragma solidity ^0.5.0;

import "@openzeppelin/contracts/math/SafeMath.sol";
import "@openzeppelin/contracts/token/ERC20/ERC20.sol";
import "./UniversalERC20.sol";


contract Minterest is ERC20 {

    using SafeMath for uint256;
    using UniversalERC20 for IERC20;

    struct MovingPrice {
        uint256 time;
        uint256 value;
        uint256 speed;
    }

    uint256 public constant MIN_INTEREST = 2e18;
    uint256 public constant MAX_INTEREST = 22e18;

    IERC20 public token;
    MovingPrice public movingPrice;
    uint256 public totalLiquidity;

    constructor(IERC20 _token) public {
        token = _token;
    }

    // View methods

    function price() public view returns(uint256) {
        return priceForTime(now);
    }

    function priceForTime(uint256 time) public view returns(uint256) {
        return movingPrice.value.add(time.sub(movingPrice.time).mul(movingPrice.speed));
    }

    function interest() public returns(uint256) {
        return interestForLiquidityAndBalance(totalLiquidity, token.universalBalanceOf(address(this)));
    }

    function interestForLiquidityAndBalance(uint256 liquidity, uint256 balance) public pure returns(uint256) {
        return MIN_INTEREST.add(MAX_INTEREST.sub(MIN_INTEREST).mul(balance).div(liquidity));
    }

    // Non-view methods

    function deposit(uint256 value) public payable {
        _mint(msg.sender, value.mul(price()).div(1e18));
        token.universalTransferFrom(msg.sender, address(this), value);

        _updateSpeed(movingPrice.speed.mul(totalLiquidity).div(totalLiquidity.add(value)));
        totalLiquidity = totalLiquidity.add(value);
    }

    function withdraw(uint256 amount) public {
        uint256 value = amount.mul(1e18).div(price());
        _burn(msg.sender, amount);
        token.universalTransfer(msg.sender, value);

        _updateSpeed(movingPrice.speed.mul(totalLiquidity).div(totalLiquidity.sub(value)));
        totalLiquidity = totalLiquidity.sub(value);
    }

    function borrow(uint256 amount) public {
        // TODO: Fix
        // ...

        // TODO: Fix
        // _updateSpeed(movingPrice.speed.mul(totalLiquidity).div(totalLiquidity.sub(value)));
    }

    function repay(uint256 amount) public payable {
        // TODO: Fix
        // ...

        // TODO: Fix
        // _updateSpeed(movingPrice.speed.mul(totalLiquidity).div(totalLiquidity.add(value)));
    }

    // Internal methods

    function _updateSpeed(uint256 newSpeed) internal {
        movingPrice = MovingPrice({
            time: now,
            value: price(),
            speed: newSpeed
        });
    }
}