refactor DPP & DVM

contracts/lib/DODOMath.sol

@@ -34,6 +34,7 @@ library DODOMath {
         uint256 i,
         uint256 k
     ) internal pure returns (uint256) {
+        require(V0 > 0, "TARGET_IS_ZERO");
         uint256 fairAmount = DecimalMath.mul(i, V1.sub(V2)); // i*delta
         uint256 V0V0V1V2 = DecimalMath.divCeil(V0.mul(V0).div(V1), V2);
         uint256 penalty = DecimalMath.mul(k, V0V0V1V2); // k(V0^2/V1/V2)
@@ -62,6 +63,7 @@ library DODOMath {
         bool deltaBSig,
         uint256 k
     ) internal pure returns (uint256) {
+        require(Q0 > 0, "TARGET_IS_ZERO");
         // calculate -b value and sig
         // -b = (1-k)Q1-kQ0^2/Q1+i*deltaB
         uint256 kQ02Q1 = DecimalMath.mul(k, Q0).mul(Q0).div(Q1); // kQ0^2/Q1

contracts/lib/ExternalValue.sol

@@ -0,0 +1,29 @@
+/*
+
+    Copyright 2020 DODO ZOO.
+    SPDX-License-Identifier: Apache-2.0
+
+*/
+
+pragma solidity 0.6.9;
+pragma experimental ABIEncoderV2;
+
+import {Ownable} from "./Ownable.sol";
+
+interface IExternalValue {
+    function set(uint256) external;
+
+    function get() external view returns (uint256);
+}
+
+contract ExternalValue is IExternalValue, Ownable {
+    uint256 public _VALUE_;
+
+    function set(uint256 value) external override {
+        _VALUE_ = value;
+    }
+
+    function get() external override view returns (uint256) {
+        return _VALUE_;
+    }
+}

contracts/lib/GasPriceSource.sol

@@ -1,29 +0,0 @@
-/*
-
-    Copyright 2020 DODO ZOO.
-    SPDX-License-Identifier: Apache-2.0
-
-*/
-
-pragma solidity 0.6.9;
-pragma experimental ABIEncoderV2;
-
-import {Ownable} from "./Ownable.sol";
-
-interface IGasPriceSource {
-    function setGasPrice(uint256) external;
-
-    function getGasPrice() external view returns (uint256);
-}
-
-contract GasPriceSource is IGasPriceSource, Ownable {
-    uint256 public _GAS_PRICE_;
-
-    function setGasPrice(uint256 gasPrice) external override {
-        _GAS_PRICE_ = gasPrice;
-    }
-
-    function getGasPrice() external override view returns (uint256) {
-        return _GAS_PRICE_;
-    }
-}

contracts/lib/PMMPricing.sol

@@ -213,6 +213,7 @@ library PMMPricing {
 
     // ============ Helper functions ============
 
+    // todo 我不确定这个函数是不是能改state的状态
     function adjustedTarget(PMMState memory state) public pure {
         if (state.R == RState.BELOW_ONE) {
             uint256 fairAmount = DecimalMath.mulFloor(state.B.sub(state.B0), state.i);