Add PotHelper with view function for simulating chi if dripped now.

src/Interfaces.sol

@@ -28,3 +28,5 @@ import { PotAbstract } from "./dss/PotAbstract.sol";
 import { SpotAbstract } from "./dss/SpotAbstract.sol";
 import { VatAbstract } from "./dss/VatAbstract.sol";
 import { VowAbstract } from "./dss/VowAbstract.sol";
+
+import { PotHelper } from "./dss/PotHelper.sol";

src/dss/PotAbstract.sol

@@ -5,25 +5,25 @@ import { VatAbstract } from "./VatAbstract.sol";
 // https://github.com/makerdao/dss/blob/master/src/pot.sol
 contract PotAbstract {
     // mapping (address => uint256) public wards;
-    function wards(address) public returns (uint256);
+    function wards(address) public view returns (uint256);
     function rely(address) external;
     function deny(address) external;
     // mapping (address => uint256) public pie;  // user Savings Dai
-    function pie(address) public returns (uint256);
+    function pie(address) public view returns (uint256);
     // uint256 public Pie;  // total Savings Dai
-    function Pie() public returns (uint256);
+    function Pie() public view returns (uint256);
     // uint256 public dsr;  // the Dai Savings Rate
-    function dsr() public returns (uint256);
+    function dsr() public view returns (uint256);
     // uint256 public chi;  // the Rate Accumulator
-    function chi() public returns (uint256);
+    function chi() public view returns (uint256);
     // VatAbstract public vat;  // CDP engine
-    function vat() public returns (VatAbstract);
+    function vat() public view returns (VatAbstract);
     // address public vow;  // debt engine
-    function vow() public returns (address);
+    function vow() public view returns (address);
     // uint256 public rho;  // time of last drip
-    function rho() public returns (uint256);
+    function rho() public view returns (uint256);
     // uint256 public live;  // Access Flag
-    function live() public returns (uint256);
+    function live() public view returns (uint256);
     function file(bytes32, uint256) external;
     function file(bytes32, address) external;
     function cage() external;

src/dss/PotHelper.sol

@@ -0,0 +1,59 @@
+pragma solidity ^0.5.12;
+
+import { PotAbstract } from "./PotAbstract.sol";
+
+// https://github.com/makerdao/dss/blob/master/src/pot.sol
+contract PotHelper {
+
+    PotAbstract pa;
+
+    constructor(address pot) public {
+        pa = PotAbstract(pot);
+    }
+
+    // https://github.com/makerdao/dss/blob/master/src/pot.sol#L79
+    uint256 constant ONE = 10 ** 27;
+
+    function mul(uint x, uint y) internal pure returns (uint z) {
+        require(y == 0 || (z = x * y) / y == x);
+    }
+
+    function rmul(uint x, uint y) internal pure returns (uint z) {
+        z = mul(x, y) / ONE;
+    }
+
+    function rpow(uint x, uint n, uint base) internal pure returns (uint z) {
+        assembly {
+            switch x case 0 {switch n case 0 {z := base} default {z := 0}}
+            default {
+                switch mod(n, 2) case 0 { z := base } default { z := x }
+                let half := div(base, 2)  // for rounding.
+                for { n := div(n, 2) } n { n := div(n,2) } {
+                    let xx := mul(x, x)
+                    if iszero(eq(div(xx, x), x)) { revert(0,0) }
+                    let xxRound := add(xx, half)
+                    if lt(xxRound, xx) { revert(0,0) }
+                    x := div(xxRound, base)
+                    if mod(n,2) {
+                        let zx := mul(z, x)
+                        if and(iszero(iszero(x)), iszero(eq(div(zx, x), z))) { revert(0,0) }
+                        let zxRound := add(zx, half)
+                        if lt(zxRound, zx) { revert(0,0) }
+                        z := div(zxRound, base)
+                    }
+                }
+            }
+        }
+    }
+
+    // View function for calculating value of chi iff drip() is called in the same block.
+    function drop() external view returns (uint256) {
+        if (now == pa.rho()) return pa.chi();
+        return rmul(rpow(pa.dsr(), now - pa.rho(), ONE), pa.chi());
+    }
+
+    // Pass the Pot Abstract for additional operations
+    function pot() external view returns (PotAbstract) {
+        return pa;
+    }
+}