fix & added tests

Author: dovgopoly

contracts/libs/math/DSMath.sol

@@ -63,7 +63,7 @@ library DSMath {
      * @notice The function that computes square roots using the Babylonian method
      * @dev The original implementation can be found here github.com/abdk-consulting/abdk-libraries-solidity
      */
-    function sqrt(uint256 x) private pure returns (uint128) {
+    function sqrt(uint256 x) internal pure returns (uint128) {
         unchecked {
             if (x == 0) {
                 return 0;

contracts/mock/libs/math/DSMathMock.sol

@@ -7,4 +7,8 @@ contract DSMathMock {
     function rpow(uint256 x_, uint256 n_, uint256 b_) external pure returns (uint256) {
         return DSMath.rpow(x_, n_, b_);
     }
+
+    function sqrt(uint256 x_) external pure returns (uint256) {
+        return DSMath.sqrt(x_);
+    }
 }

test/libs/math/DSMath.test.js

@@ -36,4 +36,27 @@ describe("DSMath", () => {
       }
     });
   });
+
+  describe("sqrt", () => {
+    it("should compute square roots properly", async () => {
+      const cases = [
+        ["0", "0"],
+        ["1", "1"],
+        ["2", "1"],
+        ["3", "1"],
+        ["4", "2"],
+        ["15", "3"],
+        ["16", "4"],
+        ["17", "4"],
+        ["105", "10"],
+        ["1787926567434221169891766654055630785600812306453167569", "1337133713371337133713371337"],
+        [toBN(2).pow(254), toBN(2).pow(127)],
+        [toBN(2).pow(256).minus(1), toBN(2).pow(128).minus(1)],
+      ];
+
+      for (const [square, root] of cases) {
+        assertBNequal(await dsMath.sqrt(square), root);
+      }
+    });
+  });
 });