Unsigned conversion #2111

contracts/mocks/SafeCastMock.sol

@@ -4,6 +4,11 @@ import "../utils/SafeCast.sol";
 
 contract SafeCastMock {
     using SafeCast for uint;
+    using SafeCast for int;
+
+    function toUint256(int a) public pure returns (uint256) {
+        return a.toUint256();
+    }
 
     function toUint128(uint a) public pure returns (uint128) {
         return a.toUint128();

contracts/utils/SafeCast.sol

@@ -94,4 +94,16 @@ library SafeCast {
         require(value < 2**8, "SafeCast: value doesn\'t fit in 8 bits");
         return uint8(value);
     }
+
+    /**
+     * @dev Converts an unsigned uint256 into a signed int256.
+     *
+     * Requirements:
+     *
+     * - input must be greater than or equal to 0.
+     */
+    function toUint256(int256 value) internal pure returns (uint256) {
+        require(value >= 0, "SafeCast: value must be positive");
+        return uint256(value);
+    }
 }

test/utils/SafeCast.test.js

@@ -42,5 +42,44 @@ describe('SafeCast', async () => {
     });
   }
 
+  describe('toUint256', () => {
+    const maxInt256 = new BN('1').shln(255).notn(256);
+    const minInt256 = new BN('1').shln(255);
+    const maxUint256 = new BN('0').notn(256);
+
+    it('downcasts 0', async function () {
+      expect(await this.safeCast.toUint256(0)).to.be.bignumber.equal('0');
+    });
+
+    it('downcasts 1', async function () {
+      expect(await this.safeCast.toUint256(1)).to.be.bignumber.equal('1');
+    });
+
+    it(`downcasts INT256_MAX ${maxInt256}`, async function () {
+      expect(await this.safeCast.toUint256(maxInt256)).to.be.bignumber.equal(maxInt256);
+    });
+
+    it('reverts when downcasting -1', async function () {
+      await expectRevert(
+        this.safeCast.toUint256(-1),
+        'SafeCast: value must be positive'
+      );
+    });
+
+    it(`reverts when downcasting INT256_MIN ${minInt256}`, async function () {
+      await expectRevert(
+        this.safeCast.toUint256(minInt256),
+        'SafeCast: value must be positive'
+      );
+    });
+
+    it(`reverts when downcasting UINT256_MAX ${maxUint256}`, async function () {
+      await expectRevert(
+        this.safeCast.toUint256(maxUint256),
+        'SafeCast: value must be positive'
+      );
+    });
+  });
+
   [8, 16, 32, 64, 128].forEach(bits => testToUint(bits));
 });