Add a Strings.toHexString function (#2504)

Co-authored-by: Francisco Giordano <frangio.1@gmail.com>

CHANGELOG.md

@@ -5,6 +5,7 @@
 * Now targeting the 0.8.x line of Solidity compilers. For 0.6.x (resp 0.7.x) support, use version 3.4.0 (resp 3.4.0-solc-0.7) of OpenZeppelin.
 * `Context`: making `_msgData` return `bytes calldata` instead of `bytes memory` ([#2492](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2492))
 * `ERC20`: Removed the `_setDecimals` function and the storage slot associated to decimals. ([#2502](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2502))
+* `Strings`: addition of a `toHexString` function.  ([#2504](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2504))
 
 ## 3.4.0 (2021-02-02)
 

contracts/mocks/StringsMock.sol

@@ -8,4 +8,10 @@ contract StringsMock {
     function fromUint256(uint256 value) public pure returns (string memory) {
         return Strings.toString(value);
     }
+    function fromUint256Hex(uint256 value) public pure returns (string memory) {
+        return Strings.toHexString(value);
+    }
+    function fromUint256HexFixed(uint256 value, uint256 length) public pure returns (string memory) {
+        return Strings.toHexString(value, length);
+    }
 }

contracts/utils/Strings.sol

@@ -6,8 +6,10 @@ pragma solidity ^0.8.0;
  * @dev String operations.
  */
 library Strings {
+    bytes16 private constant alphabet = "0123456789abcdef";
+
     /**
-     * @dev Converts a `uint256` to its ASCII `string` representation.
+     * @dev Converts a `uint256` to its ASCII `string` decimal representation.
      */
     function toString(uint256 value) internal pure returns (string memory) {
         // Inspired by OraclizeAPI's implementation - MIT licence
@@ -23,12 +25,43 @@ library Strings {
             temp /= 10;
         }
         bytes memory buffer = new bytes(digits);
-        uint256 index = digits;
-        temp = value;
+        while (value != 0) {
+            digits -= 1;
+            buffer[digits] = bytes1(uint8(48 + uint256(value % 10)));
+            value /= 10;
+        }
+        return string(buffer);
+    }
+
+    /**
+     * @dev Converts a `uint256` to its ASCII `string` hexadecimal representation.
+     */
+    function toHexString(uint256 value) internal pure returns (string memory) {
+        if (value == 0) {
+            return "0x00";
+        }
+        uint256 temp = value;
+        uint256 length = 0;
         while (temp != 0) {
-            buffer[--index] = bytes1(uint8(48 + uint256(temp % 10)));
-            temp /= 10;
+            length++;
+            temp >>= 8;
+        }
+        return toHexString(value, length);
+    }
+
+    /**
+     * @dev Converts a `uint256` to its ASCII `string` hexadecimal representation with fixed length.
+     */
+    function toHexString(uint256 value, uint256 length) internal pure returns (string memory) {
+        bytes memory buffer = new bytes(2 * length + 2);
+        buffer[0] = "0";
+        buffer[1] = "x";
+        for (uint256 i = 2 * length + 1; i > 1; --i) {
+            buffer[i] = alphabet[value & 0xf];
+            value >>= 4;
         }
+        require(value == 0, "Strings: hex length insufficient");
         return string(buffer);
     }
+
 }

test/utils/Strings.test.js

@@ -1,4 +1,4 @@
-const { constants } = require('@openzeppelin/test-helpers');
+const { constants, expectRevert } = require('@openzeppelin/test-helpers');
 
 const { expect } = require('chai');
 
@@ -9,7 +9,7 @@ contract('Strings', function (accounts) {
     this.strings = await StringsMock.new();
   });
 
-  describe('from uint256', function () {
+  describe('from uint256 - decimal format', function () {
     it('converts 0', async function () {
       expect(await this.strings.fromUint256(0)).to.equal('0');
     });
@@ -22,4 +22,38 @@ contract('Strings', function (accounts) {
       expect(await this.strings.fromUint256(constants.MAX_UINT256)).to.equal(constants.MAX_UINT256.toString());
     });
   });
+
+  describe('from uint256 - hex format', function () {
+    it('converts 0', async function () {
+      expect(await this.strings.fromUint256Hex(0)).to.equal('0x00');
+    });
+
+    it('converts a positive number', async function () {
+      expect(await this.strings.fromUint256Hex(0x4132)).to.equal('0x4132');
+    });
+
+    it('converts MAX_UINT256', async function () {
+      expect(await this.strings.fromUint256Hex(constants.MAX_UINT256))
+        .to.equal(web3.utils.toHex(constants.MAX_UINT256));
+    });
+  });
+
+  describe('from uint256 - fixed hex format', function () {
+    it('converts a positive number (long)', async function () {
+      expect(await this.strings.fromUint256HexFixed(0x4132, 32))
+        .to.equal('0x0000000000000000000000000000000000000000000000000000000000004132');
+    });
+
+    it('converts a positive number (short)', async function () {
+      await expectRevert(
+        this.strings.fromUint256HexFixed(0x4132, 1),
+        'Strings: hex length insufficient',
+      );
+    });
+
+    it('converts MAX_UINT256', async function () {
+      expect(await this.strings.fromUint256HexFixed(constants.MAX_UINT256, 32))
+        .to.equal(web3.utils.toHex(constants.MAX_UINT256));
+    });
+  });
 });