Optimize toString (#3573)

Co-authored-by: Hadrien Croubois <hadrien.croubois@gmail.com>
Co-authored-by: Francisco Giordano <frangio.1@gmail.com>

contracts/mocks/StringsMock.sol

@@ -5,19 +5,19 @@ pragma solidity ^0.8.0;
 import "../utils/Strings.sol";
 
 contract StringsMock {
-    function fromUint256(uint256 value) public pure returns (string memory) {
+    function toString(uint256 value) public pure returns (string memory) {
         return Strings.toString(value);
     }
 
-    function fromUint256Hex(uint256 value) public pure returns (string memory) {
+    function toHexString(uint256 value) public pure returns (string memory) {
         return Strings.toHexString(value);
     }
 
-    function fromUint256HexFixed(uint256 value, uint256 length) public pure returns (string memory) {
+    function toHexString(uint256 value, uint256 length) public pure returns (string memory) {
         return Strings.toHexString(value, length);
     }
 
-    function fromAddressHexFixed(address addr) public pure returns (string memory) {
+    function toHexString(address addr) public pure returns (string memory) {
         return Strings.toHexString(addr);
     }
 }

contracts/utils/Strings.sol

@@ -7,48 +7,99 @@ pragma solidity ^0.8.0;
  * @dev String operations.
  */
 library Strings {
-    bytes16 private constant _HEX_SYMBOLS = "0123456789abcdef";
+    bytes16 private constant _SYMBOLS = "0123456789abcdef";
     uint8 private constant _ADDRESS_LENGTH = 20;
 
     /**
      * @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
-        // https://github.com/oraclize/ethereum-api/blob/b42146b063c7d6ee1358846c198246239e9360e8/oraclizeAPI_0.4.25.sol
+        unchecked {
+            uint256 length = 1;
 
-        if (value == 0) {
-            return "0";
+            // compute log10(value), and add it to length
+            uint256 valueCopy = value;
+            if (valueCopy >= 10**64) {
+                valueCopy /= 10**64;
+                length += 64;
+            }
+            if (valueCopy >= 10**32) {
+                valueCopy /= 10**32;
+                length += 32;
+            }
+            if (valueCopy >= 10**16) {
+                valueCopy /= 10**16;
+                length += 16;
+            }
+            if (valueCopy >= 10**8) {
+                valueCopy /= 10**8;
+                length += 8;
+            }
+            if (valueCopy >= 10**4) {
+                valueCopy /= 10**4;
+                length += 4;
+            }
+            if (valueCopy >= 10**2) {
+                valueCopy /= 10**2;
+                length += 2;
+            }
+            if (valueCopy >= 10**1) {
+                length += 1;
+            }
+            // now, length is log10(value) + 1
+
+            string memory buffer = new string(length);
+            uint256 ptr;
+            /// @solidity memory-safe-assembly
+            assembly {
+                ptr := add(buffer, add(32, length))
+            }
+            while (true) {
+                ptr--;
+                /// @solidity memory-safe-assembly
+                assembly {
+                    mstore8(ptr, byte(mod(value, 10), _SYMBOLS))
+                }
+                value /= 10;
+                if (value == 0) break;
+            }
+            return buffer;
         }
-        uint256 temp = value;
-        uint256 digits;
-        while (temp != 0) {
-            digits++;
-            temp /= 10;
-        }
-        bytes memory buffer = new bytes(digits);
-        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";
+        unchecked {
+            uint256 length = 1;
+
+            // compute log256(value), and add it to length
+            uint256 valueCopy = value;
+            if (valueCopy >= 1 << 128) {
+                valueCopy >>= 128;
+                length += 16;
+            }
+            if (valueCopy >= 1 << 64) {
+                valueCopy >>= 64;
+                length += 8;
+            }
+            if (valueCopy >= 1 << 32) {
+                valueCopy >>= 32;
+                length += 4;
+            }
+            if (valueCopy >= 1 << 16) {
+                valueCopy >>= 16;
+                length += 2;
+            }
+            if (valueCopy >= 1 << 8) {
+                valueCopy >>= 8;
+                length += 1;
+            }
+            // now, length is log256(value) + 1
+
+            return toHexString(value, length);
         }
-        uint256 temp = value;
-        uint256 length = 0;
-        while (temp != 0) {
-            length++;
-            temp >>= 8;
-        }
-        return toHexString(value, length);
     }
 
     /**
@@ -59,7 +110,7 @@ library Strings {
         buffer[0] = "0";
         buffer[1] = "x";
         for (uint256 i = 2 * length + 1; i > 1; --i) {
-            buffer[i] = _HEX_SYMBOLS[value & 0xf];
+            buffer[i] = _SYMBOLS[value & 0xf];
             value >>= 4;
         }
         require(value == 0, "Strings: hex length insufficient");