⚡️ Optimize toString(int256) (#351)

Co-authored-by: Vectorized <[email protected]>

.gas-snapshot

@@ -279,12 +279,12 @@ FixedPointMathLibTest:testSqrt(uint256) (runs: 256, μ: 997, ~: 1013)
 FixedPointMathLibTest:testSqrtBack(uint256) (runs: 256, μ: 15210, ~: 340)
 FixedPointMathLibTest:testSqrtBackHashed(uint256) (runs: 256, μ: 59066, ~: 59500)
 FixedPointMathLibTest:testSqrtBackHashedSingle() (gas: 58937)
-LibStringTest:testDifferentiallyFuzzToString(uint256,bytes) (runs: 256, μ: 20875, ~: 8988)
-LibStringTest:testDifferentiallyFuzzToStringInt(int256,bytes) (runs: 256, μ: 20129, ~: 8360)
+LibStringTest:testDifferentiallyFuzzToString(uint256,bytes) (runs: 256, μ: 20892, ~: 8988)
+LibStringTest:testDifferentiallyFuzzToStringInt(int256,bytes) (runs: 256, μ: 20081, ~: 8356)
 LibStringTest:testToString() (gas: 10069)
 LibStringTest:testToStringDirty() (gas: 8145)
-LibStringTest:testToStringIntNegative() (gas: 11096)
-LibStringTest:testToStringIntPositive() (gas: 10509)
+LibStringTest:testToStringIntNegative() (gas: 9634)
+LibStringTest:testToStringIntPositive() (gas: 10481)
 LibStringTest:testToStringOverwrite() (gas: 506)
 MerkleProofLibTest:testValidProofSupplied() (gas: 2153)
 MerkleProofLibTest:testVerifyEmptyMerkleProofSuppliedLeafAndRootDifferent() (gas: 1458)

src/utils/LibString.sol

@@ -5,9 +5,24 @@ pragma solidity >=0.8.0;
 /// @author Solmate (https://github.com/transmissions11/solmate/blob/main/src/utils/LibString.sol)
 /// @author Modified from Solady (https://github.com/Vectorized/solady/blob/main/src/utils/LibString.sol)
 library LibString {
-    function toString(int256 value) internal pure returns (string memory) {
+    function toString(int256 value) internal pure returns (string memory str) {
+        if (value >= 0) return toString(uint256(value));
+
         unchecked {
-            return value >= 0 ? toString(uint256(value)) : string(abi.encodePacked("-", toString(uint256(-value))));
+            str = toString(uint256(-value));
+
+            /// @solidity memory-safe-assembly
+            assembly {
+                // Note: This is only safe because we over-allocate memory
+                // and write the string from right to left in toString(uint256),
+                // and thus can be sure that sub(str, 1) is an unused memory location.
+
+                let length := mload(str) // Load the string length.
+                // Put the - character at the start of the string contents.
+                mstore(str, 45) // 45 is the ASCII code for the - character.
+                str := sub(str, 1) // Move back the string pointer by a byte.
+                mstore(str, add(length, 1)) // Update the string length.
+            }
         }
     }