fix(ctb): clean up BytesUtils (#2964)

Cleans up BytesUtils and properly attributes the slice code (although
it's licensed as Unlicense so I don't think strictly necessary, but
attribution is always nice and I think we just missed it the first time
around).
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

.changeset/beige-eggs-jog.md

+---
+'@eth-optimism/contracts-bedrock': patch
+---
+
+Clean up BytesUtils

packages/contracts-bedrock/contracts/libraries/BytesUtils.sol

@@ -3,20 +3,31 @@ pragma solidity ^0.8.9;
 
 /**
  * @title BytesUtils
+ * @notice BytesUtils is a library for manipulating byte arrays.
  */
 library BytesUtils {
-    /**********************
-     * Internal Functions *
-     **********************/
-
+    /**
+     * @custom:attribution https://github.com/GNSPS/solidity-bytes-utils
+     * @notice Slices a byte array with a given starting index and length. Returns a new byte array
+     *         as opposed to a pointer to the original array. Will throw if trying to slice more
+     *         bytes than exist in the array.
+     *
+     * @param _bytes Byte array to slice.
+     * @param _start Starting index of the slice.
+     * @param _length Length of the slice.
+     *
+     * @return Slice of the input byte array.
+     */
     function slice(
         bytes memory _bytes,
         uint256 _start,
         uint256 _length
     ) internal pure returns (bytes memory) {
-        require(_length + 31 >= _length, "slice_overflow");
-        require(_start + _length >= _start, "slice_overflow");
-        require(_bytes.length >= _start + _length, "slice_outOfBounds");
+        unchecked {
+            require(_length + 31 >= _length, "slice_overflow");
+            require(_start + _length >= _start, "slice_overflow");
+            require(_bytes.length >= _start + _length, "slice_outOfBounds");
+        }
 
         bytes memory tempBytes;
 
@@ -76,51 +87,63 @@ library BytesUtils {
         return tempBytes;
     }
 
+    /**
+     * @notice Slices a byte array with a given starting index up to the end of the original byte
+     *         array. Returns a new array rathern than a pointer to the original.
+     *
+     * @param _bytes Byte array to slice.
+     * @param _start Starting index of the slice.
+     *
+     * @return Slice of the input byte array.
+     */
     function slice(bytes memory _bytes, uint256 _start) internal pure returns (bytes memory) {
         if (_start >= _bytes.length) {
             return bytes("");
         }
-
         return slice(_bytes, _start, _bytes.length - _start);
     }
 
-    function toBytes32(bytes memory _bytes) internal pure returns (bytes32) {
-        if (_bytes.length < 32) {
-            bytes32 ret;
-            assembly {
-                ret := mload(add(_bytes, 32))
-            }
-            return ret;
-        }
-
-        return abi.decode(_bytes, (bytes32)); // will truncate if input length > 32 bytes
-    }
-
-    function toUint256(bytes memory _bytes) internal pure returns (uint256) {
-        return uint256(toBytes32(_bytes));
-    }
-
+    /**
+     * @notice Converts a byte array into a nibble array by splitting each byte into two nibbles.
+     *         Resulting nibble array will be exactly twice as long as the input byte array.
+     *
+     * @param _bytes Input byte array to convert.
+     *
+     * @return Resulting nibble array.
+     */
     function toNibbles(bytes memory _bytes) internal pure returns (bytes memory) {
         bytes memory nibbles = new bytes(_bytes.length * 2);
-
         for (uint256 i = 0; i < _bytes.length; i++) {
             nibbles[i * 2] = _bytes[i] >> 4;
             nibbles[i * 2 + 1] = bytes1(uint8(_bytes[i]) % 16);
         }
-
         return nibbles;
     }
 
+    /**
+     * @notice Generates a byte array from a nibble array by joining each set of two nibbles into a
+     *         single byte. Resulting byte array will be half as long as the input byte array.
+     *
+     * @param _bytes Input nibble array to convert.
+     *
+     * @return Resulting byte array.
+     */
     function fromNibbles(bytes memory _bytes) internal pure returns (bytes memory) {
         bytes memory ret = new bytes(_bytes.length / 2);
-
         for (uint256 i = 0; i < ret.length; i++) {
             ret[i] = (_bytes[i * 2] << 4) | (_bytes[i * 2 + 1]);
         }
-
         return ret;
     }
 
+    /**
+     * @notice Compares two byte arrays by comparing their keccak256 hashes.
+     *
+     * @param _bytes First byte array to compare.
+     * @param _other Second byte array to compare.
+     *
+     * @return True if the two byte arrays are equal, false otherwise.
+     */
     function equal(bytes memory _bytes, bytes memory _other) internal pure returns (bool) {
         return keccak256(_bytes) == keccak256(_other);
     }

packages/contracts-bedrock/contracts/libraries/trie/MerkleTrie.sol

@@ -167,7 +167,7 @@ library MerkleTrie {
             } else {
                 // Nodes smaller than 31 bytes aren't hashed.
                 require(
-                    BytesUtils.toBytes32(currentNode.encoded) == currentNodeID,
+                    bytes32(currentNode.encoded) == currentNodeID,
                     "Invalid internal node hash"
                 );
             }
@@ -272,7 +272,7 @@ library MerkleTrie {
             nodeID = RLPReader.readBytes(_node);
         }
 
-        return BytesUtils.toBytes32(nodeID);
+        return bytes32(nodeID);
     }
 
     /**