Clean up PreimageOracle

packages/contracts-bedrock/contracts/cannon/PreimageOracle.sol

 // SPDX-License-Identifier: MIT
 pragma solidity 0.8.15;
 
+/// @title PreimageOracle
+/// @notice A contract for storing permissioned pre-images.
 contract PreimageOracle {
+    /// @notice Mapping of pre-image keys to pre-image lengths.
     mapping(bytes32 => uint256) public preimageLengths;
+
+    /// @notice Mapping of pre-image keys to pre-image parts.
     mapping(bytes32 => mapping(uint256 => bytes32)) public preimageParts;
+
+    /// @notice Mapping of pre-image keys to pre-image part offsets.
     mapping(bytes32 => mapping(uint256 => bool)) public preimagePartOk;
 
-    function readPreimage(bytes32 key, uint256 offset)
+    /// @notice Reads a pre-image from the oracle.
+    /// @param _key The key of the pre-image to read.
+    /// @param _offset The offset of the pre-image to read.
+    /// @return dat_ The pre-image data.
+    /// @return datLen_ The length of the pre-image data.
+    function readPreimage(bytes32 _key, uint256 _offset)
         external
         view
-        returns (bytes32 dat, uint256 datLen)
+        returns (bytes32 dat_, uint256 datLen_)
     {
-        require(preimagePartOk[key][offset], "preimage must exist");
-        datLen = 32;
-        uint256 length = preimageLengths[key];
-        // add 8 for the length-prefix part
-        if (offset + 32 >= length + 8) {
-            datLen = length + 8 - offset;
+        require(preimagePartOk[_key][_offset], "pre-image must exist");
+
+        // Calculate the length of the pre-image data
+        // Add 8 for the length-prefix part
+        datLen_ = 32;
+        uint256 length = preimageLengths[_key];
+        if (_offset + 32 >= length + 8) {
+            datLen_ = length + 8 - _offset;
         }
-        dat = preimageParts[key][offset];
+
+        // Retrieve the pre-image data
+        dat_ = preimageParts[_key][_offset];
     }
 
     // TODO(CLI-4104):
@@ -37,17 +53,47 @@ contract PreimageOracle {
         preimageLengths[key] = size;
     }
 
-    // loadKeccak256PreimagePart prepares the pre-image to be read by keccak256 key,
-    // starting at the given offset, up to 32 bytes (clipped at preimage length, if out of data).
-    function loadKeccak256PreimagePart(uint256 partOffset, bytes calldata preimage) external {
+    /// @notice Computes and returns the key for a pre-image.
+    /// @param _preimage The pre-image.
+    /// @return key_ The pre-image key.
+    function computePreimageKey(bytes calldata _preimage) external pure returns (bytes32 key_) {
+        uint256 size;
+        assembly {
+            size := calldataload(0x24)
+
+            // Leave slots 0x40 and 0x60 untouched,
+            // and everything after as scratch-memory.
+            let ptr := 0x80
+
+            // Store size as a big-endian uint64 at the start of pre-image
+            mstore(ptr, shl(192, size))
+            ptr := add(ptr, 8)
+
+            // Copy preimage payload into memory so we can hash and read it.
+            calldatacopy(ptr, _preimage.offset, size)
+
+            // Compute the pre-image keccak256 hash (aka the pre-image key)
+            let h := keccak256(ptr, size)
+
+            // Mask out prefix byte, replace with type 2 byte
+            key_ := or(and(h, not(shl(248, 0xFF))), shl(248, 2))
+        }
+    }
+
+    /// @notice Prepares a pre-image to be read by keccak256 key, starting at
+    ///         the given offset and up to 32 bytes (clipped at pre-image length, if out of data).
+    /// @param _partOffset The offset of the pre-image to read.
+    /// @param _preimage The preimage data.
+    function loadKeccak256PreimagePart(uint256 _partOffset, bytes calldata _preimage) external {
         uint256 size;
         bytes32 key;
         bytes32 part;
         assembly {
             // len(sig) + len(partOffset) + len(preimage offset) = 4 + 32 + 32 = 0x44
             size := calldataload(0x44)
+
             // revert if part offset > size+8 (i.e. parts must be within bounds)
-            if gt(partOffset, add(size, 8)) {
+            if gt(_partOffset, add(size, 8)) {
                 revert(0, 0)
             }
             // we leave solidity slots 0x40 and 0x60 untouched,
@@ -57,16 +103,16 @@ contract PreimageOracle {
             mstore(ptr, shl(192, size))
             ptr := add(ptr, 8)
             // copy preimage payload into memory so we can hash and read it.
-            calldatacopy(ptr, preimage.offset, size)
+            calldatacopy(ptr, _preimage.offset, size)
             // Note that it includes the 8-byte big-endian uint64 length prefix.
             // this will be zero-padded at the end, since memory at end is clean.
-            part := mload(add(sub(ptr, 8), partOffset))
+            part := mload(add(sub(ptr, 8), _partOffset))
             let h := keccak256(ptr, size) // compute preimage keccak256 hash
             // mask out prefix byte, replace with type 2 byte
             key := or(and(h, not(shl(248, 0xFF))), shl(248, 2))
         }
-        preimagePartOk[key][partOffset] = true;
-        preimageParts[key][partOffset] = part;
+        preimagePartOk[key][_partOffset] = true;
+        preimageParts[key][_partOffset] = part;
         preimageLengths[key] = size;
     }
 }