✨ LibBytes truncate

src/utils/LibBytes.sol

@@ -405,6 +405,34 @@ library LibBytes {
         result = slice(subject, start, type(uint256).max);
     }
 
+    /// @dev Reduces the size of `subject` to `n`.
+    /// If `n` is greater than the size of `subject`, this will be a no-op.
+    function truncate(bytes memory subject, uint256 n)
+        internal
+        pure
+        returns (bytes memory result)
+    {
+        /// @solidity memory-safe-assembly
+        assembly {
+            result := subject
+            mstore(mul(lt(n, mload(result)), result), n)
+        }
+    }
+
+    /// @dev Returns a copy of `subject`, with the length reduced to `n`.
+    /// If `n` is greater than the size of `subject`, this will be a no-op.
+    function truncatedCalldata(bytes calldata subject, uint256 n)
+        internal
+        pure
+        returns (bytes calldata result)
+    {
+        /// @solidity memory-safe-assembly
+        assembly {
+            result.offset := subject.offset
+            result.length := xor(n, mul(xor(n, subject.length), lt(subject.length, n)))
+        }
+    }
+
     /// @dev Returns all the indices of `needle` in `subject`.
     /// The indices are byte offsets.
     function indicesOf(bytes memory subject, bytes memory needle)

src/utils/g/LibBytes.sol

@@ -409,6 +409,20 @@ library LibBytes {
         result = slice(subject, start, type(uint256).max);
     }
 
+    /// @dev Reduces the size of `subject` to `n`.
+    /// If `n` is greater than the size of `subject`, this will be a no-op.
+    function truncate(bytes memory subject, uint256 n)
+        internal
+        pure
+        returns (bytes memory result)
+    {
+        /// @solidity memory-safe-assembly
+        assembly {
+            result := subject
+            mstore(mul(lt(n, mload(result)), result), n)
+        }
+    }
+
     /// @dev Returns all the indices of `needle` in `subject`.
     /// The indices are byte offsets.
     function indicesOf(bytes memory subject, bytes memory needle)

test/LibBytes.t.sol

@@ -18,4 +18,17 @@ contract LibBytesTest is SoladyTest {
         bytes memory expected = LibBytes.slice(a, o, o + 32);
         assertEq(abi.encode(LibBytes.loadCalldata(a, o)), expected);
     }
+
+    function testTruncate(bytes memory a, uint256 n) public {
+        bytes memory sliced = LibBytes.slice(a, 0, n);
+        bytes memory truncated = LibBytes.truncate(a, n);
+        assertEq(truncated, sliced);
+        assertEq(a, sliced);
+    }
+
+    function testTruncatedCalldata(bytes calldata a, uint256 n) public {
+        bytes memory sliced = LibBytes.slice(a, 0, n);
+        bytes memory truncated = LibBytes.truncatedCalldata(a, n);
+        assertEq(truncated, sliced);
+    }
 }