Port Base64 tests to truffle (#4926) (#4929)

Co-authored-by: Hadrien Croubois <hadrien.croubois@gmail.com>

contracts/mocks/Base64Dirty.sol

@@ -0,0 +1,19 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import {Base64} from "../utils/Base64.sol";
+
+contract Base64Dirty {
+    struct A {
+        uint256 value;
+    }
+
+    function encode(bytes memory input) public pure returns (string memory) {
+        A memory unused = A({value: type(uint256).max});
+        // To silence warning
+        unused;
+
+        return Base64.encode(input);
+    }
+}

contracts/utils/Base64.sol

@@ -41,12 +41,19 @@ library Base64 {
             let tablePtr := add(table, 1)
 
             // Prepare result pointer, jump over length
-            let resultPtr := add(result, 32)
+            let resultPtr := add(result, 0x20)
+            let dataPtr := data
+            let endPtr := add(data, mload(data))
+
+            // In some cases, the last iteration will read bytes after the end of the data. We cache the value, and
+            // set it to zero to make sure no dirty bytes are read in that section.
+            let afterPtr := add(endPtr, 0x20)
+            let afterCache := mload(afterPtr)
+            mstore(afterPtr, 0x00)
 
             // Run over the input, 3 bytes at a time
             for {
-                let dataPtr := data
-                let endPtr := add(data, mload(data))
+
             } lt(dataPtr, endPtr) {
 
             } {
@@ -54,13 +61,12 @@ library Base64 {
                 dataPtr := add(dataPtr, 3)
                 let input := mload(dataPtr)
 
-                // To write each character, shift the 3 bytes (18 bits) chunk
+                // To write each character, shift the 3 byte (24 bits) chunk
                 // 4 times in blocks of 6 bits for each character (18, 12, 6, 0)
-                // and apply logical AND with 0x3F which is the number of
-                // the previous character in the ASCII table prior to the Base64 Table
-                // The result is then added to the table to get the character to write,
-                // and finally write it in the result pointer but with a left shift
-                // of 256 (1 byte) - 8 (1 ASCII char) = 248 bits
+                // and apply logical AND with 0x3F to bitmask the least significant 6 bits.
+                // Use this as an index into the lookup table, mload an entire word
+                // so the desired character is in the least significant byte, and
+                // mstore8 this least significant byte into the result and continue.
 
                 mstore8(resultPtr, mload(add(tablePtr, and(shr(18, input), 0x3F))))
                 resultPtr := add(resultPtr, 1) // Advance
@@ -75,6 +81,9 @@ library Base64 {
                 resultPtr := add(resultPtr, 1) // Advance
             }
 
+            // Reset the value that was cached
+            mstore(afterPtr, afterCache)
+
             // When data `bytes` is not exactly 3 bytes long
             // it is padded with `=` characters at the end
             switch mod(mload(data), 3)