fix: dynamic array encoding

[xj894976677]

Dynamic Array Encoding Fix in Web3j

Problem Statement

The original implementation did not correctly handle dynamic arrays according to Solidity's ABI encoding specification. In Solidity, when using abi.encode() with array parameters, the output includes dynamic offsets pointing to the actual array data.

Example in Solidity

// Solidity contract
function encodeExample(uint256[] memory tokenIds, uint256[] memory amounts) public pure returns (bytes memory) {
    return abi.encode(tokenIds, amounts);
}

When calling this function with:

tokenIds = [1, 2]
amounts = [100, 200, 300]

The encoded output structure should be:

[offset_1][offset_2][array1_length][array1_data...][array2_length][array2_data...]

Issue

The original Web3j implementation was concatenating array data directly without proper offset handling:

// Original problematic implementation
ByteArrayOutputStream concatenatedArrayEncodingBuffer = new ByteArrayOutputStream();
for (Object arrayItem : arrayItems) {
    // Directly writing array data without offsets
    concatenatedArrayEncodingBuffer.write(arrayItemEncoding, 0, arrayItemEncoding.length);
}

Solution

Modified the implementation to follow Solidity's ABI encoding specification:

// Fixed implementation
// 1. Calculate and write offsets in the header
encValues.add(BigInteger.valueOf(headSize + dynamicDataSize));

// 2. Store dynamic data separately
ByteArrayOutputStream dynamicBuffer = new ByteArrayOutputStream();
// Write array length
byte[] lengthBytes = Numeric.toBytesPadded(BigInteger.valueOf(arrayItems.size()), 32);
dynamicBuffer.write(lengthBytes, 0, lengthBytes.length);
// Write array elements
for (Object arrayItem : arrayItems) {
    byte[] itemBytes = Numeric.toBytesPadded(convertToBigInt(arrayItem), 32);
    dynamicBuffer.write(itemBytes, 0, itemBytes.length);
}

Impact

• Ensures compatibility with Solidity's abi.encode() function
• Fixes signature verification issues in contracts using EIP-712
• Correctly handles multiple dynamic arrays in structured data

This change aligns Web3j's implementation with Ethereum's ABI specification for dynamic array encoding.