feat: dynamic array encode

[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.

[subhramit]

Hi, I am not a maintainer (in fact, I just started exploring this project) and was having a hard time finding good first issues to solve myself, so thought of beginning to contribute by reviewing code.

I don't know if the PR is correct or not, as I don't have a lot of context - so pretty sure there will be many more changes when the maintainers take a look, but I left some comments below on code quality which would help you regardless of whether the PR goes in.

[subhramit]

Redundant comment

[subhramit]

Why the extra newlines?

[subhramit]

Unnecessary change?

[subhramit]

You are not calculating the header size, but hardcoding it - so the comment is misleading.
It's better to convert this into a constant like static final HEADER_SIZE just after the class declaration above the constructor at and use it wherever needed.
If you name the constant descriptively enough, the comment can also be let go of.

[subhramit]

Redundant comment

[subhramit]

This seems to be AI-generated.
Nevertheless, if this is correct (I don't have much context), the comment here is useless as it talks about a change, which makes sense in the context of this PR, but won't once merged. Should be removed.
Think - how would the code look like once merged?

[subhramit]

Redundant comment.

[subhramit]

Extract magic number to a constant

[subhramit]

Same here ^

[subhramit]

Redundant comment - possibly because of AI