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I’m trying to understand ABI decoding of multicall RPC calls so I can create stubbed tests using WireMock for my Rust EVM application.

The multicall function signature is:

function aggregate(Call[] calldata calls) external payable returns (uint256 blockNumber, bytes[] memory returnData);

The two ERC-20 functions I’m calling inside it are:

function symbol() public view returns (string); function decimals() public view returns (uint8);

What I Tried

I attempted to manually construct the encoded response.

For three tokens I tried this:

0x 1 - 000000000000000000000000000000000000000000000000000000000378cc49 // block number 2 - 0000000000000000000000000000000000000000000000000000000000000040 // Offset to start of array 3 - 0000000000000000000000000000000000000000000000000000000000000006 // Length of array 4 - 0000000000000000000000000000000000000000000000000000000000000080 // Offset to string 1 5 - 00000000000000000000000000000000000000000000000000000000000000e0 // Offset to string 2 6 - 0000000000000000000000000000000000000000000000000000000000000140 // Offset to string 3 7 - 0000000000000000000000000000000000000000000000000000000000000012 // Decimals for coin 1 8 - 0000000000000000000000000000000000000000000000000000000000000004 // Length of string 1 9 - 57424e4200000000000000000000000000000000000000000000000000000000 // String contents 10 - 0000000000000000000000000000000000000000000000000000000000000012 // Decimals for coin 11 - 0000000000000000000000000000000000000000000000000000000000000004 // Length of string 12 - 43616b6500000000000000000000000000000000000000000000000000000000 // String contents 13 - 0000000000000000000000000000000000000000000000000000000000000012 // Decimals for coin 14 - 0000000000000000000000000000000000000000000000000000000000000004 // Length of string 15 - 5553444300000000000000000000000000000000000000000000000000000000 // String contents

Then this failed so I tried to do this for a single return:

 "0x 000000000000000000000000000000000000000000000000000000000378cc49 // Block number 0000000000000000000000000000000000000000000000000000000000000040 // Offset to array 0000000000000000000000000000000000000000000000000000000000000002 // Length of array 0000000000000000000000000000000000000000000000000000000000000040 // Offset to string 0000000000000000000000000000000000000000000000000000000000000012 // Decimals 0000000000000000000000000000000000000000000000000000000000000004 // String Length 57424e4200000000000000000000000000000000000000000000000000000000" // String contents

But when I run these through WireMock and hit them from Rust, I get:

type check failed for "offset (usize)" with data: 0000000000000000000000000040000000000000000000000000000000000000

What Actually Happens

When I make a real multicall, the response looks like this:

aggregateReturn { blockNumber: 58256789, returnData: [ "0x0000000000000000000000000000000000000000000000000000000000000012", "0x0000000000000000000000000000000000000000000000000000000000000020 0000000000000000000000000000000000000000000000000000000000000004 57424e4200000000000000000000000000000000000000000000000000000000", "0x0000000000000000000000000000000000000000000000000000000000000012", "0x0000000000000000000000000000000000000000000000000000000000000020 0000000000000000000000000000000000000000000000000000000000000004 43616b6500000000000000000000000000000000000000000000000000000000", "0x0000000000000000000000000000000000000000000000000000000000000012", "0x0000000000000000000000000000000000000000000000000000000000000020 0000000000000000000000000000000000000000000000000000000000000004 5553444300000000000000000000000000000000000000000000000000000000" ] }

My Question

How should I correctly construct a stubbed multicall response for testing (e.g. for 3 tokens with decimals = 18 and symbols "BNB", "Cake", "USDC")?

Thanks in advance

Edit: I tried to follow this as a tutorial so really can't see what I am missing:

ABI examples

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1 Answer 1

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 import { encodeAbiParameters, decodeAbiParameters, parseAbiParameters } from 'viem' const blockNumber = 58256789n const returnData = [ encodeAbiParameters(parseAbiParameters('string'), ['BNB']), encodeAbiParameters(parseAbiParameters('uint8'), [18]), encodeAbiParameters(parseAbiParameters('string'), ['Cake']), encodeAbiParameters(parseAbiParameters('uint8'), [18]), encodeAbiParameters(parseAbiParameters('string'), ['USDC']), encodeAbiParameters(parseAbiParameters('uint8'), [18]) ] const encoded = encodeAbiParameters( parseAbiParameters('uint256, bytes[]'), [blockNumber, returnData] ) console.log('Encoded for WireMock:', encoded) const [decodedBlock, decodedData] = decodeAbiParameters( parseAbiParameters('uint256, bytes[]'), encoded ) console.log('Block:', decodedBlock) // Decode each return value decodedData.forEach((data, i) => { if (i % 2 === 0) { const [symbol] = decodeAbiParameters(parseAbiParameters('string'), data) console.log(`Token ${i/2 + 1} symbol:`, symbol) } else { const [decimals] = decodeAbiParameters(parseAbiParameters('uint8'), data) console.log(`Token ${Math.floor(i/2) + 1} decimals:`, decimals) } })

You can try this script with viem library:

https://viem.sh/docs/getting-started

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