Abi encode / decode mystery additional 32 byte field Uniswap v2

Question

When I encode my abi data in web3js or web3.py for a uni v2 trade I get the following:

0x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But when I use remix to encode/decode I get an extra 32 byte field added. Solidity will not decode on chain without this field.

0x
0000000000000000000000000000000000000000000000000000000000000020 // this guy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What is this extra 32 byte field 0000000000000000000000000000000000000000000000000000000000000020 and why is it only added with an encode through solidity? I'm assuming it has something to do with the array of addresses but I'm not sure what.

Thanks!

---

Code to reproduce ethers/web3js

const ethers = require('ethers'); // "ethers": "^5.7.2",
const Web3 = require("web3"); // "web3": "^1.8.1"
const web3 = new Web3();
let types = [ 'address[]', 'uint256', 'uint256', 'uint256', 'uint160' ]
let values = [
  [
    "0x8AC76a51cc950d9822D68b83fE1Ad97B32Cd580d",
    "0xbb4CdB9CBd36B01bD1cBaEBF2De08d9173bc095c",
  ],
  1690831269,
  ethers.BigNumber.from("200000000000000000000"),
  0,
  ethers.BigNumber.from("5078959830289813231285622442"),
];
web3.eth.abi.encodeParameters(types, values)

Code to reproduce solidity

pragma solidity 0.8.18;

contract Test {
    struct ExactInputSingleParams {
        address[] path;
        uint256 deadline;
        uint256 amountIn;
        uint256 amountOutMinimum;
        uint160 sqrtPriceLimitX96;
    }

    function encode() external pure returns (bytes memory) {
        address[] memory addresses = new address[](2);
        addresses[0] = 0x8AC76a51cc950d9822D68b83fE1Ad97B32Cd580d;
        addresses[1] = 0xbb4CdB9CBd36B01bD1cBaEBF2De08d9173bc095c;
        ExactInputSingleParams memory _params = ExactInputSingleParams({
            path: addresses,
            deadline: 1690831269,
            amountIn: 200000000000000000000,
            amountOutMinimum: 0,
            sqrtPriceLimitX96: 5078959830289813231285622442
        });
        return abi.encode(_params);
    }

    function decode(bytes memory message) external pure returns(uint256) {
        ExactInputSingleParams memory _params = abi.decode(
            message,
            (ExactInputSingleParams)
        );
        return _params.deadline;
    }
}

Answer 1

"that guy" indicates where the data start, and it is hex 0x20 so data starts at byte 32. (it seems this appears only whenever there is any dynamic value)

word 1 is 0xa0, which is the offset where the array data starts, at byte 160, after the static values.
word 2 is 0x64c809a5, which is 1690831269
word 3 is 0xad78ebc5ac6200000, which is 200000000000000000000
word 4 is 0x00
word 5 is 0x10693794192ce3e67394d6aa, which is 5078959830289813231285622442
word 6 is 0x02, which is the length of the dynamic array
And the last 2 words are the values of the array.

ethers/web3js do not implement the first pointer because its not really neccesary. I don´t know why Solidity implemented it, but its not a problem as long as you use the native functions to encode and decode.