Question about the best Uniswap path

Question

How the best path is calculated ? Is it a fonction in the smart contract or on the website ?

Because I checked the function swapExactETHForTokens and the path need to be set by the function caller.

function swapExactETHForTokens(uint amountOutMin, address[] calldata path, address to, uint deadline)
        external
        virtual
        override
        payable
        ensure(deadline)
        returns (uint[] memory amounts)
    {
        require(path[0] == WETH, 'UniswapV2Router: INVALID_PATH');
        amounts = UniswapV2Library.getAmountsOut(factory, msg.value, path);
        require(amounts[amounts.length - 1] >= amountOutMin, 'UniswapV2Router: INSUFFICIENT_OUTPUT_AMOUNT');
        IWETH(WETH).deposit{value: amounts[0]}();
        assert(IWETH(WETH).transfer(UniswapV2Library.pairFor(factory, path[0], path[1]), amounts[0]));
        _swap(amounts, path, to);
    }

If I want to swap programatically, how I'm supposed to know the best path ?

Answer 1

The best path is not calculated within the contracts. You must provide the exact path when calling the contract, so you must know it in advance.

We can infer from this that it is calculated off-chain by the website. I do not know if the code for the website is public, but you could check the uniswap github repo and see if they published it there.