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function sortTokens(address tokenA, address tokenB) internal pure returns (address token0, address token1) {
    require(tokenA != tokenB, 'UniswapV2Library: IDENTICAL_ADDRESSES');
    (token0, token1) = tokenA < tokenB ? (tokenA, tokenB) : (tokenB, tokenA);
    require(token0 != address(0), 'UniswapV2Library: ZERO_ADDRESS');
}

In this function we are sorting the tokens and why we are comparing the data type address

function _swapSupportingFeeOnTransferTokens(address[] memory path, address _to) internal virtual {//ex[usdt,weth,usdc]
    for (uint i; i < path.length - 1; i++) {
        (address input, address output) = (path[i], path[i + 1]);//(input-->usdt,output-->weth)
        (address token0,) = UniswapV2Library.sortTokens(input, output);
        IUniswapV2Pair pair = IUniswapV2Pair(UniswapV2Library.pairFor(factory, input, output));//getting the pair address for selecteed pair
        uint amountInput;
        uint amountOutput;
        //reserve0 is the amount of token0 we have in the pair contract(liquidity pool)balance of usdt in the pair contract
        //reserve1 is the amount of token1 we have i the pair contract(liquidity pool)balance of weth in the pair contract
        { // scope to avoid stack too deep errors
        (uint reserve0, uint reserve1,) = pair.getReserves();//get the amounts of token0 and token1
        (uint reserveInput, uint reserveOutput) = input == token0 ? (reserve0, reserve1) : (reserve1, reserve0);//reserves of input tokens and output tokens
        amountInput = IERC20(input).balanceOf(address(pair)).sub(reserveInput);//our input amount
        //-->we need to take 2% of amount input
        amountOutput = UniswapV2Library.getAmountOut(amountInput, reserveInput, reserveOutput);//maximum amount of output token you will get (cutting the percentage)
        //we will get the output amount as 98% estimated
        }
        (uint amount0Out, uint amount1Out) = input == token0 ? (uint(0), amountOutput) : (amountOutput, uint(0));
        address to = i < path.length - 2 ? UniswapV2Library.pairFor(factory, output, path[i + 2]) : _to;
        pair.swap(amount0Out, amount1Out, to, new bytes(0));
    }
}
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If the tokens weren't sorted, it would result in a distinct pair address for each sort order:

  • Tokens: [A,B] -> Pair address: X
  • Tokens: [B,A] -> Pair address: Y

By sorting the tokens first, the address of any given pair is pre-determined, and can be computed without any on-chain lookups (using CREATE2).

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