Source

SWAP ERC20

• deploy erc20mock contract , 2 times for token a & token b

• deploy swap contract , put that 2 address as value's

• don't forget to approve the swap address on that 2 mock as spender

• run createValuetion function as liquidity pool

• with your test account or other account run swap function

• for clear liquidity run clearValuetion function

ERC20 mock token for test here

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Review

• Create pool

function createValuetion(uint _amountA, uint _amountB) external returns (uint variant) {
    require(_amountA > 0 && _amountB > 0, "why you want to spend gas!?");
    pairA.transferFrom(_msgSender(), _this(), _amountA);
    pairB.transferFrom(_msgSender(), _this(), _amountB);
    require(holdA * _amountB == holdB * _amountA, "rule is rule, read mathematicals");
    totalSupply == 0 ? variant = sqrt_(_amountA * _amountB) : 
        variant = _min(
            (_amountA * totalSupply) / holdA,
            (_amountB * totalSupply) / holdB
        );
    require(variant > 0, "imposible thing happend");
    _lock(_msgSender(), variant);
    _hold(pairA.balanceOf(_this()), pairB.balanceOf(_this()));
    emit CreateValue(_msgSender(), _amountA, _amountB, block.timestamp);
}

• Remove pool

function clearValuetion(uint _variant) external returns (uint amountA, uint amountB) {
    uint balanceA = pairA.balanceOf(_this());
    uint balanceB = pairB.balanceOf(_this());
    amountA = (_variant * balanceA) / totalSupply;
    amountB = (_variant * balanceB) / totalSupply;
    require(amountA > 0 && amountB > 0, "imposible thing happend, not match value on tokens, one of them is zero");
    _release(_msgSender(), _variant);
    _hold(balanceA - amountA, balanceB - amountB);
    pairA.transfer(_msgSender(), amountA);
    pairB.transfer(_msgSender(), amountB);
    emit ClearValue(_msgSender(), _variant, block.timestamp);
}

• Swap 2 pairs

function swap(address _tokenIn, uint _amountIn) external isValidToken(_tokenIn) returns (uint amountOut) {
    require(_amountIn > 0, "can not posibble the amount be zero!");
    bool isPairA = _tokenIn == address(pairA);
    (IERC20 pairIn, IERC20 pairOut, uint holdIn, uint holdOut) = isPairA
        ? (pairA, pairB, holdA, holdB)
        : (pairB, pairA, holdB, holdA);
    pairIn.transferFrom(_msgSender(), _this(), _amountIn);
    amountOut = (holdOut * _amountIn) / (holdIn + _amountIn);
    pairOut.transfer(_msgSender(), amountOut);
    _hold(pairA.balanceOf(_this()), pairB.balanceOf(_this()));
    emit Swap(_msgSender(), _amountIn, amountOut, block.timestamp);
}