UniswapV2Library

pairFor -> calculates the CREATE2 address of the liquidity pool

• getReserves → fetches and sorts the reserves for a pair

• getAmountsOut -> Iterates over the path of exchanges needed and performs chained getAmountOut calculations on any number of pairs

• getAmountOut → given an input amount of an asset and pair reserves, returns the maximum output amount of the other asset considering a 0.3% fee

sortTokens

// returns sorted token addresses, used to handle return values from pairs sorted in this order
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");
}

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).

You have to sort the tokens first, otherwise you will get 2 different pair addresses for each pair.

Regardless of scenario, token0 will be the smaller address:

First Tab

first address always smallest

Code

pairFor

compute pair (liquidity pool) addresses without any on-chain lookups because of CREATE2. The following values are required for this technique:

address	The factory address
salt	keccak256(abi.encodePacked(token0, token1))
keccak256(init_code)	0x96e8ac4277198ff8b6f785478aa9a39f403cb768dd02cbee326c3e7da348845f

• token0 must be strictly less than token1 by sort order.

• UniswapV2Factory is deployed at 0x5C69bEe701ef814a2B6a3EDD4B1652CB9cc5aA6f

• factory creates liquidity pools via createPair

https://docs.uniswap.org/contracts/v2/guides/smart-contract-integration/getting-pair-addresses