Uniswap – How to Calculate Swap Price from a Uniswap V2 Swap Event

eventsswapsuniswap

Consider the following swap event as listed here:

amount0In :4003282810462813624841310246
amount1In :947647544007674247
amount0Out :729688943313804828976594873545
amount1Out :0

To calculate the swap price (token1/token0)( without accounting for potential transfer tax) you would just do amount1/amount0, and depending on buy or sell this would translate to amount1Out/amount0In or amount1In/amount0Out. Please let me know if I have this part wrong.

However, the above event has both amount0In and amount1In as non-zero. What does this even mean (i.e. what happened here) and how to calculate the swap price in this situation?

Best Answer

Sometimes Uniswap pool reserves as tracked by the smart contract goes out of sync with the token balances in the pool's contract - for example, when someone donates a token in the pool and fails call sync() afterwards.

The result is these weird swap events, where one or both input token amounts are more than what was provided by the swapper. In the general case this means that swap event is not a 100% reliable way how to get the price in the pool.

If the transaction has come through the Uniswap router or some other sensible contract, then its pretty safe to assume that only one token was actually intended to be used for the input. Which one - that depends on the amount0Out or amount1Out not being zero.

In your example amount1Out is zero, so token 1 was probably the input token. To compute the price, you must further subtract the 0.3% fee from the input amount, like this:

amount1InWithFee = 947647544007674247 * 997 / 1000
price = amount1InWithFee / amount0Out

Now the real horror is transactions where both outputs are zero... Luckily with the newer routers they are much less frequent than in the early years of Uniswap.

Related Solutions

uniswap – Understanding Price Differences Between Pair Contracts and Swap Pair Contracts

The pair price is the price that you will get if you trade an infinitesimally small amount. It is a theoretical equilibrium price that you will never get in practice.

The swap price is the price that you will get if you swap the given amount (1 Eth in your screenshot). The swap price will always be lower than the pair price because of the "price impact" (something referred to as slippage), which is inherent to how AMM work. The bigger the swap in relation to the size of the pool, the worst price you will get. You can learn more about it here. The swap price most likely also include the swap fee that you pay to the liquidity providers (often 0.2% or 0.3%).

Uniswap – How Does Uniswap Interact with Token Contract When Users Swap?

When you swap an ERC20 token for ETH on Uniswap, the token's transfer function is called - see IERC20.transfer. IIRC when you "buy" your wallet is the recipient and the Token-WETH pair contract (created by the Uniswap Factory) is the sender. On the other hand, when you "sell" your wallet is the sender and the pair is the recipient.

If you'd like to get the address of the pair (your token-WETH) you can use the factory to get it:

IUniswapV2Router router = IUniswapV2Router(uniswapV2RouterAddress);
IUniswapV2Pair pair = IUniswapV2Factory(router.factory())
    .getPair(address(this), router.WETH());

See Uniswap interface code and this answer (for example) to find the Uniswap router address (or google it)

Best Answer

Related Solutions

uniswap – Understanding Price Differences Between Pair Contracts and Swap Pair Contracts

Uniswap – How Does Uniswap Interact with Token Contract When Users Swap?

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