SwapDeposit

class defipy.process.SwapDeposit()

Single-sided deposit, also called a zap-in. The caller arrives with only one token from the pair; the primitive computes the optimal swap fraction α, swaps that fraction for the other token, then performs a balanced two-sided deposit. No leftover dust.

The closed-form V2 zap formula is f·α²·dx + r·α·(1+f) − r = 0 where f = 0.997 is the fee multiplier, r is the input-token reserve, and dx is the deposit amount. The primitive solves the quadratic, executes the swap leg, then deposits the remainder.

Available on Uniswap V2 and Uniswap V3 only. Balancer and Stableswap support is tracked for a later release.

Methods

apply

amount_out = SwapDeposit().apply(pool, token_in, user, amount_in)

Deposit amount_in of token_in into pool on behalf of user, swapping the optimal portion internally so the deposit balances cleanly.

Parameters

Name	Type	Description
`pool`	exchange	Pool instance to deposit into. V2 or V3 only.
`token_in`	`ERC20`	The single token the user is depositing. The other token of the pair is acquired via the internal swap.
`user`	`str`	User address (string identifier) who provides liquidity and receives LP tokens.
`amount_in`	`float`	Amount of `token_in` to zap in.

Returns

Name	Type	Description
`amount_out`	`float`	Amount of the other token that was acquired internally and deposited alongside the remainder of `token_in`.

Example

Zap exact token1 into the pool

from defipy import *

user_nm = 'user'
eth_amount = 1000
tkn_amount = 100000

tkn = ERC20("TKN", "0x111")
eth = ERC20("ETH", "0x09")
exchg_data = UniswapExchangeData(tkn0 = eth, tkn1 = tkn, symbol="LP", address="0x011")

factory = UniswapFactory("ETH pool factory", "0x2")
lp = factory.deploy(exchg_data)

Join().apply(lp, user_nm, eth_amount, tkn_amount)
SwapDeposit().apply(lp, tkn, user_nm, 100)
lp.summary()

Exchange ETH-TKN (LP) Reserves: ETH = 1000.0, TKN = 100100.0 Liquidity: 10004.991241237401

Zap exact token0 into the pool

amount_out = SwapDeposit().apply(lp, eth, user_nm, 1)
lp.summary()

Exchange ETH-TKN (LP) Reserves: ETH = 1001.0, TKN = 100000.0 Liquidity: 10004.991241237401

Notes

The optimal swap fraction is not 0.5. The closed-form solution gives α → 1/(1+f) ≈ 0.50075 in the zero-deposit limit (slight upward bias from the fee asymmetry), and α decreases with deposit size. A larger zap moves the price more, so each unit swapped buys less, so the optimal fraction shrinks.
For a non-mutating projection of what SwapDeposit would do — useful for previewing α and the expected LP-token mint — see the OptimalDepositSplit agentic primitive.
The mechanism is: deposit incoming token → perform 50%-side swap → perform 50/50 deposit with the remaining input plus the swap output.