AMM Pools: XSDWETHpool & BankXWETHpool

Overview

BankX operates two custom AMM pools forked from UniswapV2: XSDWETHpool (XSD ↔ WETH) and BankXWETHpool (BankX ↔ WETH). Both pools use the constant-product invariant (k = reserve0 * reserve1) for pricing but diverge in fee structure and access control. Swaps are gated exclusively to the Router contract. Liquidity provision does not mint LP tokens — instead, liquidity is tracked and rewarded through the RewardManager.

Key Concepts

• No LP tokens. Unlike standard UniswapV2 pools, these pools do not issue LP tokens on liquidity provision. WETH and token balances are sent directly to the pool and tracked through reserve0/reserve1 updates via sync().
• Router-gated swaps. The swap() function checks msg.sender == router_address and reverts for any other caller.
• reserve_residue. Both pools enforce a minimum residual liquidity (reserve0_residue, reserve1_residue) that cannot be drained. This prevents the pool from being fully emptied, maintaining price continuity.
• Cumulative price oracles. price0CumulativeLast and price1CumulativeLast are updated each time _update() runs, enabling TWAP (time-weighted average price) calculations by external integrators.
• sync() — Forces pool reserves to match current token balances. Called by RewardManager after liquidity provision, and by XSDStablecoin/BankXToken after pool burns.
• skim() — Transfers any token balance above current reserves to a specified address. Used for balance recovery.

Fee Structure

The XSDWETHpool fee is enforced in the K invariant check:

uint balance0Adjusted = balance0 * 1000 - amount0In * 2;
uint balance1Adjusted = balance1 * 1000 - amount1In * 2;
require(balance0Adjusted * balance1Adjusted >= uint(_reserve0) * uint(_reserve1) * (1000**2));

XSDWETHpool

Token Pair

State Variables

Functions

Events

BankXWETHpool

Token Pair

Fee Difference

The BankX/WETH pool uses a simpler K invariant — it does not apply any fee:

require((balance0 * balance1) >= uint(_reserve0) * (_reserve1), 'BankXWETH: K');

This means swaps are fee-free. The economic incentive for liquidity providers comes from the BankX staking and reward system rather than swap fees.

Notable Difference from XSDWETHpool

BankXWETHpool tracks bankxamount — the BankX input amount on XSD-to-WETH swaps — which is used by BankXToken.burnpoolBankX() for deflationary accounting. XSDWETHpool tracks xsdamount for the same reason.

Security Considerations

• lock modifier prevents reentrancy within pools. Both pools use a uint private unlocked = 1 mutex, equivalent to OpenZeppelin's nonReentrant guard. Note that BankXWETHpool also inherits ReentrancyGuard from OpenZeppelin, providing double protection.
• Swap router restriction. Only router_address can call swap(). Integrators cannot bypass the Router's circuit breakers by calling the pool directly.
• reserve_residue protects against full drain. Setting both residues to 0 would allow a swap to drain the pool entirely to the residue floor. Residues should be set to non-trivial amounts at deployment.
• No LP token = no standard DEX composability. These pools cannot be integrated with DEX aggregators or other DeFi protocols that expect ERC-20 LP tokens. Liquidity is managed entirely through the BankX Router/RewardManager system.
• sync() is permissionless. Anyone can call sync() to update reserves to match balances. This is used intentionally by the RewardManager after liquidity deposits, but it also means unexpected token transfers to the pool will be reflected in reserves on the next sync().

Integration Guide

// Read XSD/WETH pool reserves for price calculation:
IXSDWETHpool pool = IXSDWETHpool(XSDWETH_POOL_ADDRESS);
(uint112 xsdReserve, uint112 wethReserve, ) = pool.getReserves();
 
// Spot XSD price in USD (1e6 precision):
uint256 ethPrice = XSD.eth_usd_price(); // from XSDStablecoin
uint256 xsdPriceVsEth = (uint256(xsdReserve) * 1e6) / uint256(wethReserve);
uint256 xsdPriceUsd = (ethPrice * 1e6) / xsdPriceVsEth;
 
// Note: always use the PID Controller's rolling average price for
// protocol-facing logic rather than the spot price:
uint256 xsdAvgPrice = IPIDController(PID_ADDRESS).xsd_updated_price();