QuantDEX

A constant-product AMM (automated market maker) DEX built in Solidity. Supports multi-pool token swaps, liquidity provision, and LP share tracking — all governed by the x * y = k invariant with a 0.3% swap fee.

Rebuilt from my original 2017 DEX experiment — same vision, now with constant-product AMM and proper test coverage.

How it works

Pool state

Each token pair maps to a pool via keccak256(abi.encodePacked(tokenA, tokenB)). The pool stores reserveA, reserveB, and totalShares.

Adding liquidity

addLiquidity(tokenA, tokenB, amountA, amountB) pulls both tokens from the caller and mints LP shares proportional to the deposit. The first deposit bootstraps the pool at the geometric mean (sqrt(amountA * amountB)). Subsequent deposits must respect the existing ratio — the contract mints shares based on the lesser of the two pro-rata amounts.

Removing liquidity

removeLiquidity(tokenA, tokenB, shares) burns LP shares and returns a pro-rata slice of both reserves to the caller.

Swapping

swap(tokenIn, tokenOut, amountIn, minAmountOut) routes through the pool in either direction. The output amount is computed as:

amountOut = (amountIn * 997 * reserveOut) / (reserveIn * 1000 + amountIn * 997)

The 0.3% fee stays in the pool, accruing to LPs. The minAmountOut parameter acts as a slippage guard — the call reverts if the output falls below it.

Security notes

• Reserves are internal accounting (pool.reserveA/pool.reserveB), never balanceOf. This structurally closes the first-depositor donation/inflation attack: a raw transfer to the contract isn't counted, so it can't move share price. (The sqrt bootstrap is only for ratio-independence — Uniswap v2 §3.4 — not the inflation defense.)
• Transfers use OpenZeppelin SafeERC20, so tokens that return false or no value (e.g. USDT) revert rather than silently failing.
• Standard ERC20s only. Reserves are credited by the requested amount, so fee-on-transfer / rebasing tokens are unsupported.

Full attack-surface analysis: SECURITY.md. Status: unaudited, educational.

Stack

• Solidity ^0.8.20
• Foundry

Build

forge build

Test

forge test -vv

Tests cover:

• Bootstrap liquidity (addLiquidity)
• Second-deposit liquidity at existing ratio
• Full removeLiquidity round-trip
• Swap A→B and B→A
• Slippage revert
• Constant-product invariant holds after swap (k_after >= k_before)