FeeDiscountHook (Uniswap v4) — Fee Discount Hook

A Uniswap v4 Hook built with Foundry that applies a discounted swap fee for whitelisted users.

Features

• ✅ Whitelisted users get a lower fee override (0.1% in this implementation)
• ✅ Non-whitelisted users pay the normal pool fee
• ✅ Includes Foundry tests using Uniswap v4 core test utilities (Deployers, PoolSwapTest, HookMiner)

What this hook does

This hook implements the beforeSwap hook and dynamically returns a fee override:

• If tx.origin is whitelisted → feeOverride = DISCOUNTED_FEE
• Otherwise → feeOverride = 0 (no override, default pool fee applies)

⚠️ Note: using tx.origin is intentional here to support router-based swaps in tests. In production systems you might want to use msg.sender, signed payloads, or more robust user identity checks.

Files

Hook Contract

• src/feeDiscount.sol

Key elements:

• FeeDiscountHook inherits BaseHook
• owner-controlled setWhitelist
• implements _beforeSwap(...)
• returns BeforeSwapDeltaLibrary.ZERO_DELTA and a fee override

Tests

• test/feeDiscount.t.sol

Tests include:

• only owner can whitelist users
• whitelisted swaps pay less than non-whitelisted swaps
• revoked whitelist pays full fee again
• zero amount swap reverts (SwapAmountCannotBeZero)

Tech Stack

• Solidity ^0.8.24
• Foundry
• Uniswap v4 core + periphery
• HookMiner for deterministic hook deployment with correct permissions flags

Setup

Prerequisites

• Install Foundry

curl -L https://foundry.paradigm.xyz | bash
foundryup

Installation

# Install dependencies
forge install

# Build
forge build

# Test
forge test -vv

# Format
forge fmt

How it works (high level)

1. Hook permissions

   The hook declares only beforeSwap as enabled:

   beforeSwap: true

2. Hook deployed using HookMiner

   Uniswap v4 hooks must be deployed at an address that matches the hook permissions flags.

   The tests use:

   HookMiner.find(...)
   new FeeDiscountHook{salt: salt}(manager)

   so that the hook address matches the required flags.

3. Discount fee override

   The hook applies:

   uint24 public constant DISCOUNTED_FEE = 1000; // 0.1%

   Then in _beforeSwap:

   uint24 feeOverride = whitelist[tx.origin] ? DISCOUNTED_FEE : 0;

Example expected output (tests)

When running:

forge test -vv

You should see logs like:

Whitelisted user input paid < Non-whitelisted input paid
Savings shown in wei

Security Notes

This is a learning project.

• Access control is owner-based and simple.
• Using tx.origin is generally discouraged in production but useful in router-based swaps & testing.

Contributing

Contributions are welcome! Please open an issue or submit a pull request.

License

MIT License