Build a v4 hook

// SPDX-License-Identifier: MIT
pragma solidity 0.8.26;

// FairLaunch: a Uniswap v4 hook, written by v4 Builder (v4builder/2) from the recipe below.
// Callbacks: afterInitialize, beforeRemoveLiquidity, beforeSwap. Its address carries them in its low bits.
// Open its pools with the dynamic fee flag (0x800000). Every swap pays at least 0.3%.
//
// - The fee starts at 25% when the pool opens and falls in a straight line to zero over 10 minutes.
// - A wallet that just swapped in this pool waits 30 seconds before its next swap.
// - Liquidity added to the pool cannot be removed for 30 days after the pool opens.

import {BaseHook} from "@openzeppelin/uniswap-hooks/src/base/BaseHook.sol";
import {IPoolManager} from "@uniswap/v4-core/src/interfaces/IPoolManager.sol";
import {Hooks} from "@uniswap/v4-core/src/libraries/Hooks.sol";
import {PoolKey} from "@uniswap/v4-core/src/types/PoolKey.sol";
import {PoolId, PoolIdLibrary} from "@uniswap/v4-core/src/types/PoolId.sol";
import {SwapParams, ModifyLiquidityParams} from "@uniswap/v4-core/src/types/PoolOperation.sol";
import {BeforeSwapDelta, BeforeSwapDeltaLibrary} from "@uniswap/v4-core/src/types/BeforeSwapDelta.sol";
import {LPFeeLibrary} from "@uniswap/v4-core/src/libraries/LPFeeLibrary.sol";

contract FairLaunch is BaseHook {
    using PoolIdLibrary for PoolKey;
    using LPFeeLibrary for uint24;

    uint24 public constant BASE_FEE = 3000; // 0.3%, the floor of every swap's fee
    uint256 public constant LAUNCH_FEE = 250000; // 25% at the open
    uint256 public constant LAUNCH_WINDOW = 600; // seconds for it to fall to zero
    uint256 public constant COOLDOWN = 30; // seconds between two swaps of one wallet
    uint256 public constant LP_LOCK = 2592000; // 30 days after the open
    uint8 internal constant COOLING_DOWN = 1;

    struct PoolState {
        uint64 openedAt;
    }