What DeFi Actually Is

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

import "@openzeppelin/contracts/token/ERC20/ERC20.sol";
import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
import "@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol";

// A minimum-viable DeFi vault.
// Anyone deposits USDC, gets shares representing pro-rata ownership.
// No authority can take user funds — only deposit/withdraw logic governs.

contract MiniVault is ERC20 {
    using SafeERC20 for IERC20;

    IERC20 public immutable asset;

    constructor(IERC20 _asset) ERC20("MiniVault Share", "mvUSDC") {
        asset = _asset;
    }

    function deposit(uint256 amount, address receiver) external returns (uint256 shares) {
        uint256 totalAssets = asset.balanceOf(address(this));
        uint256 totalShares = totalSupply();

        shares = totalShares == 0
            ? amount
            : (amount * totalShares) / totalAssets;

        asset.safeTransferFrom(msg.sender, address(this), amount);
        _mint(receiver, shares);
    }

    function withdraw(uint256 shares, address receiver) external returns (uint256 amount) {
        uint256 totalAssets = asset.balanceOf(address(this));
        amount = (shares * totalAssets) / totalSupply();

        _burn(msg.sender, shares);
        asset.safeTransfer(receiver, amount);
    }
}

// Key DeFi properties illustrated:
//   - Custody: contract holds funds, not a person or company
//   - Pro-rata: shares track ownership precisely
//   - Permissionless: no whitelist; anyone can deposit/withdraw
//   - Composable: another contract can deposit on a user's behalf