The Complete DeFi and Blockchain Guide for Developers: From Smart Contracts to Profit Strategies

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

// Simple token swap contract
contract SimpleSwap {
    mapping(address => uint256) public balanceA;
    mapping(address => uint256) public balanceB;

    uint256 public reserveA;
    uint256 public reserveB;

    // Add liquidity
    function addLiquidity(uint256 amountA, uint256 amountB) external {
        // Transfer tokens to the contract (in practice, ERC20 transferFrom)
        reserveA += amountA;
        reserveB += amountB;
    }

    // Swap Token A -> B (AMM: x * y = k)
    function swapAforB(uint256 amountA) external returns (uint256 amountB) {
        uint256 k = reserveA * reserveB;  // Constant product invariant
        uint256 newReserveA = reserveA + amountA;
        uint256 newReserveB = k / newReserveA;
        amountB = reserveB - newReserveB;

        reserveA = newReserveA;
        reserveB = newReserveB;

        return amountB;
    }
}

User Wallet (MetaMask)
    ↓ Transaction Signing
Node (Infura/Alchemy)
    ↓ Broadcast
Mempool
    ↓ Block Inclusion (Gas Fee Competition)
Block Finalization
    ↓ State Change
Smart Contract Execution

# AMM Simulation
class ConstantProductAMM:
    def __init__(self, reserve_a: float, reserve_b: float, fee: float = 0.003):
        self.reserve_a = reserve_a
        self.reserve_b = reserve_b
        self.fee = fee
        self.k = reserve_a * reserve_b

    def get_price(self) -> float:
        """Price of Token A (in Token B terms)"""
        return self.reserve_b / self.reserve_a

    def swap_a_for_b(self, amount_a: float) -> float:
        """Swap Token A for Token B"""
        # Deduct fees
        amount_a_after_fee = amount_a * (1 - self.fee)

        # Apply x * y = k formula
        new_reserve_a = self.reserve_a + amount_a_after_fee
        new_reserve_b = self.k / new_reserve_a
        amount_b_out = self.reserve_b - new_reserve_b

        self.reserve_a = new_reserve_a
        self.reserve_b = new_reserve_b
        self.k = self.reserve_a * self.reserve_b

        return amount_b_out

    def calculate_price_impact(self, amount_a: float) -> float:
        """Calculate price impact"""
        price_before = self.get_price()
        # Temporary swap
        amount_b = self.swap_a_for_b(amount_a)
        price_after = self.get_price()
        # Rollback
        self.reserve_a -= amount_a * (1 - self.fee)
        self.reserve_b += amount_b
        self.k = self.reserve_a * self.reserve_b

        return abs(price_after - price_before) / price_before * 100

# Usage example
pool = ConstantProductAMM(1_000_000, 1_000_000)  # 1:1 pool
print(f"Initial price: {pool.get_price():.4f}")
# Initial price: 1.0000

amount_out = pool.swap_a_for_b(10_000)
print(f"10,000 A -> {amount_out:.2f} B")
# 10,000 A -> 9,940.18 B (slippage occurs)

print(f"Price after swap: {pool.get_price():.4f}")
# Price after swap: 1.0202 (price movement)

# Lending protocol mechanism simulation
class LendingPool:
    def __init__(self):
        self.deposits = {}      # Deposits per user
        self.borrows = {}       # Borrows per user
        self.total_deposits = 0
        self.total_borrows = 0
        self.base_rate = 0.02   # Base interest rate 2%
        self.slope1 = 0.04      # Slope below optimal utilization
        self.slope2 = 0.75      # Slope above optimal utilization
        self.optimal_utilization = 0.80  # Optimal utilization 80%

    def utilization_rate(self) -> float:
        """Utilization = Total Borrows / Total Deposits"""
        if self.total_deposits == 0:
            return 0
        return self.total_borrows / self.total_deposits

    def borrow_rate(self) -> float:
        """Variable borrow interest rate based on utilization"""
        u = self.utilization_rate()
        if u <= self.optimal_utilization:
            return self.base_rate + (u / self.optimal_utilization) * self.slope1
        else:
            excess = (u - self.optimal_utilization) / (1 - self.optimal_utilization)
            return self.base_rate + self.slope1 + excess * self.slope2

    def supply_rate(self) -> float:
        """Supply interest rate = Borrow rate * Utilization"""
        return self.borrow_rate() * self.utilization_rate()

    def deposit(self, user: str, amount: float):
        self.deposits[user] = self.deposits.get(user, 0) + amount
        self.total_deposits += amount

    def borrow(self, user: str, amount: float, collateral_value: float):
        # LTV (Loan-to-Value) check
        max_borrow = collateral_value * 0.75  # 75% LTV
        current_borrow = self.borrows.get(user, 0)
        if current_borrow + amount > max_borrow:
            raise ValueError(f"LTV exceeded: max {max_borrow}, current {current_borrow}")

        self.borrows[user] = current_borrow + amount
        self.total_borrows += amount

# Simulation
pool = LendingPool()
pool.deposit("Alice", 1_000_000)  # Deposit $1 million
pool.borrow("Bob", 600_000, 1_000_000)  # Borrow $600K

print(f"Utilization rate: {pool.utilization_rate():.1%}")
# Utilization rate: 60.0%
print(f"Borrow rate: {pool.borrow_rate():.2%}")
# Borrow rate: 5.00%
print(f"Supply rate: {pool.supply_rate():.2%}")
# Supply rate: 3.00%

# Yield Farming profit calculation
class YieldFarm:
    def __init__(self, pool_name: str, tvl: float, daily_rewards: float):
        self.pool_name = pool_name
        self.tvl = tvl  # Total Value Locked
        self.daily_rewards = daily_rewards

    def apr(self) -> float:
        """Annual Percentage Rate (simple interest)"""
        return (self.daily_rewards * 365) / self.tvl * 100

    def apy(self, compounds_per_year: int = 365) -> float:
        """Annual Percentage Yield (compound interest)"""
        daily_rate = self.daily_rewards / self.tvl
        return ((1 + daily_rate) ** compounds_per_year - 1) * 100

    def impermanent_loss(self, price_ratio: float) -> float:
        """Impermanent loss calculation
        price_ratio: current price / initial price
        """
        il = 2 * (price_ratio ** 0.5) / (1 + price_ratio) - 1
        return il * 100  # Percentage

# ETH-USDC pool example
farm = YieldFarm("ETH-USDC", tvl=10_000_000, daily_rewards=5_000)
print(f"APR: {farm.apr():.1f}%")
# APR: 18.2%
print(f"APY: {farm.apy():.1f}%")
# APY: 20.0%

# If ETH price doubles
il = farm.impermanent_loss(2.0)
print(f"Impermanent loss (2x price): {il:.2f}%")
# Impermanent loss (2x price): -5.72%

// Vulnerable code example - Reentrancy attack
// Never use this pattern!
contract Vulnerable {
    mapping(address => uint256) public balances;

    // Dangerous: state change after external call
    function withdraw() external {
        uint256 amount = balances[msg.sender];
        (bool success, ) = msg.sender.call{value: amount}("");
        require(success);
        balances[msg.sender] = 0;  // State change after external call!
    }
}

// Safe pattern: Checks-Effects-Interactions
contract Safe {
    mapping(address => uint256) public balances;

    function withdraw() external {
        uint256 amount = balances[msg.sender];
        balances[msg.sender] = 0;  // State change first!
        (bool success, ) = msg.sender.call{value: amount}("");
        require(success);
    }
}

Risk: ★☆☆☆☆
Expected return: 3-8% APY

Method:
1. Deposit USDC/USDT on Aave
2. Earn lending interest
3. Bonus: Aave token rewards

Note: Stablecoins also carry depegging risk

Risk: ★★★☆☆
Expected return: 10-25% APY

Method:
1. Provide liquidity to correlated asset pair pools (e.g., ETH-stETH)
2. Earn trading fees + reward tokens
3. Low impermanent loss (since prices move similarly)

Note: Smart contract risk still exists

Risk: ★★★★★
Expected return: 30-100%+ APY

Method:
1. Deposit assets
2. Borrow against deposited assets
3. Provide liquidity again with borrowed funds (leverage)
4. Fees + rewards exceed interest

Note: Liquidation risk, complex position management required

# Hardhat - Smart contract development environment
npm install --save-dev hardhat
npx hardhat init

# Foundry - Fast development tool based on Rust
curl -L https://foundry.paradigm.xyz | bash
foundryup

# ethers.js - Frontend integration
npm install ethers

// Querying DeFi protocols with ethers.js
import { ethers } from 'ethers'

const provider = new ethers.JsonRpcProvider('https://eth.llamarpc.com')

// Uniswap V3 pool price query
const poolAddress = '0x8ad599c3A0ff1De082011EFDDc58f1908eb6e6D8' // USDC/ETH
const poolABI = [
  'function slot0() view returns (uint160, int24, uint16, uint16, uint16, uint8, bool)',
]
const pool = new ethers.Contract(poolAddress, poolABI, provider)

const [sqrtPriceX96] = await pool.slot0()
const price = (Number(sqrtPriceX96) / 2 ** 96) ** 2 * 1e12 // Price calculation
console.log(`ETH/USDC: $${price.toFixed(2)}`)

# Transaction record management example
import csv
from datetime import datetime

def log_transaction(tx_type, asset, amount, usd_value, tx_hash):
    with open('defi_transactions.csv', 'a', newline='') as f:
        writer = csv.writer(f)
        writer.writerow([
            datetime.now().isoformat(),
            tx_type,  # deposit, withdraw, swap, harvest
            asset,
            amount,
            usd_value,
            tx_hash,
        ])