import Foundation
import CryptoKit

struct Block {
    let index: Int
    let previousHash: String
    let timestamp: Date
    let data: String
    let hash: String
    
    init(index: Int, previousHash: String, data: String) {
        self.index = index
        self.previousHash = previousHash
        self.timestamp = Date()
        self.data = data
        self.hash = Block.calculateHash(index: index, previousHash: previousHash, timestamp: self.timestamp, data: data)
    }
    
    static func calculateHash(index: Int, previousHash: String, timestamp: Date, data: String) -> String {
        let input = "\(index)\(previousHash)\(timestamp)\(data)"
        let hash = SHA256.hash(data: input.data(using: .utf8)!)
        return hash.compactMap { String(format: "%02x", $0) }.joined()
    }
}

struct Blockchain {
    var blocks: [Block]
    
    init() {
        self.blocks = [Block(index: 0, previousHash: "0", data: "Genesis Block")]
    }
    
    mutating func addBlock(data: String) {
        let previousBlock = blocks.last!
        let newBlock = Block(index: previousBlock.index + 1, previousHash: previousBlock.hash, data: data)
        blocks.append(newBlock)
    }
}

struct Transaction {
    let sender: String
    let recipient: String
    let amount: Double
}

import CryptoKit

func signTransaction(transaction: Transaction, privateKey: P256.Signing.PrivateKey) -> String {
    let transactionData = "\(transaction.sender)\(transaction.recipient)\(transaction.amount)"
    let signature = try! privateKey.signature(for: Data(transactionData.utf8))
    return signature.compactMap { String(format: "%02x", $0) }.joined()
}

func verifyTransaction(transaction: Transaction, signature: String, publicKey: P256.Signing.PublicKey) -> Bool {
    let transactionData = "\(transaction.sender)\(transaction.recipient)\(transaction.amount)"
    let signatureData = Data(hex: signature)
    return publicKey.isValidSignature(signatureData, for: Data(transactionData.utf8))
}