node1.py

import datetime
import hashlib
import json
from urllib.parse import urlparse
from uuid import uuid4

import requests
from flask import Flask, jsonify, request


class Blockchain:
    def __init__(self):
        self.chain = []
        self.transactions = []
        self.create_block(
            proof=1, previous_hash="0"
        )  # the proof here (PoW) is the nonce.
        self.nodes = set()

    def create_block(self, proof, previous_hash):
        block = {
            "index": len(self.chain) + 1,
            "timestamp": str(datetime.datetime.now()),
            "proof": proof,
            "previous_hash": previous_hash,
            "transactions": self.transactions,
        }
        self.transactions = []
        self.chain.append(block)
        return block

    def get_previous_block(self):
        return self.chain[-1]

    # a proof of work is a number or piece of date the miners have to solve or find in order to mine a new block
    # this number have to be hard to find (for the currency not to lose its value) but easy to verify!

    def proof_of_work(self, previous_proof):
        new_proof = 1
        check_proof = False
        while check_proof is False:
            hash_operation = hashlib.sha256(
                str(new_proof**2 - previous_proof**2).encode()
            ).hexdigest()
            # Here the only prerequisite for the operation between prev and new hashes is that the operation cannot be simetrical (X operation Y == Y operation X)
            if hash_operation[:4] == "0000":
                check_proof = True
            else:
                new_proof += 1
        return new_proof

    def hash(self, block):
        encoded_block = json.dumps(block, sort_keys=True).encode()
        return hashlib.sha256(encoded_block).hexdigest()

    def is_chain_valid(self, chain):
        previous_block = chain[0]
        block_index = 1
        while block_index < len(chain):
            block = chain[block_index]
            if block["previous_hash"] != self.hash(previous_block):
                return False
            previous_proof = previous_block["proof"]
            proof = block["proof"]
            hash_operation = hashlib.sha256(
                str(proof**2 - previous_proof**2).encode()
            ).hexdigest()
            if hash_operation[:4] != "0000":
                return False
            previous_block = block
            block_index += 1
        return True

    def add_transactions(self, sender, receiver, amount):
        self.transactions.append(
            {"sender": sender, "receiver": receiver, "amount": amount}
        )
        previous_block = self.get_previous_block()
        return previous_block["index"] + 1

    def add_node(self, address):
        parsed_url = urlparse(address)
        self.nodes.add(parsed_url.netloc)

    def replace_chain(self):
        network = self.nodes
        longest_chain = None
        max_length = len(self.chain)
        for node in network:
            response = requests.get(f"http://{node}/get_chain")
            if response.status_code == 200:
                length = response.json()["length"]
                chain = response.json()["chain"]
                if length > max_length and self.is_chain_valid(chain):
                    max_length = length
                    longest_chain = chain
        if longest_chain:
            self.chain = longest_chain
            return True
        return False


### Mining the blockchain

# create a Web App
app = Flask(__name__)

# create an address on port 5000
node_address = str(uuid4()).replace("-", "")

# Creates the blockchain
blockchain = Blockchain()

# via GET method send command to mine a block
@app.route("/mine_block/", methods=["GET"])
def mine_block():
    previous_block = blockchain.get_previous_block()
    previous_proof = previous_block["proof"]
    block_proof = blockchain.proof_of_work(previous_proof)
    previous_hash = blockchain.hash(previous_block)
    blockchain.add_transactions(sender=node_address, receiver="node1", amount=1)
    block = blockchain.create_block(block_proof, previous_hash)
    response = {
        "message": "Congratulations, you just mined a block.",
        "index": block["index"],
        "timestamp": block["timestamp"],
        "proof": block["proof"],
        "previous_hash": block["previous_hash"],
        "transactions": block["transactions"],
    }
    return jsonify(response), 200


# display the full blockchain
@app.route("/get_chain/", methods=["GET"])
def get_chain():
    response = {"chain": blockchain.chain, "length": len(blockchain.chain)}
    return jsonify(response), 200


# check if blockchain is working fine.
@app.route("/is_chain_valid/", methods=["GET"])
def is_chain_valid():
    response = {"Is chain valid?": str(blockchain.is_chain_valid(blockchain.chain))}
    return jsonify(response), 200


# adds new transaction to the blockchain
@app.route("/add_transaction/", methods=["POST"])
def add_transaction():
    json = request.get_json()
    transaction_keys = ["sender", "receiver", "amount"]
    if not all(key in json for key in transaction_keys):
        return "some elements of the transaction are missing", 400
    index = blockchain.add_transactions(
        json["sender"], json["receiver"], json["amount"]
    )
    response = {"message": f"this transaction will be added to block {index}."}
    return jsonify(response), 201


# Connects new nodes
@app.route("/connect_node/", methods=["POST"])
def connect_node():
    json = request.get_json()
    nodes = json.get("nodes")
    if nodes is None:
        return "list of nodes is empty.", 400
    for node in nodes:
        blockchain.add_node(node)
    response = {
        "message": "All the nodes are connected.",
        "total_nodes": list(blockchain.nodes),
    }
    return jsonify(response), 201


# Replaces the chain for the largest one, if needed
@app.route("/replace_chain/", methods=["GET"])
def replace_chain():
    is_chain_replaced = blockchain.replace_chain()
    if is_chain_replaced:
        response = {
            "message": "The nodes had different chains. The chain was replaced by the largest one.",
            "new_chain": blockchain.chain,
        }
    else:
        response = {
            "message": "The chain is already the largest one.",
            "actual_chain": blockchain.chain,
        }
    return jsonify(response), 200


# run the app
app.run(host="0.0.0.0", port=5001)