blockchain.py

import hashlib
import json
from time import time
from uuid import uuid4
from flask import Flask, jsonify, request
from urllib.parse import urlparse

class Blockchain:
    def __init__(self):
        self.chain = []
        self.current_transactions = []

        #Create the genesis block
        self.new_block(proof=100,previous_hash=1)
        self.nodes = set()

    def register_node(self,address):
        #add a new node to the list of nodes
        parsed_url = urlparse(address)
        self.nodes.add(parsed_url.netloc)

    def valid_chain(self,chain):
        #determine if given blockchain is valid
        last_block = chain[0]
        current_index = 1
        while current_index < len(chain):
            block = chain[current_index]
            print(f'{last_block}')
            print(f'{block}')
            print("\n---------------\n")

            if block['previous_hash'] != self.hash(last_block):
                return False
            if not self.valid_proof(last_block['proof'],block['proof']):
                return False

            last_block = block
            current_index += 1
        return True

    def resolve_conflicts(self):
        #our consensus algorithm
        neighbours = self.nodes
        new_chain = None
        #looking for chains longer than ours
        max_length = len(self.chain)

        for node in neighbours:
            response = requests.get(f'http://{node}/chain')
            if response.status_code == 200:
                length = response.json()['length']
                chain = response.json()['chain']
                if length > max_length and self.valid_chain(chain):
                    max_length = length
                    new_chain = chain

        if new_chain:
            self.chain = new_chain
            return True
        
        return False
        

    def new_block(self,proof,previous_hash=None):
        block = {
            'index': len(self.chain)+1,
            'timestamp': time(),
            'transactions': self.current_transactions,
            'proof': proof,
            'previous_hash': previous_hash if previous_hash!=None else self.hash(self.chain[-1]),
            }

        self.current_transactions = []

        self.chain.append(block)
        return block

    def new_transaction(self,sender,recipient,amount):
        self.current_transactions.append({'sender':sender,'recipient':recipient,'amount':amount,})

        #index of the block that will hold this transaction
        return self.last_block['index'] + 1

    @staticmethod
    def hash(block):
        #creates a SHA-256 hash of a Block
        #dump json object as string and encode it utf-8(default)
        block_string = json.dumps(block,sort_keys=True).encode()

        return hashlib.sha256(block_string).hexdigest()

    @property
    def last_block(self):
        return self.chain[-1]


    def proof_of_work(self,last_proof):
        #Find a p' such that hash(pp') has 4 leading zeros
        #p is the previous proof , p' is the new proof
        proof = 0

        while self.valid_proof(last_proof,proof) == False:
            proof += 1
        return proof

    @staticmethod
    def valid_proof(last_proof,proof):
        guess = f'{last_proof}{proof}'.encode()
        guess_hash = hashlib.sha256(guess).hexdigest()
        return (guess_hash[:4] == "0000")

    
    
#Instantiate our Node(server)
app = Flask(__name__)
#unique identifier for this node
node_identifier = str(uuid4()).replace('-','')
#instantiate the Blockchain
blockchain = Blockchain()

@app.route('/')
def hello_world():
    return 'Hello, Blockchain!'


@app.route('/mine',methods=['GET'])
def mine():
    #run the proof of work algorithm
    last_block = blockchain.last_block
    last_proof = last_block['proof']
    proof = blockchain.proof_of_work(last_proof)

    #receive reward for mining
    blockchain.new_transaction(
        sender="0",
        recipient=node_identifier,
        amount=1,
        )

    previous_hash = blockchain.hash(last_block)
    block = blockchain.new_block(proof,previous_hash)

    response = {
        'message': "New Block Forged",
        'index': block['index'],
        'transactions': block['transactions'],
        'proof': block['proof'],
        'previous_hash': block['previous_hash'],
        }

    return jsonify(response), 200


@app.route('/transaction/new',methods=['POST'])
def new_transaction():
    values = request.get_json()
    
    #check the required fields
    required = ['sender','recipient','amount']
    if not all(k in values for k in required):
        return 'Missing values', 400

    #create a new transaction
    index = blockchain.new_transaction(values['sender'],values['recipient'],values['amount'])
    response = {'message': f'Transaction will be added to Block {index}'}

    return jsonify(response), 201


@app.route('/chain',methods=['GET'])
def full_chain():
    response = {
        'chain': blockchain.chain,
        'length': len(blockchain.chain),
        }

    return jsonify(response), 200


@app.route('/nodes/register',methods=['POST'])
def register_nodes():
    values = request.get_json()
    nodes = values.get('nodes')

    if nodes is None:
        return "Error: Please supply a valid list of nodes", 400

    for node in nodes:
        blockchain.register_node(node)

    response = {
            'message': 'New nodes have been added',
            'total_nodes': list(blockchain.nodes),
        }
    return jsonify(response), 201


@app.route('/nodes/response',methods=['GET'])
def consensus():
    replaced = blockchain.resolve_conflicts()
    if replaced:
        response = {
                'message': 'Our chain was replaced',
                'new_chain': blockchain.chain
            }
    else:
        response = {
                'message': 'Our chain is authoritative',
                'chain': blockchain.chain
            }
    return jsonify(response), 200

    
if __name__ == '__main__':
    app.run(host='0.0.0.0',port=5000)