text: # Module 2 - Create a Cryptocurrency
  
  # To be installed:
  # Flask==0.12.2: pip install Flask==0.12.2
  # Postman HTTP Client: https://www.getpostman.com/
  # requests==2.18.4: pip install requests==2.18.4
  
  # Importing the libraries
  import datetime
  import hashlib
  import json
  from flask import Flask, jsonify, request
  import requests
  from uuid import uuid4
  from urllib.parse import urlparse
  
  # Part 1 - Building a Blockchain
  
  
  class Blockchain:
  
      def __init__(self):
          self.chain = []
          self.transactions = []
          self.create_block(proof=1, previous_hash='0')
          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]
  
      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()
              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_transaction(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
  
  # Part 2 - Mining our Blockchain
  
  
  # Creating a Web App
  app = Flask(__name__)
  
  # Creating an address for the node on Port 5001
  node_address = str(uuid4()).replace('-', '')
  
  # Creating a Blockchain
  blockchain = Blockchain()
  
  # Mining a new block
  
  
  @app.route('/mine_block', methods=['GET'])
  def mine_block():
      previous_block = blockchain.get_previous_block()
      previous_proof = previous_block['proof']
      proof = blockchain.proof_of_work(previous_proof)
      previous_hash = blockchain.hash(previous_block)
      blockchain.add_transaction(
          sender=node_address, receiver='Hadelin', amount=1)
      block = blockchain.create_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
  
  # Getting the full Blockchain
  
  
  @app.route('/get_chain', methods=['GET'])
  def get_chain():
      response = {'chain': blockchain.chain,
                  'length': len(blockchain.chain)}
      return jsonify(response), 200
  
  # Checking if the Blockchain is valid
  
  
  @app.route('/is_valid', methods=['GET'])
  def is_valid():
      is_valid = blockchain.is_chain_valid(blockchain.chain)
      if is_valid:
          response = {'message': 'All good. The Blockchain is valid.'}
      else:
          response = {
              'message': 'Houston, we have a problem. The Blockchain is not valid.'}
      return jsonify(response), 200
  
  # Adding a 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_transaction(
          json['sender'], json['receiver'], json['amount'])
      response = {'message': f'This transaction will be added to Block {index}'}
      return jsonify(response), 201
  
  # Part 3 - Decentralizing our Blockchain
  
  # Connecting new nodes
  
  
  @app.route('/connect_node', methods=['POST'])
  def connect_node():
      json = request.get_json()
      nodes = json.get('nodes')
      if nodes is None:
          return "No node", 400
      for node in nodes:
          blockchain.add_node(node)
      response = {'message': 'All the nodes are now connected. The Hadcoin Blockchain now contains the following nodes:',
                  'total_nodes': list(blockchain.nodes)}
      return jsonify(response), 201
  
  # Replacing the chain by the longest chain 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 so the chain was replaced by the longest one.',
                      'new_chain': blockchain.chain}
      else:
          response = {'message': 'All good. The chain is the largest one.',
                      'actual_chain': blockchain.chain}
      return jsonify(response), 200
  
  
  # Running the app
  app.run(host='0.0.0.0', port=5002)
  