blockchain.py

import hashlib
import json
from time import time
#from textwrap import dedent
import requests
from uuid import uuid4
from urllib.parse import urlparse

from flask import Flask, jsonify, request

class Blockchain(object):
    def __init__(self):
        self.chain = []
        self.current_transactions = []
        # set() is used tonhold the list of nodes
        self.nodes = set()

        # create the first block
        self.new_block(previous_hash=1, proof=100)

    def register_node(self, address):
        """
        Add a new node to the list of nodes
        :param address: Address of node. Eg. 'http://192.168.0.5:5000'
        """

        parsed_url = urlparse(address)
        if parsed_url.netloc:
            self.nodes.add(parsed_url.netloc)
        elif parsed_url.path:
            # Accepts an URL without scheme like '192.168.0.5:5000'.
            self.nodes.add(parsed_url.path)
        else:
            raise ValueError('Invalid URL')

    def valid_chain(self, chain):
        """
        Determine if a given blockchain is valid

        :param chain: <list> A blockchain
        :return: <bool> True if valid else False
        """

        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")
            # Chekc if the hash of the block is correct
            if block['previous_hash'] != self.hash(last_block):
                return False

            # Check of the Proof of Work is correct
            if not self.valid_proof(last_block['proof'], block['proof']):
                return False

            last_block = block
            current_index += 1

        return True

    def resolve_conflicts(self):
        """
        This is our Consensus Algorithm for resolving conflicts
        by replacing a chain with the longest one in the network.

        :return: <bool> True is our chain was replaced else False
        """

        neighbours = self.nodes
        new_chain = None

        # We're looking for chains longer than ours
        max_length = len(self.chain)

        # Grab and verify the chains from all the nodes in our network
        for node in neighbours:
            response = requests.get(f'http://{node}/chain')

            if response.status_code == 200:
                length = response.json()['length']
                chain = response.json()['chain']

                # Check if the length is longer and the chain is valid
                if length > max_length and self.valid_chain(chain):
                    max_length = length
                    new_chain = chain

        # Replace our chain if we discovered a new, valid chain longer than ours
        if new_chain:
            self.chain = new_chain
            return True
        return False

    def new_block(self, proof, previous_hash=None):
        """
        creates a new block and adds it to the chain

        :param proof: <int> the proof given by the Proof of Work Algorithm
        :param previous_hash: (Optional) <str> hash of previous block
        :return: <dict> new Block
        """
        block = {
            'index': len(self.chain) + 1,
            'timestamp': time(),
            'transactions': self.current_transactions,
            'proof': proof,
            'previous_hash': previous_hash or self.hash(self.chain[-1]),
        }

        # reset the current list of transactions
        self.current_transactions = []

        self.chain.append(block)
        return block

    def new_transaction(self, sender, recipient, amount):
        """
        Creates a new transaction to go into the next mined  Block
        :param sender: <str> Address of the sender
        :param recipient: <str> Address of the recipient
        :param amount: <int> Amount
        :return: <int> the index of the block that will hold this transaction
        """
        self.current_transactions.append({
            'sender': sender,
            'recipient': recipient,
            'amount': amount,
        })

        return self.last_block['index'] + 1

    @staticmethod
    def hash(block):
        """
        Creates a SHA-256 hash of a Block
        :param block: <dict> Block
        :return: <str>

        The dict must be ordered, or else we'll have inconsistent hashes
        """
        block_string = json.dumps(block, sort_keys=True).encode()
        return hashlib.sha256(block_string).hexdigest()

    @property
    def last_block(self):
        # returns the last block in the chain
        return self.chain[-1]

    def proof_of_work(self, last_proof):
        """
        Proof of Work Algorithm:
        - Find a number x such that hash(xx') contains 4 leading zeroes, where x is the previous x'
        - x is the previous proof and x' is the new proof

        :param last_proof: <int>
        :return: <int>
        """

        proof = 0
        while self.valid_proof(last_proof, proof) is False:
            proof += 1
        return proof

    @staticmethod
    def valid_proof(last_proof, proof):
        """
        Validates the Proof: Does hash(last_proof, proof) contain 4 leading zeroes?

        :param last_proof: <int> Previous Proof
        :param proof: <int> Current Proof
        :return: <bool> True if correct, else False
        """

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