chain.py

from hashlib import sha256
from block import Block
import time


class Blockchain:
    # difficulty of our PoW algorithm
    difficulty = 1

    def __init__(self):
        self.unconfirmed_transactions = []
        self.chain = []

    def create_genesis_block(self):
        """
        A function to generate genesis block and appends it to
        the chain. The block has index 0, previous_hash as 0, and
        a valid hash.
        """
        genesis_block = Block(0, [], 0, "0")
        genesis_block.hash = genesis_block.compute_hash()
        self.chain.append(genesis_block)

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

    def add_block(self, block, proof):
        """
        A function that adds the block to the chain after verification.
        
        Verification includes:
        - Checking if the proof is valid.
        - The previous_hash referred in the block and the hash of latest block
          in the chain match.
        """
        previous_hash = self.last_block.hash

        if previous_hash != block.previous_hash:
            return False

        if not Blockchain.is_valid_proof(block, proof):
            return False

        block.hash = proof
        self.chain.append(block)
        return True

    @staticmethod
    def proof_of_work(block):
        """
        Function that tries different values of nonce to get a hash
        that satisfies our difficulty criteria.
        """
        block.nonce = 0

        computed_hash = block.compute_hash()
        while not computed_hash.startswith("0" * Blockchain.difficulty):
            block.nonce += 1
            computed_hash = block.compute_hash()

        return computed_hash

    def add_new_transaction(self, transaction):
        self.unconfirmed_transactions.append(transaction)

    @classmethod
    def is_valid_proof(cls, block, block_hash):
        """
        Check if block_hash is valid hash of block and satisfies
        the difficulty criteria.
        """
        return (
            block_hash.startswith("0" * Blockchain.difficulty)
            and block_hash == block.compute_hash()
        )

    @classmethod
    def check_chain_validity(cls, chain):
        result = True
        previous_hash = "0"

        for block in chain:
            block_hash = block.hash
            # remove the hash field to recompute the hash again
            # using `compute_hash` method.
            delattr(block, "hash")

            if (
                not cls.is_valid_proof(block, block.hash)
                or previous_hash != block.previous_hash
            ):
                result = False
                break

            block.hash, previous_hash = block_hash, block_hash

        return result

    def mine(self):
        """
        This function serves as an interface to add the pending
        transactions to the blockchain by adding them to the block
        and figuring out Proof Of Work.
        """
        if not self.unconfirmed_transactions:
            return False

        last_block = self.last_block

        new_block = Block(
            index=last_block.index + 1,
            transactions=self.unconfirmed_transactions,
            timestamp=time.time(),
            previous_hash=last_block.hash,
        )

        proof = self.proof_of_work(new_block)
        self.add_block(new_block, proof)

        self.unconfirmed_transactions = []

        return True