Add simple blockchain mining algorithm with PoW (#13114)

* Add simple blockchain mining algorithm with PoW

* Add type hints for __init__ methods

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Added compute hash doctest

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Added init return

* Fix type hints and add doctests for simple blockchain

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: LetitiaGilbert

blockchain/simple_blockchain.py

@@ -0,0 +1,168 @@
+"""
+A simple blockchain implementation with Proof-of-Work (PoW).
+
+This educational example demonstrates:
+- Block structure with index, timestamp, data, previous hash, nonce, and hash
+- Mining via Proof-of-Work
+- Chain integrity verification
+
+Author: Letitia Gilbert
+"""
+
+import hashlib
+from time import time
+
+
+class Block:
+    """
+    Represents a single block in a blockchain.
+
+    Attributes:
+        index (int): Position of the block in the chain.
+        timestamp (float): Creation time of the block.
+        data (str): Data stored in the block.
+        previous_hash (str): Hash of the previous block.
+        nonce (int): Number used for mining.
+        hash (str): SHA256 hash of the block's content.
+    """
+
+    def __init__(
+        self, index: int, data: str, previous_hash: str, difficulty: int = 2
+    ) -> None:
+        self.index = index
+        self.timestamp = time()
+        self.data = data
+        self.previous_hash = previous_hash
+        self.nonce, self.hash = self.mine_block(difficulty)
+
+    def compute_hash(self, nonce: int) -> str:
+        """
+        Compute SHA256 hash of the block with given nonce.
+
+        Args:
+            nonce (int): Nonce to include in the hash.
+
+        Returns:
+            str: Hexadecimal hash string.
+
+        >>> block = Block(0, "Genesis", "0", difficulty=2)
+        >>> len(block.compute_hash(0)) == 64
+        True
+        >>> isinstance(block.compute_hash(0), str)
+        True
+        """
+        block_string = (
+            f"{self.index}{self.timestamp}{self.data}{self.previous_hash}{nonce}"
+        )
+        return hashlib.sha256(block_string.encode()).hexdigest()
+
+    def mine_block(self, difficulty: int) -> tuple[int, str]:
+        """
+        Simple Proof-of-Work mining algorithm.
+
+        Args:
+            difficulty (int): Number of leading zeros required in the hash.
+
+        Returns:
+            Tuple[int, str]: Valid nonce and resulting hash that satisfies difficulty.
+
+        >>> block = Block(0, "Genesis", "0", difficulty=2)
+        >>> block.hash.startswith('00')
+        True
+        """
+        if difficulty < 1:
+            raise ValueError("Difficulty must be at least 1")
+        nonce = 0
+        target = "0" * difficulty
+        while True:
+            hash_result = self.compute_hash(nonce)
+            if hash_result.startswith(target):
+                return nonce, hash_result
+            nonce += 1
+
+
+class Blockchain:
+    """
+    Simple blockchain class maintaining a list of blocks.
+
+    Attributes:
+        chain (List[Block]): List of blocks forming the chain.
+    """
+
+    def __init__(self, difficulty: int = 2) -> None:
+        self.difficulty = difficulty
+        self.chain: list[Block] = [self.create_genesis_block()]
+
+    def create_genesis_block(self) -> Block:
+        """
+        Create the first block in the blockchain.
+
+        Returns:
+            Block: Genesis block.
+
+        >>> bc = Blockchain()
+        >>> bc.chain[0].index
+        0
+        >>> bc.chain[0].hash.startswith('00')
+        True
+        """
+        return Block(0, "Genesis Block", "0", self.difficulty)
+
+    def add_block(self, data: str) -> Block:
+        """
+        Add a new block to the blockchain with given data.
+
+        Args:
+            data (str): Data to store in the block.
+
+        Returns:
+            Block: Newly added block.
+
+        >>> bc = Blockchain()
+        >>> new_block = bc.add_block("Test Data")
+        >>> new_block.index
+        1
+        >>> new_block.previous_hash == bc.chain[0].hash
+        True
+        >>> new_block.hash.startswith('00')
+        True
+        >>> bc.is_valid()
+        True
+        """
+        prev_hash = self.chain[-1].hash
+        new_block = Block(len(self.chain), data, prev_hash, self.difficulty)
+        self.chain.append(new_block)
+        return new_block
+
+    def is_valid(self) -> bool:
+        """
+        Verify the integrity of the blockchain.
+
+        Returns:
+            bool: True if chain is valid, False otherwise.
+
+        >>> bc = Blockchain()
+        >>> new_block = bc.add_block("Test")
+        >>> new_block.index
+        1
+        >>> new_block.previous_hash == bc.chain[0].hash
+        True
+        >>> new_block.hash.startswith('00')
+        True
+        >>> bc.is_valid()
+        True
+        >>> bc.chain[1].previous_hash = "tampered"
+        >>> bc.is_valid()
+        False
+
+        """
+        for i in range(1, len(self.chain)):
+            current = self.chain[i]
+            prev = self.chain[i - 1]
+            if current.previous_hash != prev.hash:
+                return False
+            if not current.hash.startswith("0" * self.difficulty):
+                return False
+            if current.hash != current.compute_hash(current.nonce):
+                return False
+        return True