TheAlgorithms · anurags10 · Oct 3, 2024 · Oct 3, 2024 · Oct 3, 2024 · Oct 3, 2024
diff --git a/DIRECTORY.md b/DIRECTORY.md
@@ -22,6 +22,7 @@
   * [Rat In Maze](backtracking/rat_in_maze.py)
   * [Sudoku](backtracking/sudoku.py)
   * [Sum Of Subsets](backtracking/sum_of_subsets.py)
+  * [Word Break](backtracking/word_break.py)
   * [Word Ladder](backtracking/word_ladder.py)
   * [Word Search](backtracking/word_search.py)
 
@@ -54,6 +55,7 @@
 
 ## Blockchain
   * [Diophantine Equation](blockchain/diophantine_equation.py)
+  * [Pow Algorithm](blockchain/pow_algorithm.py)
 
 ## Boolean Algebra
   * [And Gate](boolean_algebra/and_gate.py)

diff --git a/blockchain/pow_algorithm.py b/blockchain/pow_algorithm.py
@@ -0,0 +1,189 @@
+# Title: Proof of Work Algorithm for Blockchain
+
+## Algorithm Statement:
+# The algorithm implements the Proof of Work (PoW) consensus mechanism used in
+# blockchain to validate blocks. PoW ensures participants (miners) perform a
+# computational task to create a valid block and add it to the blockchain. The
+# difficulty is defined by the number of leading zeros required in the block hash.
+
+import hashlib
+import time
+
+
+class Block:
+    def __init__(
+        self,
+        index: int,
+        previous_hash: str,
+        transactions: str,
+        timestamp: float,
+        difficulty: int,
+    ) -> None:
+        """
+        Initializes a Block object with the specified parameters.
+
+        Parameters:
+        - index (int): The index of the block in the blockchain.
+        - previous_hash (str): The hash of the previous block.
+        - transactions (str): The list of transactions in the block.
+        - timestamp (float): The time when the block was created
+          (in Unix timestamp format).
+        - difficulty (int): The difficulty level for mining this block.
+        """
+        self.index = index
+        self.previous_hash = previous_hash
+        self.transactions = transactions
+        self.timestamp = timestamp
+        self.nonce = 0  # Start with nonce 0
+        self.difficulty = difficulty
+        self.hash = self.compute_hash()
+
+    def compute_hash(self) -> str:
+        """
+        Generates the hash of the block content.
+        Combines index, previous hash, transactions, timestamp, and nonce into
+        a string, which is then hashed using SHA-256.
+
+        Returns:
+        - str: The hash of the block.
+        """
+        block_string = (
+            f"{self.index}{self.previous_hash}{self.transactions}{self.timestamp}"
+            f"{self.nonce}"
+        )
+        return hashlib.sha256(block_string.encode()).hexdigest()
+
+    def mine_block(self) -> None:
+        """
+        Performs Proof of Work by adjusting the nonce until a valid hash is found.
+        A valid hash has the required number of leading zeros based on the
+        difficulty level.
+
+        Returns:
+        - None
+        """
+        target = (
+            "0" * self.difficulty
+        )  # Target hash should start with 'difficulty' zeros
+        while self.hash[: self.difficulty] != target:
+            self.nonce += 1
+            self.hash = self.compute_hash()
+
+        print(f"Block mined with nonce {self.nonce}, hash: {self.hash}")
+
+
+class Blockchain:
+    def __init__(self, difficulty: int) -> None:
+        """
+        Initializes the blockchain with a given difficulty level.
+
+        Parameters:
+        - difficulty (int): The difficulty level for mining blocks in this blockchain.
+
+        Returns:
+        - None
+        """
+        self.chain: list[Block] = []  # Adding type hint for the list of blocks
+        self.difficulty = difficulty
+        self.create_genesis_block()
+
+    def create_genesis_block(self) -> None:
+        """
+        Creates the first block in the blockchain (the Genesis block).
+
+        Returns:
+        - None
+        """
+        genesis_block = Block(0, "0", "Genesis Block", time.time(), self.difficulty)
+        genesis_block.mine_block()
+        self.chain.append(genesis_block)
+
+    def add_block(self, transactions: str) -> None:
+        """
+        Adds a new block to the blockchain after performing Proof of Work.
+
+        Parameters:
+        - transactions (str): The list of transactions to be added in the new block.
+
+        Returns:
+        - None
+        """
+        previous_block = self.chain[-1]
+        new_block = Block(
+            len(self.chain),
+            previous_block.hash,
+            transactions,
+            time.time(),
+            self.difficulty,
+        )
+        new_block.mine_block()
+        self.chain.append(new_block)
+
+    def is_chain_valid(self) -> bool:
+        """
+        Verifies the integrity of the blockchain by ensuring each block's previous
+        hash matches and that all blocks meet the Proof of Work requirement.
+
+        Returns:
+        - bool: True if the blockchain is valid, False otherwise.
+        """
+        for i in range(1, len(self.chain)):
+            current_block = self.chain[i]
+            previous_block = self.chain[i - 1]
+
+            if current_block.hash != current_block.compute_hash():
+                print(f"Invalid block at index {i}. Hash mismatch.")
+                return False
+
+            if current_block.previous_hash != previous_block.hash:
+                print(f"Invalid chain at index {i}. Previous hash mismatch.")
+                return False
+
+        return True
+
+
+# Test cases
+
+
+## Test Case 1: Blockchain Initialization and Genesis Block
+# This test verifies if the blockchain is correctly initialized with a Genesis block
+# and if the block is successfully mined.
+def test_blockchain() -> None:
+    """
+    Test cases for the Blockchain proof of work algorithm.
+
+    Returns:
+    - None
+    """
+    # Create blockchain with difficulty level of 4 (hash should start with 4 zeros)
+    blockchain = Blockchain(difficulty=4)
+
+    ## Test Case 2: Add a block and verify the block is mined
+    # This test adds a new block with transactions and ensures it's mined according
+    # to the proof of work mechanism.
+    blockchain.add_block("Transaction 1: Alice pays Bob 5 BTC")
+    blockchain.add_block("Transaction 2: Bob pays Charlie 3 BTC")
+
+    ## Test Case 3: Verify blockchain integrity
+    # This test checks that the blockchain remains valid after adding new blocks
+    assert blockchain.is_chain_valid(), "Blockchain should be valid"
+
+    ## Test Case 4: Tampering with the blockchain
+    # This test simulates tampering with the blockchain and checks that the validation
+    # correctly detects the tampering.
+    blockchain.chain[
+        1
+    ].transactions = "Transaction 1: Alice pays Bob 50 BTC"  # Tampering
+    assert (
+        not blockchain.is_chain_valid()
+    ), "Blockchain should be invalid due to tampering"
+
+    ## Test Case 5: Correct blockchain validation
+    # This test checks if the blockchain becomes invalid after tampering and verifies
+    # if the PoW still holds after tampering is done.
+
+    print("All test cases passed.")
+
+
+if __name__ == "__main__":
+    test_blockchain()