import struct
import hashlib

def reverse_hex(hex_str):
    return bytes.fromhex(hex_str)[::-1].hex()

def serialize_header(version, prev_hash, merkle_root, timestamp, bits, nonce):
    # Pack the fields into a single byte stream
    # 'I' = 4-byte unsigned int (Little-Endian '<')
    # '32s' = 32-byte string

    # Note: Hash fields are already little-endian in raw data,
    # but often shown as big-endian in explorers.

    header = struct.pack('<I', version)
    header += bytes.fromhex(reverse_hex(prev_hash))
    header += bytes.fromhex(reverse_hex(merkle_root))
    header += struct.pack('<I', timestamp)
    header += bytes.fromhex(reverse_hex(bits)) # Bits is already compact hex
    header += struct.pack('<I', nonce)

    return header

# --- Simulation Data (Block 800,000) ---
version = 0x20000000        # BIP 9 Base
prev_hash = "000000000000000000052d314a292751d9044a835a6662de370f7221d604646a"
merkle_root = "5231c6298642289c02d645938986a7d5e69e46a9a08e7514a34f4929a0e698a9"
timestamp = 1689254345      # 2023-07-13
bits = "17066f2a"           # Compact Target
nonce = 2894348574

# 1. Serialize
raw_header = serialize_header(version, prev_hash, merkle_root, timestamp, bits, nonce)

# 2. Output
print(f"[*] Raw 80-byte Header (Hex): {raw_header.hex()}")
print(f"[*] Header Length: {len(raw_header)} bytes")

# 3. Calculate Block Hash
# Block Hash = SHA256(SHA256(header))
first_pass = hashlib.sha256(raw_header).digest()
second_pass = hashlib.sha256(first_pass).digest()
block_hash = second_pass[::-1].hex() # Convert to Big-Endian for display

print(f"\n[*] Calculated Block Hash: {block_hash}")