import struct
from datetime import datetime

def decode_nlocktime(tx_hex):
    # nLockTime is ALWAYS the last 4 bytes of a transaction
    tx_bytes = bytes.fromhex(tx_hex)
    raw_locktime = tx_bytes[-4:]

    # Unpack as little-endian uint32
    nLockTime = struct.unpack('<I', raw_locktime)[0]

    print(f"--- Decoding nLockTime: {nLockTime} ---")

    if nLockTime == 0:
        print("[*] Status: Final (No Lock)")
        return

    # Apply the 500,000,000 threshold
    if nLockTime < 500000000:
        print(f"[*] Type: Block Height")
        print(f"[*] Condition: Valid at or after block {nLockTime:,}")
    else:
        print(f"[*] Type: Unix Timestamp")
        # Convert to human-readable date
        date_str = datetime.utcfromtimestamp(nLockTime).strftime('%Y-%m-%d %H:%M:%S UTC')
        print(f"[*] Condition: Valid after {date_str}")

# --- Simulation ---

# Scenario A: Block Height 700,000 (0x000aad0a -> 0aaad000)
# Last 4 bytes: 60ae0a00
print("Scenario A:")
decode_nlocktime("010000000000000000000000000060ae0a00")

# Scenario B: Unix Timestamp for Jan 1, 2024 (1704067200)
# Hex: 0x6591ea80 -> 80ea9165
print("\nScenario B:")
decode_nlocktime("010000000000000000000000000080ea9165")

# Scenario C: Default (No Lock)
print("\nScenario C:")
decode_nlocktime("010000000000000000000000000000000000")