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#!/usr/bin/env python3
import sys
def compute_checksum(data: bytes) -> int:
# PSA checksum excludes first 4 bytes ("MEL\0")
relevant = data[4:]
total = sum(relevant)
folded = (total & 0xFF) + (total >> 8)
return (~folded) & 0xFF
def interactive_mode():
print("PSA Checksum Generator (Melody 0x3001 packets)")
print("Paste the hex payload WITHOUT the checksum byte. Type 'exit' to quit.")
while True:
user_input = input("Payload> ").strip()
if user_input.lower() in ("exit", "quit"):
break
try:
data = bytes.fromhex(user_input)
checksum = compute_checksum(data)
completed = data + bytes([checksum])
print("Checksum: {:02X}".format(checksum))
print("Full Packet: {}".format(completed.hex()))
except ValueError:
print("Invalid hex input. Try again.")
if __name__ == "__main__":
if len(sys.argv) == 2 and sys.argv[1] == "--batch":
print("Batch mode not implemented. Run without arguments for interactive mode.")
else:
interactive_mode()
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