EMV TLV Builder

# BER-TLV Builder — encode EMV tag-length-value structures
# No external dependencies required

def encode_length(n: int) -> bytes:
    if n <= 127:
        return bytes([n])
    length_bytes = n.to_bytes((n.bit_length() + 7) // 8, "big")
    return bytes([0x80 | len(length_bytes)]) + length_bytes

def build_tlv(entries: list) -> str:
    result = b""
    for tag_hex, value_hex in entries:
        tag   = bytes.fromhex(tag_hex.replace(" ", ""))
        value = bytes.fromhex(value_hex.replace(" ", ""))
        result += tag + encode_length(len(value)) + value
    return result.hex().upper()

# Example
entries = [
    ("9F02", "000000002500"),   # Amount Authorised
    ("9A",   "260101"),         # Transaction Date
    ("9C",   "00"),             # Transaction Type
]
tlv = build_tlv(entries)
print(f"TLV: {tlv}")
# TLV: 9F0206000000002500 9A03260101 9C0100  (spaces added for clarity)