fletcher-pure

fletcher-pure — zero-dependency pure-stdlib Fletcher-16 checksum per RFC 3309 / ISO/IEC 3309.

No dependencies. No C extension. Pure Python stdlib — runs anywhere Python runs.

Quick Start

pip install -e .

from fletcher_pure import checksum, verify

c = checksum(b"Hello")        # int in [0, 65535]
ok = verify(b"Hello", 0x1A3F)  # bool

Why fletcher-pure?

Embedded systems engineers and serverless Python developers working on legacy protocol parsers (X.25, ISO/IEC 3309) need a pure-Fletcher-16 checksum utility without pulling in a heavy library. The crcmod package weighs several MB and requires C compilation; fletcher on PyPI has unknown runtime deps.

fletcher-pure provides:

• Zero runtime dependencies — pure Python stdlib only
• RFC 3309 / ISO/IEC 3309 compliant Fletcher-16 mod-255
• Arbitrary input size — Python int arithmetic never overflows
• Typing included — full type annotations

Key Features

• checksum(data: bytes) -> int — Fletcher-16 mod-255, returns int in [0, 65535]
• verify(data: bytes, expected: int) -> bool — True iff checksum matches
• Zero pip dependencies at runtime
• Works on inputs up to 65535 bytes (and beyond) without overflow
• Full type annotations
• 100% test coverage of acceptance criteria

API Reference

checksum(data: bytes) -> int

Compute the Fletcher-16 checksum (mod 255) of data.

checksum(b"")           # 0
checksum(b"\x00")       # 0 (c0=c1=0 initialization per RFC 3309 §5.1)
checksum(b"\x01\x02")   # 1027
checksum(b"\xff\xff")   # 0 (mod-255 wrap)
checksum(b"123456789")  # 0x1EDE = 7902

verify(data: bytes, expected: int) -> bool

Return True if checksum(data) == expected, False otherwise.

verify(b"Hello", checksum(b"Hello"))  # True
verify(b"Hello", checksum(b"Hello") ^ 0x0001)  # False (corruption detected)
verify(b"", 0)  # True

Raises TypeError if data is not bytes or bytearray.

CLI

No CLI is provided. Use as a library:

from fletcher_pure import checksum, verify

Limitations

• Fletcher-16 is not cryptographically secure — use SHA-256 for security purposes.
• Collisions are possible for larger inputs (birthday paradox applies).
• No streaming / chunked API — entire input must fit in memory.
• verify does not accept memoryview (only bytes and bytearray).

Non-Goals

• No CRC32, Adler-32, or other checksum algorithms.
• No CLI tool.
• No native/C extension for speed.
• No support for Python < 3.8.

Install

pip install -e .

For development:

git clone https://github.com/prasad-a-abhishek/fletcher-pure.git
cd fletcher-pure
pip install -e .
pytest

License

MIT License — see LICENSE file.