Variable width bytes representation of uint64_t

gistfile1.txt

'''
Variable width bytes representation of uint64_t.

Inspired by Ruby interpreter。
https://techlife.cookpad.com/entry/2019/09/26/143000

> 0x0000000000000000 - 0x000000000000007f: 1byte  | 1XXXXXXX |
> 0x0000000000000080 - 0x0000000000003fff: 2bytes | 01XXXXXX | XXXXXXXX |
> 0x0000000000004000 - 0x00000000001fffff: 3bytes | 001XXXXX | XXXXXXXX | XXXXXXXX |
> 0x0000000000020000 - 0x000000000fffffff: 4bytes | 0001XXXX | XXXXXXXX | XXXXXXXX | XXXXXXXX |
> ...
> 0x0001000000000000 - 0x00ffffffffffffff: 8bytes | 00000001 | XXXXXXXX | XXXXXXXX | XXXXXXXX | XXXXXXXX | XXXXXXXX | XXXXXXXX | XXXXXXXX |
> 0x0100000000000000 - 0xffffffffffffffff: 9bytes | 00000000 | XXXXXXXX | XXXXXXXX | XXXXXXXX | XXXXXXXX | XXXXXXXX | XXXXXXXX | XXXXXXXX | XXXXXXXX |
'''

from typing import List, Tuple


def pack(n: int) -> bytes:
    if n < 0:
        raise ValueError("negative number is not supported")
    l = n.bit_length()
    if l > 64:
        raise ValueError("too large")
    s = (l - 1) // 7 + 1
    if s > 8:
        return b"\0" + n.to_bytes(8, "big")
    else:
        packed = bytearray(n.to_bytes(s, "big"))
        packed[0] = packed[0] | (0x100 >> s)
        return bytes(packed)


def unpack(packed: bytes) -> Tuple[int, int]:
    size = 8 - packed[0].bit_length()
    b = bytearray(packed[:size])
    b[0] = packed[0] & (0xFF >> size)
    return int.from_bytes(b, "big"), size


def test():
    cases = [
        (1, 0b10000001 .to_bytes(1, "big")),
        (0b01111111, 0b11111111 .to_bytes(1, "big")),
        (0b00111111_11111111, 0b01111111_11111111 .to_bytes(2, "big")),
        (
            0b00000000_11111111_11111111_11111111_11111111_11111111_11111111_11111111,
            0b00000001_11111111_11111111_11111111_11111111_11111111_11111111_11111111 .to_bytes(
                8, "big"
            ),
        ),
        (
            0b00000000_11111111_11111111_11111111_11111111_11111111_11111111_11111111_11111111,
            0b00000000_11111111_11111111_11111111_11111111_11111111_11111111_11111111_11111111 .to_bytes(
                9, "big"
            ),
        ),
    ]
    for n, packed in cases:
        assert pack(n) == packed
        assert unpack(packed) == (n, len(packed))