vlq.py

"""
Source taken from: https://gist.github.com/mjpieters/86b0d152bb51d5f5979346d11005588b
"""

from typing import Tuple

_b64chars = b"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
_b64table = [None] * (max(_b64chars) + 1)
for i, b in enumerate(_b64chars):
    _b64table[b] = i

_encode = _b64chars.decode().__getitem__
_shiftsize, _flag, _mask = 5, 1 << 5, (1 << 5) - 1


def base64vlq_decode(vlqval: str) -> Tuple[int]:
    """Decode Base64 VLQ value"""
    results = []
    add = results.append
    shiftsize, flag, mask = _shiftsize, _flag, _mask
    shift = value = 0
    # use byte values and a table to go from base64 characters to integers
    for v in map(_b64table.__getitem__, vlqval.encode("ascii")):
        value += (v & mask) << shift
        if v & flag:
            shift += shiftsize
            continue
        # determine sign and add to results
        add((value >> 1) * (-1 if value & 1 else 1))
        shift = value = 0
    return results


def base64vlq_encode(*values: int) -> str:
    """Encode integers to a VLQ value"""
    results = []
    add = results.append
    shiftsize, flag, mask = _shiftsize, _flag, _mask
    for v in values:
        # add sign bit
        v = (abs(v) << 1) | int(v < 0)
        while True:
            toencode, v = v & mask, v >> shiftsize
            add(toencode | (v and flag))
            if not v:
                break
    return bytes(map(_b64chars.__getitem__, results)).decode()