impl/python/base64_encode.py

from typing import Sequence

_ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"


def base64_encode(bytes: Sequence[int]) -> str:
    """Bytes as standard base64 (RFC 4648 section 4), padded with "=".

    A Python ``bytes`` value is accepted as is; a list must hold integers
    0-255, and anything else is refused rather than masked to a byte.
    """
    if isinstance(bytes, (str, dict)) or not hasattr(bytes, "__len__"):
        raise TypeError("bytes must be a list of integers from 0 to 255")
    for b in bytes:
        if type(b) is not int or b < 0 or b > 255:
            raise ValueError("bytes must be a list of integers from 0 to 255")
    out = []
    n_full = len(bytes) - len(bytes) % 3
    for i in range(0, n_full, 3):
        n = (bytes[i] << 16) | (bytes[i + 1] << 8) | bytes[i + 2]
        out.append(_ALPHABET[(n >> 18) & 63] + _ALPHABET[(n >> 12) & 63] + _ALPHABET[(n >> 6) & 63] + _ALPHABET[n & 63])
    rest = len(bytes) - n_full
    if rest == 1:
        n = bytes[n_full] << 16
        out.append(_ALPHABET[(n >> 18) & 63] + _ALPHABET[(n >> 12) & 63] + "==")
    elif rest == 2:
        n = (bytes[n_full] << 16) | (bytes[n_full + 1] << 8)
        out.append(_ALPHABET[(n >> 18) & 63] + _ALPHABET[(n >> 12) & 63] + _ALPHABET[(n >> 6) & 63] + "=")
    return "".join(out)