encoding.hex

from typing import Sequence

_DIGITS = "0123456789abcdef"


def hex_encode(bytes: Sequence[int]) -> str:
    """Bytes as lowercase hexadecimal, two digits per byte.

    Bytes are a list of integers 0-255 across the registry (there is no bytes
    type); a Python ``bytes`` value is such a sequence and is accepted as is.
    A value outside the range 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")
    out = []
    for b in bytes:
        # bool is an int in Python; true is not a byte in the other two languages.
        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.append(_DIGITS[b >> 4] + _DIGITS[b & 15])
    return "".join(out)

from typing import List


def _digit_value(ch: str) -> int:
    # Compared by character range rather than int(ch, 16), which would accept
    # full-width and other scripts' digits that the other languages refuse.
    if "0" <= ch <= "9":
        return ord(ch) - 48
    if "a" <= ch <= "f":
        return ord(ch) - 87
    if "A" <= ch <= "F":
        return ord(ch) - 55
    return -1


def hex_decode(text: str) -> List[int]:
    """Hexadecimal text back to bytes, either case, strictly.

    A 0x prefix, whitespace or an odd digit count is an error, not skipped.
    """
    if not isinstance(text, str):
        raise TypeError("hex text must be a string")
    # Characters are checked before the length, so the answer for non-ASCII
    # input does not depend on how a language counts its length.
    digits = []
    for ch in text:
        d = _digit_value(ch)
        if d < 0:
            raise ValueError("hex text may only contain the digits 0-9, a-f and A-F")
        digits.append(d)
    if len(digits) % 2 != 0:
        raise ValueError("hex text must have an even number of digits, received %d" % len(digits))
    return [digits[i] * 16 + digits[i + 1] for i in range(0, len(digits), 2)]