"""Password hashing with scrypt from the standard library (no extra dependencies).

Stored format: scrypt$<n>$<r>$<p>$<salt, base64>$<hash, base64>. The parameters are stored with
the hash, so they can be raised later without invalidating existing passwords.
"""

import base64
import hashlib
import hmac
import secrets

_N, _R, _P = 2**14, 8, 1
_KEY_LENGTH = 32


def _derive(password: str, salt: bytes, n: int, r: int, p: int) -> bytes:
    return hashlib.scrypt(
        password.encode(), salt=salt, n=n, r=r, p=p, dklen=_KEY_LENGTH,
        maxmem=128 * n * r * 2,
    )


def hash_password(password: str) -> str:
    salt = secrets.token_bytes(16)
    key = _derive(password, salt, _N, _R, _P)
    encode = lambda value: base64.b64encode(value).decode()  # noqa: E731
    return f"scrypt${_N}${_R}${_P}${encode(salt)}${encode(key)}"


def verify_password(password: str, stored: str) -> bool:
    try:
        scheme, n, r, p, salt, key = stored.split("$")
        if scheme != "scrypt":
            return False
        expected = base64.b64decode(key)
        actual = _derive(password, base64.b64decode(salt), int(n), int(r), int(p))
    except ValueError:
        return False
    return hmac.compare_digest(actual, expected)