"""Password hashing (ADR-0001): PBKDF2-SHA256 over an HMAC pepper, plus loading that pepper."""

import base64
import functools
import hashlib
import hmac
import logging
import os
import secrets
from pathlib import Path

log = logging.getLogger(__name__)

ITERATIONS = 600_000
SALT_BYTES = 16
MIN_PEPPER_CHARS = 32


def hash_password(
    password: str,
    pepper: bytes,
    *,
    salt: bytes | None = None,
    iterations: int = ITERATIONS,
) -> str:
    salt = secrets.token_bytes(SALT_BYTES) if salt is None else salt
    derived = _derive(password, pepper, salt, iterations)
    return f"pbkdf2_sha256${iterations}${_b64(salt)}${_b64(derived)}"


def verify_password(password: str, stored: str, pepper: bytes) -> bool:
    _, iterations, salt, expected = stored.split("$")
    derived = _derive(password, pepper, base64.b64decode(salt), int(iterations))
    return hmac.compare_digest(derived, base64.b64decode(expected))


def needs_rehash(stored: str) -> bool:
    return int(stored.split("$")[1]) < ITERATIONS


def _derive(password: str, pepper: bytes, salt: bytes, iterations: int) -> bytes:
    peppered = hmac.new(pepper, password.encode("utf-8"), hashlib.sha256).digest()
    return hashlib.pbkdf2_hmac("sha256", peppered, salt, iterations)


def _b64(raw: bytes) -> str:
    return base64.b64encode(raw).decode()


@functools.cache
def dummy_hash() -> str:
    # Verified against for unknown usernames so their response time matches a real account.
    return hash_password(secrets.token_urlsafe(16), b"\0" * MIN_PEPPER_CHARS)


def load_pepper(pepper_file: Path) -> bytes:
    from_env = os.environ.get("PASSWORD_PEPPER")
    if from_env is not None:
        if len(from_env) < MIN_PEPPER_CHARS:
            raise RuntimeError(
                f"PASSWORD_PEPPER must be at least {MIN_PEPPER_CHARS} characters. "
                'Generate one with: python -c "import secrets; print(secrets.token_hex(32))"'
            )
        return from_env.encode()
    try:
        fd = os.open(pepper_file, os.O_WRONLY | os.O_CREAT | os.O_EXCL, 0o600)
    except FileExistsError:
        pass
    else:
        with os.fdopen(fd, "w") as f:
            f.write(secrets.token_hex(32))
    log.warning(
        "PASSWORD_PEPPER is not set; using %s. Development only: production must supply the pepper "
        "from a secrets manager.",
        pepper_file,
    )
    pepper = pepper_file.read_text().strip()
    if len(pepper) < MIN_PEPPER_CHARS:
        raise RuntimeError(
            f"{pepper_file} holds fewer than {MIN_PEPPER_CHARS} characters. Set PASSWORD_PEPPER, or delete "
            "the file to regenerate it (existing passwords will stop verifying)."
        )
    return pepper.encode()