"""Time-based one-time passwords (RFC 6238) — the 6-digit codes an
authenticator app (Google Authenticator, Microsoft Authenticator, 1Password,
…) shows every 30 seconds. SHA-1, 6 digits, 30-second steps: the defaults
every app supports. A code is accepted for the step before and after the
current one too, for clock drift; the caller keeps the last step it
accepted so the same code can't be used twice."""

import base64
import hashlib
import hmac
import secrets
import struct
import time
from urllib.parse import quote, urlencode

STEP_SECONDS = 30
DIGITS = 6
DRIFT_STEPS = 1


def new_secret() -> str:
    """160 random bits, base32 — what an authenticator app is given."""
    return base64.b32encode(secrets.token_bytes(20)).decode().rstrip("=")


def _key(secret: str) -> bytes:
    return base64.b32decode(secret + "=" * (-len(secret) % 8), casefold=True)


def code_at(secret: str, step: int) -> str:
    digest = hmac.new(_key(secret), struct.pack(">Q", step), hashlib.sha1).digest()
    offset = digest[-1] & 0x0F
    value = struct.unpack(">I", digest[offset : offset + 4])[0] & 0x7FFFFFFF
    return str(value % 10**DIGITS).zfill(DIGITS)


def current_step(now: float | None = None) -> int:
    return int((time.time() if now is None else now) // STEP_SECONDS)


def verify(secret: str, code: str, *, last_step: int | None = None, now: float | None = None) -> int | None:
    """The step `code` belongs to, if it's valid now and newer than
    last_step — else None."""
    code = "".join(ch for ch in code if ch.isdigit())
    if len(code) != DIGITS:
        return None
    step = current_step(now)
    for candidate in range(step - DRIFT_STEPS, step + DRIFT_STEPS + 1):
        if last_step is not None and candidate <= last_step:
            continue
        if hmac.compare_digest(code_at(secret, candidate), code):
            return candidate
    return None


def provisioning_uri(secret: str, account: str, issuer: str) -> str:
    """The otpauth:// link an authenticator app reads from the QR code."""
    label = quote(f"{issuer}:{account}")
    return f"otpauth://totp/{label}?" + urlencode({"secret": secret, "issuer": issuer, "algorithm": "SHA1", "digits": DIGITS, "period": STEP_SECONDS})


def qr_svg(uri: str) -> str:
    """The link as an SVG QR code, ready to show."""
    import io

    import segno

    out = io.BytesIO()
    segno.make(uri, error="m").save(out, kind="svg", scale=5, border=2, xmldecl=False, svgns=True, dark="#0f172a", light="#ffffff")
    return out.getvalue().decode()