"""OS-level scrolling of the focused window via pyautogui.

We drive real scroll input rather than the DOM, so it works over a plain
screenshot pipeline on any OS. Needs Accessibility permission on macOS and an
X11 session on Linux.
"""
from __future__ import annotations

import time

from config import SCROLL_AMOUNT, SCROLL_PAUSE


class ScrollError(RuntimeError):
    pass


def _pyautogui():
    try:
        import pyautogui

        pyautogui.FAILSAFE = False
        return pyautogui
    except Exception as e:  # pragma: no cover
        raise ScrollError(
            f"pyautogui not available ({e}). On macOS grant Accessibility; "
            f"on Linux use X11."
        )


def scroll_to_top() -> None:
    pg = _pyautogui()
    # A large upward scroll reliably returns most pages to the top.
    for _ in range(10):
        pg.scroll(SCROLL_AMOUNT)
    time.sleep(SCROLL_PAUSE)


def scroll_down(amount: int = SCROLL_AMOUNT) -> None:
    pg = _pyautogui()
    pg.scroll(-abs(amount))
    time.sleep(SCROLL_PAUSE)