"""Page numbering of the merged document.

Two ways to number pages:

css
    CSS counters do the work during the render. Correct and cheap, but it only
    works when the whole document is rendered in one pass, because each chunk
    restarts the page counter.

overlay
    A transparent numbering layer with the same page size is rendered once and
    merged onto the finished PDF. This is the only option for a chunked document
    and for Chromium, which has no usable page counters.
"""

from __future__ import annotations

import logging
from pathlib import Path

from ..errors import EngineUnavailableError
from ..models import PageNumbers, PageSettings
from .styles import build_overlay_css

logger = logging.getLogger(__name__)


def build_overlay(
    total_pages: int,
    width_pt: float,
    height_pt: float,
    page: PageSettings,
    page_numbers: PageNumbers,
    output_path: Path,
) -> Path:
    """Render the numbering layer, one empty page per page of the document."""
    try:
        from weasyprint import HTML
    except ImportError as exc:
        raise EngineUnavailableError(
            "Cislovani stranek vyzaduje nainstalovany WeasyPrint, ktery kresli cislovaci vrstvu.",
        ) from exc

    css = build_overlay_css(width_pt, height_pt, page, page_numbers, total_pages)
    slots = '<div class="pdf-page-slot"></div>' * total_pages
    html = (
        "<!DOCTYPE html><html><head><meta charset=\"utf-8\">"
        f"<style>{css}</style></head><body>{slots}</body></html>"
    )

    HTML(string=html).write_pdf(target=str(output_path))
    logger.info("Numbering overlay rendered", extra={"pages": total_pages})
    return output_path


def apply_overlay(document_path: Path, overlay_path: Path, output_path: Path) -> None:
    """Stamp the numbering layer onto every page of the document."""
    from pypdf import PdfReader, PdfWriter

    writer = PdfWriter(clone_from=str(document_path))
    try:
        with overlay_path.open("rb") as handle:
            overlay = PdfReader(handle)
            available = len(overlay.pages)

            if available < len(writer.pages):
                logger.warning(
                    "Numbering overlay has fewer pages than the document, tail will stay unnumbered",
                    extra={"overlay_pages": available, "document_pages": len(writer.pages)},
                )

            for index, page in enumerate(writer.pages):
                if index >= available:
                    break
                page.merge_page(overlay.pages[index])

            with output_path.open("wb") as target:
                writer.write(target)
    finally:
        writer.close()