# === PDF GENERATION ===
# Converts a Django HTML template into a PDF file using WeasyPrint.
# Used for payslip, receipt, and payroll report PDFs (both email and
# browser download).
#
# Why WeasyPrint?
# ----------------
# Migrated from xhtml2pdf in Nov 2026. WeasyPrint is a browser-grade
# HTML-to-PDF renderer — it supports modern CSS features that xhtml2pdf
# could not: flexbox, grid, @font-face (custom web fonts), box-shadow,
# border-radius, transforms, and proper CSS cascade handling.
#
# WeasyPrint has system dependencies (Pango, Cairo, GDK-PixBuf) that
# xhtml2pdf did not need. On Flatlogic's Linux environment these are
# already installed system-wide — no extra setup. On Windows dev
# machines, we install the GTK3 runtime (via `winget install
# tschoonj.GTKForWindows`) and then the `_ensure_gtk_on_windows()`
# helper below tells Python where to find the DLLs.
#
# Why the Windows DLL dance?
# --------------------------
# Since Python 3.8, `PATH` alone is no longer sufficient to load native
# DLLs — Python requires explicit `os.add_dll_directory()` calls for
# security reasons. This helper walks common GTK3 install locations
# and registers the first one found. On Linux it's a no-op.
#
# IMPORTANT: WeasyPrint is imported LAZILY (inside the function, not
# at the top of the file). This is intentional — if WeasyPrint or its
# system libraries are missing, the rest of the app still works;
# only PDF generation fails gracefully and returns None.

import logging
import os
import sys
from django.conf import settings
from django.template.loader import get_template

logger = logging.getLogger(__name__)


# Common install locations for the GTK3 runtime on Windows.
# Checked in order; first hit wins.
_WINDOWS_GTK_PATHS = (
    r"C:\Program Files\GTK3-Runtime Win64\bin",
    r"C:\Program Files (x86)\GTK3-Runtime Win64\bin",
)

# Module-level flag so we only add the DLL directory once per process.
_gtk_registered = False


def _ensure_gtk_on_windows():
    """Register the GTK3 runtime DLL directory on Windows.

    WeasyPrint loads DLLs two ways:
      1. `ctypes.util.find_library('gobject-2.0-0')` — reads os.environ['PATH']
      2. `ctypes.CDLL(...)` — uses Windows DLL search, affected by
         `os.add_dll_directory()` since Python 3.8

    We hit both paths so the library is discoverable regardless of
    which route WeasyPrint chooses. This is a no-op on Linux/macOS
    and on Windows when GTK is already registered.
    """
    global _gtk_registered
    if _gtk_registered or sys.platform != "win32":
        return
    for path in _WINDOWS_GTK_PATHS:
        if os.path.isdir(path):
            # Prepend to PATH so find_library() locates the DLLs.
            current_path = os.environ.get("PATH", "")
            if path.lower() not in current_path.lower():
                os.environ["PATH"] = path + os.pathsep + current_path
            # Register via add_dll_directory for ctypes.CDLL() loads.
            if hasattr(os, "add_dll_directory"):
                os.add_dll_directory(path)
            _gtk_registered = True
            logger.debug("Registered GTK3 runtime directory: %s", path)
            return
    logger.debug(
        "No GTK3 runtime directory found on Windows in %s", _WINDOWS_GTK_PATHS,
    )


def render_to_pdf(template_src, context_dict=None):
    """
    Render a Django template to PDF bytes using WeasyPrint.

    Args:
        template_src:  Path to the template
                       (e.g. 'core/pdf/payslip_pdf.html')
        context_dict:  Template context variables

    Returns:
        PDF content as bytes, or None if there was an error.
    """
    if context_dict is None:
        context_dict = {}

    # On Windows we need to tell Python where the GTK3 DLLs live
    # BEFORE importing weasyprint. Harmless no-op on Linux/macOS.
    _ensure_gtk_on_windows()

    # --- Lazy import: only load WeasyPrint when actually generating a PDF ---
    # ImportError covers the Python package being missing.
    # OSError covers missing native libs (Pango, Cairo) at runtime.
    try:
        from weasyprint import HTML
    except (ImportError, OSError) as e:
        logger.error(
            "WeasyPrint is not available — cannot generate PDF. "
            "Install with: pip install weasyprint. "
            "On Windows, also install the GTK3 runtime via "
            "`winget install tschoonj.GTKForWindows`. "
            "Underlying error: %s", e,
        )
        return None

    # Render the Django template to HTML first
    template = get_template(template_src)
    html = template.render(context_dict)

    # Convert HTML to PDF bytes.
    # base_url lets WeasyPrint resolve relative paths to static files
    # (e.g. fonts in static/fonts/, images in static/img/). We fall back
    # to "." so the call still succeeds when STATIC_ROOT isn't set
    # (e.g. during local dev before collectstatic has run).
    base = getattr(settings, "STATIC_ROOT", None) or "."
    try:
        return HTML(string=html, base_url=base).write_pdf()
    except Exception as e:
        logger.exception("WeasyPrint render failed: %s", e)
        return None