"""Moteur Delivretiser — convertir un livret en pages sequentielles."""

from pypdf import PdfReader, PdfWriter, PageObject, Transformation
from .pdf_utils import get_page_size
import math


def unbooklet(input_path, output_path):
    """Delivretiser un PDF en format livret.

    Chaque page du livret contient 2 pages cote a cote.
    On les coupe en 2 et on les remet dans l'ordre sequentiel.
    """
    reader = PdfReader(input_path)
    num_sheets = len(reader.pages)
    total_pages = num_sheets * 2

    # Extraire toutes les demi-pages
    half_pages = []  # (sheet_index, "left" ou "right")
    for i in range(num_sheets):
        half_pages.append((i, "left"))
        half_pages.append((i, "right"))

    # Ordre du livret (piqure a cheval) :
    # Feuille 0 recto : page N, page 1
    # Feuille 0 verso : page 2, page N-1
    # Feuille 1 recto : page N-2, page 3
    # etc.
    # On reconstruit l'ordre
    page_map = {}  # page_number (1-based) -> (sheet_index, side)
    n = total_pages
    for i in range(num_sheets):
        if i % 2 == 0:
            # Recto : gauche = N - i, droite = i + 1
            page_map[n - i] = (i, "left")
            page_map[i + 1] = (i, "right")
        else:
            # Verso : gauche = i + 1, droite = N - i
            page_map[i + 1] = (i, "left")
            page_map[n - i] = (i, "right")

    writer = PdfWriter()

    for page_num in range(1, total_pages + 1):
        if page_num not in page_map:
            continue
        sheet_idx, side = page_map[page_num]
        sheet = reader.pages[sheet_idx]
        sheet_w, sheet_h = get_page_size(sheet)
        half_w = sheet_w / 2

        # Creer une nouvelle page avec la moitie
        new_page = PageObject.create_blank_page(width=half_w, height=sheet_h)

        if side == "left":
            # Copier la moitie gauche
            overlay = PageObject.create_blank_page(width=half_w, height=sheet_h)
            overlay.merge_page(sheet)
            overlay.mediabox.upper_right = (half_w, sheet_h)
            new_page.merge_page(overlay)
        else:
            # Copier la moitie droite
            overlay = PageObject.create_blank_page(width=half_w, height=sheet_h)
            overlay.merge_page(sheet)
            overlay.add_transformation(Transformation().translate(-half_w, 0))
            overlay.mediabox.upper_right = (half_w, sheet_h)
            new_page.merge_page(overlay)

        writer.add_page(new_page)

    with open(output_path, "wb") as f:
        writer.write(f)
    return output_path