"""generator layout + pagination.

Per BR2.1/BR2.3, US1/NFR2: flows articles into a classic column-based
multi-page A4 layout with page breaks at boundaries, page numbers and running
headers. Deterministic, stdlib-only.
"""

from __future__ import annotations

from .model import Article, Column, ColumnItem, Page


class ColumnFlow:
    """Flows placed items into columns/pages with running headers."""

    def __init__(self, columns_per_page: int = 3, items_per_column: int = 4) -> None:
        self.columns_per_page = columns_per_page
        self.items_per_column = items_per_column

    def flow(self, article_items: list[ColumnItem]) -> list[Page]:
        pages: list[Page] = []
        current: Column | None = None
        page: Page | None = None
        idx = 0
        for item in article_items:
            if page is None or current is None or len(current.items) >= self.items_per_column:
                current = Column(column_index=0 if current is None else ((current.column_index + 1) % self.columns_per_page))
                if page is None or (current.column_index == 0 and len(page.columns) >= self.columns_per_page):
                    page_number = len(pages) + 1
                    page = Page(page_number=page_number, columns=[], running_header=f"Page {page_number}")
                    pages.append(page)
                page.columns.append(current)
                idx += 1
            current.items.append(item)
        return pages or [Page(page_number=1, columns=[Column(column_index=0)], running_header="Page 1")]


def build_layout(articles: list[Article]) -> list[Page]:
    """Map articles into column items and flow them (BR2.1)."""
    items: list[ColumnItem] = []
    for a in articles:
        items.append(ColumnItem(item_id=a.article_id, kind="article", source_ref=a.article_id))
    flow = ColumnFlow()
    return flow.flow(items)