"""Line-API scroll view for the continuous reader — renders only the rows
actually visible on screen (via Widget.render_line), reading from the flat
Strip list diora_tui.layout.build_layout() precomputed once. This is what
keeps very large books responsive: nothing here scales with book size at
paint time, only with viewport height.
"""

from __future__ import annotations

from contextlib import contextmanager
from typing import Callable

from textual.geometry import Size
from textual.scroll_view import ScrollView
from textual.strip import Strip

from .layout import BookLayout


class ContinuousBookView(ScrollView):
    # We always wrap text to fit exactly, so a horizontal scrollbar should
    # never be needed — and "scroll" (not "auto") for the vertical one keeps
    # its gutter reserved from the very first (still-empty) layout pass, so
    # the width we wrap text at later never has to guess whether a scrollbar
    # will appear and steal columns out from under already-wrapped lines.
    DEFAULT_CSS = """
    ContinuousBookView {
        overflow-x: hidden;
        overflow-y: scroll;
    }
    """

    def __init__(self, book_layout: BookLayout, on_read_scroll: Callable[[], None] | None = None) -> None:
        super().__init__()
        self.book_layout = book_layout
        self.virtual_size = Size(book_layout.width, book_layout.total_rows)
        # Called when the reader moves the view (keys, wheel, scrollbar), not when the program does
        # (restoring a position, re-wrapping after a resize, jumping on an offer): only the first
        # is "arriving at a place by reading" for the reading position (docs/progress-sync.md).
        self.on_read_scroll = on_read_scroll
        self._program_scrolls = 0

    @contextmanager
    def program_scroll(self):
        self._program_scrolls += 1
        try:
            yield
        finally:
            self._program_scrolls -= 1

    def watch_scroll_y(self, old_value: float, new_value: float) -> None:
        super().watch_scroll_y(old_value, new_value)
        if old_value != new_value and not self._program_scrolls and self.on_read_scroll is not None:
            self.on_read_scroll()

    def render_line(self, y: int) -> Strip:
        _scroll_x, scroll_y = self.scroll_offset
        row = scroll_y + y
        strips = self.book_layout.row_strips
        width = self.scrollable_content_region.width
        if row < 0 or row >= len(strips):
            return Strip.blank(width, self.rich_style)
        return strips[row].crop_extend(0, width, self.rich_style)