use std::cell::RefCell;
use std::collections::{HashMap, HashSet, VecDeque};
use std::rc::Rc;
use std::sync::mpsc::{self, TryRecvError};
use std::thread;
use std::time::Duration;

use gtk::gdk;
use gtk::glib;
use gtk::prelude::*;

use crate::archive::{ArchivePage, DecodedPage, decode_page, list_pages};
use crate::database::Database;
use crate::zip_cache::ArchiveSource;

const CACHE_LIMIT_BYTES: usize = 256 * 1024 * 1024;
const CACHE_LIMIT_PAGES: usize = 16;
const FAST_CACHE_LIMIT_BYTES: usize = 1024 * 1024 * 1024;
const FAST_CACHE_LIMIT_PAGES: usize = 64;

pub struct ViewerWindow;

impl ViewerWindow {
    pub fn open(
        application: &gtk::Application,
        source: ArchiveSource,
        database: Rc<Database>,
        fast_image_loading: bool,
    ) {
        let title = source
            .local_path
            .file_name()
            .map(|name| name.to_string_lossy().into_owned())
            .unwrap_or_else(|| "Comicviewer".into());
        let window = gtk::ApplicationWindow::builder()
            .application(application)
            .title(&title)
            .default_width(1100)
            .default_height(850)
            .build();
        let header = gtk::HeaderBar::new();
        header.set_title_widget(Some(&gtk::Label::new(Some(&title))));
        window.set_titlebar(Some(&header));

        let root = gtk::Box::new(gtk::Orientation::Vertical, 0);
        let toolbar = gtk::Box::new(gtk::Orientation::Horizontal, 6);
        toolbar.set_margin_top(8);
        toolbar.set_margin_bottom(8);
        toolbar.set_margin_start(8);
        toolbar.set_margin_end(8);
        let previous_button = gtk::Button::builder()
            .icon_name("go-previous-symbolic")
            .tooltip_text("이전")
            .build();
        let next_button = gtk::Button::builder()
            .icon_name("go-next-symbolic")
            .tooltip_text("다음")
            .build();
        let shift_back_button = gtk::Button::with_label("한 장 이전");
        let shift_forward_button = gtk::Button::with_label("한 장 다음");
        let page_label = gtk::Label::new(Some("- / -"));
        page_label.set_hexpand(true);
        let direction_dropdown = gtk::DropDown::from_strings(&["좌→우", "우→좌"]);
        direction_dropdown.set_selected(1);
        let mode_dropdown = gtk::DropDown::from_strings(&[
            "100%",
            "폭 맞춤",
            "높이 맞춤",
            "2페이지",
            "이어서 보기",
        ]);
        mode_dropdown.set_selected(1);
        toolbar.append(&previous_button);
        toolbar.append(&shift_back_button);
        toolbar.append(&shift_forward_button);
        toolbar.append(&next_button);
        toolbar.append(&page_label);
        toolbar.append(&direction_dropdown);
        toolbar.append(&mode_dropdown);

        let single_picture = page_picture();
        let single_scrolled = gtk::ScrolledWindow::builder()
            .hexpand(true)
            .vexpand(true)
            .child(&single_picture)
            .build();
        let left_picture = page_picture();
        let right_picture = page_picture();
        let spread_box = gtk::Box::new(gtk::Orientation::Horizontal, 8);
        spread_box.set_homogeneous(true);
        spread_box.append(&left_picture);
        spread_box.append(&right_picture);

        let continuous_box = gtk::Box::new(gtk::Orientation::Vertical, 8);
        continuous_box.set_halign(gtk::Align::Fill);
        let continuous_scrolled = gtk::ScrolledWindow::builder()
            .hexpand(true)
            .vexpand(true)
            .child(&continuous_box)
            .build();
        let stack = gtk::Stack::new();
        stack.set_hexpand(true);
        stack.set_vexpand(true);
        stack.add_named(&single_scrolled, Some("single"));
        stack.add_named(&spread_box, Some("spread"));
        stack.add_named(&continuous_scrolled, Some("continuous"));

        let spinner = gtk::Spinner::new();
        spinner.set_halign(gtk::Align::Center);
        spinner.set_valign(gtk::Align::Center);
        let overlay = gtk::Overlay::new();
        overlay.set_child(Some(&stack));
        overlay.add_overlay(&spinner);
        let status_label = gtk::Label::builder()
            .label("ZIP 파일을 읽는 중...")
            .xalign(0.0)
            .margin_top(6)
            .margin_bottom(6)
            .margin_start(8)
            .margin_end(8)
            .build();
        root.append(&toolbar);
        root.append(&overlay);
        root.append(&status_label);
        window.set_child(Some(&root));

        let controller = Rc::new(ViewerController {
            window,
            source,
            database,
            stack,
            single_picture,
            left_picture,
            right_picture,
            continuous_box,
            continuous_scrolled,
            spinner,
            page_label,
            status_label,
            previous_button,
            next_button,
            shift_back_button,
            shift_forward_button,
            mode_dropdown,
            direction_dropdown,
            fast_image_loading,
            state: RefCell::new(ViewerState {
                pages: Vec::new(),
                anchor: 0,
                cache: HashMap::new(),
                requested: HashSet::new(),
                pending: VecDeque::new(),
                in_flight: 0,
                failed: HashMap::new(),
                access_clock: 0,
                continuous_rows: Vec::new(),
                continuous_bound: HashSet::new(),
            }),
        });
        controller.connect_signals();
        controller.apply_view_mode();
        controller.window.present();
        controller.load_archive();
    }
}

#[derive(Clone, Copy, PartialEq, Eq)]
enum ViewMode {
    ActualSize,
    FitWidth,
    FitHeight,
    TwoPage,
    Continuous,
}

struct CachedPage {
    texture: gdk::MemoryTexture,
    cost: usize,
    last_used: u64,
}

struct ContinuousRow {
    frame: gtk::AspectFrame,
    picture: gtk::Picture,
    ratio: f32,
}

struct ViewerState {
    pages: Vec<ArchivePage>,
    anchor: usize,
    cache: HashMap<usize, CachedPage>,
    requested: HashSet<usize>,
    pending: VecDeque<usize>,
    in_flight: usize,
    failed: HashMap<usize, String>,
    access_clock: u64,
    continuous_rows: Vec<ContinuousRow>,
    continuous_bound: HashSet<usize>,
}

struct ViewerController {
    window: gtk::ApplicationWindow,
    source: ArchiveSource,
    database: Rc<Database>,
    stack: gtk::Stack,
    single_picture: gtk::Picture,
    left_picture: gtk::Picture,
    right_picture: gtk::Picture,
    continuous_box: gtk::Box,
    continuous_scrolled: gtk::ScrolledWindow,
    spinner: gtk::Spinner,
    page_label: gtk::Label,
    status_label: gtk::Label,
    previous_button: gtk::Button,
    next_button: gtk::Button,
    shift_back_button: gtk::Button,
    shift_forward_button: gtk::Button,
    mode_dropdown: gtk::DropDown,
    direction_dropdown: gtk::DropDown,
    fast_image_loading: bool,
    state: RefCell<ViewerState>,
}

impl ViewerController {
    fn connect_signals(self: &Rc<Self>) {
        let controller = Rc::clone(self);
        self.previous_button
            .connect_clicked(move |_| controller.navigate(false));
        let controller = Rc::clone(self);
        self.next_button
            .connect_clicked(move |_| controller.navigate(true));
        let controller = Rc::clone(self);
        self.shift_back_button
            .connect_clicked(move |_| controller.shift_pairing(-1));
        let controller = Rc::clone(self);
        self.shift_forward_button
            .connect_clicked(move |_| controller.shift_pairing(1));
        let controller = Rc::clone(self);
        self.mode_dropdown
            .connect_selected_notify(move |_| controller.apply_view_mode());
        let controller = Rc::clone(self);
        self.direction_dropdown
            .connect_selected_notify(move |_| controller.render());

        let adjustment = self.continuous_scrolled.vadjustment();
        let controller = Rc::clone(self);
        adjustment.connect_value_changed(move |_| controller.continuous_scrolled());
        let controller = Rc::clone(self);
        adjustment.connect_changed(move |_| controller.continuous_scrolled());
        let controller = Rc::clone(self);
        self.continuous_scrolled
            .connect_notify_local(Some("width"), move |_, _| {
                controller.resize_continuous_rows();
            });

        let scroll_controller = gtk::EventControllerScroll::new(
            gtk::EventControllerScrollFlags::VERTICAL | gtk::EventControllerScrollFlags::DISCRETE,
        );
        scroll_controller.set_propagation_phase(gtk::PropagationPhase::Capture);
        let controller = Rc::clone(self);
        scroll_controller.connect_scroll(move |_, _, delta_y| {
            if controller.view_mode() == ViewMode::Continuous || delta_y == 0.0 {
                return glib::Propagation::Proceed;
            }
            controller.navigate(delta_y > 0.0);
            glib::Propagation::Stop
        });
        self.stack.add_controller(scroll_controller);

        let key_controller = gtk::EventControllerKey::new();
        let controller = Rc::clone(self);
        key_controller.connect_key_pressed(move |_, key, _, _| match key {
            gdk::Key::Left | gdk::Key::Page_Up => {
                controller.navigate(false);
                glib::Propagation::Stop
            }
            gdk::Key::Right | gdk::Key::Page_Down | gdk::Key::space => {
                controller.navigate(true);
                glib::Propagation::Stop
            }
            _ if key
                .to_unicode()
                .is_some_and(|character| character.eq_ignore_ascii_case(&'f')) =>
            {
                controller.toggle_fullscreen();
                glib::Propagation::Stop
            }
            gdk::Key::Escape if controller.window.is_fullscreen() => {
                controller.window.unfullscreen();
                glib::Propagation::Stop
            }
            _ => glib::Propagation::Proceed,
        });
        self.window.add_controller(key_controller);
    }

    fn toggle_fullscreen(&self) {
        if self.window.is_fullscreen() {
            self.window.unfullscreen();
        } else {
            self.window.fullscreen();
        }
    }

    fn load_archive(self: &Rc<Self>) {
        self.spinner.start();
        let path = self.source.local_path.clone();
        let (sender, receiver) = mpsc::channel();
        thread::spawn(move || {
            let _ = sender.send(list_pages(&path).map_err(|error| format!("{error:#}")));
        });
        let controller = Rc::downgrade(self);
        glib::timeout_add_local(Duration::from_millis(20), move || {
            match receiver.try_recv() {
                Ok(result) => {
                    if let Some(controller) = controller.upgrade() {
                        controller.finish_archive_load(result);
                    }
                    glib::ControlFlow::Break
                }
                Err(TryRecvError::Empty) => glib::ControlFlow::Continue,
                Err(TryRecvError::Disconnected) => glib::ControlFlow::Break,
            }
        });
    }

    fn finish_archive_load(self: &Rc<Self>, result: Result<Vec<ArchivePage>, String>) {
        match result {
            Ok(pages) if pages.is_empty() => {
                self.spinner.stop();
                self.status_label
                    .set_text("지원하는 이미지가 ZIP 파일에 없습니다.");
            }
            Ok(pages) => {
                let saved = self
                    .database
                    .load_progress(&self.source.source_uri)
                    .ok()
                    .flatten()
                    .unwrap_or(0)
                    .min(pages.len() - 1);
                let mut state = self.state.borrow_mut();
                state.pages = pages;
                state.anchor = saved;
                drop(state);
                self.build_continuous_rows();
                self.render();
            }
            Err(error) => {
                self.spinner.stop();
                self.status_label.set_text(&error);
            }
        }
    }

    fn build_continuous_rows(self: &Rc<Self>) {
        while let Some(child) = self.continuous_box.first_child() {
            self.continuous_box.remove(&child);
        }
        let count = self.state.borrow().pages.len();
        let mut rows = Vec::with_capacity(count);
        let default_ratio = 2.0 / 3.0;
        let width = self.continuous_content_width();
        for _ in 0..count {
            let picture = gtk::Picture::new();
            picture.set_content_fit(gtk::ContentFit::Contain);
            picture.set_hexpand(true);
            picture.set_vexpand(false);
            picture.set_can_shrink(true);
            picture.set_size_request(1, 1);
            let frame = gtk::AspectFrame::new(0.5, 0.5, default_ratio, false);
            frame.set_hexpand(true);
            frame.set_size_request(-1, row_height(width, default_ratio));
            frame.set_child(Some(&picture));
            self.continuous_box.append(&frame);
            rows.push(ContinuousRow {
                frame,
                picture,
                ratio: default_ratio,
            });
        }
        let mut state = self.state.borrow_mut();
        state.continuous_rows = rows;
        state.continuous_bound.clear();
    }

    fn view_mode(&self) -> ViewMode {
        match self.mode_dropdown.selected() {
            0 => ViewMode::ActualSize,
            2 => ViewMode::FitHeight,
            3 => ViewMode::TwoPage,
            4 => ViewMode::Continuous,
            _ => ViewMode::FitWidth,
        }
    }

    fn apply_view_mode(self: &Rc<Self>) {
        let mode = self.view_mode();
        self.shift_back_button
            .set_visible(mode == ViewMode::TwoPage);
        self.shift_forward_button
            .set_visible(mode == ViewMode::TwoPage);
        self.direction_dropdown
            .set_visible(mode == ViewMode::TwoPage);
        match mode {
            ViewMode::TwoPage => self.stack.set_visible_child_name("spread"),
            ViewMode::Continuous => self.stack.set_visible_child_name("continuous"),
            _ => self.stack.set_visible_child_name("single"),
        }
        if mode == ViewMode::Continuous {
            self.single_picture.set_paintable(gdk::Paintable::NONE);
            self.left_picture.set_paintable(gdk::Paintable::NONE);
            self.right_picture.set_paintable(gdk::Paintable::NONE);
            self.schedule_scroll_to_anchor();
        } else {
            self.clear_continuous_paintables();
        }
        let actual = mode == ViewMode::ActualSize;
        self.single_picture.set_can_shrink(!actual);
        self.single_picture.set_hexpand(mode == ViewMode::FitWidth);
        self.single_picture.set_vexpand(mode == ViewMode::FitHeight);
        self.render();
    }

    fn navigate(self: &Rc<Self>, forward: bool) {
        let step = if self.view_mode() == ViewMode::TwoPage {
            2
        } else {
            1
        };
        let mut state = self.state.borrow_mut();
        if state.pages.is_empty() {
            return;
        }
        let next = if forward {
            state.anchor.saturating_add(step).min(state.pages.len() - 1)
        } else {
            state.anchor.saturating_sub(step)
        };
        if next == state.anchor {
            return;
        }
        state.anchor = next;
        drop(state);
        self.render();
        if self.view_mode() == ViewMode::Continuous {
            self.schedule_scroll_to_anchor();
        }
    }

    fn shift_pairing(self: &Rc<Self>, offset: isize) {
        let mut state = self.state.borrow_mut();
        if state.pages.is_empty() {
            return;
        }
        state.anchor = state
            .anchor
            .saturating_add_signed(offset)
            .min(state.pages.len() - 1);
        drop(state);
        self.render();
    }

    fn render(self: &Rc<Self>) {
        let visible = self.visible_indices();
        if visible.is_empty() {
            return;
        }
        match self.view_mode() {
            ViewMode::TwoPage => self.render_spread(&visible),
            ViewMode::Continuous => self.render_continuous(&visible),
            _ => self.render_single(visible[0]),
        }
        self.update_navigation();
        self.request_pages(self.prefetch_indices(&visible));
        let state = self.state.borrow();
        let _ =
            self.database
                .save_progress(&self.source.source_uri, state.anchor, state.pages.len());
    }

    fn visible_indices(&self) -> Vec<usize> {
        let state = self.state.borrow();
        if state.pages.is_empty() {
            return Vec::new();
        }
        if self.view_mode() == ViewMode::TwoPage {
            spread_indices(state.anchor, state.pages.len()).to_vec()
        } else if self.view_mode() == ViewMode::Continuous {
            drop(state);
            self.continuous_visible_indices()
        } else {
            vec![state.anchor]
        }
    }

    fn prefetch_indices(&self, visible: &[usize]) -> Vec<usize> {
        let count = self.state.borrow().pages.len();
        prefetch_window(visible, count, self.fast_image_loading)
    }

    fn request_pages(self: &Rc<Self>, indices: Vec<usize>) {
        let mut state = self.state.borrow_mut();
        let mut prioritized = VecDeque::new();
        for index in indices {
            if index >= state.pages.len()
                || state.cache.contains_key(&index)
                || state.failed.contains_key(&index)
            {
                continue;
            }
            if state.requested.contains(&index) {
                if state.pending.contains(&index) {
                    prioritized.push_back(index);
                }
            } else {
                state.requested.insert(index);
                prioritized.push_back(index);
            }
        }
        let prioritized_set: HashSet<_> = prioritized.iter().copied().collect();
        state
            .pending
            .retain(|index| !prioritized_set.contains(index));
        prioritized.append(&mut state.pending);
        state.pending = prioritized;
        drop(state);
        self.pump_requests();
        self.update_spinner();
    }

    fn pump_requests(self: &Rc<Self>) {
        let concurrency = if self.fast_image_loading { 4 } else { 2 };
        while let Some((index, page)) = {
            let mut state = self.state.borrow_mut();
            if state.in_flight >= concurrency {
                None
            } else {
                state.pending.pop_front().map(|index| {
                    state.in_flight += 1;
                    (index, state.pages[index].clone())
                })
            }
        } {
            let path = self.source.local_path.clone();
            let (sender, receiver) = mpsc::channel();
            thread::spawn(move || {
                let result = decode_page(&path, &page).map_err(|error| format!("{error:#}"));
                let _ = sender.send(result);
            });
            let controller = Rc::downgrade(self);
            glib::timeout_add_local(Duration::from_millis(20), move || {
                match receiver.try_recv() {
                    Ok(result) => {
                        if let Some(controller) = controller.upgrade() {
                            controller.finish_page_load(index, result);
                        }
                        glib::ControlFlow::Break
                    }
                    Err(TryRecvError::Empty) => glib::ControlFlow::Continue,
                    Err(TryRecvError::Disconnected) => glib::ControlFlow::Break,
                }
            });
        }
    }

    fn finish_page_load(self: &Rc<Self>, index: usize, result: Result<DecodedPage, String>) {
        let mut state = self.state.borrow_mut();
        state.requested.remove(&index);
        state.in_flight = state.in_flight.saturating_sub(1);
        let mut decoded_ratio = None;
        match result {
            Ok(page) => {
                let ratio = page.width as f32 / page.height as f32;
                decoded_ratio = Some(ratio);
                let cost = page.rgba.len();
                let bytes = glib::Bytes::from_owned(page.rgba);
                let texture = gdk::MemoryTexture::new(
                    page.width as i32,
                    page.height as i32,
                    gdk::MemoryFormat::R8g8b8a8,
                    &bytes,
                    page.width as usize * 4,
                );
                state.access_clock = state.access_clock.wrapping_add(1);
                let last_used = state.access_clock;
                state.cache.insert(
                    index,
                    CachedPage {
                        texture,
                        cost,
                        last_used,
                    },
                );
            }
            Err(error) => {
                state.failed.insert(index, error);
            }
        }
        drop(state);
        if let Some(ratio) = decoded_ratio {
            let frame = {
                let mut state = self.state.borrow_mut();
                state.continuous_rows.get_mut(index).map(|row| {
                    row.ratio = ratio;
                    row.frame.clone()
                })
            };
            if let Some(frame) = frame {
                frame.set_ratio(ratio);
                frame.set_size_request(-1, row_height(self.continuous_content_width(), ratio));
            }
        }
        self.evict_cache();
        self.render_loaded_pages();
        self.update_spinner();
        self.pump_requests();
    }

    fn render_loaded_pages(&self) {
        let visible = self.visible_indices();
        match self.view_mode() {
            ViewMode::TwoPage => self.render_spread(&visible),
            ViewMode::Continuous => self.render_continuous(&visible),
            _ if !visible.is_empty() => self.render_single(visible[0]),
            _ => {}
        }
    }

    fn render_single(&self, index: usize) {
        let state = self.state.borrow();
        self.single_picture
            .set_paintable(state.cache.get(&index).map(|page| &page.texture));
        self.status_label.set_text(&state.pages[index].name);
    }

    fn render_spread(&self, indices: &[usize]) {
        let state = self.state.borrow();
        let first = indices.first().and_then(|index| state.cache.get(index));
        let second = indices.get(1).and_then(|index| state.cache.get(index));
        if self.direction_dropdown.selected() == 1 {
            self.left_picture
                .set_paintable(second.map(|page| &page.texture));
            self.right_picture
                .set_paintable(first.map(|page| &page.texture));
        } else {
            self.left_picture
                .set_paintable(first.map(|page| &page.texture));
            self.right_picture
                .set_paintable(second.map(|page| &page.texture));
        }
        self.status_label.set_text(
            &indices
                .iter()
                .map(|index| state.pages[*index].name.as_str())
                .collect::<Vec<_>>()
                .join(" · "),
        );
    }

    fn render_continuous(&self, visible: &[usize]) {
        let new_bound: HashSet<_> = visible.iter().copied().collect();
        let mut state = self.state.borrow_mut();
        let old_bound = state.continuous_bound.clone();
        let mut updates = Vec::new();
        for index in old_bound.difference(&new_bound) {
            updates.push((state.continuous_rows[*index].picture.clone(), None));
        }
        for index in &new_bound {
            state.access_clock = state.access_clock.wrapping_add(1);
            let clock = state.access_clock;
            let texture = state.cache.get_mut(index).map(|page| {
                page.last_used = clock;
                page.texture.clone()
            });
            updates.push((state.continuous_rows[*index].picture.clone(), texture));
        }
        state.continuous_bound = new_bound;
        let status = visible
            .first()
            .map(|index| state.pages[*index].name.clone());
        drop(state);
        for (picture, texture) in updates {
            picture.set_paintable(texture.as_ref());
        }
        if let Some(status) = status {
            self.status_label.set_text(&status);
        }
    }

    fn continuous_scrolled(self: &Rc<Self>) {
        if self.view_mode() != ViewMode::Continuous || self.state.borrow().pages.is_empty() {
            return;
        }
        let visible = self.visible_indices();
        let new_anchor = visible.first().copied();
        let changed = new_anchor.is_some_and(|index| index != self.state.borrow().anchor);
        if let Some(index) = new_anchor {
            self.state.borrow_mut().anchor = index;
        }
        self.render_continuous(&visible);
        self.request_pages(self.prefetch_indices(&visible));
        self.update_navigation();
        self.update_spinner();
        if changed {
            let state = self.state.borrow();
            let _ = self.database.save_progress(
                &self.source.source_uri,
                state.anchor,
                state.pages.len(),
            );
        }
    }

    fn continuous_visible_indices(&self) -> Vec<usize> {
        let state = self.state.borrow();
        if state.pages.is_empty() {
            return Vec::new();
        }
        let adjustment = self.continuous_scrolled.vadjustment();
        let top = adjustment.value();
        let bottom = top + adjustment.page_size();
        if adjustment.page_size() <= 0.0 {
            return vec![state.anchor];
        }
        let bounds: Vec<_> = state
            .continuous_rows
            .iter()
            .map(|row| {
                row.frame
                    .compute_bounds(&self.continuous_box)
                    .map(|bounds| {
                        let row_top = f64::from(bounds.y());
                        (row_top, row_top + f64::from(bounds.height()))
                    })
            })
            .collect();
        let mut visible = intersecting_rows(&bounds, top, bottom);
        if visible.is_empty() {
            visible.push(state.anchor);
        }
        visible
    }

    fn schedule_scroll_to_anchor(self: &Rc<Self>) {
        if self.view_mode() != ViewMode::Continuous || self.state.borrow().pages.is_empty() {
            return;
        }
        let controller = Rc::downgrade(self);
        glib::idle_add_local_once(move || {
            if let Some(controller) = controller.upgrade() {
                controller.scroll_to_anchor();
            }
        });
    }

    fn scroll_to_anchor(&self) {
        let state = self.state.borrow();
        let Some(row) = state.continuous_rows.get(state.anchor) else {
            return;
        };
        let Some(bounds) = row.frame.compute_bounds(&self.continuous_box) else {
            return;
        };
        let adjustment = self.continuous_scrolled.vadjustment();
        let maximum = (adjustment.upper() - adjustment.page_size()).max(adjustment.lower());
        adjustment.set_value(f64::from(bounds.y()).clamp(adjustment.lower(), maximum));
    }

    fn clear_continuous_paintables(&self) {
        let mut state = self.state.borrow_mut();
        let indices: Vec<_> = state.continuous_bound.drain().collect();
        let pictures: Vec<_> = indices
            .into_iter()
            .map(|index| state.continuous_rows[index].picture.clone())
            .collect();
        drop(state);
        for picture in pictures {
            picture.set_paintable(gdk::Paintable::NONE);
        }
    }

    fn resize_continuous_rows(&self) {
        let width = self.continuous_content_width();
        let rows: Vec<_> = self
            .state
            .borrow()
            .continuous_rows
            .iter()
            .map(|row| (row.frame.clone(), row.ratio))
            .collect();
        for (frame, ratio) in rows {
            frame.set_size_request(-1, row_height(width, ratio));
        }
    }

    fn continuous_content_width(&self) -> i32 {
        (self.continuous_scrolled.allocated_width() - 24).max(320)
    }

    fn evict_cache(&self) {
        let protected: HashSet<_> = self.visible_indices().into_iter().collect();
        let (byte_limit, page_limit) = if self.fast_image_loading {
            (FAST_CACHE_LIMIT_BYTES, FAST_CACHE_LIMIT_PAGES)
        } else {
            (CACHE_LIMIT_BYTES, CACHE_LIMIT_PAGES)
        };
        let mut state = self.state.borrow_mut();
        loop {
            let bytes: usize = state.cache.values().map(|page| page.cost).sum();
            if bytes <= byte_limit && state.cache.len() <= page_limit {
                break;
            }
            let candidate = state
                .cache
                .iter()
                .filter(|(index, _)| !protected.contains(index))
                .min_by_key(|(_, page)| page.last_used)
                .map(|(index, _)| *index);
            let Some(candidate) = candidate else {
                break;
            };
            state.cache.remove(&candidate);
        }
    }

    fn update_spinner(&self) {
        let state = self.state.borrow();
        let waiting = self
            .visible_indices()
            .iter()
            .any(|index| !state.cache.contains_key(index) && !state.failed.contains_key(index));
        if waiting {
            self.spinner.start();
        } else {
            self.spinner.stop();
        }
    }

    fn update_navigation(&self) {
        let state = self.state.borrow();
        let count = state.pages.len();
        if count == 0 {
            self.page_label.set_text("- / -");
            return;
        }
        let visible = self.visible_indices();
        let end = visible.last().copied().unwrap_or(state.anchor) + 1;
        if visible.len() > 1 && self.view_mode() == ViewMode::TwoPage {
            self.page_label
                .set_text(&format!("{}-{end} / {count}", state.anchor + 1));
        } else {
            self.page_label
                .set_text(&format!("{} / {count}", state.anchor + 1));
        }
        self.previous_button.set_sensitive(state.anchor > 0);
        self.next_button.set_sensitive(end < count);
        self.shift_back_button.set_sensitive(state.anchor > 0);
        self.shift_forward_button
            .set_sensitive(state.anchor + 1 < count);
    }
}

fn page_picture() -> gtk::Picture {
    let picture = gtk::Picture::new();
    picture.set_content_fit(gtk::ContentFit::Contain);
    picture.set_hexpand(true);
    picture.set_vexpand(true);
    picture
}

fn spread_indices(anchor: usize, count: usize) -> Vec<usize> {
    if count == 0 {
        return Vec::new();
    }
    let anchor = anchor.min(count - 1);
    let mut pages = vec![anchor];
    if anchor + 1 < count {
        pages.push(anchor + 1);
    }
    pages
}

fn row_height(width: i32, ratio: f32) -> i32 {
    ((width as f32 / ratio).round() as i32).clamp(1, 30_000)
}

fn prefetch_window(visible: &[usize], count: usize, fast: bool) -> Vec<usize> {
    if visible.is_empty() || count == 0 {
        return Vec::new();
    }
    let (ahead, behind) = if fast { (12, 4) } else { (2, 1) };
    let mut indices = visible.to_vec();
    let last = *visible.last().unwrap();
    for offset in 1..=ahead {
        let index = last + offset;
        if index >= count {
            break;
        }
        indices.push(index);
    }
    let first = visible[0];
    for offset in 1..=behind.min(first) {
        indices.push(first - offset);
    }
    indices
}

fn intersecting_rows(bounds: &[Option<(f64, f64)>], top: f64, bottom: f64) -> Vec<usize> {
    bounds
        .iter()
        .enumerate()
        .filter_map(|(index, bounds)| {
            bounds
                .filter(|(row_top, row_bottom)| *row_bottom > top && *row_top < bottom)
                .map(|_| index)
        })
        .collect()
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn spread_handles_last_odd_page() {
        assert_eq!(spread_indices(0, 3), [0, 1]);
        assert_eq!(spread_indices(2, 3), [2]);
    }

    #[test]
    fn shifted_pairing_starts_at_requested_page() {
        assert_eq!(spread_indices(1, 5), [1, 2]);
    }

    #[test]
    fn continuous_row_height_preserves_image_ratio() {
        assert_eq!(row_height(1200, 2.0 / 3.0), 1800);
        assert_eq!(row_height(1200, 1.5), 800);
    }

    #[test]
    fn continuous_visibility_uses_real_unequal_row_bounds() {
        let bounds = [
            Some((0.0, 1800.0)),
            Some((1808.0, 2608.0)),
            Some((2616.0, 4416.0)),
        ];
        assert_eq!(intersecting_rows(&bounds, 1000.0, 1900.0), [0, 1]);
        assert_eq!(intersecting_rows(&bounds, 3000.0, 3800.0), [2]);
    }

    #[test]
    fn fast_loading_prefetches_a_larger_window() {
        assert_eq!(prefetch_window(&[10], 30, false), [10, 11, 12, 9]);
        assert_eq!(
            prefetch_window(&[10], 30, true),
            [
                10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 9, 8, 7, 6
            ]
        );
    }
}