yog_book/

renderer.rs

//! Book renderer: yog-ui lays the book out and draws its text and
//! buttons; everything else goes through `draw2d` too — the sprite
//! sheets, item icons, SVG pages, entities — so the game and Yog-IDLE's
//! book designer run the same code.

use std::collections::HashMap;
use yog_gfx::GfxContext;
use yog_ui::{Align, FlexDir, Stretch, UiRoot, widget};

use crate::state::BookViewState;
use crate::theme::BookTheme;
use crate::{Book, BookPage};

// Patchouli-compatible book texture layout (512×256 sprite sheet).
// UV coordinates are in pixels on a 512×256 sheet.
// The full open-book background occupies 272×180 at UV (0,0).
pub const BOOK_TEX_W: f32 = 512.0;
pub const BOOK_TEX_H: f32 = 256.0;
// Open-book dimensions in texture-space
pub const BK_W: f32 = 272.0;
pub const BK_H: f32 = 180.0;
// Per-page dimensions and offsets (in book-local coords)
pub const PAGE_W: f32 = 116.0;
pub const PAGE_H: f32 = 156.0;
pub const TOP_PAD: f32 = 18.0; // vertical space above page text area
pub const LEFT_X: f32 = 15.0; // left page X inside book
pub const RIGHT_X: f32 = 141.0; // right page X inside book
// Separator strip UV origin on the sprite sheet
pub const SEP_U: f32 = 140.0;
pub const SEP_V: f32 = 180.0;
pub const SEP_W: f32 = 110.0;
pub const SEP_H: f32 = 3.0;
/// Room (GUI px) the book keeps free around it before it scales down.
const SCREEN_MARGIN: f32 = 16.0;
/// Smallest the book scales on a tiny screen.
const MIN_SCALE: f32 = 0.4;

// ── Embedded textures ─────────────────────────────────────────────────────────

/// The book's sprite sheet (cover, pages, separators, nameplate).
pub static BOOK_PNG: &[u8] = include_bytes!("../assets/book_brown.png");
/// The crafting-grid sprite sheet.
pub static CRAFTING_PNG: &[u8] = include_bytes!("../assets/crafting.png");
/// The image source the book draws [`BOOK_PNG`] as: the loader ships it
/// as `assets/yog/textures/gui/book/book.png` (Yog-IDLE resolves it to
/// the bytes).
pub const BOOK_SHEET: &str = "yog:gui/book/book";
/// The image source the book draws [`CRAFTING_PNG`] as.
pub const CRAFTING_SHEET: &str = "yog:gui/book/crafting";

// ── Background sprite (texture blit, drawn before yog-ui) ────────────────────

#[derive(Clone, Copy, PartialEq)]
pub(crate) enum SpriteSheet {
    Book,
    Crafting,
}

/// A subrect blit from one of the embedded sprite sheets, drawn before yog-ui.
#[derive(Clone)]
pub(crate) struct BgSprite {
    pub sheet: SpriteSheet,
    /// Source rect in pixels on the sheet.
    pub u: f32,
    pub v: f32,
    pub uw: f32,
    pub uh: f32,
    /// Destination on screen.
    pub x: f32,
    pub y: f32,
    pub w: f32,
    pub h: f32,
}

// ── Pending overlay draw commands ─────────────────────────────────────────────

#[derive(Clone)]
pub(crate) enum OverlayCmd {
    Svg {
        data: String,
        x: f32,
        y: f32,
        w: f32,
        h: f32,
    },
    /// A living entity standing in the box.
    Entity {
        entity: String,
        x: f32,
        y: f32,
        w: f32,
        h: f32,
    },
    /// MC default-font text rendered via draw2d (e.g. nameplate title/subtitle).
    McText {
        text: String,
        x: f32,
        y: f32,
        color: u32,
    },
    /// Item icon rendered through our GL pipeline from an MC-managed texture.
    McItem {
        item_id: String,
        x: f32,
        y: f32,
        w: f32,
        h: f32,
    },
    /// Flat texture icon blitted as-is (16×16), for icon strings that name a
    /// texture resource directly rather than an item — matches Patchouli's
    /// `BookIcon.from`: any icon string ending in `.png` is a raw texture,
    /// everything else is parsed as an item stack.
    McTexture {
        path: String,
        x: f32,
        y: f32,
        w: f32,
        h: f32,
    },
}

/// Push a book-author-supplied icon (category/entry icon field) as the right
/// overlay kind. Matches Patchouli's `BookIcon.from(String)`: `.png` → raw
/// texture blit, anything else → item stack render.
fn push_icon(overlays: &mut Vec<OverlayCmd>, icon: &str, x: f32, y: f32, w: f32, h: f32) {
    if icon.ends_with(".png") {
        overlays.push(OverlayCmd::McTexture {
            path: icon.to_string(),
            x,
            y,
            w,
            h,
        });
    } else {
        overlays.push(OverlayCmd::McItem {
            item_id: icon.to_string(),
            x,
            y,
            w,
            h,
        });
    }
}

// ── Recipe visuals (parsed from registered recipe JSON) ───────────────────────

#[derive(Clone)]
pub(crate) enum RecipeVis {
    Shaped {
        grid: Vec<Option<String>>,
        width: usize,
        output: String,
        count: u32,
    },
    Shapeless {
        ingredients: Vec<String>,
        output: String,
        count: u32,
    },
    Smelting {
        input: String,
        output: String,
    },
}

fn parse_recipe(json: &str) -> Option<RecipeVis> {
    let v: serde_json::Value = serde_json::from_str(json).ok()?;
    match v.get("type")?.as_str()? {
        "minecraft:crafting_shaped" => {
            let pattern: Vec<&str> = v
                .get("pattern")?
                .as_array()?
                .iter()
                .filter_map(|p| p.as_str())
                .collect();
            let width = pattern.iter().map(|r| r.chars().count()).max()?.max(1);
            let key = v.get("key")?.as_object()?;
            let mut grid = Vec::new();
            for row in &pattern {
                let mut chars: Vec<char> = row.chars().collect();
                chars.resize(width, ' ');
                for ch in chars {
                    grid.push(
                        key.get(&ch.to_string())
                            .and_then(|k| k.get("item"))
                            .and_then(|i| i.as_str())
                            .map(str::to_owned),
                    );
                }
            }
            let result = v.get("result")?;
            Some(RecipeVis::Shaped {
                grid,
                width,
                output: result.get("item")?.as_str()?.to_owned(),
                count: result.get("count").and_then(|c| c.as_u64()).unwrap_or(1) as u32,
            })
        }
        "minecraft:crafting_shapeless" => {
            let ingredients = v
                .get("ingredients")?
                .as_array()?
                .iter()
                .filter_map(|i| i.get("item").and_then(|x| x.as_str()).map(str::to_owned))
                .collect();
            let result = v.get("result")?;
            Some(RecipeVis::Shapeless {
                ingredients,
                output: result.get("item")?.as_str()?.to_owned(),
                count: result.get("count").and_then(|c| c.as_u64()).unwrap_or(1) as u32,
            })
        }
        "minecraft:smelting"
        | "minecraft:blasting"
        | "minecraft:smoking"
        | "minecraft:campfire_cooking" => Some(RecipeVis::Smelting {
            input: v.get("ingredient")?.get("item")?.as_str()?.to_owned(),
            output: v.get("result")?.as_str()?.to_owned(),
        }),
        _ => None,
    }
}

/// Normalize icon ids like "yog:item/ruby" / "yog:block/x" to registry item
/// ids ("yog:ruby") accepted by the MC item renderer.
fn normalize_item_id(id: &str) -> String {
    let (ns, path) = id.split_once(':').unwrap_or(("minecraft", id));
    let name = path
        .trim_start_matches("item/")
        .trim_start_matches("block/");
    format!("{ns}:{name}")
}

/// "yog:ruby_block" → "Ruby Block" (fallback display name from an item id).
fn pretty_item_name(id: &str) -> String {
    let name = id.rsplit(':').next().unwrap_or(id);
    name.split('_')
        .map(|w| {
            let mut c = w.chars();
            match c.next() {
                Some(f) => f.to_uppercase().collect::<String>() + c.as_str(),
                None => String::new(),
            }
        })
        .collect::<Vec<_>>()
        .join(" ")
}

// ── BookRenderer ──────────────────────────────────────────────────────────────

pub struct BookRenderer {
    pub book: Book,
    pub state: BookViewState,
    pub theme: BookTheme,
    recipes: HashMap<String, RecipeVis>,
    pub ui: Option<UiRoot>,
    bg_sprites: Vec<BgSprite>,
    overlays: Vec<OverlayCmd>,
    dirty: bool,
    last_sw: f32,
    last_sh: f32,
    /// i18n books: the untranslated book, the translator and the language
    /// probe (the book name's text) the shown book was made for.
    source: Option<Book>,
    translator: Option<fn(&str) -> Option<String>>,
    probe: Option<Option<String>>,
}

impl BookRenderer {
    pub fn new(book: Book) -> Self {
        let theme = BookTheme::default().with_nameplate(&book.nameplate_color);
        Self {
            book: flow(book),
            state: BookViewState::default(),
            theme,
            recipes: HashMap::new(),
            ui: None,
            bg_sprites: Vec::new(),
            overlays: Vec::new(),
            dirty: true,
            last_sw: 0.0,
            last_sh: 0.0,
            source: None,
            translator: None,
            probe: None,
        }
    }

    /// For [`Book::i18n`] books: the book's texts are translation keys,
    /// looked up with `tr` (unknown keys stay as they are) and re-looked up
    /// whenever the language changes.
    pub fn set_translator(&mut self, tr: fn(&str) -> Option<String>) {
        if self.book.i18n {
            self.source.get_or_insert_with(|| self.book.clone());
            // (Already flowed: flowing again only splits what still overflows.)
            self.translator = Some(tr);
            self.probe = None;
        }
    }

    /// Re-translates the book when the language changed since last time.
    fn localize(&mut self) {
        let (Some(src), Some(tr)) = (&self.source, self.translator) else {
            return;
        };
        let probe = Some(tr(&src.name));
        if probe == self.probe {
            return;
        }
        self.book = flow(src.localized(|k| tr(k).unwrap_or_else(|| k.to_string())));
        self.probe = probe;
        self.dirty = true;
    }

    pub fn handle_event(&mut self, ev: &str) {
        if self.state.handle(ev, &self.book) {
            self.dirty = true;
        }
    }

    /// Register a recipe (vanilla recipe JSON) so Crafting/Smelting pages can
    /// render it visually. Unknown recipe types are ignored.
    pub fn add_recipe(&mut self, id: impl Into<String>, json: &str) {
        if let Some(vis) = parse_recipe(json) {
            self.recipes.insert(id.into(), vis);
            self.dirty = true;
        }
    }

    /// Lays the book out for a `sw`×`sh` screen (when something changed);
    /// the book's rect.
    fn prepare(&mut self, sw: f32, sh: f32) -> (f32, f32, f32, f32) {
        // Render at Patchouli-native size (272×180 GUI pixels). Scale DOWN only
        // if the screen is smaller; never scale up (MC font is designed for 1×).
        let scale = ((sw - SCREEN_MARGIN) / BK_W)
            .min((sh - SCREEN_MARGIN) / BK_H)
            .clamp(MIN_SCALE, 1.0);
        let bw = BK_W * scale;
        let bh = BK_H * scale;
        let bx = (sw - bw) / 2.0;
        let by = (sh - bh) / 2.0;

        self.localize();
        // Rebuild widget tree if dirty or screen resized.
        if self.dirty || sw != self.last_sw || sh != self.last_sh {
            let (root, bg_sprites, overlays) = build_ui(
                &self.book,
                &self.state,
                &self.theme,
                &self.recipes,
                bx,
                by,
                bw,
                bh,
            );
            self.ui = Some(root);
            self.bg_sprites = bg_sprites;
            self.overlays = overlays;
            self.dirty = false;
            self.last_sw = sw;
            self.last_sh = sh;
        }
        if let Some(ui) = &mut self.ui
            && ui.needs_layout
        {
            ui.layout(sw, sh);
        }
        (bx, by, bw, bh)
    }

    /// Draws the book into a `sw`×`sh` screen — all of it through
    /// `draw2d` (its sprite sheets as the image sources [`BOOK_SHEET`] and
    /// [`CRAFTING_SHEET`]).
    pub fn render(&mut self, ctx: &GfxContext, sw: f32, sh: f32) {
        let (bx, by, bw, bh) = self.prepare(sw, sh);
        let d2 = ctx.draw2d();
        d2.image_region(BOOK_SHEET, bx, by, bw, bh, [0.0, 0.0, BK_W, BK_H], 0xFFFF_FFFF);
        for spr in &self.bg_sprites {
            let sheet = match spr.sheet {
                SpriteSheet::Book => BOOK_SHEET,
                SpriteSheet::Crafting => CRAFTING_SHEET,
            };
            d2.image_region(sheet, spr.x, spr.y, spr.w, spr.h, [spr.u, spr.v, spr.uw, spr.uh], 0xFFFF_FFFF);
        }
        if let Some(ui) = &mut self.ui {
            ui.render(ctx);
        }
        for ov in &self.overlays {
            match *ov {
                OverlayCmd::Svg { ref data, x, y, w, h } => d2.svg(data, x, y, w, h),
                OverlayCmd::Entity { ref entity, x, y, w, h } => d2.entity(entity, x, y, w, h, 30.0, 0.0),
                _ => {}
            }
        }
        // Item stacks and MC-font text last: item rendering changes GL state.
        self.draw_mc_overlays(ctx);
    }

    fn draw_mc_overlays(&self, ctx: &GfxContext) {
        for ov in &self.overlays {
            match *ov {
                OverlayCmd::McItem { ref item_id, x, y, w, h } => {
                    ctx.draw2d().item(&normalize_item_id(item_id), x, y, w.min(h));
                }
                OverlayCmd::McText { ref text, x, y, color } => {
                    ctx.draw2d().text(text, x, y, color, false);
                }
                OverlayCmd::McTexture { ref path, x, y, w, h } => {
                    // Standalone icon texture, not an atlas region — drawn
                    // whole, scaled into the icon box.
                    ctx.draw2d().image(path, x, y, w, h);
                }
                _ => {}
            }
        }
    }

    /// A click at `(x, y)` (screen GUI pixels, as last rendered): what it
    /// hit does its thing. The action it was (`"next_page"`, `"entry:3"`,
    /// …), if any.
    pub fn click(&mut self, x: f32, y: f32) -> Option<String> {
        let ui = self.ui.as_ref()?;
        let hit = yog_ui::layout::hit_test(&ui.layout_root, x, y)?;
        let action = hit.on_click.clone()?;
        self.handle_event(&action);
        Some(action)
    }

    /// Shows `entry` (an entry id) at `page`, as a reader would have got
    /// there; `false` if there's no such entry.
    pub fn show(&mut self, entry: &str, page: usize) -> bool {
        let Some(e) = self.book.entries.iter().find(|e| e.id == entry) else { return false };
        let Some(cat) = self.book.categories.iter().position(|c| c.id == e.category) else { return false };
        self.state.cat = cat;
        self.state.at_home = false;
        let in_cat = self.state.entries_in_cat(&self.book);
        let Some(i) = in_cat.iter().position(|x| x.id == entry) else { return false };
        self.state.entry = i;
        self.state.page = page.min(e.pages.len().saturating_sub(1));
        self.dirty = true;
        true
    }

    /// The entry shown (its id) and the page of it, or `None` on the
    /// landing and list pages.
    pub fn shown(&self) -> Option<(String, usize)> {
        if self.state.at_home {
            return None;
        }
        self.state.current_entry(&self.book).map(|e| (e.id.clone(), self.state.page))
    }

    /// Replaces the book (an edit), keeping the reader where they were.
    pub fn set_book(&mut self, book: Book) {
        let at = self.shown();
        self.book = flow(book);
        self.theme = BookTheme::default().with_nameplate(&self.book.nameplate_color);
        self.dirty = true;
        match at {
            Some((entry, page)) if self.show(&entry, page) => {}
            _ => {
                if self.state.cat >= self.book.categories.len() {
                    self.state = BookViewState::default();
                }
            }
        }
    }
}

// ── UI builder ────────────────────────────────────────────────────────────────

thread_local! {
    /// The scale the book is drawn at (≤ 1): its text is drawn at it too,
    /// so it stays on the page however small the book gets.
    static TEXT_SCALE: std::cell::Cell<f32> = const { std::cell::Cell::new(1.0) };
}

fn text_scale() -> f32 {
    TEXT_SCALE.with(|s| s.get())
}

/// Book label: MC font, no drop-shadow (Patchouli draws book text shadowless).
fn lbl(text: impl Into<String>) -> widget::Widget {
    widget::label(text).shadow(false).font_scale(text_scale())
}
/// Book button: no drop-shadow.
fn btn(text: impl Into<String>) -> widget::Widget {
    widget::button(text).shadow(false).font_scale(text_scale())
}

/// A title: one line, shrunk (down to half size, then cut short) to fit
/// the page's width.
fn title_lbl(text: impl Into<String>) -> widget::Widget {
    let text = text.into();
    let fit = ((PAGE_W - 6.0) / yog_ui::text::str_width(&text, 1.0)).clamp(0.5, 1.0);
    widget::label(text).shadow(false).font_scale(text_scale() * fit).no_wrap().ellipsis()
}

// ── Flowing text ──────────────────────────────────────────────────────────────

/// Width of a page's text, in book pixels.
const TEXT_W: f32 = PAGE_W - 8.0;
/// Height a page's body has for text, in book pixels: the page less its
/// title and separator, the page navigation and the padding.
const BODY_H: f32 = PAGE_H - 22.0 - 12.0 - 14.0;

/// How much of the body a page's own content takes above its text.
fn content_above_text(page: &BookPage) -> Option<f32> {
    Some(match page {
        BookPage::Text { title: Some(_), .. } => 22.0,
        BookPage::Text { .. } => 0.0,
        BookPage::Spotlight { .. } => 40.0,
        BookPage::Crafting { .. } => 80.0,
        BookPage::Smelting { .. } => 40.0,
        BookPage::Image { .. } | BookPage::Entity { .. } => 104.0,
        BookPage::Relations { entries, .. } => 12.0 + entries.len() as f32 * 10.0,
        _ => return None,
    })
}

fn page_text_mut(page: &mut BookPage) -> Option<&mut String> {
    match page {
        BookPage::Text { text, .. } => Some(text),
        BookPage::Spotlight { text, .. }
        | BookPage::Crafting { text, .. }
        | BookPage::Smelting { text, .. }
        | BookPage::Image { text, .. }
        | BookPage::Entity { text, .. }
        | BookPage::Relations { text, .. } => text.as_mut(),
        _ => None,
    }
}

/// Text that doesn't fit its page goes on to new text pages after it.
fn flow(mut book: Book) -> Book {
    let line_h = yog_ui::text::LINE_H + yog_ui::text::LINE_GAP;
    let fits = |h: f32| (((h + yog_ui::text::LINE_GAP) / line_h).floor() as usize).max(1);
    for entry in &mut book.entries {
        let mut pages = Vec::with_capacity(entry.pages.len());
        for mut page in std::mem::take(&mut entry.pages) {
            let Some(above) = content_above_text(&page) else {
                pages.push(page);
                continue;
            };
            let Some(text) = page_text_mut(&mut page) else {
                pages.push(page);
                continue;
            };
            let lines: Vec<String> = text
                .split('\n')
                .flat_map(|para| {
                    if para.is_empty() { vec![String::new()] } else { yog_ui::text::wrap_text(para, TEXT_W, 1.0) }
                })
                .collect();
            let first = fits(BODY_H - above);
            if lines.len() <= first {
                pages.push(page);
                continue;
            }
            *text = lines[..first].join("\n");
            pages.push(page);
            for chunk in lines[first..].chunks(fits(BODY_H)) {
                pages.push(BookPage::Text { text: chunk.join("\n"), title: None });
            }
        }
        entry.pages = pages;
    }
    book
}

/// Build the yog-ui widget tree + bg sprites + overlay commands for the current book state.
/// `bx/by/bw/bh` are the screen-space book rect (same values used to blit the bg texture).
fn build_ui(
    book: &Book,
    state: &BookViewState,
    theme: &BookTheme,
    recipes: &HashMap<String, RecipeVis>,
    bx: f32,
    by: f32,
    bw: f32,
    bh: f32,
) -> (UiRoot, Vec<BgSprite>, Vec<OverlayCmd>) {
    let mut bg_sprites: Vec<BgSprite> = Vec::new();
    let mut overlays: Vec<OverlayCmd> = Vec::new();

    // Scale from Patchouli's fixed BK_W×BK_H coordinate space → actual bw×bh.
    let sx = bw / BK_W;
    TEXT_SCALE.with(|s| s.set(sx));
    let sy = bh / BK_H;

    // In book-local: left page starts at x=LEFT_X, right at RIGHT_X, both y=TOP_PAD.
    let pw = PAGE_W * sx;
    let ph = PAGE_H * sy;
    let spine_gap = (RIGHT_X - LEFT_X - PAGE_W) * sx; // ≈10px scaled

    // Content x/y in screen space (for SVG overlay positioning).
    let lx = bx + LEFT_X * sx;
    let rx = bx + RIGHT_X * sx;
    let py = by + TOP_PAD * sy;

    // Separator x offset within a page: centered = PAGE_W/2 - SEP_W/2 = 58-55 = 3.
    let sep_cx = (PAGE_W / 2.0 - SEP_W / 2.0) * sx; // = 3*sx

    if state.at_home {
        // Nameplate banner: book-local (-8, 12) from LEFT_PAGE_X, size 140×31.
        bg_sprites.push(BgSprite {
            sheet: SpriteSheet::Book,
            u: 0.0,
            v: 180.0,
            uw: 140.0,
            uh: 31.0,
            x: bx + (LEFT_X - 8.0) * sx,
            y: by + 12.0 * sy,
            w: 140.0 * sx,
            h: 31.0 * sy,
        });
    }
    // Separator on the right page below its header ("Categories" or the
    // category name; y=12 page-local), centered.
    bg_sprites.push(BgSprite {
        sheet: SpriteSheet::Book,
        u: SEP_U,
        v: SEP_V,
        uw: SEP_W,
        uh: SEP_H,
        x: rx + sep_cx,
        y: by + (TOP_PAD + 12.0) * sy,
        w: SEP_W * sx,
        h: (SEP_H * sy).max(1.0),
    });

    let (left_page, right_page) = if state.at_home {
        let l = build_landing_left(book, theme, pw, ph, sx, sy, bx, by, &mut overlays);
        let r = build_categories_right(book, state, theme, pw, ph, rx, py, sx, sy, &mut overlays);
        (l, r)
    } else {
        let l = build_entry_left(
            book,
            state,
            theme,
            recipes,
            pw,
            ph,
            lx,
            py,
            sx,
            sy,
            sep_cx,
            &mut bg_sprites,
            &mut overlays,
        );
        let r = build_entries_right(book, state, theme, pw, ph, rx, py, sx, sy, &mut overlays);
        (l, r)
    };

    // Root row spans the full book width.
    // padding-left = LEFT_X*sx offsets both pages from the left cover edge.
    // padding-top  = TOP_PAD*sy offsets from the header banner.
    // spine_gap spacer sits between the two pages.
    // gap must be 0: analytic overlay positions assume pages sit exactly at
    // LEFT_X / RIGHT_X — any flex gap would shift widgets (and their focus
    // highlights) away from the absolutely-positioned icons.
    let root_widget = widget::panel(FlexDir::Row)
        .w(bw)
        .h(bh)
        .gap(0.0)
        .padding(TOP_PAD * sy, 0.0, 0.0, LEFT_X * sx)
        .child(left_page)
        .child(widget::spacer().w(spine_gap)) // spine gap
        .child(right_page);

    // Outer shell: positions the book on screen via a full-screen transparent Row.
    // We use a Column + Row arrangement to apply top/left offsets without margin support.
    let outer = widget::panel(FlexDir::Column)
        .w(bw)
        .h(bh)
        .child(root_widget);

    // Screen-level container: positions book at (bx, by).
    // bx offset via padding, by offset via padding-top.
    // Note: layout computes from (0,0), so we pass bx/by into the UiRoot manually
    // by wrapping in a full-screen panel with the right padding.
    let screen_root = widget::panel(FlexDir::Row)
        .padding(by, 0.0, 0.0, bx)
        .child(outer);

    let ui = UiRoot::new(&book.id, screen_root);
    (ui, bg_sprites, overlays)
}

// ── Left page: landing (home view) ───────────────────────────────────────────

fn build_landing_left(
    book: &Book,
    theme: &BookTheme,
    page_w: f32,
    page_h: f32,
    sx: f32,
    sy: f32,
    bx: f32,
    by: f32,
    overlays: &mut Vec<OverlayCmd>,
) -> widget::Widget {
    // Patchouli: title at book-local (13, 16), subtitle at (24, 24).
    // These overlap the nameplate sprite which is above/within the page top.
    overlays.push(OverlayCmd::McText {
        text: book.name.clone(),
        x: bx + 13.0 * sx,
        y: by + 16.0 * sy,
        color: theme.nameplate,
    });
    if let Some(author) = &book.author {
        overlays.push(OverlayCmd::McText {
            text: format!("by {}", author),
            x: bx + 24.0 * sx,
            y: by + 24.0 * sy,
            color: theme.nameplate,
        });
    }

    // Widget content: landing text + entry count.
    // Landing text starts at page-local y=25 (= book-local y=43).
    // We push it down with top padding = 25*sy.
    let mut col = widget::panel(FlexDir::Column)
        .w(page_w)
        .h(page_h)
        .padding(25.0 * sy, 6.0, 4.0, 4.0)
        .gap(0.0);

    for para in book.landing_text.split('\n') {
        if para.is_empty() {
            col = col.child(widget::spacer().h(4.0 * sy));
        } else {
            col = col.child(lbl(para).color(theme.text));
        }
    }

    let total = book.entries.len();
    col = col.child(widget::spacer().flex(1.0));
    col = col.child(
        lbl(format!("{} entries", total))
            .color(theme.nav)
            .h(9.0 * sy)
            .align(Align::Center),
    );
    col
}

// ── Right page: category list (home view) ────────────────────────────────────

fn build_categories_right(
    book: &Book,
    state: &BookViewState,
    theme: &BookTheme,
    page_w: f32,
    page_h: f32,
    rx: f32,
    py: f32,
    sx: f32,
    sy: f32,
    overlays: &mut Vec<OverlayCmd>,
) -> widget::Widget {
    // Patchouli right-page layout (landing):
    //   "Categories" header at page-local y=0, centered
    //   Separator at page-local y=12 (drawn as bg sprite, not here)
    //   4-column 24×24 icon grid starting at page-local (10, 25)
    let cell_w = 24.0 * sx;
    let cell_h = 24.0 * sy;
    let grid_pad_left = 10.0 * sx;
    let header_h = 9.0 * sy;

    let mut col = widget::panel(FlexDir::Column)
        .w(page_w)
        .h(page_h)
        .padding(0.0, 4.0, 4.0, 0.0)
        .gap(0.0);

    col = col.child(
        lbl("Categories")
            .color(theme.divider)
            .h(header_h)
            .align(Align::Center)
            .no_wrap(),
    );
    // Gap covering the separator region up to grid start (page-local y=25).
    col = col.child(widget::spacer().h(25.0 * sy - header_h));

    // 4-column icon grid; icons drawn as GL overlays at exact Patchouli positions.
    let cats = &book.categories;
    let icon_s = 16.0 * sx.min(sy);
    let mut row_i = 0usize;
    loop {
        let row_start = row_i * 4;
        if row_start >= cats.len() {
            break;
        }
        let mut row =
            widget::panel(FlexDir::Row)
                .h(cell_h)
                .gap(0.0)
                .padding(0.0, 0.0, 0.0, grid_pad_left);
        for col_i in 0..4 {
            let idx = row_start + col_i;
            if idx >= cats.len() {
                break;
            }
            let cat = &cats[idx];
            let selected = !state.at_home && idx == state.cat;
            let bg = if selected { theme.nav_selected_bg } else { 0 };

            if let Some(icon_id) = &cat.icon {
                push_icon(
                    overlays,
                    icon_id,
                    rx + (10.0 + col_i as f32 * 24.0 + 4.0) * sx,
                    py + (25.0 + row_i as f32 * 24.0 + 4.0) * sy,
                    icon_s,
                    icon_s,
                );
            }
            let cell = widget::panel(FlexDir::Column)
                .w(cell_w)
                .h(cell_h)
                .bg(bg)
                .on_click(format!("cat:{}", idx))
                .id(format!("book_cat_{}", idx));
            row = row.child(cell);
        }
        col = col.child(row);
        row_i += 1;
    }
    col
}

// ── Left page: entry content (entry view) ────────────────────────────────────

fn build_entry_left(
    book: &Book,
    state: &BookViewState,
    theme: &BookTheme,
    recipes: &HashMap<String, RecipeVis>,
    page_w: f32,
    page_h: f32,
    ox: f32,
    oy: f32,
    sx: f32,
    sy: f32,
    sep_cx: f32,
    bg_sprites: &mut Vec<BgSprite>,
    overlays: &mut Vec<OverlayCmd>,
) -> widget::Widget {
    let entry = state.current_entry(book);
    let page = entry.and_then(|e| e.pages.get(state.page));
    let page_count = state.page_count(book);
    let title_text = entry.map(|e| e.name.as_str()).unwrap_or("");

    // Patchouli PageText page 0:
    //   title centered at page-local (PAGE_W/2, 0)
    //   separator at page-local (0, 12)  → centered ≡ sep_cx offset
    //   text body at page-local (0, 22)
    let title_h = 9.0 * sy;
    let sep_h_px = (SEP_H * sy).max(1.0);
    let sep_y = oy + 12.0 * sy;
    let body_oy = oy + 22.0 * sy;

    bg_sprites.push(BgSprite {
        sheet: SpriteSheet::Book,
        u: SEP_U,
        v: SEP_V,
        uw: SEP_W,
        uh: SEP_H,
        x: ox + sep_cx,
        y: sep_y,
        w: SEP_W * sx,
        h: sep_h_px,
    });

    let page_label = format!("{}/{}", state.page + 1, page_count);
    let nav = widget::panel(FlexDir::Row)
        .h(12.0 * sy)
        .gap(4.0)
        .padding(0.0, 2.0, 0.0, 2.0)
        .child(
            btn("◀")
                .w(12.0 * sx)
                .h(12.0 * sy)
                .color(theme.nav)
                .on_click("prev_page")
                .id("prev_page"),
        )
        .child(
            lbl(&page_label)
                .color(theme.nav)
                .flex(1.0)
                .align(Align::Center),
        )
        .child(
            btn("⌂")
                .w(12.0 * sx)
                .h(12.0 * sy)
                .color(theme.nav)
                .on_click("home")
                .id("book_home"),
        )
        .child(
            btn("▶")
                .w(12.0 * sx)
                .h(12.0 * sy)
                .color(theme.nav)
                .on_click("next_page")
                .id("next_page"),
        );

    let page_body = build_page(
        page, state.page, theme, recipes, bg_sprites, overlays, ox, body_oy, sx, sy,
    );

    widget::panel(FlexDir::Column)
        .w(page_w)
        .h(page_h)
        .padding(0.0, 6.0, 4.0, 4.0)
        .gap(0.0)
        .child(
            title_lbl(title_text)
                .color(theme.title)
                .h(title_h)
                .align(Align::Center)
                .no_wrap(),
        )
        .child(widget::spacer().h((12.0 - 9.0) * sy)) // gap: title end → sep start
        .child(widget::spacer().h(sep_h_px)) // height occupied by sep sprite
        .child(widget::spacer().h((22.0 - 12.0 - SEP_H) * sy)) // gap: sep end → body
        .child(page_body)
        .child(nav)
}

// ── Right page: entry list for selected category (entry view) ─────────────────

fn build_entries_right(
    book: &Book,
    state: &BookViewState,
    theme: &BookTheme,
    page_w: f32,
    page_h: f32,
    rx: f32,
    py: f32,
    sx: f32,
    sy: f32,
    overlays: &mut Vec<OverlayCmd>,
) -> widget::Widget {
    let entries = state.entries_visible(book);
    let cat_name = book
        .categories
        .get(state.cat)
        .map(|c| c.name.as_str())
        .unwrap_or("Entries");
    let spread_count = state.list_spread_count(book);

    // Patchouli GuiBookEntryList layout (right page):
    //   category name at page-local y=0, centered
    //   separator at page-local y=12 (drawn as bg sprite in build_ui)
    //   entries start at page-local y=20, each h=11
    let header_h = 9.0 * sy;
    let row_h = 11.0 * sy;
    let icon_size = 8.0 * sx.min(sy);

    let mut col = widget::panel(FlexDir::Column)
        .w(page_w)
        .h(page_h)
        .padding(0.0, 4.0, 4.0, 0.0)
        .gap(0.0);

    col = col.child(
        lbl(cat_name)
            .color(theme.divider)
            .h(header_h)
            .align(Align::Center)
            .no_wrap(),
    );
    // sep region: y=9..15 (sep h=3), then gap to entries start at y=20
    col = col.child(widget::spacer().h((12.0 - 9.0) * sy)); // gap to sep
    col = col.child(widget::spacer().h((SEP_H * sy).max(1.0))); // sep height
    col = col.child(widget::spacer().h((20.0 - 12.0 - SEP_H) * sy)); // gap to entries

    let abs_start = state.list_spread_start();
    for (i, entry) in entries.iter().enumerate() {
        let abs_i = abs_start + i;
        let selected = abs_i == state.entry;
        let bg = if selected { theme.nav_selected_bg } else { 0 };
        let color = if selected {
            theme.nav_selected
        } else {
            theme.nav
        };

        // Icon at page-local (1, 20 + i*11 + 1), 8×8 (Patchouli renders entry
        // icons at 0.5× scale).
        if let Some(icon_id) = &entry.icon {
            push_icon(
                overlays,
                icon_id,
                rx + 1.0 * sx,
                py + (20.0 + i as f32 * 11.0 + 1.0) * sy,
                icon_size,
                icon_size,
            );
        }

        let mut row = widget::panel(FlexDir::Row)
            .h(row_h)
            .gap(2.0)
            .bg(bg)
            .on_click(format!("entry:{}", abs_i))
            .id(format!("book_entry_{}", abs_i))
            .padding(1.0, 2.0, 1.0, 2.0);

        row = row.child(widget::spacer().w(icon_size + 2.0));
        row = row.child(lbl(&entry.name).color(color).flex(1.0).no_wrap());
        col = col.child(row);
    }

    if spread_count > 1 {
        col = col.child(widget::spacer().flex(1.0));
        let spread_label = format!("{}/{}", state.list_spread + 1, spread_count);
        col = col.child(
            widget::panel(FlexDir::Row)
                .h(12.0 * sy)
                .gap(2.0)
                .child(
                    btn("◀")
                        .w(12.0 * sx)
                        .h(12.0 * sy)
                        .color(theme.nav)
                        .on_click("prev_list")
                        .id("prev_list"),
                )
                .child(
                    lbl(&spread_label)
                        .color(theme.nav)
                        .flex(1.0)
                        .align(Align::Center),
                )
                .child(
                    btn("▶")
                        .w(12.0 * sx)
                        .h(12.0 * sy)
                        .color(theme.nav)
                        .on_click("next_list")
                        .id("next_list"),
                ),
        );
    }
    col
}

/// Draw a Patchouli-style crafting recipe: title, 100×62 grid background
/// (UV 0,0 on the crafting sheet), 3×N ingredient icons, output item.
/// `rx0`/`ry` are body-local recipe origin; `ox`/`oy` the body screen origin.
#[allow(clippy::too_many_arguments)]
fn draw_crafting_grid(
    theme: &BookTheme,
    bg_sprites: &mut Vec<BgSprite>,
    overlays: &mut Vec<OverlayCmd>,
    col: &mut widget::Widget,
    grid: &[Option<String>],
    width: usize,
    output: &str,
    count: u32,
    shapeless: bool,
    ox: f32,
    oy: f32,
    rx0: f32,
    ry: f32,
    sx: f32,
    sy: f32,
) {
    let mut c = std::mem::replace(col, widget::panel(FlexDir::Column));
    c = c.child(
        title_lbl(pretty_item_name(output))
            .color(theme.title)
            .h(10.0 * sy)
            .align(Align::Center),
    );

    // Grid background: blit at (rx0-2, ry-2) size 100×62, UV(0,0).
    bg_sprites.push(BgSprite {
        sheet: SpriteSheet::Crafting,
        u: 0.0,
        v: 0.0,
        uw: 100.0,
        uh: 62.0,
        x: ox + (rx0 - 2.0) * sx,
        y: oy + (ry - 2.0) * sy,
        w: 100.0 * sx,
        h: 62.0 * sy,
    });
    // Shapeless marker: UV(0,64) 11×11 at (rx0+62, ry+2).
    if shapeless {
        bg_sprites.push(BgSprite {
            sheet: SpriteSheet::Crafting,
            u: 0.0,
            v: 64.0,
            uw: 11.0,
            uh: 11.0,
            x: ox + (rx0 + 62.0) * sx,
            y: oy + (ry + 2.0) * sy,
            w: 11.0 * sx,
            h: 11.0 * sy,
        });
    }

    let icon_s = 16.0 * sx.min(sy);
    let width = width.max(1);
    for (i, slot) in grid.iter().enumerate() {
        if let Some(item) = slot {
            overlays.push(OverlayCmd::McItem {
                item_id: item.clone(),
                x: ox + (rx0 + 3.0 + (i % width) as f32 * 19.0) * sx,
                y: oy + (ry + 3.0 + (i / width) as f32 * 19.0) * sy,
                w: icon_s,
                h: icon_s,
            });
        }
    }
    // Output item at (rx0+79, ry+22); stack count below it when > 1.
    overlays.push(OverlayCmd::McItem {
        item_id: output.to_owned(),
        x: ox + (rx0 + 79.0) * sx,
        y: oy + (ry + 22.0) * sy,
        w: icon_s,
        h: icon_s,
    });
    if count > 1 {
        overlays.push(OverlayCmd::McText {
            text: format!("x{}", count),
            x: ox + (rx0 + 81.0) * sx,
            y: oy + (ry + 40.0) * sy,
            color: theme.title,
        });
    }
    // Patchouli's "toast symbol" under the output: the crafting table.
    overlays.push(OverlayCmd::McItem {
        item_id: "minecraft:crafting_table".into(),
        x: ox + (rx0 + 79.0) * sx,
        y: oy + (ry + 41.0) * sy,
        w: icon_s,
        h: icon_s,
    });

    // Spacer from title end (10) to below the grid (ry+62+4).
    c = c.child(widget::spacer().h((ry + 62.0 + 4.0 - 10.0) * sy));
    *col = c;
}

// ── Page content builder ──────────────────────────────────────────────────────

/// Build the content widget for a single page.
/// `ox/oy` = screen top-left of the page body area.
/// `sx/sy` = book-local-to-screen scale factors.
fn build_page(
    page: Option<&BookPage>,
    page_num: usize,
    theme: &BookTheme,
    recipes: &HashMap<String, RecipeVis>,
    bg_sprites: &mut Vec<BgSprite>,
    overlays: &mut Vec<OverlayCmd>,
    ox: f32,
    oy: f32,
    sx: f32,
    sy: f32,
) -> widget::Widget {
    let mut col = widget::panel(FlexDir::Column).flex(1.0).gap(4.0);

    let Some(page) = page else {
        return col.child(lbl("No entries yet.").color(theme.nav));
    };

    match page {
        BookPage::Text { text, title } => {
            // On non-first pages, show an optional section title + separator.
            if page_num > 0
                && let Some(t) = title
            {
                let sep_h_px = (SEP_H * sy).max(1.0);
                let title_h = 9.0 * sy;
                col = col.child(
                    title_lbl(t.as_str())
                        .color(theme.title)
                        .h(title_h)
                        .align(Align::Center)
                        .no_wrap(),
                );
                // sep at y=12 page-local, centered
                bg_sprites.push(BgSprite {
                    sheet: SpriteSheet::Book,
                    u: SEP_U,
                    v: SEP_V,
                    uw: SEP_W,
                    uh: SEP_H,
                    x: ox + (PAGE_W / 2.0 - SEP_W / 2.0) * sx,
                    y: oy + 12.0 * sy,
                    w: SEP_W * sx,
                    h: sep_h_px,
                });
                col = col.child(widget::spacer().h((12.0 - 9.0) * sy));
                col = col.child(widget::spacer().h(sep_h_px));
                col = col.child(widget::spacer().h((22.0 - 12.0 - SEP_H) * sy));
            }
            for para in text.split('\n') {
                col = col.child(lbl(para).color(theme.text));
            }
        }

        BookPage::Spotlight { item, title, text } => {
            // Patchouli PageSpotlight (page-body-local coordinates):
            //   title at y=0 centered, item frame at (PAGE_W/2-33, 10) 66×26
            //   (UV 0,102 on the 128×256 crafting sheet), item at (PAGE_W/2-8, 15),
            //   text at y=40.
            bg_sprites.push(BgSprite {
                sheet: SpriteSheet::Crafting,
                u: 0.0,
                v: 102.0,
                uw: 66.0,
                uh: 26.0,
                x: ox + (PAGE_W / 2.0 - 33.0) * sx,
                y: oy + 10.0 * sy,
                w: 66.0 * sx,
                h: 26.0 * sy,
            });

            let item_name = title
                .as_deref()
                .or(item.name.as_deref())
                .unwrap_or(item.id.as_str());
            col = col.child(
                title_lbl(item_name)
                    .color(theme.title)
                    .h(10.0 * sy)
                    .align(Align::Center),
            );

            let icon_s = 16.0 * sx.min(sy);
            overlays.push(OverlayCmd::McItem {
                item_id: item.id.clone(),
                x: ox + (PAGE_W / 2.0 - 8.0) * sx,
                y: oy + 15.0 * sy,
                w: icon_s,
                h: icon_s,
            });

            // Spacer from title end (y=10) to text start (y=40).
            col = col.child(widget::spacer().h(30.0 * sy));

            if let Some(t) = text {
                col = col.child(lbl(t.as_str()).color(theme.text));
            }
        }

        BookPage::Crafting { recipe_id, text } => {
            // Patchouli PageCrafting: recipe origin at page-local
            // (PAGE_W/2-49, 4); we place it in body-local coords below a title.
            let rx0 = PAGE_W / 2.0 - 49.0; // = 9
            let ry = 12.0; // body-local recipe y (title occupies 0..10)
            match recipes.get(recipe_id.as_str()) {
                Some(RecipeVis::Shaped {
                    grid,
                    width,
                    output,
                    count,
                }) => {
                    draw_crafting_grid(
                        theme, bg_sprites, overlays, &mut col, grid, *width, output, *count, false,
                        ox, oy, rx0, ry, sx, sy,
                    );
                }
                Some(RecipeVis::Shapeless {
                    ingredients,
                    output,
                    count,
                }) => {
                    let grid: Vec<Option<String>> = ingredients.iter().cloned().map(Some).collect();
                    draw_crafting_grid(
                        theme, bg_sprites, overlays, &mut col, &grid, 3, output, *count, true, ox,
                        oy, rx0, ry, sx, sy,
                    );
                }
                _ => {
                    col = col.child(lbl(format!("[Recipe: {}]", recipe_id)).color(theme.nav));
                }
            }
            if let Some(t) = text {
                col = col.child(lbl(t.as_str()).color(theme.text));
            }
        }

        BookPage::Smelting { recipe_id, text } => {
            // Patchouli PageSmelting: bg UV(11,71) 96×24, input at +4,+4,
            // output at +76,+4.
            let rx0 = PAGE_W / 2.0 - 49.0;
            let ry = 12.0;
            match recipes.get(recipe_id.as_str()) {
                Some(RecipeVis::Smelting { input, output }) => {
                    col = col.child(
                        title_lbl(pretty_item_name(output))
                            .color(theme.title)
                            .h(10.0 * sy)
                            .align(Align::Center),
                    );
                    bg_sprites.push(BgSprite {
                        sheet: SpriteSheet::Crafting,
                        u: 11.0,
                        v: 71.0,
                        uw: 96.0,
                        uh: 24.0,
                        x: ox + rx0 * sx,
                        y: oy + ry * sy,
                        w: 96.0 * sx,
                        h: 24.0 * sy,
                    });
                    let icon_s = 16.0 * sx.min(sy);
                    overlays.push(OverlayCmd::McItem {
                        item_id: input.clone(),
                        x: ox + (rx0 + 4.0) * sx,
                        y: oy + (ry + 4.0) * sy,
                        w: icon_s,
                        h: icon_s,
                    });
                    // Center: the furnace itself (Patchouli's "toast symbol").
                    overlays.push(OverlayCmd::McItem {
                        item_id: "minecraft:furnace".into(),
                        x: ox + (rx0 + 40.0) * sx,
                        y: oy + (ry + 4.0) * sy,
                        w: icon_s,
                        h: icon_s,
                    });
                    overlays.push(OverlayCmd::McItem {
                        item_id: output.clone(),
                        x: ox + (rx0 + 76.0) * sx,
                        y: oy + (ry + 4.0) * sy,
                        w: icon_s,
                        h: icon_s,
                    });
                    // Spacer from title end (10) to below the furnace strip (ry+24+4).
                    col = col.child(widget::spacer().h((ry + 24.0 + 4.0 - 10.0) * sy));
                }
                _ => {
                    col = col.child(lbl(format!("[Recipe: {}]", recipe_id)).color(theme.nav));
                }
            }
            if let Some(t) = text {
                col = col.child(lbl(t.as_str()).color(theme.text));
            }
        }

        BookPage::Image {
            texture,
            title,
            text,
            ..
        } => {
            if let Some(t) = title {
                col = col.child(title_lbl(t.as_str()).color(theme.title));
            }
            col = col.child(
                widget::image(texture)
                    .region([0.0, 0.0, 80.0, 80.0])
                    .stretch(Stretch::Fill)
                    .w(80.0)
                    .h(80.0),
            );
            if let Some(t) = text {
                col = col.child(lbl(t.as_str()).color(theme.text));
            }
        }

        BookPage::Svg { data, title, text } => {
            let top = if let Some(t) = title {
                col = col.child(title_lbl(t.as_str()).color(theme.title));
                12.0
            } else {
                0.0
            };
            overlays.push(OverlayCmd::Svg {
                data: data.clone(),
                x: ox + (PAGE_W / 2.0 - 32.0) * sx,
                y: oy + top * sy,
                w: 64.0 * sx,
                h: 64.0 * sy,
            });
            col = col.child(widget::spacer().h(68.0 * sy));
            if let Some(t) = text {
                col = col.child(lbl(t.as_str()).color(theme.text));
            }
        }

        BookPage::CustomText { text, font, color } => {
            // A resource-pack font (assets/<ns>/font/<name>.json — a `ttf`
            // provider, say), at `size_px` for its 9-pixel line.
            col = col.child(
                lbl(text.as_str())
                    .font(font.font_id.as_str())
                    .font_scale(text_scale() * font.size_px / yog_ui::text::LINE_H)
                    .color(*color),
            );
        }

        BookPage::Relations { entries, text } => {
            if let Some(t) = text {
                col = col.child(lbl(t.as_str()).color(theme.text));
            }
            col = col.child(title_lbl("See also:").color(theme.title));
            for e in entries {
                col = col.child(lbl(format!("• {}", e)).color(theme.nav));
            }
        }

        BookPage::Entity {
            entity_type,
            name,
            text,
        } => {
            let display = name.as_deref().unwrap_or(entity_type.as_str());
            col = col.child(title_lbl(display).color(theme.title));
            overlays.push(OverlayCmd::Entity {
                entity: entity_type.clone(),
                x: ox + (PAGE_W / 2.0 - 40.0) * sx,
                y: oy + 12.0 * sy,
                w: 80.0 * sx,
                h: 80.0 * sy,
            });
            col = col.child(widget::spacer().h(84.0 * sy));
            if let Some(t) = text {
                col = col.child(lbl(t.as_str()).color(theme.text));
            }
        }

        BookPage::Pattern {
            op_id,
            input,
            output,
            text,
            ..
        } => {
            col = col.child(title_lbl(op_id.as_str()).color(theme.title));
            col = col.child(lbl(format!("{} → {}", input, output)).color(theme.nav));
            if !text.is_empty() {
                col = col.child(lbl(text.as_str()).color(theme.text));
            }
        }

        BookPage::Empty => {}
    }

    col
}

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

    fn book(pages: Vec<BookPage>) -> Book {
        let mut book = Book::new("t:b", "B");
        book.entries.push(BookEntry { id: "e".into(), name: "E".into(), pages, ..Default::default() });
        book
    }

    #[test]
    fn long_text_flows_onto_new_pages() {
        let long = "word ".repeat(400);
        let flowed = flow(book(vec![BookPage::Text { text: long.clone(), title: None }, BookPage::Empty]));
        let pages = &flowed.entries[0].pages;
        assert!(pages.len() > 3, "{}", pages.len());
        assert!(matches!(pages.last(), Some(BookPage::Empty)), "what came after stays after");
        let line_h = yog_ui::text::LINE_H + yog_ui::text::LINE_GAP;
        for p in &pages[..pages.len() - 1] {
            let BookPage::Text { text, .. } = p else { panic!("{p:?}") };
            assert!(text.lines().count() as f32 * line_h <= BODY_H + 1.0);
            assert!(text.lines().all(|l| yog_ui::text::str_width(l, 1.0) <= TEXT_W));
        }
        let words: usize = pages.iter().map(|p| match p {
            BookPage::Text { text, .. } => text.split_whitespace().count(),
            _ => 0,
        }).sum();
        assert_eq!(words, 400, "nothing lost");
    }

    #[test]
    fn edits_keep_the_reader_where_they_were() {
        let mut b = book(vec![BookPage::Text { text: "one".into(), title: None }, BookPage::Text { text: "two".into(), title: None }]);
        b.categories.push(crate::BookCategory { id: "c".into(), name: "C".into(), ..Default::default() });
        b.entries[0].category = "c".into();
        let mut r = BookRenderer::new(b.clone());
        assert_eq!(r.shown(), None, "opens on the landing page");
        assert!(r.show("e", 1));
        assert_eq!(r.shown(), Some(("e".into(), 1)));
        r.prepare(800.0, 600.0);
        assert!(r.ui.is_some());

        b.entries[0].pages.push(BookPage::Empty);
        r.set_book(b);
        assert_eq!(r.shown(), Some(("e".into(), 1)));
        assert!(!r.show("missing", 0));
    }

    #[test]
    fn short_text_stays_put() {
        let pages = vec![BookPage::Text { text: "Hello".into(), title: Some("T".into()) }];
        let flowed = flow(book(pages.clone()));
        assert_eq!(flowed.entries[0].pages.len(), 1);
    }
}