odox-ui 0.5.0

//! The window around a document: opening one, saving it, saying what went
//! wrong, and the menu and keys that are the same in all three applications.
//!
//! The shell owns every read from and write to the disk. A view is handed
//! bytes and hands bytes back, so the one place that knows a path is here, and
//! so is the one place that asks before throwing changes away.
//
// Author: David M. Anderson
// Built with AI assistance (Claude, Anthropic)

use std::path::{Path, PathBuf};

use eframe::egui::{self, Key, KeyboardShortcut, Modifiers, Ui};
use odox_core::Document;

use crate::edit::{Caret, Editing};
use crate::find_bar::{FindBar, Step};
use crate::i18n::{fill, t};
use crate::settings::Settings;

/// What an application tells the shell about itself.
///
/// The identifiers are not translated: a desktop entry, a window class and a file
/// extension are the same in every language.
pub struct Product {
    /// The binary's name, which is also the `.desktop` entry's basename and the
    /// Wayland application id. The compositor matches the three to find the
    /// window's icon, so they have to agree.
    pub id: &'static str,
    /// The file extension the application opens.
    pub extension: &'static str,
    /// What the file dialog and the empty window call that kind of file.
    ///
    /// The English, which is the message id: it is looked up through [`t`] where
    /// it is shown, because a `Product` is built before `run` puts a catalogue in
    /// force and a translation looked up here would be the English every time.
    pub format: &'static str,
    /// The application's icon directory, for the one platform with no other way
    /// to give a window its icon.
    ///
    /// Windows takes a window's icon from a resource compiled into the
    /// executable, and there is no resource compiler in this build, so the file
    /// travels in the binary instead. Empty everywhere else, and empty in a
    /// build made from a published crate, where the file is not there to stage:
    /// see each application's `build.rs`.
    pub icon: &'static [u8],
}

/// What the shell needs from the view that draws a particular format.
pub trait View {
    /// Take a document's bytes. The path is for the window's title and for
    /// reloading, and is never read from here: this is given the bytes.
    ///
    /// The context is for the work a new document makes necessary before it can
    /// be drawn — loading the font faces its styles name, which rebuilds egui's
    /// glyph atlas and so belongs to opening rather than to drawing.
    ///
    /// # Errors
    ///
    /// A message to show the person, already translated.
    fn open(&mut self, ctx: &egui::Context, bytes: &[u8], path: &Path) -> Result<(), String>;

    /// Forget the document.
    fn close(&mut self);

    /// Whether a document is open.
    fn is_open(&self) -> bool;

    /// What the document calls itself, for the window's title.
    fn title(&self) -> Option<String>;

    /// The document, for saving and for undo. `None` while nothing is open.
    fn document_mut(&mut self) -> Option<&mut Document>;

    /// The content tree was replaced under the document, by undo or redo.
    /// A view that derives anything from the tree rebuilds it here.
    fn reindex(&mut self) {}

    /// Where the caret is, for a view that has one, so that an undo can
    /// bring it back there.
    fn caret(&self) -> Option<Caret> {
        None
    }

    /// Put the caret where an undo or a redo says it was, after
    /// [`Self::reindex`].
    fn restore_caret(&mut self, _caret: Caret) {}

    /// Whether the view is showing the document full screen and nothing else,
    /// as a slideshow does: the shell takes its menu, its panels and its keys
    /// away until it is not.
    fn presenting(&self) -> bool {
        false
    }

    /// Look for text in the document and keep what is found, to draw
    /// highlighted; an empty query forgets the search. Answers how many
    /// matches there are. Asked again whenever the query or the text changes,
    /// and the current match is kept if there are still that many.
    fn find(&mut self, _query: &str) -> usize {
        0
    }

    /// Make one of the matches the current one, and bring it into view.
    fn show_match(&mut self, _index: usize) {}

    /// Draw the document. The shell has already put a scroll area or a panel
    /// around whatever this needs. The editing state says whether edit mode is
    /// on and takes the snapshot an edit records before it changes the tree.
    fn central(&mut self, ui: &mut Ui, zoom: f32, editing: &mut Editing);

    /// Draw the panel beside the document — an outline, a sheet list, a slide
    /// list — and answer whether there is one to draw.
    fn side(&mut self, _ui: &mut Ui) -> bool {
        false
    }

    /// Add the application's own items to the View menu.
    fn view_menu(&mut self, _ui: &mut Ui) {}
}

/// The window: chrome, keys, errors, and one view inside it.
pub struct Shell<V: View> {
    view: V,
    product: Product,
    path: Option<PathBuf>,
    error: Option<String>,
    /// Set when a document was opened during a frame, cleared at the end of it.
    ///
    /// See the comment where it is read in [`eframe::App::ui`].
    settling: bool,
    zoom: f32,
    show_side: bool,
    editing: Editing,
    settings: Settings,
    find: FindBar,
    /// Whether the window has been made to fill the screen for a slideshow.
    screen: Screen,
    /// What was asked for while the document had unsaved changes, waiting on
    /// the person's answer.
    pending: Option<Pending>,
    /// The person has answered that the window may close, so the next request
    /// to close it is not asked about again.
    closing: bool,
}

/// How much of the screen the window has.
#[derive(Clone, Copy, PartialEq, Eq)]
enum Screen {
    Window,
    Full,
}

/// Something that would throw away unsaved changes, held until the person
/// says whether to save them, discard them, or not do it after all.
#[derive(Clone, Copy)]
enum Pending {
    AskForAFile,
    Reload,
    Close,
    Quit,
}

/// How far a zoom step moves, and the limits.
const ZOOM_STEP: f32 = 1.1;
const ZOOM_MIN: f32 = 0.4;
const ZOOM_MAX: f32 = 4.0;

impl<V: View> Shell<V> {
    /// A window with nothing open.
    pub fn new(product: Product, view: V) -> Self {
        Self {
            view,
            product,
            path: None,
            error: None,
            settling: false,
            zoom: 1.0,
            show_side: true,
            editing: Editing::default(),
            settings: Settings::load(),
            find: FindBar::default(),
            screen: Screen::Window,
            pending: None,
            closing: false,
        }
    }

    /// Open a path, reading it here so that the view never touches the file
    /// system.
    ///
    /// Unconditional: the document that was open is gone. [`Self::request`]
    /// is the way in for anything a person asked for, and it asks first.
    pub fn open(&mut self, ctx: &egui::Context, path: &Path) {
        match std::fs::read(path) {
            Ok(bytes) => match self.view.open(ctx, &bytes, path) {
                Ok(()) => {
                    self.path = Some(path.to_path_buf());
                    self.error = None;
                    self.settling = true;
                    self.editing.reset();
                    self.editing.on = self.settings.open_in_edit_mode;
                }
                Err(message) => {
                    self.view.close();
                    self.path = None;
                    self.error = Some(message);
                    self.editing.reset();
                }
            },
            Err(e) => {
                self.error = Some(fill(
                    t("{file} could not be read: {reason}"),
                    &[("file", &name_of(path)), ("reason", &e.to_string())],
                ));
            }
        }
    }

    /// Do something that would lose unsaved changes, or ask first.
    ///
    /// With nothing to lose it is done now. Otherwise it waits on the answer
    /// to the question [`Self::ask_about_changes`] draws, and is done, or not,
    /// from there.
    fn request(&mut self, ctx: &egui::Context, action: Pending) {
        if self.editing.modified() {
            self.pending = Some(action);
        } else {
            self.perform(ctx, action);
        }
    }

    fn perform(&mut self, ctx: &egui::Context, action: Pending) {
        match action {
            Pending::AskForAFile => self.ask_for_a_file(ctx),
            Pending::Reload => {
                if let Some(path) = self.path.clone() {
                    self.open(ctx, &path);
                }
            }
            Pending::Close => self.close(),
            Pending::Quit => {
                self.closing = true;
                ctx.send_viewport_cmd(egui::ViewportCommand::Close);
            }
        }
    }

    fn ask_for_a_file(&mut self, ctx: &egui::Context) {
        let file = rfd::FileDialog::new()
            .add_filter(t(self.product.format), &[self.product.extension])
            .pick_file();
        if let Some(path) = file {
            self.open(ctx, &path);
        }
    }

    fn close(&mut self) {
        self.view.close();
        self.path = None;
        self.error = None;
        self.editing.reset();
    }

    /// Write the document over the file it was read from.
    ///
    /// Answers whether it was written, which is what a pending action waits
    /// on: a save that failed is not a reason to go on and close the window.
    fn save(&mut self) -> bool {
        match self.path.clone() {
            Some(path) => self.write_to(&path),
            None => self.save_as(),
        }
    }

    /// Ask where to write the document, and write it there.
    fn save_as(&mut self) -> bool {
        let mut dialog =
            rfd::FileDialog::new().add_filter(t(self.product.format), &[self.product.extension]);
        if let Some(path) = &self.path {
            dialog = dialog.set_file_name(name_of(path));
            if let Some(directory) = path.parent() {
                dialog = dialog.set_directory(directory);
            }
        }
        let Some(mut path) = dialog.save_file() else {
            return false;
        };
        // A name typed without an extension gets the format's, because the
        // desktop finds the application by the extension and a file without
        // one opens nowhere.
        if path.extension().is_none() {
            path.set_extension(self.product.extension);
        }
        if self.write_to(&path) {
            self.path = Some(path);
            true
        } else {
            false
        }
    }

    /// Serialize the document, prove it reads back, and put it on disk.
    /// DESIGN.md §11.
    fn write_to(&mut self, path: &Path) -> bool {
        let Some(document) = self.view.document_mut() else {
            return false;
        };
        let written = document
            .write_verified()
            .map_err(|e| e.to_string())
            .and_then(|bytes| replace_file(path, &bytes).map_err(|e| e.to_string()));
        match written {
            Ok(()) => {
                self.editing.mark_saved();
                self.error = None;
                true
            }
            Err(reason) => {
                self.error = Some(fill(
                    t("{file} could not be saved: {reason}"),
                    &[("file", &name_of(path)), ("reason", &reason)],
                ));
                false
            }
        }
    }

    fn undo(&mut self) {
        let caret = self.view.caret();
        if let Some(document) = self.view.document_mut()
            && let Some((previous, caret)) = self.editing.undo(&document.content, caret)
        {
            document.content = previous;
            self.after_history(caret);
        }
    }

    fn redo(&mut self) {
        let caret = self.view.caret();
        if let Some(document) = self.view.document_mut()
            && let Some((next, caret)) = self.editing.redo(&document.content, caret)
        {
            document.content = next;
            self.after_history(caret);
        }
    }

    /// The tree was replaced by undo or redo: the view rebuilds from it, and
    /// the caret goes where the state it returned to had it.
    fn after_history(&mut self, caret: Option<Caret>) {
        self.view.reindex();
        if let Some(caret) = caret {
            self.view.restore_caret(caret);
        }
    }

    /// The window's title: the document's own title, or its file name, marked
    /// when it has changes the file does not.
    fn window_title(&self) -> String {
        let document = self
            .view
            .title()
            .filter(|title| !title.trim().is_empty())
            .or_else(|| self.path.as_deref().map(name_of));
        let mark = if self.editing.modified() { "● " } else { "" };
        match document {
            Some(name) => format!("{mark}{name} — {}", self.product.id),
            None => self.product.id.to_owned(),
        }
    }

    /// The question a pending action waits on, drawn over the window.
    fn ask_about_changes(&mut self, ctx: &egui::Context) {
        if self.pending.is_none() {
            return;
        }
        let file = self.path.as_deref().map(name_of).unwrap_or_default();
        // Save, discard, or neither. `None` until the person answers.
        let mut answer = None;
        egui::Modal::new(egui::Id::new("unsaved")).show(ctx, |ui| {
            ui.heading(fill(t("Save changes to {file}?"), &[("file", &file)]));
            ui.label(t("The document has changes that are not saved."));
            ui.add_space(8.0);
            ui.horizontal(|ui| {
                if ui.button(t("Save")).clicked() || ui.input(|i| i.key_pressed(Key::Enter)) {
                    answer = Some(Some(true));
                }
                if ui.button(t("Discard")).clicked() {
                    answer = Some(Some(false));
                }
                if ui.button(t("Cancel")).clicked() || ui.input(|i| i.key_pressed(Key::Escape)) {
                    answer = Some(None);
                }
            });
        });
        let Some(answer) = answer else {
            return;
        };
        let Some(action) = self.pending.take() else {
            return;
        };
        match answer {
            Some(true) => {
                if self.save() {
                    self.perform(ctx, action);
                }
            }
            Some(false) => self.perform(ctx, action),
            None => {}
        }
    }
}

impl<V: View> eframe::App for Shell<V> {
    fn ui(&mut self, ui: &mut Ui, _frame: &mut eframe::Frame) {
        let ctx = ui.ctx().clone();

        // The window's own close button, which asks like Quit does. The close
        // is cancelled and asked for again when the person has answered.
        if ctx.input(|i| i.viewport().close_requested()) && !self.closing && self.editing.modified()
        {
            ctx.send_viewport_cmd(egui::ViewportCommand::CancelClose);
            self.pending = Some(Pending::Quit);
        }

        // Nothing is taken while the question is up, so an answer typed at it
        // reaches it and nothing else; and nothing is taken while a text field
        // has the focus, so Ctrl+Z inside a cell undoes the typing and not the
        // document. The page editor is not such a field: its typing is the
        // document's, and so is its Ctrl+Z.
        let presenting = self.view.presenting();
        let screen = if presenting {
            Screen::Full
        } else {
            Screen::Window
        };
        if screen != self.screen {
            self.screen = screen;
            ctx.send_viewport_cmd(egui::ViewportCommand::Fullscreen(presenting));
        }
        self.editing.asking = self.pending.is_some();
        let field_focused = ctx
            .memory(egui::Memory::focused)
            .is_some_and(|id| id != crate::flow_model::page_editor_id());
        if self.pending.is_none() && !presenting {
            self.find_keys(&ctx);
        }
        if self.pending.is_none() && !field_focused && !presenting {
            self.keys(&ctx);
        }

        // A document macOS asked for, which reaches here rather than through
        // the command line. Taken rather than read, so one event opens one
        // document instead of reopening it on every frame after.
        #[cfg(target_os = "macos")]
        if let Some(path) = crate::opened_document::taken() {
            self.open(&ctx, &path);
        }

        ctx.send_viewport_cmd(egui::ViewportCommand::Title(self.window_title()));

        if !presenting {
            self.chrome(ui, &ctx);
        }

        // **A document opened during this frame is not drawn until the next
        // one.** Opening one registers the font families it names, and
        // `Context::set_fonts` takes effect at the start of the following pass,
        // so laying the document out now would ask for a family the definitions
        // still in force do not carry. egui does not fall back for that: it
        // panics, and a panic inside the macOS event callback cannot unwind, so
        // the process aborts.
        //
        // Every way of opening a document but one goes through a frame:
        // Ctrl+O, Reload, and the Apple Event. The exception is the path on the
        // command line, which is opened in eframe's creation closure before any
        // pass has begun, and is why this went unnoticed until a runner opened a
        // document through Launch Services. `tests/fonts_midframe.rs` pins the
        // hazard.
        //
        // One frame, and the repaint is asked for rather than waited for, so
        // the document appears immediately rather than when the pointer next
        // moves.
        let settling = self.settling;
        if settling {
            self.settling = false;
            ctx.request_repaint();
        }

        let central = if presenting {
            egui::CentralPanel::default().frame(egui::Frame::NONE.fill(egui::Color32::BLACK))
        } else {
            egui::CentralPanel::default_margins()
        };
        central.show(ui, |ui| {
            if settling {
                // Deliberately blank, and for one frame. Drawing the
                // nothing-open message here instead would flash it between a
                // double-click and the document.
            } else if self.view.is_open() {
                self.view.central(ui, self.zoom, &mut self.editing);
            } else {
                self.nothing_open(ui);
            }
        });

        self.ask_about_changes(&ctx);
    }
}

impl<V: View> Shell<V> {
    /// What is around the document: the menu, the search bar, an error and
    /// the view's side panel.
    fn chrome(&mut self, ui: &mut Ui, ctx: &egui::Context) {
        egui::Panel::top("menu").show(ui, |ui| self.menu_bar(ui, ctx));
        self.search(ui);

        if let Some(message) = self.error.clone() {
            egui::Panel::bottom("error").show(ui, |ui| {
                ui.horizontal_wrapped(|ui| {
                    ui.colored_label(ui.visuals().error_fg_color, "\u{26a0}");
                    ui.label(message);
                    if ui.button(t("Dismiss")).clicked() {
                        self.error = None;
                    }
                });
            });
        }

        if self.show_side && self.view.is_open() {
            // The view answers whether it has anything to put beside the
            // document, so that a format with nothing there gets no empty panel
            // rather than a blank one a person has to close.
            let mut drew = false;
            egui::Panel::left("side")
                .resizable(true)
                .default_size(220.0)
                .min_size(120.0)
                .show(ui, |ui| {
                    egui::ScrollArea::both().show(ui, |ui| {
                        drew = self.view.side(ui);
                    });
                });
            if !drew {
                self.show_side = false;
            }
        }
    }

    /// The menu bar: the File and Edit menus that are the same in every
    /// application, the View menu with the application's own items after the
    /// shell's, and on the right what mode the window is in and how far it is
    /// zoomed.
    fn menu_bar(&mut self, ui: &mut Ui, ctx: &egui::Context) {
        egui::MenuBar::new().ui(ui, |ui| {
            ui.menu_button(t("File"), |ui| {
                if ui.button(t("Open…")).clicked() {
                    ui.close();
                    self.request(ctx, Pending::AskForAFile);
                }
                let open = self.path.is_some();
                if ui
                    .add_enabled(open, egui::Button::new(t("Reload")))
                    .clicked()
                {
                    ui.close();
                    self.request(ctx, Pending::Reload);
                }
                if ui
                    .add_enabled(open, egui::Button::new(t("Close")))
                    .clicked()
                {
                    ui.close();
                    self.request(ctx, Pending::Close);
                }
                ui.separator();
                let modified = open && self.editing.modified();
                if ui
                    .add_enabled(modified, egui::Button::new(t("Save")))
                    .clicked()
                {
                    ui.close();
                    self.save();
                }
                if ui
                    .add_enabled(open, egui::Button::new(t("Save as…")))
                    .clicked()
                {
                    ui.close();
                    self.save_as();
                }
                ui.separator();
                if ui.button(t("Quit")).clicked() {
                    ui.close();
                    self.request(ctx, Pending::Quit);
                }
            });
            ui.menu_button(t("Edit"), |ui| self.edit_menu(ui));
            ui.menu_button(t("View"), |ui| {
                if ui.button(t("Zoom in")).clicked() {
                    self.zoom = (self.zoom * ZOOM_STEP).min(ZOOM_MAX);
                }
                if ui.button(t("Zoom out")).clicked() {
                    self.zoom = (self.zoom / ZOOM_STEP).max(ZOOM_MIN);
                }
                if ui.button(t("Actual size")).clicked() {
                    self.zoom = 1.0;
                }
                ui.separator();
                ui.checkbox(&mut self.show_side, t("Show the side panel"));
                self.view.view_menu(ui);
            });
            ui.with_layout(egui::Layout::right_to_left(egui::Align::Center), |ui| {
                #[allow(clippy::cast_possible_truncation, clippy::cast_sign_loss)]
                let percent = (self.zoom * 100.0).round() as u32;
                ui.label(fill(t("{percent}%"), &[("percent", &percent.to_string())]));
                if self.editing.on && self.view.is_open() {
                    ui.separator();
                    ui.strong(t("Editing"));
                }
            });
        });
    }

    /// The Edit menu: undo and redo, search, and the two preferences about
    /// editing.
    fn edit_menu(&mut self, ui: &mut Ui) {
        if ui
            .add_enabled(self.editing.can_undo(), egui::Button::new(t("Undo")))
            .clicked()
        {
            ui.close();
            self.undo();
        }
        if ui
            .add_enabled(self.editing.can_redo(), egui::Button::new(t("Redo")))
            .clicked()
        {
            ui.close();
            self.redo();
        }
        ui.separator();
        if ui
            .add_enabled(self.view.is_open(), egui::Button::new(t("Find…")))
            .clicked()
        {
            ui.close();
            self.find.open();
        }
        ui.separator();
        ui.checkbox(&mut self.editing.on, t("Edit mode"));
        if ui
            .checkbox(
                &mut self.settings.open_in_edit_mode,
                t("Open documents in edit mode"),
            )
            .changed()
            && let Err(e) = self.settings.save()
        {
            self.error = Some(fill(
                t("The setting could not be saved: {reason}"),
                &[("reason", &e.to_string())],
            ));
        }
    }

    /// The search bar, when it is open, and the view's matches kept in step
    /// with what it says and with the document.
    fn search(&mut self, ui: &mut Ui) {
        if !self.find.is_open() || !self.view.is_open() {
            if self.find.searching {
                self.find.searching = false;
                self.find.count = 0;
                self.view.find("");
            }
            return;
        }
        let mut step = None;
        egui::Panel::top("find").show(ui, |ui| step = self.find.ui(ui));
        if !self.find.is_open() {
            return;
        }
        if let Some(query_changed) = self.find.due(self.editing.revision()) {
            self.find.searching = true;
            self.find.count = self.view.find(self.find.query());
            if query_changed {
                self.find.current = 0;
                if self.find.count > 0 {
                    self.view.show_match(0);
                }
            } else {
                self.find.current = self.find.current.min(self.find.count.saturating_sub(1));
            }
        }
        if let Some(step) = step {
            self.step_match(step);
        }
    }

    fn step_match(&mut self, step: Step) {
        let count = self.find.count;
        if count == 0 {
            return;
        }
        self.find.current = match step {
            Step::Next => (self.find.current + 1) % count,
            Step::Previous => (self.find.current + count - 1) % count,
        };
        self.view.show_match(self.find.current);
    }

    /// What the window says before a document is opened.
    fn nothing_open(&mut self, ui: &mut Ui) {
        ui.vertical_centered(|ui| {
            ui.add_space(ui.available_height() * 0.3);
            // The application's own name, which is not translated, and the name
            // of the format, which is: Comma's German has said
            // `OpenDocument-Tabellendokument` since it shipped.
            ui.heading(self.product.id);
            ui.label(t(self.product.format));
            ui.add_space(12.0);
            if ui.button(t("Open a document…")).clicked() {
                let ctx = ui.ctx().clone();
                self.ask_for_a_file(&ctx);
            }
        });
    }

    /// Ctrl+F and F3, which a text field has no use for and so are taken
    /// whether or not one has the keyboard.
    fn find_keys(&mut self, ctx: &egui::Context) {
        if !self.view.is_open() {
            return;
        }
        let pressed = |modifiers, key| {
            ctx.input_mut(|input| input.consume_shortcut(&KeyboardShortcut::new(modifiers, key)))
        };
        if pressed(Modifiers::COMMAND, Key::F) {
            self.find.open();
        }
        if pressed(Modifiers::SHIFT, Key::F3) {
            self.step_match(Step::Previous);
        }
        if pressed(Modifiers::NONE, Key::F3) {
            if self.find.is_open() {
                self.step_match(Step::Next);
            } else {
                self.find.open();
            }
        }
    }

    fn keys(&mut self, ctx: &egui::Context) {
        let pressed = |key| {
            ctx.input_mut(|input| {
                input.consume_shortcut(&KeyboardShortcut::new(Modifiers::COMMAND, key))
            })
        };
        let shifted = |key| {
            ctx.input_mut(|input| {
                input.consume_shortcut(&KeyboardShortcut::new(
                    Modifiers::COMMAND | Modifiers::SHIFT,
                    key,
                ))
            })
        };
        // The shifted pair first, because Ctrl+Shift+S is not Ctrl+S.
        if shifted(Key::S) {
            self.save_as();
        }
        if shifted(Key::Z) || pressed(Key::Y) {
            self.redo();
        }
        if pressed(Key::S) && self.editing.modified() {
            self.save();
        }
        if pressed(Key::Z) {
            self.undo();
        }
        if pressed(Key::E) && self.view.is_open() {
            self.editing.on = !self.editing.on;
        }
        if pressed(Key::O) {
            self.request(ctx, Pending::AskForAFile);
        }
        if pressed(Key::R) {
            self.request(ctx, Pending::Reload);
        }
        if pressed(Key::W) {
            self.request(ctx, Pending::Close);
        }
        // `Plus` is what the key sends with shift held and `Equals` without, and
        // a person pressing the same physical key means the same thing either way.
        if pressed(Key::Plus) || pressed(Key::Equals) {
            self.zoom = (self.zoom * ZOOM_STEP).min(ZOOM_MAX);
        }
        if pressed(Key::Minus) {
            self.zoom = (self.zoom / ZOOM_STEP).max(ZOOM_MIN);
        }
        if pressed(Key::Num0) {
            self.zoom = 1.0;
        }
    }
}

/// A file's name without its directory, for a title or a message.
fn name_of(path: &Path) -> String {
    path.file_name().map_or_else(
        || path.display().to_string(),
        |name| name.to_string_lossy().into_owned(),
    )
}

/// Put bytes where a file is, so that the file is either what it was or what
/// was written and never half of each.
///
/// The bytes go into a temporary file beside the target, which is then renamed
/// over it: a rename within one filesystem is atomic, and a crash before it
/// leaves the original untouched and a stray `.part` file to delete. The
/// target's permissions are carried over, because the rename replaces the
/// inode and would otherwise leave a document writable by whoever the
/// temporary file's default said.
///
/// **macOS writes in place.** The sandbox grant a person gives by choosing a
/// file covers the file and not its directory, so a temporary file beside it
/// is refused with *Operation not permitted*. The safe rewrite there goes
/// through `NSItemReplacementDirectory` and `replaceItemAtURL:`, which is a
/// platform arm this build cannot verify and does not carry yet.
fn replace_file(path: &Path, bytes: &[u8]) -> std::io::Result<()> {
    #[cfg(target_os = "macos")]
    {
        std::fs::write(path, bytes)
    }
    #[cfg(not(target_os = "macos"))]
    {
        use std::io::Write as _;
        let temporary = path.with_file_name(format!("{}.part", name_of(path)));
        let mut file = std::fs::File::create(&temporary)?;
        let written = file
            .write_all(bytes)
            .and_then(|()| file.sync_all())
            .and_then(|()| match std::fs::metadata(path) {
                Ok(existing) => std::fs::set_permissions(&temporary, existing.permissions()),
                Err(_) => Ok(()),
            })
            .and_then(|()| std::fs::rename(&temporary, path));
        if written.is_err() {
            // Best effort: the error the person sees is the one that stopped
            // the write, not the one about tidying up after it.
            let _ = std::fs::remove_file(&temporary);
        }
        written
    }
}

/// Open a window for a product, with the paths given on the command line.
///
/// # Errors
///
/// The window could not be created, which is eframe's answer and not this
/// application's.
pub fn run<V: View + 'static>(
    product: Product,
    build: impl FnOnce(&egui::Context) -> V + 'static,
) -> eframe::Result {
    crate::i18n::start();

    let id = product.id;
    let viewport = egui::ViewportBuilder::default()
        // How a Wayland compositor finds the window's icon and its name: it
        // matches this against the basename of the `.desktop` entry.
        .with_app_id(id)
        .with_title(id)
        .with_inner_size([1000.0, 760.0])
        .with_min_inner_size([420.0, 320.0]);

    // **Naming no icon is not neutral on macOS, and it costs the Dock.** The
    // bundle carries the `.icns` and `CFBundleIconFile` points at it, which is
    // where a macOS application's icon comes from, so this looks like an arm
    // with nothing to do. It has something to do: eframe substitutes its own
    // logo for a viewport that names no icon and hands that to
    // `setApplicationIconImage:`, which outranks the bundle. Finder, Launch
    // Services and every API still resolve the right drawing, so nothing short
    // of a person looking at the Dock finds it, and it caught two of the
    // sibling applications before it was written down.
    //
    // An empty `IconData` declines the icon rather than replacing it:
    // eframe turns one into `None` and the macOS arm only calls the selector
    // where there is an image. Handing the drawing over again would also work
    // and would carry a second copy of it in the binary to overwrite the
    // bundle's with a worse-scaled equal.
    //
    // One line for three windows, because all three come through here.
    // **Unverified from Linux**: it compiles for the target and nothing else
    // about it can be checked without looking at a Dock.
    #[cfg(target_os = "macos")]
    let viewport = viewport.with_icon(egui::IconData::default());

    // Windows, which is the other half of the same question. Here an icon has
    // to be given, because the shell reads one out of a resource and this build
    // compiles no resource.
    #[cfg(target_os = "windows")]
    let viewport = match window_icon(product.icon) {
        Some(icon) => viewport.with_icon(icon),
        None => viewport,
    };

    let options = eframe::NativeOptions {
        viewport,
        ..eframe::NativeOptions::default()
    };

    // Before `eframe`, because macOS dispatches the document that launched this
    // process before the creation closure is reached and AppKit's own handler
    // refuses it there. `opened_document` has the measurement.
    #[cfg(target_os = "macos")]
    crate::opened_document::watch();

    let first = std::env::args_os().nth(1).map(PathBuf::from);
    eframe::run_native(
        id,
        options,
        Box::new(move |cc| {
            crate::system_theme::follow(&cc.egui_ctx);
            // Now that there is a context, a document that arrives has
            // somewhere to wake.
            #[cfg(target_os = "macos")]
            crate::opened_document::wake_with(&cc.egui_ctx);
            let mut shell = Shell::new(product, build(&cc.egui_ctx));
            if let Some(path) = first {
                shell.open(&cc.egui_ctx, &path);
            }
            Ok(Box::new(shell))
        }),
    )
}

/// The 64-pixel entry of an icon directory, as a window icon.
///
/// 64 because it is the largest size no scaling has to enlarge, and every
/// smaller one the shell wants is a reduction of it. `None` where the directory
/// is empty, which is what a build from a published crate has, or where it does
/// not decode, which is a broken artefact rather than a reason to refuse to
/// open a window.
#[cfg(target_os = "windows")]
fn window_icon(bytes: &'static [u8]) -> Option<egui::IconData> {
    if bytes.is_empty() {
        return None;
    }
    let directory = ico::IconDir::read(std::io::Cursor::new(bytes)).ok()?;
    let entry = directory
        .entries()
        .iter()
        .find(|entry| entry.width() == 64)
        .or_else(|| directory.entries().last())?;
    let image = entry.decode().ok()?;
    Some(egui::IconData {
        width: image.width(),
        height: image.height(),
        rgba: image.rgba_data().to_vec(),
    })
}

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

    /// The document on disk is what was written, with the permissions it had,
    /// and nothing is left beside it.
    #[test]
    fn a_file_is_replaced_whole() {
        let directory = std::env::temp_dir().join(format!("odox-replace-{}", std::process::id()));
        std::fs::create_dir_all(&directory).expect("a scratch directory");
        let path = directory.join("document.odt");
        std::fs::write(&path, b"before").expect("the original");
        let mode = std::fs::metadata(&path)
            .expect("its metadata")
            .permissions();

        replace_file(&path, b"after").expect("the replacement");

        assert_eq!(std::fs::read(&path).expect("the file"), b"after");
        assert_eq!(
            std::fs::metadata(&path)
                .expect("its metadata")
                .permissions(),
            mode
        );
        let left: Vec<_> = std::fs::read_dir(&directory)
            .expect("the directory")
            .filter_map(Result::ok)
            .map(|entry| entry.file_name())
            .collect();
        assert_eq!(left, vec![std::ffi::OsString::from("document.odt")]);
        std::fs::remove_dir_all(directory).expect("tidy");
    }

    /// A target that cannot be written leaves nothing behind and says so.
    #[test]
    fn a_failed_write_leaves_no_part_file() {
        let directory = std::env::temp_dir().join(format!("odox-refuse-{}", std::process::id()));
        std::fs::create_dir_all(&directory).expect("a scratch directory");
        let missing = directory.join("nowhere").join("document.odt");
        assert!(replace_file(&missing, b"x").is_err());
        assert_eq!(
            std::fs::read_dir(&directory)
                .expect("the directory")
                .count(),
            0
        );
        std::fs::remove_dir_all(directory).expect("tidy");
    }
}