//! Selection and tree state (`CollectionsController`), and the pure
//! decisions a drop, a delete, or a hold-release comes down to.
//!
//! `decide_drop`, `decide_delete` and `decide_release` are kept apart from
//! the callbacks that call them for the reason the crate-level doc gives:
//! the gesture cannot be driven from a test, but the decision can.

use std::cell::RefCell;
use std::collections::BTreeSet;
use std::rc::Rc;

use dr_types::{CollectionId, ImageId};

use super::press::PressUndo;

/// TRACES: FR-CAT-5
/// How a run of grid ordinals is turned into the ids it names.
///
/// See [`CollectionsController::span_source`] for why this is supplied rather
/// than reached for.
type SpanIds = Rc<dyn Fn(usize, usize) -> Vec<ImageId>>;

/// Selection, drag, and tree state for the running window.
///
/// Everything is `RefCell` because Slint callbacks are `Fn`, not `FnMut`, and
/// they all run on the one event-loop thread — the same shape
/// [`crate::library_ui::LibraryController`] uses.
#[derive(Default)]
pub struct CollectionsController {
    /// Selected images, by catalog id. A `BTreeSet` rather than a `Vec` so
    /// membership tests are cheap during a rubber-band and the order a drop
    /// applies in is stable across runs.
    pub(super) selection: RefCell<BTreeSet<ImageId>>,
    /// Where a shift-click extends from. The last cell *clicked*, not the last
    /// added — extending from the far end of a previous range is not what the
    /// gesture means anywhere else.
    ///
    /// An **image ordinal in the library**, not a row of the loaded window.
    /// The grid is a window over the catalog, so row 7 names a different
    /// photograph after every scroll: held as a row, an anchor set before a
    /// scroll described a range from wherever that row had since drifted to.
    /// Selection is by id for the same reason (see the preamble); this is the
    /// same argument applied to the one index that has to survive a move.
    pub(super) anchor: RefCell<Option<usize>>,
    /// TRACES: FR-UI-2 | FR-UI-4
    /// TRACES: FR-CAT-7 | FR-UI-4
    /// A photograph the most recent press asked to take *out* of the
    /// selection, held until the release says the press was a tap.
    ///
    /// See [`Press::Deferred`] for why removal cannot happen on the press:
    /// this is the whole of what stops a drag of forty photographs carrying
    /// one. Overwritten by the next press and dropped by the drag that
    /// consumes it, so at most one is ever pending.
    pub(super) pending_toggle: std::cell::Cell<Option<ImageId>>,
    /// TRACES: FR-UI-4
    /// What the selection was immediately before the most recent press, so a
    /// gesture that turns out not to have been a press can put it back.
    ///
    /// A press has to act immediately — the drag that may follow reads the
    /// selection to build its payload, so deciding on release is too late.
    /// That is right for a drag and wrong for a pinch, which begins as an
    /// ordinary one-finger press and only becomes a pinch when the second
    /// finger lands. By then a cell has been selected that the user never
    /// meant to touch; they were reaching for the grid with two fingers.
    ///
    /// Cheap to keep: a few hundred ids at most, cloned once per press.
    pub(super) press_undo: RefCell<Option<PressUndo>>,
    /// Where the keyboard is, as an image ordinal.
    ///
    /// Distinct from the anchor, and it has to be: shift+arrow grows a range
    /// *from* the anchor *to* the cursor, so the two are the two ends and
    /// cannot be one field. `None` until the user has taken hold of the grid,
    /// so the first arrow press starts from what is on screen rather than
    /// jumping to the top of the library.
    pub(super) cursor: std::cell::Cell<Option<usize>>,
    /// Whether the press that began the current gesture carried ctrl or shift.
    ///
    /// A modified click is a *selection* gesture and must not also open the
    /// image: building a selection would otherwise throw the user into the
    /// develop view on the second ctrl-click. Slint does not report modifiers on
    /// `clicked`, so the press records them and the click consults this.
    pub(super) modified_press: std::cell::Cell<bool>,
    /// TRACES: FR-UI-2 | FR-UI-4
    /// Whether a tap in the grid selects rather than opens.
    ///
    /// Touch has no ctrl and no shift, so without a mode there is no way to
    /// select a second photograph: the first tap would open the first one. In
    /// this mode a plain press is reported as a ctrl-press and goes through the
    /// same [`apply_press`] as everything else — a separate touch policy would
    /// be a second copy of these rules to keep in step.
    pub(super) select_mode: std::cell::Cell<bool>,
    /// The timer that turns a held cell into a selection.
    ///
    /// Here rather than in `.slint` because Slint has no long-press gesture and
    /// a hand-rolled one would need a `Timer` element per visible cell — a
    /// hundred timers to answer a question about one finger. Held so that
    /// dropping it cancels: a press that ends, or is taken by the Flickable
    /// when the finger travels, must not arrive as a selection a moment later.
    pub(super) hold_timer: RefCell<Option<slint::Timer>>,
    /// Collection ids parallel to the sidebar's rows, so a hovered row index
    /// resolves to an id without another query.
    pub(super) row_ids: RefCell<Vec<CollectionId>>,
    /// Which rows are saved filters, so a drop onto one is refused *before* the
    /// release rather than after.
    pub(super) row_smart: RefCell<Vec<bool>>,
    /// Which rows have children, so the spring knows there is anything to open.
    pub(super) row_has_children: RefCell<Vec<bool>>,
    /// Collapsed collections, by id. Collapse is a view preference and
    /// deliberately not persisted to the catalog — it is not something to sync
    /// between devices.
    pub(super) collapsed: RefCell<std::collections::HashSet<CollectionId>>,
    /// Which collection scopes the grid. `None` is the whole library.
    pub(super) scope: RefCell<Option<CollectionId>>,
    /// The collection whose name is being edited in the sidebar, if any.
    ///
    /// Held here rather than in Slint because a rename can also be *started*
    /// from Rust — creating a collection opens its field — and because a
    /// commit that the catalog refuses has to leave the field open on the name
    /// the user typed rather than silently closing over a rejected edit.
    pub(super) renaming: RefCell<Option<CollectionId>>,
    /// Whether the grid is showing the trash rather than the library.
    ///
    /// Separate from `scope` because the trash is not a collection: its contents
    /// come from `trashed_at`, not from membership, and every other query in the
    /// library *excludes* exactly what this view exists to show. Folding it into
    /// `scope` as a sentinel id would put that inversion inside a type that
    /// means "a collection".
    pub(super) viewing_trash: std::cell::Cell<bool>,
    /// TRACES: FR-EXP-10
    /// The albums section below the tree, refreshed whenever the tree is:
    /// every open, scan, sync and merge that can change collections can
    /// change albums too. `None` until `lib.rs` wires it.
    pub(crate) albums: RefCell<Option<Rc<crate::albums_ui::AlbumsController>>>,
    /// The live drag: what it carries. Empty means no drag.
    pub(super) dragging: RefCell<Vec<ImageId>>,
    /// The collection being dragged, when the drag is a tree rearrangement
    /// rather than a filing of photographs.
    ///
    /// Set when the drag starts and read by the drop, the same way `dragging`
    /// works — the drop callback carries only the target's id, so what is being
    /// dropped has to be remembered rather than inspected.
    pub(super) dragging_collection: RefCell<Option<CollectionId>>,
    /// TRACES: FR-UI-3 | FR-UI-4
    /// The collection a hold has picked up, and the timer that picks it up.
    ///
    /// The tree is inside a Flickable, which claims any drag beginning inside
    /// it — so with a finger, a drag on a row is a scroll unless something says
    /// otherwise first. The hold is that something. It arms the drag *and*
    /// opens the row menu, and which of the two the user gets is decided by
    /// whether they then moved: the same fork the grid already uses to tell a
    /// hold-to-select from a drag-to-file.
    pub(super) lifted: RefCell<Option<CollectionId>>,
    /// Whether the lifted row's drag actually began. Reset by the press that
    /// arms the next one, so a release can tell a rearrangement from a menu.
    pub(super) drag_began: std::cell::Cell<bool>,
    /// The hold timer for a sidebar row. One, replaced per press, so a press
    /// that became a scroll leaves nothing queued to fire over the list the
    /// user is now scrolling.
    pub(super) row_hold_timer: RefCell<Option<slint::Timer>>,
    /// Whether each visible row has a parent, in `row_ids` order — what decides
    /// whether "All photographs" lights up as a drop target. Kept beside the
    /// rows it indexes rather than queried per drag: `refresh_tree` already has
    /// the parent in hand, and a query would answer for a tree that may have
    /// been rebuilt since.
    pub(super) row_nested: RefCell<Vec<bool>>,
    /// The collection the row menu is open on.
    ///
    /// Held here as well as in the window's title property because the two say
    /// different things: the property is what the sheet *draws*, and this is
    /// what every action *acts on*. A rename committed from the menu changes
    /// the first and must not change the second.
    pub(super) menu_for: RefCell<Option<CollectionId>>,
    /// Whether the menu's delete has been asked once and is waiting to be
    /// confirmed.
    ///
    /// A `Cell` rather than a window property alone so the decision is made in
    /// Rust: the sheet must not be able to reach the destructive branch by
    /// flipping a bit of its own.
    pub(super) menu_confirming: std::cell::Cell<bool>,
    /// The collection a drag is currently over, by id.
    ///
    /// Only the spring needs this — the *drop* is hit-tested by Slint and
    /// arrives with its own id, so nothing here has to remember where the
    /// pointer was.
    pub(super) hover_id: RefCell<Option<CollectionId>>,
    /// Spring-loaded expansion: the timer that opens a collapsed parent the
    /// pointer has been dwelling on mid-drag.
    ///
    /// One timer, restarted per row, so moving on cancels the pending
    /// expansion rather than leaving a queue of them to fire later.
    pub(super) spring_timer: RefCell<Option<slint::Timer>>,
    /// Collections the spring opened during *this* drag, so they can be closed
    /// again if the drag ends elsewhere. Without this, dragging across a deep
    /// tree leaves every parent it passed over hanging open.
    pub(super) spring_opened: RefCell<Vec<CollectionId>>,
    /// Images dropped on the trash, recorded by `dropped-on-trash` and acted on
    /// in `drag-finished` — the same deferral, for the same reason.
    pub(super) trash_requested: RefCell<Option<Vec<ImageId>>>,
    /// Drains a trash worker. Held so a second operation replaces the first
    /// rather than two timers fighting over the same model.
    pub(super) trash_timer: RefCell<Option<slint::Timer>>,
    /// Which collection a drop landed on, recorded by `dropped` and acted on in
    /// `drag-finished`.
    ///
    /// Deferred because every consequence of a drop replaces a Slint model —
    /// the tree, the grid cells — and doing that from inside the `dropped`
    /// handler destroys the elements Slint is still using to deliver the event.
    pub(super) dropped_on: RefCell<Option<CollectionId>>,
    /// TRACES: FR-CAT-5
    /// How a run of grid ordinals is turned into ids.
    ///
    /// Supplied by [`crate::library_ui`] at wiring time, because the catalog,
    /// the scope and the rating filter — everything the run depends on — belong
    /// to the grid's controller, not to this one. Held as a closure rather than
    /// reached for through a handle so the selection rules stay testable with
    /// no library open: the tests pass a run that reads a plain slice.
    ///
    /// `None` before wiring, which the caller reads as "nothing to ask" and
    /// falls back to the loaded window.
    pub(super) span_source: RefCell<Option<SpanIds>>,
    /// Where the trash workers report what they are doing.
    ///
    /// Shared with [`crate::library_ui`]: a delete and a scan are two jobs in
    /// one list, and the user asking what the application is busy with does not
    /// care which module started them.
    pub(super) activity: Rc<crate::activity::ActivityLog>,
}

/// What a release over a collection row means.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum Drop {
    /// File these photographs in the target.
    FileImages(Vec<ImageId>),
    /// Move this collection under the target.
    Reparent(CollectionId),
    /// Nothing to do — an empty drag, or a row dropped on itself.
    Nothing,
}

/// Decide what a drop on `target` should do.
///
/// Separated from the callback because the gesture cannot be driven from a
/// test — Slint owns it — while this decision is where it can actually go
/// wrong. The order matters: an image drag always fills `carried`, so
/// photographs can never be mistaken for a rearrangement, and only an empty
/// payload consults the row remembered from the press.
pub fn decide_drop(
    carried: &[ImageId],
    pressed_row: Option<CollectionId>,
    target: CollectionId,
) -> Drop {
    if !carried.is_empty() {
        return Drop::FileImages(carried.to_vec());
    }
    match pressed_row {
        // A collection cannot go inside itself. Caught here as well as in the
        // catalog so the common case is a no-op rather than an error message.
        Some(source) if source != target => Drop::Reparent(source),
        _ => Drop::Nothing,
    }
}

/// TRACES: FR-CAT-7
/// What the row menu's Delete should do on this press.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum DeleteStep {
    /// Delete it now. Nothing is lost that the user cannot see is nothing.
    Now,
    /// Say what goes, and wait to be asked again.
    Confirm,
}

/// TRACES: FR-CAT-7
/// Whether deleting this collection needs to be confirmed first.
///
/// The gesture this replaced refused outright whenever the collection held
/// anything, which made a full collection undeletable without emptying it by
/// hand — child by child, since a parent counts its descendants. Refusing is
/// not a safety property; it is the absence of one, because the user goes and
/// does the same thing the long way round.
///
/// So: an empty collection goes on the first press, because there is nothing
/// to warn about and a dialogue asking "delete this empty thing?" is the kind
/// that teaches people to dismiss dialogues. Anything else is asked once.
///
/// Split from the handler because this is the whole of the safety rule, and a
/// handler needs a window to run.
pub fn decide_delete(holds: usize, children: usize, confirmed: bool) -> DeleteStep {
    if confirmed || (holds == 0 && children == 0) {
        DeleteStep::Now
    } else {
        DeleteStep::Confirm
    }
}

/// TRACES: FR-CAT-7
/// The line under the menu's title: what this collection directly holds.
///
/// Direct members and direct children, not the sidebar's deep count, because
/// this line sits above a Delete — and delete drops *this* collection's member
/// rows while promoting its children rather than taking them. A number here
/// that counted descendants' photographs would be describing something the
/// button below it does not do.
pub fn menu_detail(holds: usize, children: usize) -> String {
    let photos = match holds {
        0 => "no photographs".to_string(),
        1 => "1 photograph".to_string(),
        n => format!("{n} photographs"),
    };
    match children {
        0 => photos,
        1 => format!("{photos} · 1 collection inside"),
        n => format!("{photos} · {n} collections inside"),
    }
}

/// TRACES: FR-CAT-7
/// What deleting this collection actually does, in full.
///
/// Every clause here is one a user has got wrong about a collections feature
/// before: that deleting a collection deletes the photographs (it does not —
/// membership is a join table), and that deleting a parent takes the
/// collections nested in it (it does not — [`dr_catalog::collections::delete`]
/// promotes them, because losing a subtree because its container was tidied
/// away is not recoverable).
pub fn delete_warning(name: &str, holds: usize, children: usize) -> String {
    let mut out = format!("Deleting “{name}” ");
    match (holds, children) {
        (0, _) => out.push_str("removes it from the sidebar."),
        (1, _) => out.push_str("takes 1 photograph out of it."),
        (n, _) => out.push_str(&format!("takes {n} photographs out of it.")),
    }
    if holds > 0 {
        out.push_str(" They stay in your library and in every other collection they are in.");
    }
    match children {
        0 => {}
        1 => {
            out.push_str(" The collection inside it moves up one level rather than going with it.")
        }
        n => out.push_str(&format!(
            " The {n} collections inside it move up one level rather than going with it."
        )),
    }
    out
}

/// TRACES: FR-UI-3 | FR-UI-4
/// What letting go of a held collection row means.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Release {
    /// The hold fired and nothing moved: the user is asking what can be done
    /// to this collection.
    OpenMenu(CollectionId),
    /// Either the hold never fired — an ordinary tap, which selects — or it
    /// did and the row was then dragged, in which case the drop has already
    /// done the work.
    Nothing,
}

/// TRACES: FR-UI-3 | FR-UI-4
/// Decide what a release does, from what the press turned into.
///
/// The whole of the fork that lets one gesture mean two things, and the reason
/// it is worth a test: get it wrong in the direction of `OpenMenu` and every
/// rearrangement ends with a sheet over the tree the user just tidied; get it
/// wrong the other way and the menu is unreachable with a finger.
pub fn decide_release(held: Option<CollectionId>, dragged: bool) -> Release {
    match held {
        Some(id) if !dragged => Release::OpenMenu(id),
        _ => Release::Nothing,
    }
}

impl CollectionsController {
    pub fn new(activity: Rc<crate::activity::ActivityLog>) -> Rc<Self> {
        Rc::new(Self {
            activity,
            ..Default::default()
        })
    }

    /// Remember which row was pressed, so a drag that follows knows what it is
    /// carrying.
    ///
    /// Recorded on the press rather than at the drag's start because Slint
    /// builds the payload through a `pure` binding, which must not have side
    /// effects — and the drop callback is handed only the *target's* id, so the
    /// source has to be remembered somewhere.
    pub fn note_row_press(&self, id: CollectionId) {
        *self.dragging_collection.borrow_mut() = Some(id);
    }

    /// Which collection the grid is scoped to, for [`crate::library_ui`] to
    /// build its query from.
    pub fn scope(&self) -> Option<CollectionId> {
        *self.scope.borrow()
    }

    /// Whether the grid should be showing the trash.
    pub fn viewing_trash(&self) -> bool {
        self.viewing_trash.get()
    }

    /// Whether the gesture in progress began with ctrl or shift held.
    ///
    /// A modified click selects and nothing more — opening the image as well
    /// would eject the user from the grid they are selecting in.
    pub fn press_was_modified(&self) -> bool {
        self.modified_press.get()
    }

    /// Selected image ids, in a stable order.
    pub fn selected(&self) -> Vec<ImageId> {
        self.selection.borrow().iter().copied().collect()
    }

    /// TRACES: FR-CAT-5
    /// The ids of grid rows `first..=last`, in the order the grid shows them.
    ///
    /// Empty when nothing has been wired in — see [`Self::span_source`].
    pub(super) fn span(&self, first: usize, last: usize) -> Vec<ImageId> {
        let source = self.span_source.borrow().clone();
        source.map(|f| f(first, last)).unwrap_or_default()
    }

    /// Drop the selection — after a scrub, or when the scope changes.
    ///
    /// Selection is by id and survives a window change, but a selection the
    /// user cannot see is a selection they will act on by accident. Clearing on
    /// a deliberate navigation is the safer of the two behaviours.
    /// TRACES: FR-EXP-10
    /// An album was chosen in the sidebar: nothing of this panel scopes the
    /// grid any more. The selection goes too, as it does on any scope change —
    /// a selection the user cannot see is one they will act on by accident.
    pub(crate) fn leave_for_album(&self) {
        self.viewing_trash.set(false);
        *self.scope.borrow_mut() = None;
        self.clear_selection();
    }

    pub fn clear_selection(&self) {
        self.selection.borrow_mut().clear();
        *self.anchor.borrow_mut() = None;
        self.cursor.set(None);
    }

    /// Where the keyboard cursor is, as an image ordinal.
    pub fn cursor(&self) -> Option<usize> {
        self.cursor.get()
    }

    pub fn set_cursor(&self, at: Option<usize>) {
        self.cursor.set(at);
    }
}

#[cfg(test)]
mod tests {
    use crate::collections_ui::test_support::*;

    use super::*;

    #[test]
    fn dragging_a_collection_onto_another_moves_it() {
        // The gesture the tree rearrangement exists for: no images carried, a
        // row remembered from the press, dropped somewhere else.
        assert_eq!(
            decide_drop(&[], Some(CollectionId(7)), CollectionId(9)),
            Drop::Reparent(CollectionId(7))
        );
    }

    #[test]
    fn photographs_are_filed_even_when_a_row_was_pressed_first() {
        // Clicking a collection and then dragging photographs into another must
        // file them, not move the collection that happens to be remembered.
        // This is the ordering the whole discrimination rests on.
        assert_eq!(
            decide_drop(&ids(3), Some(CollectionId(7)), CollectionId(9)),
            Drop::FileImages(ids(3))
        );
    }

    #[test]
    fn a_collection_dropped_on_itself_does_nothing() {
        // The catalog would refuse it as a cycle; catching it here makes the
        // commonest slip a no-op rather than an error the user has to read.
        assert_eq!(
            decide_drop(&[], Some(CollectionId(7)), CollectionId(7)),
            Drop::Nothing
        );
    }

    #[test]
    fn an_empty_drag_with_nothing_remembered_moves_nothing() {
        // A file dragged in from another application lands here with no images
        // and no pressed row. It must not disturb the tree.
        assert_eq!(decide_drop(&[], None, CollectionId(9)), Drop::Nothing);
    }

    #[test]
    fn an_empty_collection_is_deleted_without_being_asked_about() {
        // There is nothing to warn about, and a dialogue asking "delete this
        // empty thing?" is the kind that teaches people to dismiss dialogues
        // without reading them — including the one that mattered.
        assert_eq!(decide_delete(0, 0, false), DeleteStep::Now);
    }

    #[test]
    fn a_collection_holding_anything_is_asked_about_once() {
        assert_eq!(decide_delete(12, 0, false), DeleteStep::Confirm);
        assert_eq!(decide_delete(0, 1, false), DeleteStep::Confirm);
        // And once asked, it goes.
        assert_eq!(decide_delete(12, 3, true), DeleteStep::Now);
    }

    #[test]
    fn a_collection_holding_only_children_still_asks() {
        // The old gesture refused on `deep_count > 0`, which counted
        // descendants' photographs — so an empty parent of empty children was
        // deletable in one press and an empty parent of a *full* child was not
        // deletable at all. Both are now the same question, asked once.
        assert_eq!(decide_delete(0, 2, false), DeleteStep::Confirm);
    }

    #[test]
    fn the_menu_line_counts_what_delete_acts_on() {
        assert_eq!(menu_detail(0, 0), "no photographs");
        assert_eq!(menu_detail(1, 0), "1 photograph");
        assert_eq!(menu_detail(12, 1), "12 photographs · 1 collection inside");
        assert_eq!(menu_detail(0, 3), "no photographs · 3 collections inside");
    }

    #[test]
    fn the_warning_says_the_photographs_survive() {
        // The single most likely misreading of "Delete" here. If this clause
        // ever goes, a user deletes a collection expecting to lose the
        // pictures — and either panics, or does not do it at all.
        let w = delete_warning("Iceland", 12, 0);
        assert!(w.contains("12 photographs"), "{w}");
        assert!(w.contains("stay in your library"), "{w}");
    }

    #[test]
    fn the_warning_says_nested_collections_are_promoted_not_taken() {
        // `dr_catalog::collections::delete` promotes children to the deleted
        // collection's parent rather than cascading. A warning that did not
        // say so would describe a data loss that does not happen.
        let w = delete_warning("Trips", 0, 2);
        assert!(w.contains("2 collections inside it move up"), "{w}");
        // And nothing about photographs surviving, because none were lost.
        assert!(!w.contains("stay in your library"), "{w}");
    }

    #[test]
    fn a_hold_that_did_not_move_opens_the_menu() {
        assert_eq!(
            decide_release(Some(CollectionId(4)), false),
            Release::OpenMenu(CollectionId(4))
        );
    }

    #[test]
    fn a_hold_that_became_a_drag_opens_nothing() {
        // The drop has already done the work. A menu here would put a sheet
        // over the tree the user has just finished rearranging — and over the
        // row they would have to look at to see whether it worked.
        assert_eq!(
            decide_release(Some(CollectionId(4)), true),
            Release::Nothing
        );
    }

    #[test]
    fn an_ordinary_tap_opens_nothing() {
        // Nothing was ever picked up, so this is the press that selects a
        // collection and scopes the grid. A menu on every tap would make the
        // sidebar unusable.
        assert_eq!(decide_release(None, false), Release::Nothing);
    }
}