import CoreGraphics
import Foundation

// MARK: - What a click names

/// Which of the board's two selectable levels an item is — 04-interactions.md § Selection's
/// **cards XOR lanes** axis, named so the grammar can compare it rather than infer it per branch.
///
/// It is deliberately *not* stored anywhere: a selection is a set of ids and nothing else
/// (`ItemReferenceSet`), so the kind of a selection is always re-derived from the snapshot. Storing
/// it would be a second answer to a question the snapshot can always answer, and one that a reload
/// could falsify.
public enum SelectionKind: Sendable, Equatable {
    case card
    case lane
}

/// What a pointer click names: an item, its level, and the side of the live/trash boundary the
/// surface it was clicked on sits on.
///
/// The **side is the surface's, not the item's** — a card face is always `.live` and a trash row is
/// always `.trashed`, because that is what the user clicked. A click on a surface whose item flipped
/// liveness a moment ago simply selects nothing the next reload will keep, which is the ordinary
/// vanish rule and not a case for this type to model.
public struct SelectionTarget: Sendable, Equatable {
    public var id: ItemID
    public var kind: SelectionKind
    public var side: Liveness

    public init(id: ItemID, kind: SelectionKind, side: Liveness) {
        self.id = id
        self.kind = kind
        self.side = side
    }
}

/// The modifier a click carried — 04-interactions.md § Selection's three-way grammar ("click
/// selects; ⌘-click toggles; ⇧-click range-extends").
///
/// Three cases rather than an `OptionSet`, because the design gives ⌘ and ⇧ *no* combined meaning:
/// AppKit hands us both flags when both keys are down, and the grammar picks one. `ClickModifier`
/// is where that choice is made once (`ClickModifier.current`), so no call site re-decides it.
public enum ClickModifier: Sendable, Equatable {
    case plain
    case command
    case shift
}

// MARK: - SelectionGrammar

/// 04-interactions.md § Selection's pointer grammar, plus § The trash's extensions to it, as a pure
/// function of the click, the current selection, the anchor, and the snapshot
/// (`SelectionGrammarTests`).
///
/// **Homogeneity is the invariant, and it is enforced here or nowhere.** The selection is
/// homogeneous on three axes at once — cards XOR lanes (§ Selection), live XOR tombstoned, and
/// within the trash card entries XOR lane entries (§ The trash) — and every one of them is a
/// property of what a *click* is allowed to produce. So no outcome below is ever mixed: a modifier
/// that would cross an axis degrades to a replace, which is the only answer that keeps the
/// invariant true without silently dropping what the user asked for.
///
/// **Pure, for `NewCardTarget`'s reason**: the branches become lines of test rather than gestures to
/// drive, and the four surfaces that clicks arrive on (card face, lane header, lane empty space,
/// trash row) share one answer instead of four near-copies of it.
public enum SelectionGrammar {

    /// What a click leaves behind: the new selection, the anchor a subsequent ⇧-click would range
    /// from, and the head a subsequent arrow would step from.
    ///
    /// The anchor is carried *out* rather than mutated in place because it is not derivable from the
    /// selection — a ⇧-range replaces the whole set and deliberately leaves the anchor where it was,
    /// so "the last plain or ⌘ click" is a memory of a gesture and only the gesture can update it
    /// (`TransientBoardState.selectionAnchor`).
    ///
    /// **The head is always the clicked item**, in every branch below — the ⇧-branch included, where
    /// the anchor deliberately stays put. That asymmetry is the definition of the two:
    /// `TransientBoardState.selectionHead` is where the *next* step starts, and a ⇧-click moves it
    /// exactly as a plain one does.
    public struct Outcome: Sendable, Equatable {
        public var selection: ItemReferenceSet
        public var anchor: ItemID?
        public var head: ItemID?

        public init(selection: ItemReferenceSet, anchor: ItemID?, head: ItemID? = nil) {
            self.selection = selection
            self.anchor = anchor
            self.head = head
        }

        /// Nothing selected, nothing to range from, nowhere to step from — the toggle-off outcomes.
        static let cleared = Outcome(selection: .empty, anchor: nil, head: nil)
    }

    /// The grammar, one call.
    ///
    /// - Parameters:
    ///   - target: what was clicked, with the surface's liveness side (see `SelectionTarget`).
    ///   - modifier: the effective modifier, already reduced to one of three (`ClickModifier`).
    ///   - selection: the board's current selection.
    ///   - anchor: the range origin — `TransientBoardState.selectionAnchor`.
    ///   - snapshot: the board as it is now; every order list and every kind is derived from it.
    ///   - togglesOnRepeat: **the lane's click-again-to-unselect**, and only the lane's
    ///     (04-interactions.md § Selection: "single click selects the lane (click again to
    ///     unselect)", and the header "toggles like empty space (settled)"). A card face passes
    ///     `false`: Finder does not deselect a file by clicking it twice, and neither do we.
    public static func click(
        _ target: SelectionTarget,
        modifier: ClickModifier,
        selection: ItemReferenceSet,
        anchor: ItemID?,
        snapshot: BoardModel,
        togglesOnRepeat: Bool = false
    ) -> Outcome {
        switch modifier {
        case .plain:
            return plain(target, selection: selection, togglesOnRepeat: togglesOnRepeat)
        case .command:
            return command(target, selection: selection, snapshot: snapshot)
        case .shift:
            return shift(target, selection: selection, anchor: anchor, snapshot: snapshot)
        }
    }

    // MARK: - The three branches

    /// **Plain**: the selection becomes exactly what was clicked, and the click becomes the anchor.
    ///
    /// The one exception is `togglesOnRepeat`, and it tests for *sole membership* rather than mere
    /// containment: a lane click that lands inside a multi-lane selection narrows it to that lane
    /// (replace), because "click again to unselect" is about the lane the user already had, not
    /// about wiping a selection they built with ⌘.
    private static func plain(
        _ target: SelectionTarget,
        selection: ItemReferenceSet,
        togglesOnRepeat: Bool
    ) -> Outcome {
        if togglesOnRepeat, selection.liveness == target.side, selection.ids == [target.id] {
            return .cleared
        }
        return Outcome(
            selection: ItemReferenceSet(ids: [target.id], liveness: target.side),
            anchor: target.id,
            head: target.id
        )
    }

    /// **⌘-click toggles** — but only *within* a homogeneous set. Crossing either axis (a card
    /// clicked while lanes are selected, a trash row clicked while live cards are) is not a mixed
    /// selection and not a refusal: it is a **replace**, the same outcome a plain click would give,
    /// because the click unambiguously names a new set of one.
    ///
    /// The current kind is derived from the snapshot rather than remembered (`kind(of:in:)`); a
    /// selection whose members all name nothing the board renders counts as empty, so a ⌘-click
    /// after a foreign delete starts a fresh set rather than extending a ghost.
    private static func command(
        _ target: SelectionTarget,
        selection: ItemReferenceSet,
        snapshot: BoardModel
    ) -> Outcome {
        guard selection.liveness == target.side,
              let current = kind(of: selection, in: snapshot),
              current == target.kind
        else {
            return plain(target, selection: selection, togglesOnRepeat: false)
        }

        var ids = selection.ids
        if ids.remove(target.id) == nil {
            ids.insert(target.id)
        } else if ids.isEmpty {
            // The last member toggled out: nothing is selected, so there is nothing to range from.
            return .cleared
        }
        return Outcome(
            selection: ItemReferenceSet(ids: ids, liveness: target.side),
            anchor: target.id,
            head: target.id
        )
    }

    /// **⇧-click range-extends from the anchor**, Finder-list style: the whole range replaces the
    /// selection, and the anchor stays where it is so successive ⇧-clicks sweep out from the same
    /// origin rather than walking it along.
    ///
    /// The anchor is valid **iff both it and the target sit in the same order list** — which folds
    /// the nil anchor, the vanished anchor, and every axis crossing into one test, since a list is
    /// exactly one (side, kind) pair. An invalid anchor makes the click a plain one, never a no-op:
    /// the keyboard's ⇧-arrow goes inert at a boundary because its next step is ambiguous, while a
    /// click names an unambiguous target and so always has something to do.
    private static func shift(
        _ target: SelectionTarget,
        selection: ItemReferenceSet,
        anchor: ItemID?,
        snapshot: BoardModel
    ) -> Outcome {
        guard let anchor,
              let span = range(from: anchor, to: target.id, kind: target.kind, on: target.side, in: snapshot)
        else {
            return plain(target, selection: selection, togglesOnRepeat: false)
        }
        return Outcome(
            selection: ItemReferenceSet(ids: span, liveness: target.side),
            anchor: anchor,
            head: target.id
        )
    }

    /// The ids between two items in one order list, inclusive — **the span both extension gestures
    /// select**, ⇧-click and ⇧-arrow alike.
    ///
    /// It is a function rather than a branch inside `shift` because the keyboard needs the identical
    /// answer: "a ⇧-arrow extends" (04-interactions.md ▸ Grammar) means exactly the range a ⇧-click
    /// to the same item would produce, and two implementations of one span is two chances for the
    /// pointer and the keyboard to disagree about what a range is.
    ///
    /// **`nil` means the two do not share a list**, which folds the vanished endpoint, the nil
    /// anchor's caller-side absence, and every axis crossing into one test — a list is exactly one
    /// (side, kind) pair. The callers differ on what they do with that: a click degrades to a plain
    /// click (it names an unambiguous target), while a ⇧-arrow goes inert (its next step is
    /// ambiguous).
    public static func range(
        from: ItemID,
        to: ItemID,
        kind: SelectionKind,
        on side: Liveness,
        in snapshot: BoardModel
    ) -> Set<ItemID>? {
        let list = order(of: kind, on: side, in: snapshot)
        guard let start = list.firstIndex(of: from), let end = list.firstIndex(of: to) else { return nil }
        return Set(start <= end ? list[start...end] : list[end...start])
    }

    // MARK: - The order lists

    /// The list a ⇧-range walks for one (side, kind) pair — **the single place a "what's on the
    /// board, in what order" question is answered** for the pointer.
    ///
    // m5-search: the filter "is the single source of truth for what's on the board … ranges … all
    // read it" (04-interactions.md § Search). It threads in here and in `MarqueeTargetRegistry`'s
    // membership, and nowhere else — every range and every Select All is stated in terms of these
    // four lists.
    public static func order(of kind: SelectionKind, on side: Liveness, in snapshot: BoardModel) -> [ItemID] {
        switch (side, kind) {
        case (.live, .card): liveCards(in: snapshot)
        case (.live, .lane): liveLanes(in: snapshot)
        case (.trashed, _): trashEntries(of: kind, in: snapshot)
        }
    }

    /// Live cards in **flatten order** — "lane `order` first, then card `order` (a cross-lane
    /// selection flattens left-to-right, top-to-bottom)", the multi-drag order the ⌘N target rule
    /// and paste anchoring already share (04-interactions.md ▸ Drag and drop, ▸ The map).
    ///
    /// The snapshot's arrays are already in display order (`Ranks.sortedForDisplay`), so the flatten
    /// is one walk — `NewCardTarget.resolve`'s walk, in list form.
    public static func liveCards(in snapshot: BoardModel) -> [ItemID] {
        var ids: [ItemID] = []
        for lane in snapshot.lanes where !lane.isDeleted {
            for card in lane.cards where !card.isDeleted {
                ids.append(card.id)
            }
        }
        return ids
    }

    /// Live lanes, left to right. Tombstoned lanes render nowhere on the board (03-board-ui.md §
    /// Trash collapses each into one entry), so they are absent from the live lane order entirely.
    public static func liveLanes(in snapshot: BoardModel) -> [ItemID] {
        snapshot.lanes.filter { !$0.isDeleted }.map(\.id)
    }

    /// One kind of trash row, in the quasi-lane's own deterministic order (`TrashModel.entries`,
    /// whose sort is "load-bearing for input — arrow walks, ⇧-ranges, and the rubber band all read
    /// it").
    ///
    /// **Filtered to one kind, so a range skips what it cannot include.** Card and lane entries
    /// interleave in one ordering, and "a selection never mixes card entries and lane entries"
    /// (04-interactions.md ▸ The trash), so a ⇧-range between two card rows spans the sorted order
    /// and collects only the card rows — stepping over any lane row that sits between them. That is
    /// the deliberate pointer twin of the keyboard's rule: a ⇧-arrow onto a lane entry is *inert*
    /// because its next step is ambiguous, while a click names an unambiguous same-kind target and
    /// so the range simply skips.
    public static func trashEntries(of kind: SelectionKind, in snapshot: BoardModel) -> [ItemID] {
        TrashModel.entries(of: snapshot)
            .filter { $0.isLaneEntry == (kind == .lane) }
            .map(\.id)
    }

    // MARK: - The current selection's kind

    /// Which level the selection holds, or `nil` when it holds nothing the board renders on its own
    /// side.
    ///
    /// **Any member answers, because the set is homogeneous** — but the walk is the snapshot's order
    /// rather than the set's iteration order, so the answer is deterministic even for a set that
    /// somehow was not. Members that name nothing are ignored, and a set of only such members reads
    /// as empty: a selection the next reload will drop must not decide what a click does now.
    ///
    /// The membership rules are exactly the order lists': on the live side an item counts when its
    /// effective liveness is live, and on the trashed side only **rows** count — a card under a
    /// tombstoned lane has no row of its own (`TrashModel.entries`' absolute ancestor walk), so it
    /// is nobody's kind.
    public static func kind(of selection: ItemReferenceSet, in snapshot: BoardModel) -> SelectionKind? {
        guard !selection.isEmpty else { return nil }
        for lane in snapshot.lanes {
            if Liveness(isDeleted: lane.isDeleted) == selection.liveness, selection.ids.contains(lane.id) {
                return .lane
            }
            // A tombstoned lane subsumes its subtree on both sides: its cards render nowhere live
            // and have no trash row of their own.
            guard !lane.isDeleted else { continue }
            for card in lane.cards
            where Liveness(isDeleted: card.isDeleted) == selection.liveness && selection.ids.contains(card.id) {
                return .card
            }
        }
        return nil
    }

    // MARK: - Successor on delete

    /// What ⌫ selects after tombstoning `ids` — 04-interactions.md ▸ The map's Finder-style
    /// successor sibling, as a pure function of the **pre-write** snapshot.
    ///
    /// > Selection moves to the deleted item's successor sibling, Finder-style (next card in the
    /// > lane, next lane on the board; the last sibling's predecessor otherwise; empty container =
    /// > nothing selected) — repeated ⌫ walks down a lane.
    ///
    /// Three decisions the wording implies and this states:
    ///
    /// - **The container is the *last* deleted item's**, in flatten order — the same "last member"
    ///   the ⌘N target rule and paste anchoring already share. A selection spanning lanes therefore
    ///   lands in the rightmost/bottom-most one, which is where the user was working.
    /// - **The survivor search is forward first, then backward**: the first surviving sibling *after*
    ///   the last deleted position, else the last surviving sibling *before* the first deleted one.
    ///   Forward is what makes repeated ⌫ walk down a lane rather than bouncing.
    /// - **`nil` is a legitimate answer** — an emptied container selects nothing, and the caller
    ///   clears.
    ///
    /// **Deliberate deletes only.** External vanishing never picks a successor (02-architecture.md's
    /// reload-survival rule: "the selection just shrinks"), which is why this is called by
    /// `BoardStore.delete` and by nothing on the reload path.
    public static func successor(afterDeleting ids: Set<ItemID>, in snapshot: BoardModel) -> ItemID? {
        guard !ids.isEmpty else { return nil }
        let selection = ItemReferenceSet(ids: ids, liveness: .live)
        guard let kind = kind(of: selection, in: snapshot) else { return nil }

        let container: [ItemID]
        switch kind {
        case .lane:
            container = liveLanes(in: snapshot)
        case .card:
            // The last selected card in flatten order names the lane; its lane's rendered cards are
            // the container the successor is drawn from.
            guard let last = liveCards(in: snapshot).last(where: { ids.contains($0) }),
                  let lane = snapshot.lanes.first(where: { lane in
                      !lane.isDeleted && lane.cards.contains { $0.id == last && !$0.isDeleted }
                  })
            else { return nil }
            container = lane.cards.filter { !$0.isDeleted }.map(\.id)
        }

        let doomed = container.indices.filter { ids.contains(container[$0]) }
        guard let first = doomed.first, let last = doomed.last else { return nil }
        if let after = container[(last + 1)...].first(where: { !ids.contains($0) }) { return after }
        return container[..<first].last { !ids.contains($0) }
    }
}

// MARK: - The rubber band

/// One item the marquee can sweep: its identity, its level, its side, and where it is drawn.
///
/// The frame is in the board strip's coordinate space (`BoardView.stripSpace`) and is **registered
/// by the view that draws it** (`MarqueeTargetRegistry`) rather than computed here: the masonry's
/// geometry is the layout's own answer, and re-deriving it would be a second one.
public struct MarqueeTarget: Sendable, Equatable {
    public var id: ItemID
    public var kind: SelectionKind
    public var side: Liveness
    public var frame: CGRect

    public init(id: ItemID, kind: SelectionKind, side: Liveness, frame: CGRect) {
        self.id = id
        self.kind = kind
        self.side = side
        self.frame = frame
    }
}

/// What a rubber band selects, as a pure function of the band, the drawn frames, and the side the
/// band started on (`SelectionGrammarTests`).
///
/// The two rules it exists to state, both 04-interactions.md's:
///
/// - **The band stays on the side of the boundary it started on** (▸ The trash), which is why `side`
///   is a parameter rather than something derived from what the rect happens to touch: a band begun
///   on the board and dragged over the trash column selects live cards and nothing else.
/// - **The band never selects lanes** (§ Selection gives it to cards: "click-drag rubber-bands
///   across lanes" — across them, not over them). Lanes are simply never registered as targets, and
///   the live branch filters to cards anyway so the rule holds even if one were.
public enum MarqueeMath {

    /// The ids `rect` sweeps.
    ///
    /// On the **trashed** side the band must additionally stay homogeneous by *kind*, because the
    /// trash's two row kinds interleave in one column. The rule is topmost-wins: the kind of the
    /// highest intersecting row decides, and rows of the other kind are dropped — so a band pulled
    /// down from a card row keeps collecting card rows and steps over the lane rows between them,
    /// exactly as a ⇧-range does.
    public static func selection(rect: CGRect, targets: [MarqueeTarget], side: Liveness) -> Set<ItemID> {
        let hits = targets.filter { $0.side == side && rect.intersects($0.frame) }
        guard !hits.isEmpty else { return [] }

        switch side {
        case .live:
            return Set(hits.lazy.filter { $0.kind == .card }.map(\.id))
        case .trashed:
            guard let topmost = hits.min(by: isAbove) else { return [] }
            return Set(hits.lazy.filter { $0.kind == topmost.kind }.map(\.id))
        }
    }

    /// Which of two drawn rows is "higher" — top edge, then leading edge, then identity.
    ///
    /// Total rather than merely correct-for-a-column: two rows sharing a top edge must still order
    /// the same way twice, or the topmost-kind rule would pick differently on identical input.
    ///
    /// Shared with `NavigationMath`, which breaks its score ties with it for the same reason: two
    /// candidates that a metric cannot separate must still be separated the same way twice.
    static func isAbove(_ lhs: MarqueeTarget, _ rhs: MarqueeTarget) -> Bool {
        if lhs.frame.minY != rhs.frame.minY { return lhs.frame.minY < rhs.frame.minY }
        if lhs.frame.minX != rhs.frame.minX { return lhs.frame.minX < rhs.frame.minX }
        return lhs.id.rawValue < rhs.id.rawValue
    }
}