/// Where ⌘V lands (04-interactions.md ▸ Clipboard), as pure functions of the selection, the
/// last-active lane, and the snapshot (`PasteTargetTests`).
///
/// The two rules verbatim, and each clause's branch below:
///
/// > Paste lands after the anchor card (or appends to a selected lane); a multi-selection anchors at
/// > its last member in flatten order — the ⌘N target rule's shared anchor. … **A tombstoned
/// > selection never anchors paste**: ⌘V stays enabled and behaves exactly as with nothing selected —
/// > a card payload appends to the last-active lane, a lane payload lands at the board's right end.
///
/// > **Lane paste** lands after the anchor lane — the selected lane, or the selected card's lane
/// > (several selected: the last, per the shared anchor rule); nothing selected = the board's right
/// > end.
///
/// Plus 04 ▸ The map's zero-lane clause: "New Card, Return-creation, and Paste with a *card* payload
/// disable via menu validation until a lane exists … Paste with a **lane** payload … stays enabled
/// and lands at the board's right end".
///
/// **Pure, for `NewCardTarget`'s reason** — the branches become lines of test rather than gestures to
/// drive, and the menu item's `disabled` reads the *same* answer as the paste's own target rather
/// than a second, hand-kept-in-sync condition.
///
/// ### What it deliberately reuses
///
/// The anchor is `NewCardTarget.flattenAnchor`, not a second walk: 04 says creation and paste share
/// one anchor ("the ⌘N target rule's shared anchor"), and two derivations of "the last member in
/// flatten order" would be two chances for them to disagree. The card branch goes further and reuses
/// `NewCardTarget.resolve` whole, because a card paste's *fallback* is the ⌘N rule's fallback too —
/// the last-active lane, then the first lane. Only the lane branch stops at the anchor, because its
/// fallback is the board's right end instead.
enum PasteTarget {

    /// Where a card payload lands: which lane, and the position among that lane's rendered cards.
    struct Cards: Equatable {
        let laneID: ItemID
        /// A position in the lane's logical card order, counted among what it renders *now* — the
        /// convention every arrival path here takes (`BoardStore.receiveCards`).
        let index: Int
    }

    /// The card payload's target, or `nil` on a **zero-lane board** — which is therefore the menu
    /// item's `disabled` condition as well as the paste's refusal, so the two cannot disagree.
    static func cards(
        selection: ItemReferenceSet,
        lastActiveLaneID: ItemID?,
        snapshot: BoardModel
    ) -> Cards? {
        guard let resolution = NewCardTarget.resolve(
            selection: selection,
            lastActiveLaneID: lastActiveLaneID,
            snapshot: snapshot
        ),
            let lane = snapshot.lanes.first(where: { $0.id == resolution.laneID && !$0.isDeleted })
        else { return nil }

        let rendered = lane.cards.filter { !$0.isDeleted }
        // `insertionIndex` answers `nil` for "append", which is `rendered.count` — the same position
        // said two ways, and the creation path's own degradation for an anchor that has since gone.
        let index = BoardStore.insertionIndex(after: resolution.anchorCardID, among: rendered) ?? rendered.count
        return Cards(laneID: lane.id, index: index)
    }

    /// The lane payload's slot among the board's live lanes — **always an answer**, zero-lane board
    /// included, because lane paste "stays enabled and lands at the board's right end" whatever the
    /// board holds. That is what makes it the other way out of a board with no lanes.
    static func lanes(selection: ItemReferenceSet, snapshot: BoardModel) -> Int {
        let lanes = snapshot.lanes.filter { !$0.isDeleted }
        guard let anchor = NewCardTarget.flattenAnchor(selection: selection, snapshot: snapshot),
              let position = lanes.firstIndex(where: { $0.id == anchor.laneID })
        else {
            // Nothing selected, a tombstoned selection, or a stale one: the right end.
            return lanes.count
        }
        return position + 1
    }
}