import Foundation

// MARK: - TrashEntry

/// One row of the trash quasi-lane (03-board-ui.md § Trash).
///
/// **Two cases, not one**, because a tombstoned lane is not a tombstoned card wearing a different
/// symbol: it is "a single restorable entry" that *subsumes* everything beneath it, and Put Back on
/// it "returns [the lane] whole, cards and all" (04-interactions.md ▸ The trash). The card count it
/// carries is part of the entry rather than something the row re-derives, because the rule for what
/// that number means is subtle enough to want one home — see `TrashModel.entries(of:)`.
///
/// The card case carries its lane's id for the same reason `BoardStore.liveItem` returns two
/// components rather than a URL: the row's folder is `<root>/<lane>/<card>`, and the root is the
/// store's to supply — a mid-session folder rename may have moved it.
public enum TrashEntry: Identifiable, Sendable, Equatable {

    /// A card carrying its own `deleted:` under a **live** lane. A card whose lane is tombstoned
    /// never becomes one of these — see `TrashModel.entries(of:)`'s ancestor walk.
    case card(Card, laneID: ItemID)

    /// A tombstoned lane, and how many of its cards Put Back would return to the board.
    case lane(Lane, returningCardCount: Int)

    public var id: ItemID {
        switch self {
        case let .card(card, _): card.id
        case let .lane(lane, _): lane.id
        }
    }

    /// The title as written, or `nil` for an untitled item — "Untitled" is a rendering, never a
    /// value (03-board-ui.md § Card face).
    public var title: String? {
        switch self {
        case let .card(card, _): card.title.value
        case let .lane(lane, _): lane.title.value
        }
    }

    /// Which kind of row this is — the axis 04-interactions.md ▸ The trash makes a selection
    /// homogeneous over ("a selection never mixes card entries and lane entries").
    public var isLaneEntry: Bool {
        if case .lane = self { return true }
        return false
    }

    /// The `icon` field this row renders, so the row's symbol obeys the same lenient rule the board
    /// face does (`ItemSymbol`).
    public var icon: FieldValue<String> {
        switch self {
        case let .card(card, _): card.icon
        case let .lane(lane, _): lane.icon
        }
    }

    /// Where this row's folder sits under the board root.
    public var path: TrashModel.ItemPath {
        switch self {
        case let .card(card, laneID): TrashModel.ItemPath(laneID: laneID, cardID: card.id)
        case let .lane(lane, _): TrashModel.ItemPath(laneID: lane.id, cardID: nil)
        }
    }
}

// MARK: - TrashModel

/// The trash quasi-lane's contents, as a pure function of a snapshot (`TrashModelTests`) —
/// 03-board-ui.md § Trash's Contents rules with nothing else mixed in.
///
/// **Pure because the trash is a pure view.** "Tombstoned cards keep their `deleted:` key and stay
/// exactly where they are on disk; nothing about the storage schema is trash-specific", so there is
/// no trash *state* anywhere — only this derivation, re-run against whatever snapshot is current.
/// A reload therefore rebuilds the rows for free, exactly as it rebuilds the lanes.
///
/// The three rules it owns, each of which the design settles explicitly:
///
/// 1. **The absolute ancestor walk.** A tombstoned lane's entry subsumes everything beneath it — "a
///    card that carries its own `deleted:` under a tombstoned lane has **no row of its own**". There
///    is no trash carve-out from 01-storage-format.md's consumer rule. This rule is not spelled
///    here: it is `Liveness.walk`'s, because the trashed **universe** every item-referencing set is
///    held to *is* this row set — "universe and rows are one function" (02-architecture.md § Changes
///    from Kanban, settled). Everything below that asks what is in the trash asks that walk, so the
///    rows a user sees and the ids a selection may hold cannot drift apart.
/// 2. **The returning count.** A lane entry's number "counts what Put Back returns to the board —
///    cards without their own flag; individually tombstoned descendants aren't in that number, since
///    they come back to the *trash*".
/// 3. **The deterministic sort.** Newest `deleted` first; ties by folder name ascending; an
///    unparseable stamp sorts as *oldest*, after every dated entry, folder-name-ordered among its
///    kind; lane entries interleave in the same single ordering by their own stamp. The order is
///    load-bearing for input — "arrow walks, ⇧-ranges, and the rubber band all read it" — so it is
///    total, not merely stable.
public enum TrashModel {

    // MARK: - Where a row lives

    /// An item's folder, as its identity components rather than as a URL.
    ///
    /// Same shape and same reasoning as `BoardStore.liveItem`'s return: the caller builds the URL off
    /// the store's *current* `rootURL`, so a board renamed or moved mid-session writes at the new
    /// location (02-architecture.md § Write-failure surfacing).
    public struct ItemPath: Sendable, Equatable {
        public let laneID: ItemID
        /// `nil` for a lane — the path is then the lane folder itself.
        public let cardID: ItemID?

        public init(laneID: ItemID, cardID: ItemID?) {
            self.laneID = laneID
            self.cardID = cardID
        }

        public var isLane: Bool { cardID == nil }

        /// This path resolved under a board root.
        public func folder(under root: URL) -> URL {
            var url = root.appendingPathComponent(laneID.rawValue, isDirectory: true)
            if let cardID {
                url.append(component: cardID.rawValue, directoryHint: .isDirectory)
            }
            return url
        }
    }

    // MARK: - Entries

    /// The trash's rows, in the order the quasi-lane shows them.
    ///
    /// **Which items are rows is not decided here** — it is `Liveness.trashed.walk`, the same walk
    /// `ItemReferenceSet.idUniverse(of:on:)` reads, because the trashed universe and this row set are
    /// one function. Its `continue` on a tombstoned lane is the absolute ancestor walk: such a lane
    /// contributes exactly one entry and its cards contribute none, whatever their own flags say.
    ///
    /// What is left for this function is what a row *carries* — the returning count and the sort
    /// inputs — and the order it shows in, neither of which any other caller of the walk wants.
    public static func entries(of snapshot: BoardModel) -> [TrashEntry] {
        var rows: [Row] = []
        Liveness.trashed.walk(snapshot) { lane, card in
            if let card {
                rows.append(Row(
                    entry: .card(card, laneID: lane.id),
                    deleted: card.deleted.value,
                    name: card.id.rawValue
                ))
            } else {
                // Rule 2: only the cards *without* their own flag come back with the lane. The ones
                // that carry a flag stay tombstoned and get their rows back in the trash — which is
                // why Put Back on such a card is deliberately two steps.
                let returning = lane.cards.filter { !$0.isDeleted }.count
                rows.append(Row(
                    entry: .lane(lane, returningCardCount: returning),
                    deleted: lane.deleted.value,
                    name: lane.id.rawValue
                ))
            }
        }
        return rows.sorted(by: isOrdered).map(\.entry)
    }

    /// Whether the board has anything in its trash at all — the "non-empty" half of Empty Trash…'s
    /// menu validation, which "reads the board's tombstones, not the filtered view" (03 ▸ Trash).
    ///
    /// The walk's non-emptiness in **short-circuit form**, which is the one place the rule is
    /// restated and only because stopping early is the whole point: a tombstoned lane is a row
    /// outright, and under a live lane any own-flagged card is one. There is deliberately no third
    /// clause for a card beneath a tombstoned lane — the lane has already answered `true` for it.
    /// `TrashModelTests` pins the equivalence to `entries(of:).isEmpty` so the shortcut cannot drift.
    public static func isEmpty(_ snapshot: BoardModel) -> Bool {
        !snapshot.lanes.contains { lane in
            lane.isDeleted || lane.cards.contains(where: \.isDeleted)
        }
    }

    /// One entry's sort inputs, kept beside it so the comparator never re-reads the model.
    private struct Row {
        let entry: TrashEntry
        /// The parsed `deleted` stamp, or `nil` when the value is present but unparseable —
        /// 01-storage-format.md's unusable-timestamp rule, which still deletes (presence, not
        /// validity) but supplies no position in time.
        let deleted: Date?
        /// The folder name, byte-for-byte — the tie-break the loader's display order already uses
        /// (`Ranks.sortedForDisplay`, `name: { $0.id.rawValue }`), so the trash breaks ties the same
        /// way the board does.
        let name: String
    }

    /// The sort, stated once: newest first among dated entries, then every undated entry.
    ///
    /// **Undated sorts oldest, not first.** "A corrupt stamp must not outrank fresh deletions for the
    /// trash's most prominent rows" — so an unparseable value loses to every real timestamp, however
    /// old, and orders by folder name among its own kind.
    private static func isOrdered(_ lhs: Row, _ rhs: Row) -> Bool {
        switch (lhs.deleted, rhs.deleted) {
        case let (left?, right?):
            return left == right ? lhs.name < rhs.name : left > right
        case (.some, nil):
            return true
        case (nil, .some):
            return false
        case (nil, nil):
            return lhs.name < rhs.name
        }
    }

    // MARK: - Paths for the trash's writes

    /// The folders `ids` names, in display order, restricted to one liveness side.
    ///
    /// **The membership rule is not restated here** — it is `Liveness.walk`'s, the same one
    /// `ItemReferenceSet.idUniverse` and `entries(of:)` read — spelled in *paths* rather than ids
    /// because a write needs to know where the item is. So the trashed side is the trash's rows and
    /// nothing besides: a card beneath a tombstoned lane is not individually addressable, which costs
    /// nothing (it has no row for a user to act on, so no command can name it) and buys the guarantee
    /// that every path this hands a writer names something the board would draw.
    ///
    /// **A tombstoned lane still takes its subtree with it**, and that is subsumption rather than
    /// omission: its path is the lane *folder*, and removing a folder removes what is inside it. Put
    /// Back on it restores the lane and every card that rode along; Delete Immediately on it purges
    /// the whole tree, own-flag cards included — the outcome the lane entry's confirmation sentence
    /// exists to warn about (`message(lanes:unrecoverable:)`).
    ///
    /// Display order — lanes left to right, each lane then its cards — rather than the caller's set
    /// iteration order, which is not an order at all: a batch that fails partway must fail the same
    /// way twice (`BoardStore.styleSubjects` makes the same choice for the same reason). The walk
    /// visits in exactly that order, so this is a filter over it and never a sort.
    public static func paths(of ids: Set<ItemID>, on side: Liveness, in snapshot: BoardModel) -> [ItemPath] {
        guard !ids.isEmpty else { return [] }
        var result: [ItemPath] = []
        side.walk(snapshot) { lane, card in
            guard ids.contains(card?.id ?? lane.id) else { return }
            result.append(ItemPath(laneID: lane.id, cardID: card?.id))
        }
        return result
    }

    /// Every folder Empty Trash removes — "emptying purges every tombstone on the board, filter or
    /// no filter" (03 ▸ Trash).
    ///
    /// `paths(of:on:in:)` on the trashed side with **no id filter at all**, which is the strongest
    /// form of that guarantee: the command's scope is the trashed universe itself, so it cannot
    /// narrow to a selection any more than it can narrow to the search.
    ///
    /// **A tombstoned lane contributes only itself**, and that is not an omission: removing the lane
    /// folder removes the tree beneath it, own-flag cards included. Listing those cards as well would
    /// be redundant purges of paths the first removal already took (harmless — `purgeItem` treats a
    /// folder that is already gone as success — but noise) and would require a second, broader
    /// definition of "in the trash" than the one every other caller reads.
    public static func emptyTrashTargets(in snapshot: BoardModel) -> [ItemPath] {
        var result: [ItemPath] = []
        Liveness.trashed.walk(snapshot) { lane, card in
            result.append(ItemPath(laneID: lane.id, cardID: card?.id))
        }
        return result
    }

    // MARK: - Counts and phrasing

    /// How many lane entries and card entries a set of entries holds — the confirmation dialogs'
    /// only input beyond the item titles.
    public struct EntryCounts: Sendable, Equatable {
        public var lanes: Int = 0
        public var cards: Int = 0

        public var total: Int { lanes + cards }
        public var isEmpty: Bool { total == 0 }
    }

    public static func counts(of entries: [TrashEntry]) -> EntryCounts {
        var counts = EntryCounts()
        for entry in entries {
            if entry.isLaneEntry { counts.lanes += 1 } else { counts.cards += 1 }
        }
        return counts
    }

    /// The counts a set of `ItemPath`s describes — the same two numbers, from the shape the write
    /// path actually carries.
    public static func counts(of paths: [ItemPath]) -> EntryCounts {
        var counts = EntryCounts()
        for path in paths {
            if path.isLane { counts.lanes += 1 } else { counts.cards += 1 }
        }
        return counts
    }

    /// "2 lanes and 3 cards", "41 cards", "1 lane" — 06-history-undo.md's **plural folding** applied
    /// to a mixed trash selection.
    ///
    /// An empty count reads "nothing", which no caller renders: both confirmations refuse to open on
    /// an empty scope. It is spelled anyway so the function is total.
    public static func phrase(_ counts: EntryCounts) -> String {
        switch (counts.lanes, counts.cards) {
        case (0, 0): "nothing"
        case let (0, cards): plural(cards, "card")
        case let (lanes, 0): plural(lanes, "lane")
        case let (lanes, cards): "\(plural(lanes, "lane")) and \(plural(cards, "card"))"
        }
    }

    private static func plural(_ count: Int, _ noun: String) -> String {
        "\(count) \(noun)\(count == 1 ? "" : "s")"
    }

    // MARK: - Confirmations

    /// A purge confirmation's three strings, built once and rendered by the window's alert.
    ///
    /// A value rather than a view so the phrasing rules — plural folding, naming a sole item, the
    /// lane caveat, and whether the loss is actually irreversible — are testable without an alert on
    /// screen (`TrashModelTests`).
    public struct PurgePrompt: Sendable, Equatable {
        public let title: String
        public let message: String
        public let confirmTitle: String
    }

    /// Delete Immediately's alert — "the alert stands between one keystroke and unrecoverable
    /// deletion" (03-board-ui.md § Trash).
    ///
    /// A sole item is **named**; several fold into counts. `nil` when the ids name nothing
    /// tombstoned, which is also the command's own refusal — so the prompt and the action can never
    /// disagree about whether there is anything to purge.
    public static func purgePrompt(
        for ids: Set<ItemID>,
        in snapshot: BoardModel,
        unrecoverable: Bool
    ) -> PurgePrompt? {
        let targets = paths(of: ids, on: .trashed, in: snapshot)
        guard !targets.isEmpty else { return nil }

        let counts = counts(of: targets)
        let subject: String
        if targets.count == 1, let only = targets.first {
            subject = "\u{201C}\(displayName(of: only, in: snapshot))\u{201D}"
        } else {
            subject = phrase(counts)
        }
        return PurgePrompt(
            title: "Permanently delete \(subject)?",
            message: message(lanes: counts.lanes, unrecoverable: unrecoverable),
            confirmTitle: "Delete"
        )
    }

    /// Empty Trash…'s alert — **always shown** ("bulk scope, not per-item recoverability, is what it
    /// guards"), and always naming the **true count**: every tombstone on the board, never the
    /// filtered view.
    ///
    /// Counts rather than names even for a single entry, because the command is about the trash
    /// rather than about an item: "Permanently delete 41 cards" is the design's own example phrasing
    /// (06-history-undo.md's plural folding).
    public static func emptyTrashPrompt(in snapshot: BoardModel, unrecoverable: Bool) -> PurgePrompt? {
        let counts = counts(of: emptyTrashTargets(in: snapshot))
        guard !counts.isEmpty else { return nil }
        return PurgePrompt(
            title: "Permanently delete \(phrase(counts))?",
            message: message(lanes: counts.lanes, unrecoverable: unrecoverable),
            confirmTitle: "Delete"
        )
    }

    /// The alert's body: what a lane takes with it, and whether any of it comes back.
    ///
    /// The lane sentence is not decoration — a lane entry's row says "3 cards" (what Put Back would
    /// return), while purging the lane folder takes *every* card inside it, individually tombstoned
    /// ones included. That gap is exactly what a confirmation is for.
    private static func message(lanes: Int, unrecoverable: Bool) -> String {
        var parts: [String] = []
        if lanes > 0 {
            parts.append("Deleting a lane also deletes every card inside it.")
        }
        // m7-git: on a git board the content stays reachable in history, so the second sentence is
        // the honest one — and Delete Immediately does not confirm there at all
        // (`BoardStore.purgeIsUnrecoverable`).
        parts.append(unrecoverable
            ? "This can\u{2019}t be undone."
            : "The board\u{2019}s history still has them.")
        return parts.joined(separator: " ")
    }

    /// What to call an item in a prompt — its title, or the "Untitled" rendering.
    ///
    /// Total by construction: a path whose item has gone since the prompt was asked for reads
    /// "Untitled" rather than failing, which is the same shrug every other vanished-target rule in
    /// the app gives.
    private static func displayName(of path: ItemPath, in snapshot: BoardModel) -> String {
        guard let lane = snapshot.lanes.first(where: { $0.id == path.laneID }) else { return "Untitled" }
        guard let cardID = path.cardID else { return lane.title.value ?? "Untitled" }
        return lane.cards.first { $0.id == cardID }?.title.value ?? "Untitled"
    }

    // MARK: - Menu validation

    /// Whether File ▸ Delete has something to tombstone — a **live**, non-empty selection that still
    /// names something the board renders.
    ///
    /// The liveness side is the whole of the binary: "menu validation stays binary — Delete for live
    /// selections, Put Back / Delete Immediately for tombstoned ones" (04 ▸ The trash). The
    /// resolution against the snapshot is what keeps a selection the next reload will drop from
    /// enabling an item that would write nothing.
    public static func canDelete(selection: ItemReferenceSet, in snapshot: BoardModel) -> Bool {
        selection.liveness == .live && !paths(of: selection.ids, on: .live, in: snapshot).isEmpty
    }

    /// Whether File ▸ Put Back and File ▸ Delete Immediately have something to act on — the exact
    /// mirror of `canDelete`, which is what makes the two ⌘⌫ twins enable exactly one of themselves.
    public static func canActOnTrash(selection: ItemReferenceSet, in snapshot: BoardModel) -> Bool {
        selection.liveness == .trashed && !paths(of: selection.ids, on: .trashed, in: snapshot).isEmpty
    }
}