import Foundation

// MARK: - One label's standing on a board

/// One label the board actually uses, and how many cards use it.
public struct LabelTally: Sendable, Equatable {

    /// The label as it is **spelled on the board** — the first spelling the derivation met, in board
    /// order (lanes left to right, each lane's cards top to bottom, then the trash).
    ///
    /// Case-insensitive uniqueness is a per-*card* rule (`CardLabels`), so two cards may perfectly
    /// legally spell one label two ways. The universe has to pick one to offer, and the first is the
    /// only choice that does not depend on how many cards happen to carry each variant — a count-based
    /// pick would make a menu row's capitalisation flicker as cards are tagged.
    public let name: String

    /// How many **cards** carry it — never how many times it appears, which is the same number: a
    /// card's own list is deduplicated by the reading.
    public let count: Int

    public init(name: String, count: Int) {
        self.name = name
        self.count = count
    }
}

// MARK: - The universe

/// **Every label the board uses, with its frequency** — the "used labels" universe both label
/// surfaces draw from (the sidebar's autocomplete and the context menu's twelve rows plus its More…
/// dialog).
///
/// ### Derived from the snapshot, and there is no other store
///
/// The owner's ruling, and the format's own posture: the files *are* the board, so the set of labels
/// in use is a fact about the cards rather than a list the app maintains beside them. A label exists
/// because a card carries it and stops existing when the last card drops it — there is no rename, no
/// palette, no orphan to garbage-collect, and an agent that hand-writes `labels: [spike]` into an
/// `index.md` has created a label as fully as the menu can.
///
/// **The trash counts.** `BoardModel.trash` is walked beside `lanes` because a trashed card is "an
/// ordinary card in a special place" (03-board-ui.md § Trash) and, more to the point, because the
/// alternative is worse: deleting the only card carrying `spike` would silently evict the label from
/// the autocomplete, and restoring the card would bring it back — a vocabulary that flickers with the
/// trash is not a vocabulary. `trashedLanes` is deliberately *not* walked: a trashed lane is an opaque
/// unit whose cards the snapshot never loads (`TrashedLane`), so there is nothing there to count.
///
/// ### Cost
///
/// One pass over every live and trashed card, reading a field the loader already parsed — the same
/// cost class as any other whole-board derivation, and paid **once per snapshot** (`BoardStore`
/// caches it, equality-gated) rather than once per view body. That is what keeps the context menu's
/// builder free of O(board) work: everything a card face reads is this value, already built.
public struct LabelIndex: Sendable, Equatable {

    /// Every used label, **ordered by frequency descending, then case-insensitively alphabetically**
    /// — a total, snapshot-only order, so two runs over the same board produce the identical array.
    ///
    /// The alphabetical tie-break is here rather than left to the ranking because this ordering has to
    /// be deterministic on its own: it is what `universe` shows and what the ranking starts from, and
    /// a derivation whose ties fell out of dictionary iteration order would make the More… dialog
    /// reshuffle itself between reloads that changed nothing.
    public let tallies: [LabelTally]

    public init(tallies: [LabelTally]) {
        self.tallies = tallies
    }

    /// The empty board's index — no cards, or no card carrying a label.
    public static let empty = LabelIndex(tallies: [])

    /// Every used label's display name, in `tallies`' order.
    public var names: [String] { tallies.map(\.name) }

    public var isEmpty: Bool { tallies.isEmpty }

    /// The universe as the More… dialog lists it: **alphabetically, case-insensitively**, because a
    /// dialog whose whole job is "find the one you mean among all of them" is a lookup, and a
    /// frequency order is a lookup you cannot do.
    public var alphabetical: [String] {
        tallies.map(\.name).sorted { Self.precedesAlphabetically($0, $1) }
    }

    /// Whether the board already uses a case-insensitively equal name — the create field's "this is
    /// not new" test.
    public func contains(_ name: String) -> Bool {
        CardLabels.contains(name, in: names)
    }

    /// The board's own spelling of a name, when it has one — so a user who types `BUG` into the create
    /// field adds the `bug` the rest of the board already says, rather than minting a second variant.
    public func canonicalSpelling(of name: String) -> String? {
        guard let name = CardLabels.normalized(name) else { return nil }
        let key = CardLabels.canonical(name)
        return tallies.first { CardLabels.canonical($0.name) == key }?.name
    }

    // MARK: Derivation

    /// The universe of `snapshot`, live cards and trash alike.
    public static func derive(from snapshot: BoardModel) -> LabelIndex {
        var order: [String] = []
        var counts: [String: Int] = [:]
        var spellings: [String: String] = [:]

        func absorb(_ card: Card) {
            for name in card.labels.value ?? [] {
                let key = CardLabels.canonical(name)
                if spellings[key] == nil {
                    spellings[key] = name
                    order.append(key)
                }
                counts[key, default: 0] += 1
            }
        }

        for lane in snapshot.lanes {
            for card in lane.cards { absorb(card) }
        }
        for card in snapshot.trash { absorb(card) }

        let tallies = order.map { key in
            LabelTally(name: spellings[key] ?? key, count: counts[key] ?? 0)
        }
        return LabelIndex(tallies: tallies.sorted(by: precedes))
    }

    /// Frequency descending, then case-insensitively alphabetical, then by raw spelling — a **total**
    /// order, which is what makes the derivation reproducible.
    private static func precedes(_ lhs: LabelTally, _ rhs: LabelTally) -> Bool {
        if lhs.count != rhs.count { return lhs.count > rhs.count }
        return precedesAlphabetically(lhs.name, rhs.name)
    }

    /// Case-insensitive alphabetical, with the raw spelling as the last resort so `Bug` and `bug`
    /// never compare equal and the sort stays total.
    static func precedesAlphabetically(_ lhs: String, _ rhs: String) -> Bool {
        let folded = CardLabels.canonical(lhs).compare(CardLabels.canonical(rhs))
        if folded != .orderedSame { return folded == .orderedAscending }
        return lhs < rhs
    }
}

// MARK: - The menu's twelve

/// **"The most frequently and recently used labels"** — the owner's phrase for the card context
/// menu's twelve rows, turned into arithmetic.
///
/// ### The ranking, exactly
///
/// 1. **Frequency, descending** — how many of the board's cards (live and trashed) carry the label
///    (`LabelIndex`).
/// 2. **Recency, as the tie-break** — among labels used by the same number of cards, the one this
///    user applied most recently comes first (`LabelRecents`, an app-side MRU updated on every apply).
///    A label the MRU has never seen sorts behind every label it has.
/// 3. **Case-insensitively alphabetical**, then raw spelling — so the order is *total* and a menu
///    never reshuffles between two openings that changed nothing.
///
/// Then the first twelve.
///
/// ### Why frequency leads and recency follows, and what that costs
///
/// This is the ruling as given, implemented as given. It is worth being explicit about the
/// consequence, because it is visible: on a board with more than twelve labels in heavy use, a label
/// **invented today** will not appear in these rows until enough cards carry it to beat the
/// twelfth-most-common one — even though it is by far the most *recently* used. The recency half only
/// separates labels that are already tied on frequency, which on a board with an even spread of usage
/// is often most of them, and on a lopsided board is almost none.
///
/// The alternative — a blended score, or reserving a slot or two for pure recency — would surface a
/// fresh label immediately at the cost of a menu whose contents move under the user's hand. Neither
/// is obviously right, so the literal reading of the ruling is what shipped, and the observation is
/// journaled for the owner rather than quietly designed around. **More…** is the escape hatch either
/// way: every used label is one row further in, and the create field is beside it.
public enum LabelRanking {

    /// How many rows the context menu's `labels` submenu shows before its separator — the owner's
    /// "up to 12".
    public static let menuLimit = 12

    /// The top `limit` of `index`, ranked as the type comment describes.
    ///
    /// - Parameter recents: most-recent-first, `LabelRecents.labels`' own order. Entries naming labels
    ///   the board no longer uses are simply never matched — the MRU is app-wide and a board is not
    ///   obliged to know about labels from another one.
    public static func ranked(_ index: LabelIndex, recents: [String], limit: Int = menuLimit) -> [String] {
        guard limit > 0 else { return [] }
        // Position in the MRU, by canonical name — built once rather than searched per comparison, so
        // the sort stays O(n log n) instead of O(n² ) in the number of distinct labels.
        var recency: [String: Int] = [:]
        for (position, name) in recents.enumerated() {
            let key = CardLabels.canonical(name)
            if recency[key] == nil { recency[key] = position }
        }

        let ordered = index.tallies.sorted { lhs, rhs in
            if lhs.count != rhs.count { return lhs.count > rhs.count }
            let left = recency[CardLabels.canonical(lhs.name)] ?? Int.max
            let right = recency[CardLabels.canonical(rhs.name)] ?? Int.max
            if left != right { return left < right }
            return LabelIndex.precedesAlphabetically(lhs.name, rhs.name)
        }
        return ordered.prefix(limit).map(\.name)
    }
}