import Foundation

/// The styling system's vocabulary — what a style gesture *is*, before any view or any write exists
/// (03-board-ui.md § Styling). Three small value types, deliberately kept out of both the view layer
/// and the Writer: the editor builds gestures out of them, `BoardStore.applyStyle` turns them into
/// files, and every rule that can be stated without a screen or a disk is stated here where a test
/// can reach it.

// MARK: - What a gesture asks of one key

/// What one style gesture asks of one frontmatter key.
///
/// **Three cases, because "leave it alone" and "there is no value" are different instructions.** A
/// style editor always speaks about both dimensions at once — a click on a background well says
/// nothing about the symbol — so `keep` is what the untouched dimension carries, and `remove` is
/// what the two leading wells carry: "the None well … removes the `background` key" and the symbol
/// grid's "leading well is the level's default symbol and removes the `icon` key" (03 § Styling ▸
/// Controls). Writing `background: ""` instead would be a real (if blank) value and the exact
/// mistake the remove-at-default family — the empty rename, the width landing on 1 — exists to
/// avoid.
///
/// `set` carries the string that goes to frontmatter verbatim: a kebab-case palette name from the
/// grid, or an SF Symbol name. Hex never arrives here from the app ("custom hex is not pickable
/// in-app but stays fully honored from disk"), though nothing in the type forbids it.
public enum StyleChange: Sendable, Equatable {
    case keep
    case set(String)
    case remove
}

// MARK: - What a gesture is aimed at

/// What a style edit acts on — "one component, one behavior, three anchors" needs exactly one
/// vocabulary word for "who is being styled" (03-board-ui.md § Styling ▸ Controls).
///
/// **A set of items, or the board — never both, and never a mix of levels in spirit.** The editor is
/// "selection-aware: it styles the selected cards or lane, and with nothing selected, the board", and
/// 04-interactions.md's cards-XOR-lanes rule means a live selection is homogeneous by kind. Nothing
/// here enforces that (a `Set<ItemID>` cannot), because the enforcement belongs to the selection
/// model; what this type *does* guarantee is that a board target and an item target are structurally
/// different things, which is what makes "it never silently retargets to the board" (§ Controls) a
/// property of the code rather than a discipline.
public enum StyleTarget: Sendable, Equatable {
    case board
    case items(Set<ItemID>)
}

/// Which level a target sits at — the one thing the editor needs beyond the values themselves,
/// because the symbol grid's leading well is "the level's default symbol" (03 § Styling ▸ Controls)
/// and the three defaults differ (§ Capabilities).
public enum StyleLevel: Sendable, Equatable {
    case board
    case lane
    case card
}

// MARK: - The current value across a target set

/// What one style dimension currently reads across a whole target set — the editor's per-dimension
/// display, and the whole of 03-board-ui.md § Styling ▸ Controls' batch rule: "a multi-selection
/// shows per-dimension mixed state (no well selected, '—' where a value would read)".
///
/// Three states rather than an optional plus a flag, because the three mean genuinely different
/// things to the grid: `unset` selects the leading well (None / the level default), `uniform`
/// selects the matching well — or, when the value is off-palette, labels itself verbatim outside the
/// grid — and `mixed` selects nothing at all.
///
/// **Deliberately a pure function of the fields.** It takes `FieldValue`s rather than a store so the
/// interesting rules — what counts as agreement, what a malformed value reads as — are testable
/// without a board on disk.
public enum StyleFieldState: Sendable, Equatable {
    /// No target carries the key. The leading well is the current value.
    case unset
    /// Every target carries the same value, written exactly this way.
    case uniform(String)
    /// The targets disagree. Nothing is selected; the display reads "—".
    case mixed

    /// The state of one dimension over `fields`, one per target, in any order.
    ///
    /// An **empty** list is `unset` rather than a fourth case: a target set with no live members is
    /// about to dismiss the popover anyway (`StyleEditorSession.resolved(against:)`), and a frame
    /// rendered with nothing selected is the honest thing to show in the meantime.
    public static func resolve(_ fields: [FieldValue<String>]) -> StyleFieldState {
        var iterator = fields.makeIterator()
        guard let first = iterator.next() else { return .unset }
        let firstText = written(first)
        while let next = iterator.next() {
            guard written(next) == firstText else { return .mixed }
        }
        guard let firstText else { return .unset }
        return .uniform(firstText)
    }

    /// The value **as written on disk**, or `nil` when the key is absent.
    ///
    /// A malformed field — a sequence where a scalar belongs — reads as its raw text rather than as
    /// nothing, which is what makes the off-palette display honest: "off-palette values display
    /// leniently … labeled verbatim, outside the grids" (03 § Styling ▸ Controls) is about *what the
    /// author wrote*, and a value the parser could not make a string of is exactly the case where
    /// showing the bytes matters most. Agreement is judged on this same text, so two targets whose
    /// `background` is malformed in the same way read as uniform — they do carry the same value.
    public static func written(_ field: FieldValue<String>) -> String? {
        switch field {
        case .missing: nil
        case let .valid(value): value
        case let .malformed(raw): raw
        }
    }
}

// MARK: - The open Style… popover

/// The Style… popover's live target — 03-board-ui.md § Styling ▸ Controls' settled lifecycle
/// sentence, as a value: "The Style… popover tracks its target set live and dismisses when it
/// empties … its target is the selection, re-resolved across reloads by 02-architecture.md's UUID
/// rule — a member that vanishes or flips liveness leaves the set and the mixed-state display
/// recomputes; a set emptied by a foreign reload dismisses the popover … it never silently retargets
/// to the board".
///
/// **Held in `TransientBoardState`, not in a view**, and that is the whole design of it: the rule is
/// a *reload* rule, and the container that re-grounds the selection, the drag, the pending cut and
/// the rename editor on every applied snapshot is the one place a reload rule can be written once.
/// The sibling it most resembles is `RenameEditor` — an item-referencing editor that a vanish
/// discards — which is exactly the analogy the design draws ("the inline-rename discard applied
/// here").
///
/// The two embedded anchors (the card window's Style section, the board popover's styling area) need
/// none of this and get none: "the card sidebar dismisses with its card's window, and the board
/// popover's target is the board itself".
public struct StyleEditorSession: Sendable, Equatable {

    /// Who is being styled. `let`, because a session never changes target — a session whose targets
    /// all vanished is *gone*, not re-aimed.
    public let target: StyleTarget

    public init(target: StyleTarget) {
        self.target = target
    }

    /// This session re-grounded on a freshly applied snapshot, or `nil` when it has nothing left to
    /// style — which the presenting anchor reads as "dismiss".
    ///
    /// - **Items** are constrained through `ItemReferenceSet`, so liveness is **effective —
    ///   ancestor-walked** for free: a card under a lane an agent just tombstoned renders nowhere and
    ///   therefore leaves the set, exactly as it leaves the selection.
    /// - **The board** never dismisses. It cannot vanish from its own snapshot, and a board target
    ///   has no membership to lose ("a board-targeted editor has no vanish case").
    ///
    /// Nothing here can turn an item target into a board target: emptiness returns `nil`, and the
    /// only way a board session exists is for one to have been opened that way.
    public func resolved(against snapshot: BoardModel) -> StyleEditorSession? {
        switch target {
        case .board:
            return self
        case let .items(ids):
            let live = ItemReferenceSet(ids: ids, container: .board).resolved(against: snapshot).ids
            guard !live.isEmpty else { return nil }
            return live == ids ? self : StyleEditorSession(target: .items(live))
        }
    }

    /// The item the popover hangs off, or `nil` for the board window itself.
    ///
    /// **Derived, never stored.** Every candidate anchor — a card face, a lane header, the strip —
    /// asks this one question of the current snapshot, so exactly one surface presents the popover
    /// and the answer follows the target set across reloads: an anchor that vanishes hands the
    /// popover to the next live target rather than taking it down with it (only an *emptied* set
    /// dismisses). Display order — lanes left to right, cards top to bottom — is the tie-break, so
    /// the popover lands on the first thing the user's eye would find.
    public func presentationAnchor(in snapshot: BoardModel) -> ItemID? {
        guard case let .items(ids) = target else { return nil }
        for lane in snapshot.lanes {
            if ids.contains(lane.id) { return lane.id }
            if let card = lane.cards.first(where: { ids.contains($0.id) }) { return card.id }
        }
        return nil
    }
}