import Observation
import SwiftUI

// MARK: - The mode

/// Which of the body column's two surfaces is showing (05-card-window.md ▸ Mode grammar).
///
/// **Two cases, not three.** The raw-source outlet swaps the *entire content area* — title, body
/// and sidebar — so it is a state of the window, not of the body column, and it does not belong in
/// this enum. Edit Body disabling while raw source is active (11-command-nexus.md) is that
/// window-level state's rule to enforce over this one.
public enum CardBodyMode: Equatable, Sendable {
    /// The rendered, selectable preview — **the resting state**.
    case preview
    /// The raw-Markdown editor.
    case edit

    /// The mode a window opens its body in: **Preview, unless the body is empty** (05 ▸ Mode
    /// grammar: "a card opens in Preview — unless its body is empty, which opens straight into
    /// Edit with the cursor ready (a new card has nothing to preview, so ⌘↩ during creation flows
    /// title → body without a mode stop)").
    ///
    /// Whitespace is empty (`BodyMarkup.isEmpty`): a body holding one newline previews as a blank
    /// page, and stopping the user at a blank preview of a blank body is precisely the ceremony
    /// the rule removes.
    public static func opening(body: String) -> CardBodyMode {
        BodyMarkup.isEmpty(body) ? .edit : .preview
    }

    /// ⌘E — View ▸ Edit Body's checkmark toggle. Also the whole of "Return in Preview enters Edit"
    /// and "Escape in Edit returns to Preview": three gestures, one flip, so they cannot drift.
    public var toggled: CardBodyMode {
        self == .preview ? .edit : .preview
    }
}

// MARK: - The window's body surface, as a handle

/// One card window's body column, reduced to what things *outside* it need: which mode it is in,
/// and how to put a find bar over whichever surface currently holds the keyboard.
///
/// `BoardSearchPresentation`'s shape and for its reason — one per window, `@State` in the host,
/// published through the focus system so a **menu item** (Edit ▸ Find, ⌘F) can reach the frontmost
/// card window without anyone keeping a which-window-is-key register. It is deliberately not on
/// `BoardStore`: the store is the *board's*, shared by every window on it, and two card windows
/// open on two cards of one board are in two different modes.
@MainActor
@Observable
public final class CardBodyPresentation {

    /// The surface showing right now. Starts in Preview and is settled by `openIfNeeded(body:)`
    /// the first time the window has a body to judge.
    public var mode: CardBodyMode = .preview

    /// Puts the standard find bar over the focused body surface — **Edit ▸ Find (⌘F) is
    /// find-in-text here** (05 ▸ Preview; 11-command-nexus.md scopes ⌘F "Board window: board
    /// search; card window: find-in-text"). Filled in by the surface itself, which is the only
    /// thing that holds a text view to hand the action to; `nil` until one exists, which is also
    /// exactly when ⌘F has nothing to find in.
    public var findInText: (() -> Void)?

    /// Whether the opening rule has already run for this window.
    ///
    /// **Once, not per snapshot.** The rule is about *opening* a card, and the body it judges
    /// arrives with the first snapshot — but snapshots keep arriving (a watcher reload, a lane
    /// move, another window's edit). Re-running it would drag a reader back into Edit the moment
    /// someone else emptied the file, and would fight a user who had just pressed ⌘E.
    private var hasOpened = false

    public init() {}

    /// Applies the opening rule the first time it is called, and does nothing on every call after.
    @discardableResult
    public func openIfNeeded(body: String) -> CardBodyMode {
        guard !hasOpened else { return mode }
        hasOpened = true
        mode = CardBodyMode.opening(body: body)
        return mode
    }

    /// ⌘E, Return in Preview, Escape in Edit — see `CardBodyMode.toggled`.
    public func toggleMode() {
        mode = mode.toggled
    }
}

/// The focused card window's body column, beside `FocusedValues.boardSearch` — see
/// `FocusedBoardStoreKey` for why window-scoped menu items reach their window this way.
struct FocusedCardBodyKey: FocusedValueKey {
    typealias Value = CardBodyPresentation
}

extension FocusedValues {
    var cardBody: CardBodyPresentation? {
        get { self[FocusedCardBodyKey.self] }
        set { self[FocusedCardBodyKey.self] = newValue }
    }
}