import Foundation

// MARK: - NativeHistoryProvider

/// The base edition's undo substrate: one `NSUndoManager`-backed stack per board session
/// (13-native-undo.md).
///
/// ### Why `NSUndoManager` at all, when the steps are ours
///
/// Because the *command surface* is the platform's. Edit ▸ Undo and Edit ▸ Redo are the system's own
/// nil-target `undo:`/`redo:` rows (11-command-nexus.md), the toolbar pair carries the same actions
/// (`BoardToolbar`), and both light up, disable and **retitle** from whatever `UndoManager` the
/// focused window hands back — "Undo Move 3 Cards" is `NSUndoManager`'s dynamic retitling, which 13
/// names as the mechanism and 12 records as the one both editions share. Reimplementing the stack
/// over two arrays would mean reimplementing that, badly.
///
/// It stays *behind* `HistoryProviding` regardless: nothing outside this file learns that base's
/// stack is an `NSUndoManager`, which is what lets pro-m1 bind git to the same seam
/// (12 ▸ The provider seam).
///
/// ### One `register` call is exactly one step
///
/// `groupsByEvent` is turned **off** and every registration is wrapped in its own group. The default
/// (on) closes a group at the end of the run-loop turn, which would silently fold two gestures that
/// happened to land in one event into a single ⌘Z — the opposite of 13's "one gesture, one undo
/// step", and the coalescing decision belongs to the Writer call site (a multi-card move registers
/// *one* step with a plural title), never to the run loop's timing.
///
/// ### Undo flips to redo by re-registering
///
/// A step that applies pushes its own mirror image back onto the manager from inside the undo — and
/// `NSUndoManager` routes a registration made while it is undoing onto the **redo** stack, and vice
/// versa. That single rule gives the whole classic dance (undo → redo → undo …) with no second stack
/// of our own, and it is why a *skipped* step leaves nothing behind: it registers nothing, so the
/// empty group is discarded and the step is simply gone (13: "popped from the stack ... and ⌘Z falls
/// through to the next step").
@MainActor
public final class NativeHistoryProvider: HistoryProviding {

    /// The stack. `private` and never vended: see the type's note.
    private let manager = UndoManager()

    /// Set by the step just crossed when it declined to apply, read by the crossing loop below.
    /// A flag rather than a return value because the manager, not this object, calls the step.
    private var lastCrossingSkipped = false

    public init() {
        // See "One `register` call is exactly one step", above.
        manager.groupsByEvent = false
    }

    // MARK: - HistoryProviding

    public var canUndo: Bool { manager.canUndo }

    public var canRedo: Bool { manager.canRedo }

    /// `NSUndoManager` answers `""` — not `nil` — for a stack with nothing on it *and* for a step
    /// registered without a name, so the emptiness check is the honest one.
    public var undoActionName: String? {
        guard canUndo, !manager.undoActionName.isEmpty else { return nil }
        return manager.undoActionName
    }

    public var redoActionName: String? {
        guard canRedo, !manager.redoActionName.isEmpty else { return nil }
        return manager.redoActionName
    }

    public func register(_ step: HistoryStep) {
        push(step)
    }

    public func undo() {
        cross(manager.undo, while: { self.manager.canUndo })
    }

    public func redo() {
        cross(manager.redo, while: { self.manager.canRedo })
    }

    public func clear() {
        manager.removeAllActions()
    }

    // MARK: - The stack

    /// Registers `step` as one group of its own, named for the menu.
    ///
    /// The target is `self` and the payload rides in the closure, which is `NSUndoManager`'s
    /// block-based form: the manager references its target **unowned** (its own documented rule), so
    /// the provider owning the manager that references the provider is not a cycle.
    private func push(_ step: HistoryStep) {
        manager.beginUndoGrouping()
        manager.registerUndo(withTarget: self) { provider in
            provider.apply(step)
        }
        // Inside the group, deliberately: the name belongs to the group being closed, and on the
        // way back it is what retitles Redo.
        manager.setActionName(step.name)
        manager.endUndoGrouping()
    }

    /// Runs one step's action and records what happened.
    private func apply(_ step: HistoryStep) {
        switch step.undo() {
        case .applied:
            // Lands on the opposite stack — see "Undo flips to redo by re-registering".
            push(step.reversed)
        case .skipped:
            lastCrossingSkipped = true
        }
    }

    /// Crosses one step, and keeps going while the steps it crosses decline to apply — 13's
    /// fall-through: "the step is skipped, not applied ... and ⌘Z falls through to the next step".
    ///
    /// The `while` guard is the manager's own emptiness, so a stack of nothing but stale steps
    /// empties itself and stops rather than spinning.
    private func cross(_ step: () -> Void, while more: () -> Bool) {
        repeat {
            guard more() else { return }
            lastCrossingSkipped = false
            step()
        } while lastCrossingSkipped
    }
}