import Foundation

// MARK: - NativeHistoryProvider

/// The free tier's undo substrate: one stack per board session (13-native-undo.md) — **and the
/// substrate of every open card window's stack, in either tier** (re-ruled 2026-07-31, the
/// session-coarsening model): a window's fine-grained gestures are values-based inverses whatever the
/// board's own substrate is, so `CardWindowUndo` holds one of these too. Nothing below knows which of
/// the two it is; both need the same four-line grammar.
///
/// ### Two arrays, and why not `NSUndoManager`
///
/// This provider was an `NSUndoManager` for exactly one milestone, on the argument that 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. All of that is still true, and none of it lives here: the retitling is
/// `BoardUndoManager.undoMenuItemTitle` composing `undoMenuTitle(forUndoActionName:)` over the bare
/// phrase this seam vends as a `String?`. The adapter is the `UndoManager`; the substrate never
/// needed to be one.
///
/// What forced the change is the staleness milestone's third outcome. `HistoryStepOutcome.failed`
/// means **the step stays put** — a disk error is retryable, so ⌘Z must still be able to reach the
/// step it just could not write. `NSUndoManager` pops a group before running it and offers no way to
/// put it back: a registration made while undoing lands on the *redo* stack by its own documented
/// rule, and one made after the crossing returns clears the redo stack outright. Either way a failed
/// undo would have quietly destroyed something. Two arrays express all three outcomes exactly, and
/// the grammar they have to implement is four lines long.
///
/// ### One `register` call is exactly one step
///
/// Nothing here groups, coalesces, or waits for the end of a run-loop turn — 13's "one gesture, one
/// undo step" is a property of the Writer call sites (a multi-card move registers *one* step with a
/// plural title), and the substrate's job is to not have opinions about it. This is what
/// `NSUndoManager`'s `groupsByEvent = false` was buying, as an absence rather than a setting.
///
/// ### Undo flips to redo by reversing
///
/// A step that applies is pushed onto the opposite stack **reversed** — its two halves swapped
/// (`HistoryStep.reversed`) — which gives the whole classic dance (undo → redo → undo …) with one
/// rule. Both stacks therefore hold steps oriented so that *crossing them means calling `undo`*, and
/// a skipped step leaves nothing behind at all: it is popped and never re-pushed, which is 13's
/// "popped from the stack ... and ⌘Z falls through to the next step".
@MainActor
public final class NativeHistoryProvider: HistoryProviding {

    /// The two stacks, top last. Both hold steps oriented for crossing — see the type's note.
    private var undoSteps: [HistoryStep] = []
    private var redoSteps: [HistoryStep] = []

    public init() {}

    // MARK: - HistoryProviding

    public var canUndo: Bool { !undoSteps.isEmpty }

    public var canRedo: Bool { !redoSteps.isEmpty }

    /// The phrase the menu title is composed from, or `nil` when there is nothing to cross — and
    /// also `nil` for a step registered without a name, which is the emptiness the adapter's `""`
    /// contract is written against.
    public var undoActionName: String? { name(of: undoSteps.last) }

    public var redoActionName: String? { name(of: redoSteps.last) }

    /// **The steps currently crossable by ⌘Z, oldest first** — read-only, and read by exactly one
    /// caller: the card window's close, which folds the session's own stack into the one coarse board
    /// step (13-native-undo.md ▸ Rules ▸ "Window close coarsens"; `CardWindowUndo`).
    ///
    /// The undo stack is the honest input to that fold and the redo stack is not: a gesture the user
    /// undid inside the window is a gesture whose effect is no longer on disk, so it must contribute
    /// nothing to the session's net effect. Nothing here decides what a fold *means* — this is the
    /// membership question, and the two arrays already answer it.
    public var pendingSteps: [HistoryStep] { undoSteps }

    /// Records one undoable step and clears the redo stack — the classic rule, and the one every
    /// substrate shares.
    ///
    /// The cleared steps are **retired** on the way out (`HistoryStep.Retirement`): this is the
    /// "undone-and-superseded" half of the deferred purge's release condition, and it is the only
    /// moment the app can see it.
    public func register(_ step: HistoryStep) {
        undoSteps.append(step)
        retire(redoSteps)
        redoSteps.removeAll()
    }

    public func undo() { cross(.undo) }

    public func redo() { cross(.redo) }

    /// Session teardown, the add-git substrate swap, a branch reseed — every step goes, so every step
    /// retires: "the purge runs when the coarse step leaves the board stack ... or the board session
    /// ends" (13 ▸ Interaction with the trash).
    public func clear() {
        retire(undoSteps)
        retire(redoSteps)
        undoSteps.removeAll()
        redoSteps.removeAll()
    }

    private func retire(_ steps: [HistoryStep]) {
        for step in steps { step.retirement?.run() }
    }

    // MARK: - The crossing

    /// Crosses one step, and keeps going while the steps it crosses decline as **stale** — 13's
    /// fall-through: "the step is skipped, not applied ... and ⌘Z falls through to the next step".
    ///
    /// The loop's own exit is an empty stack, so a stack of nothing but stale steps empties itself
    /// and stops rather than spinning. The other two outcomes each end the crossing after one step:
    /// an applied step is the ⌘Z the user asked for, and a failed one leaves the stack exactly as it
    /// found it (`HistoryStepOutcome.failed`).
    private func cross(_ direction: HistoryDirection) {
        while let step = pop(direction) {
            switch step.undo(direction) {
            case .applied:
                push(step.reversed, onto: direction.opposite)
                return
            case .skipped:
                // Dropped for good — the third of the deferred purge's release conditions ("gone
                // stale"), and the reason a skip is reported here rather than merely counted.
                step.retirement?.run()
                continue
            case .failed:
                push(step, onto: direction)
                return
            }
        }
    }

    private func pop(_ direction: HistoryDirection) -> HistoryStep? {
        direction == .undo ? undoSteps.popLast() : redoSteps.popLast()
    }

    private func push(_ step: HistoryStep, onto direction: HistoryDirection) {
        if direction == .undo {
            undoSteps.append(step)
        } else {
            redoSteps.append(step)
        }
    }

    private func name(of step: HistoryStep?) -> String? {
        guard let name = step?.name, !name.isEmpty else { return nil }
        return name
    }
}