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 crossable behind: it is popped and never re-pushed, which is 13's
/// "popped from the stack ... and ⌘Z falls through to the next step".
///
/// ### A third collection, which is not a stack
///
/// What a stale skip leaves behind is not *nothing*, though, and that is the one place this type
/// keeps state a stack cannot express: a skipped step that was holding undo backing goes on holding
/// it until the session ends (`strandedSteps`; 13 ▸ Interaction with the trash, ruled 2026-07-31 —
/// the skip-purge decoupling). Nothing can cross a stranded step and nothing can see one in the Edit
/// menu; it is a hold outliving the history it belonged to.
@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] = []

    /// **Steps a stale skip popped that were still holding something** — off both stacks, uncrossable
    /// forever, and *not yet retired* (13-native-undo.md ▸ Interaction with the trash, ruled
    /// 2026-07-31):
    ///
    /// > "the purge runs when the coarse step leaves the board stack **cleanly** — undone-and-superseded,
    /// > or dropped off the end — or when the board session ends; **a stale-skipped step's backing
    /// > instead survives to board-session end** ... the skip banner says nothing was applied, and an
    /// > irreversible purge riding that gesture would be surprise loss — the skip is exactly when the
    /// > user may want to inspect what the collision left."
    ///
    /// **The whole step is kept, not a copy of what it was holding**, and that is the ruling's "one
    /// condition, two consumers" taken literally: the sweep's ownership question (`backedContent`) and
    /// the deferred purge's moment (`HistoryStep.Retirement`) are two readings of one object's hold,
    /// so they must not become two facts able to disagree. A skip that ran the retirement and re-armed
    /// the sweep from a side-table would be saying *released* to one consumer and *still held* to the
    /// other, for the same instant. Deferring the latch itself says it once.
    ///
    /// Only steps that owe something are stranded — see `strand(_:)`.
    private var strandedSteps: [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 }

    /// **What every step this substrate still holds is holding** — the sweep's gate
    /// (`HistoryProviding.backedContent`; 13-native-undo.md ▸ Interaction with the trash, ruled
    /// 2026-07-31).
    ///
    /// All *three* collections, unlike `pendingSteps` above, and the difference is the question: that
    /// one asks what ⌘Z would cross *now*, this one asks what has not been released. A step on the
    /// redo stack has not retired — "undone-and-superseded" is a release condition precisely because
    /// being undone is not one — so its backing is still owed; and a stranded step is the case where
    /// the two questions come furthest apart, holding backing it will never again cross for
    /// (`strandedSteps`, the skip-purge decoupling ruled 2026-07-31). The readings differing is what
    /// makes them two members.
    ///
    /// Leaving the stranded out would defeat the ruling by the back door: the step is off the stack,
    /// so the next window open's sweep would call its backing residue and purge exactly the content
    /// the skip was meant to leave standing.
    public var backedContent: Set<HistoryAnchor> {
        var backing: Set<HistoryAnchor> = []
        for step in undoSteps { backing.formUnion(step.backing) }
        for step in redoSteps { backing.formUnion(step.backing) }
        for step in strandedSteps { backing.formUnion(step.backing) }
        return backing
    }

    /// 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 — a *clean* exit in the
    /// ruling's own word — and it is the only moment the app can see it.
    ///
    /// `strandedSteps` is deliberately untouched: superseding is something that happens to a step on
    /// the redo stack, and a stranded step is on no stack to be superseded from. Its one exit is the
    /// session's end.
    public func register(_ step: HistoryStep) {
        undoSteps.append(step)
        let dropped = redoSteps
        redoSteps.removeAll()
        retire(dropped)
    }

    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).
    ///
    /// **This is where the stranded finally pay**, and the only place they can: "a stale-skipped
    /// step's backing instead survives to board-session end" (ruled 2026-07-31) names exactly this
    /// moment, and `AppModel`'s teardown reaches it with the store still alive — the `clear()` is a
    /// line above the store's release, for the reason the deferred purge needs it to be.
    public func clear() {
        let dropped = undoSteps + redoSteps + strandedSteps
        undoSteps.removeAll()
        redoSteps.removeAll()
        strandedSteps.removeAll()
        retire(dropped)
    }

    /// **Off the stack first, then retired** — the order every caller above keeps, and the one
    /// `backedContent` depends on: a retirement is where the deferred purge runs, and a step that
    /// retired while still listed would be answering that it still holds the very content it is
    /// releasing.
    private func retire(_ steps: [HistoryStep]) {
        for step in steps { step.retirement?.run() }
    }

    /// Keeps a skipped step's *hold* alive without keeping the step crossable — see `strandedSteps`.
    ///
    /// **A step holding nothing is dropped outright**, which is every board gesture: with no backing
    /// to spare and no retirement to defer there is no consumer for the entry, and stranding it would
    /// be bookkeeping that only grows. The predicate is the pair the ruling's two consumers read, so
    /// it can never spare a step neither of them would ask about.
    private func strand(_ step: HistoryStep) {
        guard step.retirement != nil || !step.backing.isEmpty else { return }
        strandedSteps.append(step)
    }

    // 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:
                // **Off the stack, but not released** — the skip-purge decoupling (13 ▸ Interaction
                // with the trash, ruled 2026-07-31). A stale skip is the one exit that is not clean:
                // "the skip banner says nothing was applied, and an irreversible purge riding that
                // gesture would be surprise loss — the skip is exactly when the user may want to
                // inspect what the collision left". So the step stops being crossable here and its
                // hold outlives it, to the session's end (`strandedSteps`).
                strand(step)
                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
    }
}