import Foundation

// MARK: - ExpectedField

/// One field a step's write left holding a known value — the unit 13-native-undo.md ▸ Rules'
/// **field-level** predicate compares.
///
/// "Each step registers both sides of its write anyway (the before-value is the inverse; the
/// after-value is what its write set), so validation compares the targeted field's current value
/// against the expected after-value" (settled, ruled 2026-07-27). These cases are therefore exactly
/// the fields the app's own inverses write and no others: a step declares what *it* set, never what
/// it merely read, so a foreign edit to a field this gesture never touched cannot skip anything.
///
/// **`nil` means the key is absent**, which is a real after-value throughout this app rather than a
/// missing one: an emptied rename removes `title`, a width of one unit removes `width`, and the None
/// well removes `background` (the remove-at-default family). A **malformed** value on disk matches
/// neither an absence nor a value — it is not something this app writes, so finding one is finding
/// somebody else's edit.
public enum ExpectedField: Sendable, Equatable {

    /// `title` — renames at every level.
    case title(String?)

    /// `order` — every reorder, every move's landing rank, and the rank half of a drag-restore.
    case order(Double)

    /// `width` — the lane resize. `nil` is the one-unit default, whose key the write removes.
    case width(Int?)

    /// `background` — the styling gesture's colour dimension.
    case background(String?)

    /// `icon` — the styling gesture's symbol dimension.
    case icon(String?)

    /// The body span, **byte for byte** — the Edit session's step, and the one inverse in the app
    /// whose fidelity is not field-level (13: "body steps compare bytes").
    case body(String)
}

// MARK: - HistoryExpectation

/// What one folder must currently hold for a step to be safe to cross — the state that step's write
/// left it in.
///
/// Two halves, both of them 13's: **existence and liveness** ("target folder gone ... → the step is
/// skipped"; "existence/liveness for create/delete/restore steps"), and the **field-level**
/// comparison above. A step carries one of these per item it touched, so a multi-card move validates
/// three targets and a single rename validates one — which is the whole of "a foreign change to an
/// unrelated item must not skip anything": an item no step named is an item no expectation mentions.
///
/// **The folder's *path* is the parent check.** A move's step expects the card at its destination
/// path; a card that a foreign writer moved elsewhere leaves nothing at that path, so the ordinary
/// existence half already answers "moved away" without a parent field of its own.
public struct HistoryExpectation: Sendable, Equatable {

    /// Where the item this step wrote to should be — the destination for a move, the item's own
    /// folder for everything else, and the board root for the board's own rename and styling.
    public let folder: URL

    /// Whether the item should be there, and if so on which side of the tombstone.
    public let presence: Presence

    /// The fields the step's write set, with the values it set them to. Empty for a step whose
    /// whole subject *is* existence — a create, a delete, a Put Back.
    public let fields: [ExpectedField]

    /// Where an item stands, as the trash's own three-way reading of it.
    public enum Presence: Sendable, Equatable {
        /// There, and rendered: no `deleted:` on the item **or on any ancestor**. Liveness is
        /// effective, the same ancestor walk `BoardStore.liveItem` and the card windows' fate rule
        /// apply — a card under a tombstoned lane renders nowhere, so it is as gone as a deleted one.
        case live

        /// There, and tombstoned — a trash row, or a card hidden under a tombstoned lane.
        case tombstoned

        /// Not there at all: the folder is gone. What an undone create leaves, and what a redone one
        /// expects to find before putting it back.
        case absent
    }

    public init(folder: URL, presence: Presence, fields: [ExpectedField]) {
        self.folder = folder
        self.presence = presence
        self.fields = fields
    }

    /// The item is live and its fields say what the step set them to.
    public static func live(_ folder: URL, _ fields: ExpectedField...) -> HistoryExpectation {
        HistoryExpectation(folder: folder, presence: .live, fields: fields)
    }

    /// The same, for a caller whose field list is computed — the styling gesture's, which varies per
    /// dimension. A label rather than a second variadic, so `.live(folder)` stays unambiguous.
    public static func live(_ folder: URL, fields: [ExpectedField]) -> HistoryExpectation {
        HistoryExpectation(folder: folder, presence: .live, fields: fields)
    }

    /// The item is tombstoned and its fields say what the step set them to.
    public static func tombstoned(_ folder: URL, _ fields: ExpectedField...) -> HistoryExpectation {
        HistoryExpectation(folder: folder, presence: .tombstoned, fields: fields)
    }

    /// Nothing is at this path.
    public static func absent(_ folder: URL) -> HistoryExpectation {
        HistoryExpectation(folder: folder, presence: .absent, fields: [])
    }

    /// The variant for a step whose target's liveness is not known until the gesture runs — the Edit
    /// session's, which is registered against a card that may have been tombstoned out from under
    /// the buffer (05-card-window.md ▸ Deletion & lifecycle).
    public static func item(
        _ folder: URL,
        tombstoned: Bool,
        _ fields: ExpectedField...
    ) -> HistoryExpectation {
        HistoryExpectation(folder: folder, presence: tombstoned ? .tombstoned : .live, fields: fields)
    }
}

// MARK: - HistoryStaleness

/// The staleness predicate: does the board still look the way this step's write left it?
/// (13-native-undo.md ▸ Rules ▸ staleness validation.)
///
/// ### It reads disk, not the snapshot
///
/// 13 says "re-checks its target against the current snapshot at ⌘Z time", and *current* is the
/// load-bearing word: the store's `snapshot` is by construction one reload behind every write the
/// app makes (the one-way flow means a write is only visible once the watcher round-trips it), so
/// validating against it would make a second ⌘Z pressed inside the debounce window compare against a
/// board that still shows the first one's *pre*-state — every rapid undo run would false-skip. Disk
/// is what "current" can honestly mean at the instant a step is crossed, and it is also what an
/// inverse is about to write to.
///
/// ### Lazily, never eagerly
///
/// Nothing here is called by the watcher, the ledger, or any background sweep: "invalidation is lazy
/// (settled — ruled 2026-07-27): staleness is discovered at ⌘Z time, never by background pruning ...
/// The stack always looks full". This type has exactly one caller, `BoardStore.cross`, one line
/// before the inverse would have been written.
public enum HistoryStaleness {

    /// Whether every target a step named still holds what that step left there.
    ///
    /// `root` is the board's current root, which the liveness walk stops at — a lane's parent.
    public static func isCurrent(_ expectations: [HistoryExpectation], under root: URL) -> Bool {
        expectations.allSatisfy { isCurrent($0, under: root) }
    }

    /// One target's answer.
    ///
    /// A file that cannot be read or parsed fails every expectation but `.absent`: an `index.md`
    /// somebody has just broken is not one holding this step's after-value, and the honest reading of
    /// "the field no longer holds it" covers a field that can no longer be read at all.
    public static func isCurrent(_ expectation: HistoryExpectation, under root: URL) -> Bool {
        guard expectation.presence != .absent else {
            return !FileManager.default.fileExists(atPath: expectation.folder.path)
        }
        guard let document = index(at: expectation.folder) else { return false }

        let tombstoned = isEffectivelyTombstoned(expectation.folder, document: document, under: root)
        guard tombstoned == (expectation.presence == .tombstoned) else { return false }

        return expectation.fields.allSatisfy { matches($0, in: document) }
    }

    // MARK: Fields

    static func matches(_ field: ExpectedField, in document: FrontmatterDocument) -> Bool {
        switch field {
        case let .title(expected): equal(document.title, expected)
        case let .order(expected): document.order.value == expected
        case let .width(expected): equal(document.width, expected)
        case let .background(expected): equal(document.background, expected)
        case let .icon(expected): equal(document.icon, expected)
        case let .body(expected): document.body == expected
        }
    }

    /// A present value matches a present expectation by value; an absence matches an absence.
    ///
    /// **A malformed field matches nothing**, deliberately: `FieldValue.malformed` is a shape this
    /// app never writes (`background: [a, b]` is somebody's hand edit), so a step that expected its
    /// own removed key must not read one as "gone" and clobber it.
    private static func equal<Value: Sendable & Equatable>(_ actual: FieldValue<Value>, _ expected: Value?) -> Bool {
        switch (actual, expected) {
        case (.missing, nil): true
        case let (.valid(value), .some(expected)): value == expected
        default: false
        }
    }

    // MARK: Liveness

    /// Whether the item at `folder` renders — **presence of `deleted:`, not its validity**
    /// (`Lane.isDeleted`'s rule), walked up through the ancestors the way every other liveness
    /// question in this app is.
    ///
    /// The board root is never tombstoned however its own frontmatter reads: a board-level `deleted:`
    /// is a tolerated load *warning* (01-storage-format.md § Deletion), not a state that hides the
    /// board from itself.
    private static func isEffectivelyTombstoned(
        _ folder: URL,
        document: FrontmatterDocument,
        under root: URL
    ) -> Bool {
        guard !isRoot(folder, root) else { return false }
        guard document.deleted.isMissing else { return true }

        // Lane and card are the only levels below the root, so this walks at most twice; the bound
        // is there so a folder that is not under this root at all (a step registered before a
        // mid-session root change) ends rather than climbing to `/`.
        var parent = folder.deletingLastPathComponent()
        for _ in 0 ..< 4 {
            guard !isRoot(parent, root) else { return false }
            guard let ancestor = index(at: parent) else { return false }
            if !ancestor.deleted.isMissing { return true }
            parent = parent.deletingLastPathComponent()
        }
        return false
    }

    private static func isRoot(_ folder: URL, _ root: URL) -> Bool {
        folder.standardizedFileURL.path == root.standardizedFileURL.path
    }

    // MARK: Reading

    /// The item's `index.md` as the app reads it, or `nil` when there is no readable, parseable one
    /// there — a folder that is gone, an indexless folder, bytes that are not UTF-8, frontmatter
    /// somebody has just broken.
    private static func index(at folder: URL) -> FrontmatterDocument? {
        let indexURL = folder.appendingPathComponent(BoardLoader.indexFileName)
        guard let data = try? Data(contentsOf: indexURL),
              let text = String(validating: data, as: UTF8.self),
              let document = try? FrontmatterDocument.parse(text)
        else { return nil }
        return document
    }
}