import Foundation
import os

// MARK: - Registration at the Writer boundary

/// The inverse-registration layer: how a `BoardStore` write turns into an undo step
/// (13-native-undo.md ▸ Rules).
///
/// ### Why it is an extension rather than lines inside each method
///
/// Every registration in `BoardStore` is the same four moves — name the gesture, hold the *values*
/// the write is about to overwrite, hold the values it is about to set, and hand the pair to the
/// session's stack — and only the middle two differ per operation. Spelling the frame once here
/// leaves each call site with the part that is actually about *that* operation: which fields it
/// touched and what they said before. It also keeps the one rule every step must obey in a single
/// place — **an inverse is performed as an ordinary app-mediated write**, through `performWrite`,
/// so an undo brackets the watcher, echoes back through the reload like any other change, refreshes
/// every window on the board, and (on git boards, pro-m1) commits. Undone changes are real writes,
/// never in-memory reverts.
///
/// ### Values, never live references
///
/// Nothing a step closes over is read from the snapshot at crossing time: every closure below
/// captures ids, folder URLs, orders, titles, style values and body bytes as **values**, computed
/// from the pre-write snapshot the store was holding when the gesture ran. That is what makes a step
/// meaningful minutes later, after any number of reloads, and it is what the next milestone's
/// field-level staleness predicate compares against — each step already carries both sides of its
/// write (13 ▸ Rules ▸ staleness validation).
@MainActor
extension BoardStore {

    // MARK: The funnel

    /// Registers one gesture's step on the board's stack — the one call every write site below makes,
    /// and the only place `HistoryStep` is built.
    ///
    /// `undo` walks the board back across the write; `redo` replays the write itself. Both are handed
    /// the store rather than capturing it, so the only reference a step holds to the board is the
    /// weak one this method installs: a stack outliving its session must not be the reason a board
    /// stays in memory.
    ///
    /// **A step that could not write answers `.skipped`.** `HistoryStepOutcome` has two cases and the
    /// honest reading of a failed inverse is the second one — nothing landed, so nothing goes on the
    /// opposite stack — and `performWrite` has already posted the banner that says why. The read-only
    /// lock, the other way a write is refused here, cannot reach this path in practice: a lock
    /// disables Undo and Redo with the rest of the mutating commands (13 ▸ Rules ▸ locks).
    ///
    /// The seam is spelled with an **untyped** `throws`, deliberately and at a cost worth recording:
    /// every inverse below is made of `BoardWriter` calls and throws nothing but `BoardWriteError`,
    /// but a multi-statement closure literal cannot inherit a typed `throws` from its context —
    /// Swift's inference stops at the brace — so a typed seam would make all twelve call sites spell
    /// `{ (_: BoardStore) throws(BoardWriteError) -> Void in }` (the same wart `performWrite`'s doc
    /// comment records, arriving from a third direction). `cross` narrows it back, and reports rather
    /// than swallows the case that cannot happen.
    func registerStep(
        _ name: String,
        undo: @escaping @MainActor (BoardStore) throws -> Void,
        redo: @escaping @MainActor (BoardStore) throws -> Void
    ) {
        guard let history else { return }
        history.register(HistoryStep(
            name: name,
            undo: { [weak self] in BoardStore.cross(self, undo) },
            redo: { [weak self] in BoardStore.cross(self, redo) }
        ))
    }

    /// Runs one side of a step as an ordinary bracketed write.
    private static func cross(
        _ store: BoardStore?,
        _ write: @MainActor (BoardStore) throws -> Void
    ) -> HistoryStepOutcome {
        // The board went away under its own stack — a session torn down between the registration and
        // the ⌘Z. Nothing to write to, so nothing ran.
        guard let store else { return .skipped }
        var foreign: (any Error)?
        let landed: Void? = try? store.performWrite { () throws(BoardWriteError) -> Void in
            do {
                try write(store)
            } catch let error as BoardWriteError {
                // The Writer's own failure: `performWrite` posts it to the banner and rethrows, which
                // is the whole of how an inverse that could not land explains itself.
                throw error
            } catch {
                foreign = error
            }
        }
        if let foreign {
            historyLogger.error("an inverse threw something that is not a write error: \(String(describing: foreign), privacy: .public)")
            return .skipped
        }
        return landed == nil ? .skipped : .applied
    }

    private static let historyLogger = Logger(subsystem: "dev.rzen.indie.Kanban", category: "history")

    // MARK: Creates

    /// One item a gesture brought into being: where it landed, the bytes it landed with, and any
    /// files the same gesture imported into it.
    ///
    /// The bytes are captured at create time because the *inverse of a create is a removal* — "create
    /// → remove the created folder" (13 ▸ Rules) — and a removal leaves nothing for a redo to read.
    /// Holding them is what lets ⇧⌘Z put the item back at its own path under its own UUID, which
    /// every step registered above this one on the stack depends on.
    struct CreatedItem {
        let folder: URL
        let indexText: String
        /// The Finder files this create imported (`createCards(fromFiles:)`), replayed on redo from
        /// the same source URLs the gesture used. Empty for every other create.
        var attachments: [URL] = []
    }

    /// Registers a create's step: undo removes the folders, redo puts them back byte-for-byte.
    ///
    /// **Removal, not a tombstone**, exactly as 13 words it: an undone create leaves *no trace*,
    /// because the item was born of the gesture being undone — a tombstone would leave a trash row
    /// for a card the user never really made. `purgeIsUnrecoverable` is untouched by this: that flag
    /// is about Delete Immediately, whose loss is the user's own final gesture, while this loss is
    /// one ⇧⌘Z away.
    ///
    /// The honest edge, recorded rather than papered over: anything that happened *inside* the
    /// created folder through an operation that registers no step of its own — an attachment added by
    /// hand, a foreign edit — goes with the folder and does not come back on redo. That follows from
    /// 13's own two rulings (remove the folder; attachment operations register nothing in v1) rather
    /// than from anything decided here.
    func registerCreation(_ items: [CreatedItem], kind: HistoryPhrase.Kind) {
        guard !items.isEmpty else { return }
        let operation: WriteOperation = kind == .lane ? .createLane : .createCard
        registerStep(HistoryPhrase.name(.add, kind: kind, count: items.count)) { _ in
            // Reversed, so a lane and a card created by one gesture unwind child-first — the same
            // order they were made in, read backwards.
            for item in items.reversed() {
                try BoardWriter.purgeItem(at: item.folder)
            }
        } redo: { _ in
            for item in items {
                try BoardWriter.recreateItem(at: item.folder, indexText: item.indexText, operation: operation)
                guard !item.attachments.isEmpty else { continue }
                _ = try BoardWriter.importAttachments(item.attachments, intoCard: item.folder)
            }
        }
    }

    /// A just-created item, ready to register — or `nil` when its bytes could not be read back, in
    /// which case the gesture simply registers nothing rather than arming an undo whose redo could
    /// not restore anything.
    func createdItem(at folder: URL, kind: HistoryPhrase.Kind, attachments: [URL] = []) -> CreatedItem? {
        let operation: WriteOperation = kind == .lane ? .createLane : .createCard
        guard let text = try? BoardWriter.readIndexText(ofItem: folder, operation: operation) else { return nil }
        return CreatedItem(folder: folder, indexText: text, attachments: attachments)
    }

    // MARK: Restoring a field to what it said before

    /// Puts a lenient string field (`title`, `background`, `icon`) back to the value it held before
    /// the write — or removes the key, which is what "before" means for a field that was not there.
    ///
    /// A **malformed** prior reads as a removal, and that is the one place an inverse is not
    /// byte-exact: the app cannot re-emit `background: [a, b]` through a document edit that only
    /// knows how to set scalars. It is also the case the forward write was designed to clear
    /// ("choosing any well replaces it" — 03-board-ui.md § Styling ▸ Controls), so the undo lands the
    /// item on the app's own reading of that field rather than resurrecting a value nothing could
    /// read.
    static func restore(_ prior: FieldValue<String>, to key: String, in document: inout FrontmatterDocument) {
        if let value = prior.value {
            document.set(key, to: .string(value))
        } else {
            document.remove(key)
        }
    }

    /// Puts a lane's `width` back — the integer it held, or no key at all.
    ///
    /// Note the asymmetry with the forward write, which removes the key at one unit (the
    /// remove-at-default family): an inverse restores the *value*, so a hand-written `width: 1` comes
    /// back as `width: 1` rather than as an absence that renders the same. Restoring what was there
    /// outranks re-deriving what the app would have written.
    static func restoreWidth(_ prior: FieldValue<Int>, in document: inout FrontmatterDocument) {
        if let value = prior.value {
            document.set(FrontmatterKeys.width, to: .int(value))
        } else {
            document.remove(FrontmatterKeys.width)
        }
    }

    /// Puts an item's tombstone back — **with the timestamp it carried**, not with `now`.
    ///
    /// The inverse of Put Back is the item returning to the trash exactly where it was, and the trash
    /// sorts by `deleted` (03-board-ui.md § Trash ▸ Contents): re-stamping would file the row under
    /// today and quietly reorder a list the user was reading. A prior that was malformed (or, by
    /// construction impossibly, missing) falls back to `now` — the item has to be tombstoned, and an
    /// unreadable timestamp is not a value to preserve.
    static func restoreTombstone(_ prior: FieldValue<Date>, in document: inout FrontmatterDocument) {
        document.set(FrontmatterKeys.deleted, to: .date(prior.value ?? Date()))
    }
}