import Foundation
import Testing
@testable import Kanban

/// `BoardStore`'s drop commits — the writes a released drag performs (04-interactions.md ▸ Drag and
/// drop, DRAG-REORDER.md § The drop commits).
///
/// Like every other write suite here these drive a **real store over a real temp board** and then
/// read back through the loader or the raw bytes, never through a snapshot the store handed out:
/// the interesting claims are about the files — which rank landed, which folder travelled, which
/// UUID was reminted, and which sibling was left alone. `WriterFixture`, `Ident` and `Item` come
/// from `WriterTestSupport.swift`.
///
/// The geometry that produces the `index` these methods take is `DropSlotMathTests`'; here the
/// index is simply given, which is the whole point of the split.

// MARK: - Fixtures

/// Three cards in the first lane, one in the second — enough room for a run of two to insert
/// between siblings without either end being the answer.
@MainActor
private func makeBoard() throws -> WriterFixture {
    let fixture = try WriterFixture()
    try fixture.item("", Item.board)
    try fixture.item(Ident.lane1, Item.rich(order: "1024", title: "Todo"))
    try fixture.item("\(Ident.lane1)/\(Ident.card1)", Item.rich(order: "1024", title: "First"))
    try fixture.item("\(Ident.lane1)/\(Ident.card2)", Item.rich(order: "2048", title: "Second"))
    try fixture.item("\(Ident.lane1)/\(Ident.card3)", Item.rich(order: "3072", title: "Third"))
    try fixture.item(Ident.lane2, Item.rich(order: "2048", title: "Doing"))
    try fixture.item("\(Ident.lane2)/\(Ident.card4)", Item.rich(order: "1024", title: "Fourth"))
    return fixture
}

/// Identities the destination board has never seen — the source board's own, for every cross-board
/// case that is *not* about the import boundary. `Ident` is shared with the writer suites and
/// deliberately small; a second board needs its own namespace to be a second board at all.
private enum Foreign {
    static let lane = "aaaaaaaa-aaaa-4aaa-8aaa-aaaaaaaaaaaa"
    static let card = "bbbbbbbb-bbbb-4bbb-8bbb-bbbbbbbbbbbb"
    static let second = "cccccccc-cccc-4ccc-8ccc-cccccccccccc"
    static let trashed = "dddddddd-dddd-4ddd-8ddd-dddddddddddd"
}

/// The board every cross-board test drags *out of*: one lane holding two cards, plus a card sitting
/// in the source board's own `.trash/` for the cross-board restore cases.
///
/// `colliding` puts the lane and its first card under identities the **destination** already holds,
/// which is the import boundary's whole question; the second card keeps its foreign identity either
/// way, so a colliding arrival can prove the degradation is per folder.
@MainActor
private func makeSourceBoard(colliding: Bool = false) throws -> WriterFixture {
    let fixture = try WriterFixture()
    try fixture.item("", Item.board)
    let laneName = colliding ? Ident.lane1 : Foreign.lane
    let cardName = colliding ? Ident.card1 : Foreign.card
    try fixture.item(laneName, Item.rich(order: "1024", title: "Imported"))
    try fixture.item("\(laneName)/\(cardName)", Item.rich(order: "1024", title: "Travelling"))
    try fixture.item("\(laneName)/\(Foreign.second)", Item.rich(order: "2048", title: "Second"))
    try fixture.item(".trash/\(Foreign.trashed)", Item.rich(order: "1024", title: "Trashed"))
    return fixture
}

private let lane1 = ItemID(rawValue: Ident.lane1)
private let lane2 = ItemID(rawValue: Ident.lane2)
private let lane3 = ItemID(rawValue: Ident.lane3)
private let card1 = ItemID(rawValue: Ident.card1)
private let card2 = ItemID(rawValue: Ident.card2)
private let card3 = ItemID(rawValue: Ident.card3)
private let card4 = ItemID(rawValue: Ident.card4)

/// The board as the loader sees it — never the store's snapshot, which a drop deliberately does not
/// touch (the one-way flow: the write lands, the watcher reloads).
private func loaded(_ fixture: WriterFixture) throws -> BoardModel {
    try BoardLoader.load(boardRoot: fixture.root).model
}

/// A lane's rendered card titles, in display order.
private func titles(_ laneID: ItemID, in fixture: WriterFixture) throws -> [String] {
    guard let lane = try loaded(fixture).lanes.first(where: { $0.id == laneID }) else { return [] }
    return lane.cards.compactMap(\.title.value)
}

/// A lane's rendered card folder names, in display order — identity, where titles would not
/// distinguish an original from its copy.
private func ids(_ laneID: ItemID, in fixture: WriterFixture) throws -> [String] {
    guard let lane = try loaded(fixture).lanes.first(where: { $0.id == laneID }) else { return [] }
    return lane.cards.map(\.id.rawValue)
}

private func order(_ fixture: WriterFixture, _ relativePath: String) throws -> FieldValue<Double> {
    try FrontmatterDocument.parse(fixture.indexText(relativePath)).order
}

/// A file's mtime — "this sibling was not rewritten", stated the way `WriteFidelityTests` states it.
private func stat(_ fixture: WriterFixture, _ relativePath: String) throws -> Date {
    let indexURL = fixture.url(relativePath).appendingPathComponent("index.md")
    let attributes = try FileManager.default.attributesOfItem(atPath: indexURL.path)
    guard let modified = attributes[.modificationDate] as? Date else {
        Issue.record("no modification date for \(relativePath)")
        return .distantPast
    }
    return modified
}

// MARK: - Within-board card moves

@MainActor
@Suite("BoardStore ▸ moveCards")
struct MoveCardsTests {

    @Test("A same-lane drop rewrites only the card that moved")
    func sameLaneReorder() throws {
        let fixture = try makeBoard()
        defer { fixture.tearDown() }
        let store = try BoardStore(rootURL: fixture.root)
        let first = try stat(fixture, "\(Ident.lane1)/\(Ident.card1)")
        let second = try stat(fixture, "\(Ident.lane1)/\(Ident.card2)")

        // Third to the head: the index is counted with the dragged card already removed, so 0 is
        // "before what is left", which is First.
        store.moveCards([card3], toLane: lane1, at: 0)

        #expect(try titles(lane1, in: fixture) == ["Third", "First", "Second"])
        // A head insert over the remaining ranks [1024, 2048].
        #expect(try order(fixture, "\(Ident.lane1)/\(Ident.card3)") == .valid(0))
        // Ranks are inserted, never permuted, so the siblings' files were never opened.
        #expect(try stat(fixture, "\(Ident.lane1)/\(Ident.card1)") == first)
        #expect(try stat(fixture, "\(Ident.lane1)/\(Ident.card2)") == second)
        #expect(store.banners.oneShots.isEmpty)
    }

    @Test("A cross-lane drop inserts the run contiguously, in flatten order")
    func crossLaneContiguousInsert() throws {
        let fixture = try makeBoard()
        defer { fixture.tearDown() }
        let store = try BoardStore(rootURL: fixture.root)

        // A Set, deliberately unordered: the run lands in *flatten* order — lane `order` first,
        // then card `order` — so Second (lane one) precedes Fourth (lane two) whatever the Set did.
        store.moveCards([card4, card2], toLane: lane1, at: 1)

        #expect(try titles(lane1, in: fixture) == ["First", "Second", "Fourth", "Third"])
        #expect(try titles(lane2, in: fixture) == [], "the arrival's folder really left lane two")
        #expect(!fixture.exists("\(Ident.lane2)/\(Ident.card4)"))
        #expect(fixture.exists("\(Ident.lane1)/\(Ident.card4)"))
        // A within-board move never remints: the UUIDs travelled unchanged.
        #expect(try ids(lane1, in: fixture) == [Ident.card1, Ident.card2, Ident.card4, Ident.card3])
        #expect(store.banners.oneShots.isEmpty)
    }

    @Test("A drop that lands where everything already is writes nothing")
    func ownSlotIsANoOp() throws {
        let fixture = try makeBoard()
        defer { fixture.tearDown() }
        let store = try BoardStore(rootURL: fixture.root)
        let stamps = [
            try stat(fixture, "\(Ident.lane1)/\(Ident.card1)"),
            try stat(fixture, "\(Ident.lane1)/\(Ident.card2)"),
            try stat(fixture, "\(Ident.lane1)/\(Ident.card3)"),
        ]

        // Second's own resting slot: with it removed the lane reads [First, Third], and 1 puts it
        // straight back between them.
        store.moveCards([card2], toLane: lane1, at: 1)

        #expect(try stat(fixture, "\(Ident.lane1)/\(Ident.card1)") == stamps[0])
        #expect(try stat(fixture, "\(Ident.lane1)/\(Ident.card2)") == stamps[1],
                "a drag that ends where it started must not stamp modified or mint a commit")
        #expect(try stat(fixture, "\(Ident.lane1)/\(Ident.card3)") == stamps[2])
        #expect(store.banners.oneShots.isEmpty)
    }

    @Test("A destination that is gone, or a set that names nothing, writes nothing")
    func noOps() throws {
        let fixture = try makeBoard()
        defer { fixture.tearDown() }
        let store = try BoardStore(rootURL: fixture.root)
        let stamp = try stat(fixture, "\(Ident.lane1)/\(Ident.card1)")

        store.moveCards([card1], toLane: ItemID(rawValue: Ident.indexless), at: 0)  // no such lane
        store.moveCards([], toLane: lane2, at: 0)                                   // nothing dragged
        store.moveCards([ItemID(rawValue: Ident.indexless)], toLane: lane2, at: 0)  // nothing there

        #expect(try stat(fixture, "\(Ident.lane1)/\(Ident.card1)") == stamp)
        #expect(try titles(lane2, in: fixture) == ["Fourth"])
        #expect(store.banners.oneShots.isEmpty)
    }

    @Test("An out-of-range index clamps rather than trapping")
    func indexClamps() throws {
        let fixture = try makeBoard()
        defer { fixture.tearDown() }
        let store = try BoardStore(rootURL: fixture.root)

        // A proposal computed against a snapshot one reload old must not trap.
        store.moveCards([card4], toLane: lane1, at: 99)

        #expect(try titles(lane1, in: fixture) == ["First", "Second", "Third", "Fourth"])
    }

    @Test("Duplicate ranks trigger a renumber, then the run places against the fresh ladder")
    func renumberFallback() throws {
        let fixture = try makeBoard()
        defer { fixture.tearDown() }
        // Two cards sharing a rank: no `Double` fits between them, which is the renumber trigger.
        try fixture.item("\(Ident.lane1)/\(Ident.card2)", Item.rich(order: "1024", title: "Second"))
        let store = try BoardStore(rootURL: fixture.root)

        store.moveCards([card3], toLane: lane1, at: 1)

        #expect(try titles(lane1, in: fixture) == ["First", "Third", "Second"])
        // The lane was compacted to the 1024 ladder first, so the interior midpoint exists again.
        #expect(try order(fixture, "\(Ident.lane1)/\(Ident.card1)") == .valid(1024))
        #expect(try order(fixture, "\(Ident.lane1)/\(Ident.card3)") == .valid(1536))
        #expect(try order(fixture, "\(Ident.lane1)/\(Ident.card2)") == .valid(2048))
        #expect(store.banners.oneShots.isEmpty)
    }

    @Test("A read-only board refuses the drop")
    func readOnlyRefuses() throws {
        let fixture = try makeBoard()
        defer { fixture.tearDown() }
        let store = try BoardStore(rootURL: fixture.root)
        store.enterVanishedRootLock()

        store.moveCards([card3], toLane: lane1, at: 0)

        #expect(try titles(lane1, in: fixture) == ["First", "Second", "Third"])
    }
}

// MARK: - Within-board ⌥-copies

/// **A copy's index is counted among the lane's full rendered cards, originals included** — unlike
/// `moveCards`', which is counted among the cards its run vacates (DRAG-REORDER.md § Resting-layout
/// zones, ruled 2026-08-01). That is not an arbitrary split: a copy leaves its originals in the
/// source board's resting layout for the whole drag (`DragSession.hiddenMembers`), so the zones that
/// produced the number counted them too, and each writer consumes the number in the space its own
/// gesture was showing. `DragRestingLayoutTests` pins the layout half; these pin the write half.
@MainActor
@Suite("BoardStore ▸ copyCards")
struct CopyCardsTests {

    @Test("A copy lands fresh-GUID duplicates at the drop and leaves the originals alone")
    func freshGUIDsAndUntouchedOriginals() throws {
        let fixture = try makeBoard()
        defer { fixture.tearDown() }
        let store = try BoardStore(rootURL: fixture.root)
        let stamp = try stat(fixture, "\(Ident.lane1)/\(Ident.card1)")

        // The lane the user is looking at reads [First, Second, Third] — the ⌥-copy's originals
        // stay in it — so 2 is "before Third".
        store.copyCards([card1], toLane: lane1, at: 2)

        #expect(try titles(lane1, in: fixture) == ["First", "Second", "First", "Third"])
        let landed = try ids(lane1, in: fixture)
        #expect(landed.count == 4)
        #expect(landed[2] != Ident.card1, "a copy mints a fresh UUID at every level")
        #expect(try stat(fixture, "\(Ident.lane1)/\(Ident.card1)") == stamp, "the original is untouched")
        #expect(store.banners.oneShots.isEmpty)
    }

    /// **The drop lands exactly where the shadow showed, below the dragged run included.** This is
    /// the case the index space is actually about: the shadow sits *after* the original, so the
    /// slot number the geometry produced counts the original on its way there, and a writer that
    /// re-read the number against the originals-removed layout would push the copy one slot too far
    /// — off the end of a three-card lane, in this fixture.
    @Test("A copy below the dragged run lands at the shadow's slot, not past it")
    func landsBelowTheRun() throws {
        let fixture = try makeBoard()
        defer { fixture.tearDown() }
        let store = try BoardStore(rootURL: fixture.root)

        // Dragging First, ⌥ held, with the shadow between Second and Third: the drawn lane is
        // [First, Second, ▮, Third], so the proposal is slot 2.
        store.copyCards([card1], toLane: lane1, at: 2)

        #expect(try titles(lane1, in: fixture) == ["First", "Second", "First", "Third"])
        let landed = try ids(lane1, in: fixture)[2]
        #expect(try order(fixture, "\(Ident.lane1)/\(landed)") == .valid(2560),
                "between Second and Third, where the shadow was")
    }

    /// The end slot, in the same space: one past the last card of the layout on screen appends.
    @Test("The terminal slot appends, counted among the originals like every other slot")
    func theTerminalSlotAppends() throws {
        let fixture = try makeBoard()
        defer { fixture.tearDown() }
        let store = try BoardStore(rootURL: fixture.root)

        store.copyCards([card1], toLane: lane1, at: 3)

        #expect(try titles(lane1, in: fixture) == ["First", "Second", "Third", "First"])
        let landed = try ids(lane1, in: fixture)[3]
        #expect(try order(fixture, "\(Ident.lane1)/\(landed)") == .valid(4096))
    }

    @Test("A copy keeps created — a copy is a fork")
    func createdIsKept() throws {
        let fixture = try makeBoard()
        defer { fixture.tearDown() }
        let store = try BoardStore(rootURL: fixture.root)

        store.copyCards([card1], toLane: lane2, at: 0)

        let landedIDs = try ids(lane2, in: fixture)
        let landed = try #require(landedIDs.first { $0 != Ident.card4 })
        let text = try fixture.indexText("\(Ident.lane2)/\(landed)")
        #expect(text.contains("created: 2026-01-01T09:00:00Z"))
        #expect(!text.contains("modified-by"), "a copy is an app write, so the foreign stamp is cleared")
    }

    @Test("A copy's ranks are placed among the originals, which are still there")
    func ranksAvoidTheOriginals() throws {
        let fixture = try makeBoard()
        defer { fixture.tearDown() }
        let store = try BoardStore(rootURL: fixture.root)

        // Index 1 is "between First and Second" in the lane as drawn, and the original First is
        // genuinely still sitting at 1024 — so the rank has to land strictly inside that gap. There
        // is no apparently-vacated slot to choose in, which is the whole point of counting the
        // index among the originals rather than around them.
        store.copyCards([card1], toLane: lane1, at: 1)

        #expect(try titles(lane1, in: fixture) == ["First", "First", "Second", "Third"])
        let landed = try ids(lane1, in: fixture)[1]
        #expect(try order(fixture, "\(Ident.lane1)/\(landed)") == .valid(1536))
        #expect(try order(fixture, "\(Ident.lane1)/\(Ident.card1)") == .valid(1024),
                "the original keeps its rank — nothing was written to it")
    }

    /// Slot 0 in the copy's space is genuinely *before* the original, which the old
    /// originals-removed space could not name at all: its 0 meant "before the first survivor",
    /// i.e. after the original.
    @Test("Slot 0 lands the copy in front of its own original")
    func slotZeroLandsInFront() throws {
        let fixture = try makeBoard()
        defer { fixture.tearDown() }
        let store = try BoardStore(rootURL: fixture.root)

        store.copyCards([card1], toLane: lane1, at: 0)

        #expect(try titles(lane1, in: fixture) == ["First", "First", "Second", "Third"])
        let landed = try ids(lane1, in: fixture)[0]
        #expect(landed != Ident.card1, "the copy is the one in front")
        let rank = try #require(order(fixture, "\(Ident.lane1)/\(landed)").value)
        #expect(rank < 1024, "a whole gap below the original, which keeps its own 1024")
        #expect(try order(fixture, "\(Ident.lane1)/\(Ident.card1)") == .valid(1024))
    }

    @Test("A multi-copy lands the run contiguously, in flatten order")
    func multiCopyIsContiguous() throws {
        let fixture = try makeBoard()
        defer { fixture.tearDown() }
        let store = try BoardStore(rootURL: fixture.root)

        store.copyCards([card3, card1], toLane: lane2, at: 1)

        #expect(try titles(lane2, in: fixture) == ["Fourth", "First", "Third"])
        #expect(try titles(lane1, in: fixture) == ["First", "Second", "Third"], "originals stay")
    }

    @Test("Nothing droppable copies nothing")
    func noOps() throws {
        let fixture = try makeBoard()
        defer { fixture.tearDown() }
        let store = try BoardStore(rootURL: fixture.root)

        store.copyCards([card1], toLane: ItemID(rawValue: Ident.lane3), at: 0)
        store.copyCards([], toLane: lane2, at: 0)

        #expect(try titles(lane2, in: fixture) == ["Fourth"])
        #expect(!fixture.exists(Ident.lane3))
        #expect(store.banners.oneShots.isEmpty)
    }
}

// MARK: - Cross-board card arrivals

@MainActor
@Suite("BoardStore ▸ receiveCards")
struct ReceiveCardsTests {

    @Test("A cross-board copy lands a fresh-GUID duplicate and leaves the source alone")
    func crossBoardCopy() throws {
        let destination = try makeBoard()
        defer { destination.tearDown() }
        let source = try makeSourceBoard()
        defer { source.tearDown() }
        let store = try BoardStore(rootURL: destination.root)

        store.receiveCards([source.url("\(Foreign.lane)/\(Foreign.card)")],
                           operation: .copy, toLane: lane2, at: 0)

        #expect(try titles(lane2, in: destination) == ["Travelling", "Fourth"])
        let landedIDs = try ids(lane2, in: destination)
        #expect(landedIDs.first != Foreign.card, "copies mint fresh UUIDs, always")
        #expect(source.exists("\(Foreign.lane)/\(Foreign.card)"), "the original stays")
        #expect(store.banners.oneShots.isEmpty)
    }

    @Test("A cross-board move carries the identity and empties the source folder")
    func crossBoardMove() throws {
        let destination = try makeBoard()
        defer { destination.tearDown() }
        let source = try makeSourceBoard()
        defer { source.tearDown() }
        let store = try BoardStore(rootURL: destination.root)

        store.receiveCards([source.url("\(Foreign.lane)/\(Foreign.card)")],
                           operation: .move, toLane: lane2, at: 1)

        #expect(try ids(lane2, in: destination) == [Ident.card4, Foreign.card], "identity travels")
        #expect(!source.exists("\(Foreign.lane)/\(Foreign.card)"))
        #expect(store.banners.oneShots.isEmpty)
    }

    @Test("A moved folder whose UUID the destination already holds arrives reminted")
    func importBoundaryRemints() throws {
        let destination = try makeBoard()
        defer { destination.tearDown() }
        // The source's card carries `card1`, which lives in the destination's first lane already.
        let source = try makeSourceBoard(colliding: true)
        defer { source.tearDown() }
        let store = try BoardStore(rootURL: destination.root)

        store.receiveCards([source.url("\(Ident.lane1)/\(Ident.card1)")],
                           operation: .move, toLane: lane2, at: 1)

        let landed = try ids(lane2, in: destination)
        #expect(landed.count == 2)
        #expect(landed[1] != Ident.card1, "a colliding UUID is repaired at the import boundary")
        #expect(destination.exists("\(Ident.lane1)/\(Ident.card1)"), "the resident keeps its identity")
        #expect(try titles(lane2, in: destination) == ["Fourth", "Travelling"])
        #expect(!source.exists("\(Ident.lane1)/\(Ident.card1)"))
    }

    @Test("A cross-board run lands contiguously at the drop, in the order given")
    func runLandsContiguously() throws {
        let destination = try makeBoard()
        defer { destination.tearDown() }
        let source = try makeSourceBoard()
        defer { source.tearDown() }
        try source.item("\(Foreign.lane)/\(Foreign.second)", Item.rich(order: "3072", title: "Second traveller"))
        let store = try BoardStore(rootURL: destination.root)

        store.receiveCards([
            source.url("\(Foreign.lane)/\(Foreign.card)"),
            source.url("\(Foreign.lane)/\(Foreign.second)"),
        ], operation: .copy, toLane: lane1, at: 1)

        #expect(try titles(lane1, in: destination)
                == ["First", "Travelling", "Second traveller", "Second", "Third"])
    }

    @Test("Nothing droppable receives nothing")
    func noOps() throws {
        let destination = try makeBoard()
        defer { destination.tearDown() }
        let source = try makeSourceBoard()
        defer { source.tearDown() }
        let store = try BoardStore(rootURL: destination.root)

        store.receiveCards([source.url("\(Foreign.lane)/\(Foreign.card)")],
                           operation: .copy, toLane: ItemID(rawValue: Ident.lane3), at: 0)
        store.receiveCards([], operation: .copy, toLane: lane2, at: 0)

        #expect(!destination.exists(Ident.lane3))
        #expect(try titles(lane2, in: destination) == ["Fourth"])
        #expect(source.exists("\(Foreign.lane)/\(Foreign.card)"))
        #expect(store.banners.oneShots.isEmpty)
    }
}

// MARK: - Within-board lane moves

/// A three-lane board, so a run of two can land at either end or between the survivors.
@MainActor
private func makeThreeLaneBoard() throws -> WriterFixture {
    let fixture = try WriterFixture()
    try fixture.item("", Item.board)
    try fixture.item(Ident.lane1, Item.rich(order: "1024", title: "Todo"))
    try fixture.item(Ident.lane2, Item.rich(order: "2048", title: "Doing"))
    try fixture.item(Ident.lane3, Item.rich(order: "3072", title: "Done"))
    return fixture
}

private func laneTitles(_ fixture: WriterFixture) throws -> [String] {
    try loaded(fixture).lanes.filter { !$0.isDeleted }.compactMap(\.title.value)
}

@MainActor
@Suite("BoardStore ▸ moveLanes")
struct MoveLanesTests {

    @Test("A run of lanes lands contiguously at the drop, in board order")
    func runLandsContiguously() throws {
        let fixture = try makeThreeLaneBoard()
        defer { fixture.tearDown() }
        let store = try BoardStore(rootURL: fixture.root)
        let untouched = try stat(fixture, Ident.lane2)

        // Todo and Done together, dropped at 0 — "before what is left", which is Doing. The two
        // arrive adjacent and in board order even though they were not adjacent to begin with.
        store.moveLanes([lane1, lane3], toIndex: 0)

        #expect(try laneTitles(fixture) == ["Todo", "Done", "Doing"])
        // Ranks are inserted, never permuted: the lane that did not move was never opened.
        #expect(try stat(fixture, Ident.lane2) == untouched)
        #expect(try order(fixture, Ident.lane2) == .valid(2048))
        #expect(store.banners.oneShots.isEmpty)
    }

    @Test("A run dropped past the survivors appends in order")
    func runAppends() throws {
        let fixture = try makeThreeLaneBoard()
        defer { fixture.tearDown() }
        let store = try BoardStore(rootURL: fixture.root)

        store.moveLanes([lane1, lane2], toIndex: 1)

        #expect(try laneTitles(fixture) == ["Done", "Todo", "Doing"])
    }

    @Test("A single lane behaves exactly as the singular command does")
    func singleLane() throws {
        let fixture = try makeThreeLaneBoard()
        defer { fixture.tearDown() }
        let store = try BoardStore(rootURL: fixture.root)

        store.moveLanes([lane3], toIndex: 0)

        #expect(try laneTitles(fixture) == ["Done", "Todo", "Doing"])
    }

    @Test("A drag that lands where everything already is writes nothing")
    func noOpDropWritesNothing() throws {
        let fixture = try makeThreeLaneBoard()
        defer { fixture.tearDown() }
        let store = try BoardStore(rootURL: fixture.root)
        let before = try [Ident.lane1, Ident.lane2, Ident.lane3].map { try stat(fixture, $0) }

        store.moveLanes([lane1], toIndex: 0)
        store.moveLanes([lane1, lane2], toIndex: 0)
        store.moveLanes([], toIndex: 1)
        store.moveLanes([ItemID(rawValue: Ident.indexless)], toIndex: 0)

        #expect(try [Ident.lane1, Ident.lane2, Ident.lane3].map { try stat(fixture, $0) } == before)
        #expect(store.banners.oneShots.isEmpty)
    }

    @Test("An out-of-range index clamps rather than trapping")
    func indexClamps() throws {
        // A store apiece: a write does not touch the snapshot (the one-way flow), so two drops
        // against one store would both be computed against the pre-drop board.
        let high = try makeThreeLaneBoard()
        defer { high.tearDown() }
        try BoardStore(rootURL: high.root).moveLanes([lane1], toIndex: 99)
        #expect(try laneTitles(high) == ["Doing", "Done", "Todo"])

        let low = try makeThreeLaneBoard()
        defer { low.tearDown() }
        try BoardStore(rootURL: low.root).moveLanes([lane3], toIndex: -5)
        #expect(try laneTitles(low) == ["Done", "Todo", "Doing"])
    }
}

// MARK: - Cross-board lane arrivals

@MainActor
@Suite("BoardStore ▸ receiveLanes")
struct ReceiveLanesTests {

    /// 04-interactions.md ▸ Drag and drop, resettled 2026-07-28: "A lane carries exactly its cards —
    /// the trash is board-level (`.trash/`), so there is nothing lane-nested to strip or carry: copy
    /// and ⌘-drag move alike transfer the lane's folder as it is; the old tombstone-stripping rule is
    /// retired with the tombstone model."
    @Test("A lane copy transfers the whole lane, minting fresh identities at every level")
    func laneCopyRemintsThroughout() throws {
        let destination = try makeBoard()
        defer { destination.tearDown() }
        let source = try makeSourceBoard()
        defer { source.tearDown() }
        let store = try BoardStore(rootURL: destination.root)

        store.receiveLanes([source.url(Foreign.lane)], operation: .copy, at: 0)

        let model = try loaded(destination)
        #expect(model.lanes.map(\.title.value) == ["Imported", "Todo", "Doing"])
        let arrived = try #require(model.lanes.first)
        #expect(arrived.id.rawValue != Foreign.lane, "a copy mints fresh UUIDs at every level")
        #expect(arrived.cards.map(\.title.value) == ["Travelling", "Second"], "nothing to strip")
        #expect(arrived.cards.allSatisfy { $0.id.rawValue != Foreign.card },
                "a copied lane's cards are new cards")
        #expect(model.trash.isEmpty, "the source's trash is board-level and never travels with a lane")
        #expect(store.banners.oneShots.isEmpty)
    }

    @Test("A lane move carries its cards whole, identity and all")
    func laneMoveCarriesItsCards() throws {
        let destination = try makeBoard()
        defer { destination.tearDown() }
        let source = try makeSourceBoard()
        defer { source.tearDown() }
        let store = try BoardStore(rootURL: destination.root)

        store.receiveLanes([source.url(Foreign.lane)], operation: .move, at: 2)

        let model = try loaded(destination)
        #expect(model.lanes.map(\.id.rawValue) == [Ident.lane1, Ident.lane2, Foreign.lane],
                "identity travels, and the drop position is honoured")
        let arrived = try #require(model.lanes.last)
        #expect(arrived.cards.map(\.id.rawValue) == [Foreign.card, Foreign.second],
                "a lane carries exactly its cards: nothing lane-nested to strip or carry")
        #expect(model.trash.isEmpty, "and nothing arrived in the destination's own trash")
        #expect(!source.exists(Foreign.lane))
        #expect(store.banners.oneShots.isEmpty)
    }

    @Test("A colliding lane move remints only the folders that collide")
    func laneMoveRemintsPerFolder() throws {
        let destination = try makeBoard()
        defer { destination.tearDown() }
        // The source lane is `lane1` holding `card1` — both already live in the destination — plus
        // one tombstoned card whose identity is foreign.
        let source = try makeSourceBoard(colliding: true)
        defer { source.tearDown() }
        let store = try BoardStore(rootURL: destination.root)

        store.receiveLanes([source.url(Ident.lane1)], operation: .move, at: 2)

        let model = try loaded(destination)
        #expect(model.lanes.count == 3)
        let arrived = try #require(model.lanes.last)
        #expect(arrived.id.rawValue != Ident.lane1, "the colliding root was repaired")
        #expect(arrived.title.value == "Imported")
        let arrivedCards = arrived.cards.map(\.id.rawValue)
        #expect(arrivedCards.count == 2)
        #expect(arrivedCards[0] != Ident.card1, "the colliding card was repaired too")
        #expect(arrivedCards[1] == Foreign.second, "and nothing else was — degradation is per folder")
        #expect(try titles(lane1, in: destination) == ["First", "Second", "Third"],
                "the residents kept their identities and their ranks")
    }

    @Test("Nothing to receive writes nothing")
    func noOps() throws {
        let destination = try makeBoard()
        defer { destination.tearDown() }
        let store = try BoardStore(rootURL: destination.root)

        store.receiveLanes([], operation: .copy, at: 0)

        #expect(try loaded(destination).lanes.count == 2)
        #expect(store.banners.oneShots.isEmpty)
    }
}

// MARK: - Restore is an ordinary move out

/// A lane holding two cards, plus one card in the board's `.trash/` — so a restore has somewhere to
/// land that is neither the head nor the tail.
///
/// **There is no tombstone anywhere in it.** "Restoring is an ordinary move out: drag a trash card
/// into any lane at any position … there is no restore-specific machinery and no Put Back"
/// (03-board-ui.md § Trash, resettled 2026-07-28), so the fixture is an ordinary board plus a
/// reserved folder.
@MainActor
private func makeTrashBoard() throws -> WriterFixture {
    let fixture = try WriterFixture()
    try fixture.item("", Item.board)
    try fixture.item(Ident.lane1, Item.rich(order: "1024", title: "Todo"))
    try fixture.item("\(Ident.lane1)/\(Ident.card1)", Item.rich(order: "1024", title: "First"))
    try fixture.item("\(Ident.lane1)/\(Ident.card3)", Item.rich(order: "3072", title: "Third"))
    try fixture.item(Ident.lane2, Item.rich(order: "2048", title: "Doing"))
    try fixture.item("\(Ident.lane2)/\(Ident.card4)", Item.rich(order: "1024", title: "Fourth"))
    try fixture.item(".trash/\(Ident.card2)", Item.rich(order: "1024", title: "Trashed"))
    return fixture
}

/// Restoring out of the trash — **through `moveCards` and `copyCards`, with no method of its own**.
///
/// That is the claim these tests exist to pin. The tombstone model needed `restoreByDrag` and
/// `receiveRestoredCards` because a restore had to clear a key as well as place a rank; a
/// materialized trash makes it "an ordinary move" (03-board-ui.md § Trash), so the ordinary drop
/// commits take a trash-side source and the retired pair is gone.
@MainActor
@Suite("BoardStore ▸ restore by move-out")
struct RestoreByMoveOutTests {

    @Test("The drop position sets the restored card's order")
    func dropPositionSetsTheOrder() throws {
        let fixture = try makeTrashBoard()
        defer { fixture.tearDown() }
        let store = try BoardStore(rootURL: fixture.root)

        // Index 0 among the lane's two cards: ahead of both.
        store.moveCards([card2], toLane: lane1, at: 0)

        #expect(try titles(lane1, in: fixture) == ["Trashed", "First", "Third"])
        #expect(try order(fixture, "\(Ident.lane1)/\(Ident.card2)") == .valid(0))
        #expect(!fixture.exists(".trash/\(Ident.card2)"), "the folder physically left the trash")
        #expect(try loaded(fixture).trash.isEmpty)
        #expect(store.banners.oneShots.isEmpty)
    }

    @Test("An out-of-range index clamps to the lane's bottom")
    func indexClamps() throws {
        let fixture = try makeTrashBoard()
        defer { fixture.tearDown() }
        let store = try BoardStore(rootURL: fixture.root)

        store.moveCards([card2], toLane: lane1, at: 99)

        #expect(try titles(lane1, in: fixture) == ["First", "Third", "Trashed"])
        #expect(try order(fixture, "\(Ident.lane1)/\(Ident.card2)") == .valid(4096))
    }

    @Test("The identity travels, exactly as it does for any move")
    func identityTravels() throws {
        let fixture = try makeTrashBoard()
        defer { fixture.tearDown() }
        let store = try BoardStore(rootURL: fixture.root)

        store.moveCards([card2], toLane: lane2, at: 0)

        #expect(try ids(lane2, in: fixture) == [Ident.card2, Ident.card4])
        #expect(!fixture.exists(".trash/\(Ident.card2)"))
        // Nothing about the restored card says it was ever deleted: there is no key to remove,
        // because there is no key.
        #expect(!(try fixture.indexText("\(Ident.lane2)/\(Ident.card2)").contains("deleted:")))
    }

    @Test("A multi-card restore lands the run contiguously, in one bracket")
    func multiRestore() throws {
        let fixture = try WriterFixture()
        defer { fixture.tearDown() }
        try fixture.item("", Item.board)
        try fixture.item(Ident.lane1, Item.rich(order: "1024", title: "Todo"))
        try fixture.item("\(Ident.lane1)/\(Ident.card1)", Item.rich(order: "1024", title: "First"))
        try fixture.item(Ident.lane2, Item.rich(order: "2048", title: "Doing"))
        try fixture.item("\(Ident.lane2)/\(Ident.card4)", Item.rich(order: "1024", title: "Fourth"))
        // The trash's own order is its display order, and since 2026-07-31 that is `modified`
        // descending (01-storage-format.md § Deletion) — `card3` above `card2`, newest-first.
        try fixture.item(
            ".trash/\(Ident.card2)",
            "---\nschema: 1\ntitle: TrashedA\norder: 2048\nmodified: 2026-05-01T09:00:00Z\n---\n")
        try fixture.item(
            ".trash/\(Ident.card3)",
            "---\nschema: 1\ntitle: TrashedB\norder: 1024\nmodified: 2026-05-03T09:00:00Z\n---\n")
        let store = try BoardStore(rootURL: fixture.root)

        store.moveCards([card3, card2], toLane: lane2, at: 0)

        #expect(try titles(lane2, in: fixture) == ["TrashedB", "TrashedA", "Fourth"],
                "the trash's own order is the landing order")
        #expect(try loaded(fixture).trash.isEmpty)
        #expect(store.banners.oneShots.isEmpty)
    }

    @Test("⌥ copies out and leaves the original in the trash")
    func copyOutLeavesTheOriginal() throws {
        let fixture = try makeTrashBoard()
        defer { fixture.tearDown() }
        let store = try BoardStore(rootURL: fixture.root)

        store.copyCards([card2], toLane: lane2, at: 0)

        #expect(try titles(lane2, in: fixture) == ["Trashed", "Fourth"])
        let landed = try #require(try ids(lane2, in: fixture).first)
        #expect(landed != Ident.card2, "a copy out of the trash is still a copy")
        #expect(fixture.exists(".trash/\(Ident.card2)"), "the original stays in the trash")
        #expect(try loaded(fixture).trash.count == 1)
    }

    @Test("A card that is not in the trash and an empty list both write nothing")
    func skipsWhatIsNotThere() throws {
        let fixture = try makeTrashBoard()
        defer { fixture.tearDown() }
        let store = try BoardStore(rootURL: fixture.root)
        let untouched = try stat(fixture, "\(Ident.lane1)/\(Ident.card1)")

        store.moveCards([ItemID(rawValue: Ident.indexless)], toLane: lane2, at: 0)
        store.moveCards([], toLane: lane2, at: 0)

        #expect(try titles(lane2, in: fixture) == ["Fourth"])
        #expect(try stat(fixture, "\(Ident.lane1)/\(Ident.card1)") == untouched)
        #expect(store.banners.oneShots.isEmpty)
    }
}

/// The cross-board half — **`receiveCards`, with no restore variant** (04-interactions.md ▸ The
/// trash: "Dropped on *another* board it follows the copy default — a live copy lands there, the
/// original stays in the source trash; ⌘-drag forces the true cross-board restore-move").
@MainActor
@Suite("BoardStore ▸ cross-board restore")
struct CrossBoardRestoreTests {

    @Test("A cross-board copy out of the trash lands a fresh identity and leaves the original")
    func copyLeavesTheOriginal() throws {
        let destination = try makeBoard()
        defer { destination.tearDown() }
        let source = try makeSourceBoard()
        defer { source.tearDown() }
        let store = try BoardStore(rootURL: destination.root)

        store.receiveCards([source.url(".trash/\(Foreign.trashed)")],
                           operation: .copy, toLane: lane2, at: 0)

        #expect(try titles(lane2, in: destination) == ["Trashed", "Fourth"])
        let landed = try #require(try ids(lane2, in: destination).first)
        #expect(landed != Foreign.trashed, "a copy out of the trash is still a copy")
        let text = try destination.indexText("\(Ident.lane2)/\(landed)")
        #expect(text.contains("created: 2026-01-01T09:00:00Z"), "a copy is a fork")

        #expect(source.exists(".trash/\(Foreign.trashed)"), "the original stays in the source trash")
        #expect(store.banners.oneShots.isEmpty)
    }

    @Test("A ⌘-drag move out of another board's trash carries the identity and empties it")
    func moveCarriesTheIdentity() throws {
        let destination = try makeBoard()
        defer { destination.tearDown() }
        let source = try makeSourceBoard()
        defer { source.tearDown() }
        let store = try BoardStore(rootURL: destination.root)

        store.receiveCards([source.url(".trash/\(Foreign.trashed)")],
                           operation: .move, toLane: lane2, at: 1)

        #expect(try ids(lane2, in: destination) == [Ident.card4, Foreign.trashed], "identity travels")
        #expect(!source.exists(".trash/\(Foreign.trashed)"), "the card left the source trash")
        #expect(try loaded(source).trash.isEmpty)
        #expect(try loaded(destination).trash.isEmpty)
        #expect(store.banners.oneShots.isEmpty)
    }
}

// MARK: - Restoring a trashed lane

/// A board whose trash holds a **lane row** — the opaque unit, subtree intact (03-board-ui.md §
/// Trash) — beside an ordinary trashed card, so the restore's rank arithmetic has a strip to land
/// on and the container has more than one kind in it.
@MainActor
private func makeTrashedLaneBoard() throws -> WriterFixture {
    let fixture = try WriterFixture()
    try fixture.item("", Item.board)
    try fixture.item(Ident.lane1, Item.rich(order: "1024", title: "Todo"))
    try fixture.item("\(Ident.lane1)/\(Ident.card1)", Item.rich(order: "1024", title: "First"))
    try fixture.item(Ident.lane2, Item.rich(order: "2048", title: "Doing"))
    try fixture.item(
        ".trash/\(Ident.lane3)",
        "---\nschema: 1\ntitle: Done\norder: 512\nkind: lane\nproject: lanework\n---\nDone body.\n"
    )
    try fixture.item(".trash/\(Ident.lane3)/\(Ident.card2)", Item.rich(order: "1024", title: "Freight"))
    try fixture.item(".trash/\(Ident.card3)", Item.rich(order: "1024", title: "Trashed"))
    return fixture
}

/// **Drag-to-restore at the lane level** (04-interactions.md ▸ The trash: "dropping … a trashed lane
/// row onto its own board's strip — is an ordinary move to the drop position"), which is
/// `BoardStore.restoreLanes` — an arrival's rank arithmetic with a within-board move's identity
/// posture and an undo step, since 13-native-undo.md's inverse inventory names "restore-by-move →
/// move back in".
@MainActor
@Suite("BoardStore ▸ restoring a trashed lane")
struct RestoreLaneTests {

    @Test("The drop slot sets the restored lane's order, and its cards ride along")
    func dropSlotSetsTheOrder() throws {
        let fixture = try makeTrashedLaneBoard()
        defer { fixture.tearDown() }
        let store = try BoardStore(rootURL: fixture.root)

        // Slot 1: between the board's two lanes.
        store.restoreLanes([lane3], toIndex: 1)

        let model = try loaded(fixture)
        #expect(model.lanes.map(\.id.rawValue) == [Ident.lane1, Ident.lane3, Ident.lane2])
        #expect(model.trashedLanes.isEmpty)
        #expect(!fixture.exists(".trash/\(Ident.lane3)"), "the folder physically left the trash")
        #expect(fixture.exists("\(Ident.lane3)/\(Ident.card2)"), "the freight came back inside it")
        #expect(model.trash.map(\.id.rawValue) == [Ident.card3], "the column's cards are untouched")
        #expect(store.banners.oneShots.isEmpty)
    }

    @Test("The identity travels — a within-board restore is a move, never an import")
    func identityTravels() throws {
        let fixture = try makeTrashedLaneBoard()
        defer { fixture.tearDown() }
        let store = try BoardStore(rootURL: fixture.root)

        store.restoreLanes([lane3], toIndex: 99)

        // The lane keeps its UUID and so does its card: the import boundary's remint is for
        // *arrivals from another board*, and a row coming out of this board's own trash is not one.
        #expect(try loaded(fixture).lanes.map(\.id.rawValue) == [Ident.lane1, Ident.lane2, Ident.lane3])
        #expect(fixture.exists("\(Ident.lane3)/\(Ident.card2)"))
        #expect(try fixture.indexText(Ident.lane3).contains("project: lanework"), "unknown keys ride along")
    }

    @Test("Its undo is the ordinary move back in, at the `order` the row was carrying")
    func undoMovesItBackIn() throws {
        let fixture = try makeTrashedLaneBoard()
        defer { fixture.tearDown() }
        let store = try BoardStore(rootURL: fixture.root)
        let history = NativeHistoryProvider()
        store.history = history

        store.restoreLanes([lane3], toIndex: 0)
        #expect(history.undoActionName == "Move Lane")

        history.undo()
        #expect(fixture.exists(".trash/\(Ident.lane3)/\(Ident.card2)"), "back in, subtree intact")
        #expect(!fixture.exists(Ident.lane3))
        #expect(try order(fixture, ".trash/\(Ident.lane3)") == .valid(512), "at the rank it was holding")

        history.redo()
        #expect(fixture.exists("\(Ident.lane3)/\(Ident.card2)"))
        #expect(try loaded(fixture).trashedLanes.isEmpty)
    }

    @Test("A row that is not in the trash, and an empty set, write nothing")
    func skipsWhatIsNotThere() throws {
        let fixture = try makeTrashedLaneBoard()
        defer { fixture.tearDown() }
        let store = try BoardStore(rootURL: fixture.root)
        let untouched = try stat(fixture, Ident.lane1)

        store.restoreLanes([lane1], toIndex: 0)
        store.restoreLanes([], toIndex: 0)

        #expect(try loaded(fixture).lanes.map(\.id.rawValue) == [Ident.lane1, Ident.lane2])
        #expect(try stat(fixture, Ident.lane1) == untouched)
        #expect(store.banners.oneShots.isEmpty)
    }

    /// The keyboard restore's write (04-interactions.md ▸ The trash: "⌘X in the trash, ⌘V … a trashed
    /// lane pastes after the anchor lane"), whose armed-cut path lands in `receiveLanes` with a
    /// source folder inside this board's own `.trash/`.
    ///
    /// **The identity has to survive that**, which is what the trash-aware source-root derivation
    /// buys: `.trash/` counts in the destination's identity scan, so a restore mistaken for an import
    /// would find the row's own UUID resident and remint the very lane it was restoring.
    @Test("A within-board paste out of the trash keeps the lane's identity")
    func pasteRestoreKeepsTheIdentity() throws {
        let fixture = try makeTrashedLaneBoard()
        defer { fixture.tearDown() }
        let store = try BoardStore(rootURL: fixture.root)

        store.receiveLanes(
            [.folder(fixture.url(".trash/\(Ident.lane3)"))],
            operation: .move,
            at: 2,
            normalizingLooseFiles: true
        )

        #expect(try loaded(fixture).lanes.map(\.id.rawValue) == [Ident.lane1, Ident.lane2, Ident.lane3])
        #expect(fixture.exists("\(Ident.lane3)/\(Ident.card2)"))
        #expect(try loaded(fixture).trashedLanes.isEmpty)
    }
}

/// The cross-board half — the ordinary arrival, exactly as for a live lane: "Dropped on *another*
/// board it follows the copy default … ⌘-drag forces the true cross-board restore-move"
/// (04-interactions.md ▸ The trash).
@MainActor
@Suite("BoardStore ▸ cross-board lane restore")
struct CrossBoardLaneRestoreTests {

    @Test("A cross-board copy out of the trash mints fresh identities and leaves the original")
    func copyLeavesTheOriginal() throws {
        let destination = try makeBoard()
        defer { destination.tearDown() }
        let source = try makeTrashedLaneBoard()
        defer { source.tearDown() }
        let store = try BoardStore(rootURL: destination.root)

        store.receiveLanes([source.url(".trash/\(Ident.lane3)")], operation: .copy, at: 99)

        let model = try loaded(destination)
        let landed = try #require(model.lanes.last)
        #expect(landed.title.value == "Done")
        #expect(landed.id.rawValue != Ident.lane3, "a copy out of the trash is still a copy")
        #expect(landed.cards.map(\.title.value) == ["Freight"])
        #expect(source.exists(".trash/\(Ident.lane3)"), "the original stays in the source trash")
    }

    @Test("A ⌘-drag move carries the lane's identity and empties the source trash")
    func moveCarriesTheIdentity() throws {
        let destination = try makeBoard()
        defer { destination.tearDown() }
        let source = try makeTrashedLaneBoard()
        defer { source.tearDown() }
        let store = try BoardStore(rootURL: destination.root)

        store.receiveLanes([source.url(".trash/\(Ident.lane3)")], operation: .move, at: 0)

        let landed = try #require(loaded(destination).lanes.first)
        #expect(landed.id.rawValue == Ident.lane3, "the lane's identity travels")
        // Its card carried `Ident.card2`, which this destination already holds — so the import
        // boundary remints that folder and only that folder, per-folder at the finest grain.
        #expect(landed.cards.map(\.title.value) == ["Freight"])
        #expect(landed.cards.map(\.id.rawValue) != [Ident.card2])
        #expect(!source.exists(".trash/\(Ident.lane3)"))
        #expect(try loaded(source).trashedLanes.isEmpty)
    }
}