import Foundation

/// What the board list renders for one `.kanban` package: enough to name it, size it and say whether
/// it is here yet — and deliberately nothing more. A summary is never the input to anything that
/// edits; opening a board mints a `BoardSession`, which walks the package properly.
struct BoardSummary: Identifiable, Sendable, Equatable {

    /// **The package root is the identity.** A board has no UUID folder name and no id key — its
    /// identity is where it is (`BoardModel.rootURL` says so), and on the phone that URL is stable
    /// for as long as nobody renames the document in Files.app.
    var id: URL { rootURL }

    let rootURL: URL

    /// The board `index.md`'s `title:`, falling back to the folder name minus `.kanban` — which is
    /// exactly the fallback 01-storage-format.md § Board naming states, and the reason a board with
    /// no `title` key is a normal board rather than an untitled one.
    let title: String

    /// The board `index.md`'s `icon:`, verbatim — an SF Symbol name, or `nil` when unset or the file
    /// couldn't be read. Unlike `title`, there is no fallback: a board with no icon shows none.
    let icon: String?

    /// The board `index.md`'s `iconColor:`, verbatim — a `CardPalette` swatch name, or `nil` on the
    /// same terms as `icon`.
    let iconColor: String?

    /// Lanes the loader would show. `nil` where the package is not materialized enough to count —
    /// see `BoardSummaryScanner` for why a number is withheld rather than guessed at zero.
    let laneCount: Int?

    /// Cards the loader would show, across every lane. Excludes `<root>/.trash/`. `nil` on the same
    /// terms as `laneCount`.
    let cardCount: Int?

    /// The package's content-change date, as the metadata query reports it. `nil` under the DEBUG
    /// local root, where it is read from the directory instead, and on any item that has none yet.
    let modified: Date?

    let download: BoardDownloadState

    /// Which of the phone's two homes this board is in.
    let location: BoardLocation
}

/// How the boards tab orders its merged list. Raw values are persisted (`AppStorage`), so they are
/// API — changing a case's raw value would silently reset every user's remembered choice.
enum BoardSortOrder: String, CaseIterable, Sendable {
    case name
    case recent
}

extension BoardSortOrder {
    /// Orders `boards` for display without touching the two-home merge that produced them.
    ///
    /// **`.name` is a no-op.** `BoardIndexStore` already publishes its list in title order —
    /// case/diacritic-insensitive, path as the tie-break — so re-sorting here would just repeat work
    /// already done upstream.
    ///
    /// **`.recent` sorts by `modified` descending, with `nil` last.** Ties — equal dates, and every
    /// `nil` among them — keep the incoming order: Swift's `sort` is not a stable sort, so the
    /// comparator breaks ties on each board's original offset rather than leaving equal rows free to
    /// swap places on every refresh.
    nonisolated func sorted(_ boards: [BoardSummary]) -> [BoardSummary] {
        switch self {
        case .name:
            return boards

        case .recent:
            return boards.enumerated().sorted { lhs, rhs in
                switch (lhs.element.modified, rhs.element.modified) {
                case let (l?, r?) where l != r:
                    return l > r
                case (nil, .some):
                    return false
                case (.some, nil):
                    return true
                default:
                    return lhs.offset < rhs.offset
                }
            }.map(\.element)
        }
    }
}

/// Where a board is stored, and therefore whether it syncs.
///
/// **Told, never sniffed.** A location is what the home an entry was enumerated from *is*, so the
/// index tags it at the point where that is a fact (`BoardIndexStore`, which holds both roots) and
/// everything downstream carries the tag. Deriving it from the URL instead would mean deciding
/// whether a path is inside a ubiquity container by looking at it — a question that has no stable
/// answer across the real container, the DEBUG stand-in root and a test's scratch directories.
enum BoardLocation: Sendable, Equatable {
    /// The `CloudHome` — syncs to the Mac and to every other device on the account.
    case icloud

    /// The `DeviceHomeResolver` home — this phone, and nowhere else.
    case local

    /// The one word the list marker, the settings sheet and its move button all name it by.
    var name: String {
        switch self {
        case .icloud: "iCloud"
        case .local: "Local"
        }
    }
}

/// Whether a board's bytes are on this device — the summary-level reading, from the metadata item's
/// own attributes.
///
/// **Coarse on purpose.** This drives one row's badge. The authoritative per-file answer, the one a
/// load actually depends on, is `PackageMaterialization.sweep` — a package can report `.current` here
/// and still be missing a card's `index.md`, which is exactly why the session sweeps rather than
/// trusting this.
enum BoardDownloadState: Sendable, Equatable {
    /// A local copy exists. Covers both `…StatusCurrent` and `…StatusDownloaded` (a local copy that
    /// may be behind the cloud's) — the distinction changes nothing the list can act on.
    case current

    /// Bytes are arriving. `fraction` is 0…1 where the daemon reports a percentage.
    case downloading(fraction: Double?)

    /// In the cloud, not here, nothing in flight. The index requests a download for every board in
    /// this state, so it is a transient the list should render as such.
    case notDownloaded

    /// No metadata to read: the DEBUG local root, or an item whose attributes have not arrived.
    case unknown

    /// Whether the shallow content walk may touch this package's files at all.
    var isReadable: Bool {
        switch self {
        case .current, .unknown: true
        case .downloading, .notDownloaded: false
        }
    }
}

/// One `.kanban` package as the index found it, before its contents were looked at — the `Sendable`
/// hand-off from the main-actor metadata read to the off-main scan.
struct BoardIndexEntry: Sendable, Equatable {
    let rootURL: URL
    let modified: Date?
    let download: BoardDownloadState

    /// Set by whoever produced the entry, from the home it enumerated — see `BoardLocation`.
    let location: BoardLocation
}

/// Turns an index entry into a summary by looking, shallowly, at the package.
///
/// **A shallow walk, not a load.** `BoardLoader` parses every `index.md` in the tree; a board list
/// showing six boards cannot afford six of those on a phone. So this counts folders through the same
/// gate the loader counts them by — UUID-shaped name (`IntegrityRules.isIdentityShaped`) holding an
/// `index.md` — and reads exactly one file, the board's own `index.md`, for its title. A folder that
/// fails the gate is a stray the loader would ignore too, so the counts agree with what the board
/// window will show without paying for the agreement.
///
/// Two known and accepted divergences from a real load, both in the direction of over-counting by at
/// most a hair: a card whose `index.md` is present but malformed is counted here and would be a
/// fail-fast defect there, and a card carrying a legacy `deleted:` key is counted here and rides
/// along flagged there. Deciding either requires parsing the file, which is the cost this walk
/// exists to avoid.
///
/// **Uncoordinated, deliberately.** These are display reads that re-run on every metadata update; a
/// torn read costs a stale title for one refresh, while an `NSFileCoordinator` bracket per board
/// would put a daemon round-trip on the path of drawing a list. The session coordinates; the index
/// does not.
enum BoardSummaryScanner {

    /// Blocking — callers run it off the main actor.
    nonisolated static func scan(_ entry: BoardIndexEntry) -> BoardSummary {
        let fallbackTitle = entry.rootURL.deletingPathExtension().lastPathComponent

        // Nothing on disk to read, and reading anyway risks a blocking materialization on whatever
        // network the phone is on. The name is still known — it is in the URL — so the row is
        // nameable while it downloads, and the next refresh fills in the rest.
        guard entry.download.isReadable else {
            return BoardSummary(
                rootURL: entry.rootURL,
                title: fallbackTitle,
                icon: nil,
                iconColor: nil,
                laneCount: nil,
                cardCount: nil,
                modified: entry.modified,
                download: entry.download,
                location: entry.location
            )
        }

        let indexURL = entry.rootURL.appendingPathComponent(IntegrityRules.indexFileName)
        let fields = readIndexFields(at: indexURL)
        let title = fields.title ?? fallbackTitle

        // An unreadable board `index.md` means this is not a board the loader would open — a package
        // still arriving, or one whose root file is genuinely broken. Either way a count would be a
        // fiction, so none is offered.
        guard FileManager.default.fileExists(atPath: indexURL.path) else {
            return BoardSummary(
                rootURL: entry.rootURL,
                title: title,
                icon: fields.icon,
                iconColor: fields.iconColor,
                laneCount: nil,
                cardCount: nil,
                modified: entry.modified,
                download: entry.download,
                location: entry.location
            )
        }

        var lanes = 0
        var cards = 0
        for lane in itemFolders(in: entry.rootURL) {
            lanes += 1
            cards += itemFolders(in: lane).count
        }

        return BoardSummary(
            rootURL: entry.rootURL,
            title: title,
            icon: fields.icon,
            iconColor: fields.iconColor,
            laneCount: lanes,
            cardCount: cards,
            modified: entry.modified,
            download: entry.download,
            location: entry.location
        )
    }

    /// The board `index.md`'s `title:`/`icon:`/`iconColor:`, read together since they come from the
    /// same one-file parse. `title` is `nil` where the file is absent, is not UTF-8, has no
    /// frontmatter, or carries no usable `title:` — every one of which is the folder-name fallback in
    /// `scan`. `icon`/`iconColor` are `nil` on the same unreadable-file terms, and also whenever the
    /// document simply doesn't set them — there is no fallback for either.
    private nonisolated static func readIndexFields(
        at indexURL: URL
    ) -> (title: String?, icon: String?, iconColor: String?) {
        guard let data = try? Data(contentsOf: indexURL),
              let text = String(data: data, encoding: .utf8),
              let document = try? FrontmatterDocument.parse(text)
        else {
            return (nil, nil, nil)
        }
        let title = document.title.value
        return (title?.isEmpty == false ? title : nil, document.icon.value, document.iconColor.value)
    }

    /// Direct subfolders that are lanes or cards by the loader's own two gates, reached through the
    /// loader's own enumeration (`BoardLoader.directoryCandidates`) so the two can never disagree
    /// about what a candidate is — hidden entries skipped, which is what keeps `<root>/.trash/` out
    /// of every count here without a second rule.
    private nonisolated static func itemFolders(in parent: URL) -> [URL] {
        guard let candidates = try? BoardLoader.directoryCandidates(in: parent) else { return [] }
        return candidates.filter { folder in
            IntegrityRules.isIdentityShaped(folder.lastPathComponent)
                && FileManager.default.fileExists(
                    atPath: folder.appendingPathComponent(IntegrityRules.indexFileName).path
                )
        }
    }
}