import Foundation

/// The **board-scale tree copy**: a per-item file walk that checks cancellation between items,
/// shared by File ▸ Duplicate (`BoardDuplicator`) and template instantiation (`TemplateEngine`).
///
/// ### Why the walk is a walk
///
/// 03-board-ui.md settles the shape for Duplicate and the reason generalizes to every copy of a
/// whole board: "the copy runs as a per-item file walk that checks cancellation between items —
/// **never one monolithic `copyItem`** — and Cancel removes the partial sibling before dismissing
/// the banner (the attachment partial-cleanup precedent)". A monolithic `copyItem` is uncancellable
/// and indivisible; a walk is both, and it is also the only shape that can *exclude* something
/// (09-templates.md's `.git` and `.trash/`).
///
/// So the walk is the promise: one `FileManager.copyItem` per file — bytes and their metadata
/// copied by the file system, never re-encoded, **symlinks copied as symlinks** (01-storage-format.md
/// § Fractal layout ▸ Rules: "Copy flows copy the link itself, never its target: Duplicate, Save as
/// Template, instantiation, cross-board copies … preserve the link verbatim") — folders recreated by
/// hand, and a cancellation read **between** items, never mid-file (a copy interrupted inside a
/// 200 MB pack file leaves rubble that is harder to reason about than one more file's wait). The
/// entries of every folder are walked in name order, so the same cancellation removes the same
/// partial tree every time and a failure names the same file twice running.
///
/// The accepted cost of building folders by hand rather than handing the tree to one `copyItem`:
/// **extended attributes on folders do not carry** (files keep theirs — each is still copied by
/// `copyItem`). Nothing the app, the storage format, or git keeps lives in a folder xattr.
///
/// ### What it does not do
///
/// It never creates the destination root, never removes a partial, and never classifies a failure.
/// Those are the caller's, because they are exactly where the two flows differ: Duplicate answers a
/// permission failure with a save panel, instantiation with a banner, and each owns its own cleanup
/// promise. This type only knows how to move a tree, one item at a time, and where to stop.
///
/// ### Not `@MainActor`
///
/// Copying a board with a year of `.git` behind it is real I/O and runs off the main actor so an
/// in-progress banner can actually spin (02-architecture.md § The banner surface). Safe by
/// construction: it touches only the two URLs it is handed, and a board's security-scoped access is
/// a process-wide grant, not a per-thread one.
enum BoardTreeCopy {

    /// Why a walk stopped and on which item — the recursion's currency, which each caller converts
    /// into its own failure vocabulary the moment it surfaces.
    enum Stop: Error {
        case cancelled
        case failed(url: URL, error: any Error)
    }

    /// Copies everything inside `source` into the already-created `destination`, one item at a time.
    ///
    /// Name order, hidden entries included (no `.skipsHiddenFiles`): `.DS_Store`, `.gitignore`,
    /// `CLAUDE.user.md` and every other dotfile or stray is part of the copy — "the copy is literal
    /// apart from the stated exclusions" (09-templates.md ▸ Save as Template) — and a deterministic
    /// order is what makes a cancellation reproducible.
    ///
    /// `excludedTopLevelNames` is compared **lowercased against the top level only**, which is the
    /// exclusions' actual scope rather than a shortcut: `.git` and `.trash/` mean something at a
    /// board root and nowhere else (06-history-undo.md's nearest-`.git`-wins detection starts at the
    /// board root; `BoardLoader.trashCandidates` looks in exactly one place), so a `.git` a template
    /// author left inside a card folder is an ordinary stray and copies verbatim like any other.
    ///
    /// `carriesFolderAttributes` decides whether a recreated folder gets the source folder's POSIX
    /// permissions and timestamps back once its contents have landed. Duplicate says yes — it is a
    /// fork, down to the mode bits. Instantiation says no: a template folder is content the app
    /// ships or the user dropped, and carrying a read-only mode (or a two-year-old date) out of it
    /// would mint a board that is read-only, or born older than itself, from a copy whose whole
    /// premise is "born today" (09-templates.md ▸ Instantiation).
    ///
    /// Each entry's type comes from `attributesOfItem`, which does **not** traverse symlinks — so a
    /// link is copied as a link (by `copyItem`, which does not follow it either) rather than being
    /// mistaken for the folder it points at and walked into.
    static func copy(
        contentsOf source: URL,
        into destination: URL,
        excludingTopLevel excludedTopLevelNames: Set<String> = [],
        carryingFolderAttributes carriesFolderAttributes: Bool = true,
        isCancelled: () -> Bool
    ) throws(Stop) {
        try copyContents(
            of: source,
            into: destination,
            excluding: excludedTopLevelNames,
            carryingFolderAttributes: carriesFolderAttributes,
            isCancelled: isCancelled
        )
    }

    /// The recursion. `excluded` is emptied one level down, which is what makes the exclusions
    /// top-level-only without the walk having to count its own depth.
    private static func copyContents(
        of source: URL,
        into destination: URL,
        excluding excluded: Set<String>,
        carryingFolderAttributes carriesFolderAttributes: Bool,
        isCancelled: () -> Bool
    ) throws(Stop) {
        let entries: [URL]
        do {
            entries = try FileManager.default.contentsOfDirectory(
                at: source,
                includingPropertiesForKeys: nil,
                options: []
            )
        } catch {
            throw .failed(url: source, error: error)
        }

        for entry in entries.sorted(by: { $0.lastPathComponent < $1.lastPathComponent }) {
            guard !excluded.contains(entry.lastPathComponent.lowercased()) else { continue }

            // Between items, never mid-item: this is the whole of "checks cancellation between
            // items", and the reason the copy is a walk at all.
            if isCancelled() { throw .cancelled }

            let attributes: [FileAttributeKey: Any]
            do {
                attributes = try FileManager.default.attributesOfItem(atPath: entry.path)
            } catch {
                throw .failed(url: entry, error: error)
            }

            let isDirectory = attributes[.type] as? FileAttributeType == .typeDirectory
            let target = destination.appendingPathComponent(entry.lastPathComponent, isDirectory: isDirectory)

            guard isDirectory else {
                // Files, symlinks, and whatever else the file system holds: `copyItem` lands the
                // bytes and the metadata that rides with them, byte-for-byte, unread.
                do {
                    try FileManager.default.copyItem(at: entry, to: target)
                } catch {
                    throw .failed(url: entry, error: error)
                }
                continue
            }

            do {
                try createDirectory(at: target)
            } catch {
                throw .failed(url: entry, error: error)
            }
            try copyContents(
                of: entry,
                into: target,
                excluding: [],
                carryingFolderAttributes: carriesFolderAttributes,
                isCancelled: isCancelled
            )
            if carriesFolderAttributes {
                restoreAttributes(from: attributes, onto: target)
            }
        }
    }

    /// Creates `url` as a plain directory, wearing the process's own default permissions until its
    /// contents have landed (`restoreAttributes(from:onto:)` puts the source's back afterwards,
    /// where the caller asked for them).
    ///
    /// **Default permissions first, the source's last**, because a folder is not only a thing being
    /// copied but the thing being copied *into*: a source folder that is read-only, or unreadable,
    /// would otherwise be reproduced as a destination this walk cannot write its own children into —
    /// and, when something later fails, as a partial tree the cleanup cannot remove either. `cp -R`
    /// defers the mode for the same reason.
    ///
    /// `withIntermediateDirectories: false` throughout: every parent either exists already (the walk
    /// just made it) or is the one the user pointed at, and inventing a missing folder would be this
    /// function deciding where a board lives.
    static func createDirectory(at url: URL) throws {
        try FileManager.default.createDirectory(at: url, withIntermediateDirectories: false)
    }

    /// Puts a folder's POSIX permissions and its creation and modification dates back, **after** its
    /// contents have landed — writing into a folder is itself a modification, and its mode may be
    /// what stops the writing (above), so both wait for the subtree to be finished.
    ///
    /// Best effort by design: a volume that will not take a date back (or a file system with no
    /// creation dates at all) is not a reason to fail a copy that otherwise worked, and neither a
    /// folder's timestamp nor its mode is something the storage format reads.
    static func restoreAttributes(from attributes: [FileAttributeKey: Any], onto url: URL) {
        var carried: [FileAttributeKey: Any] = [:]
        if let permissions = attributes[.posixPermissions] {
            carried[.posixPermissions] = permissions
        }
        if let created = attributes[.creationDate] {
            carried[.creationDate] = created
        }
        if let modified = attributes[.modificationDate] {
            carried[.modificationDate] = modified
        }
        guard !carried.isEmpty else { return }
        try? FileManager.default.setAttributes(carried, ofItemAtPath: url.path)
    }
}