import AppKit
import SwiftUI
import UniformTypeIdentifiers

// This file is a **pure extraction** from `BoardDrops.swift`, moved verbatim the moment a second
// window needed it: the card window's whole-window attachment drop (05-card-window.md ▸ Attachments)
// runs the same two rules this file holds — how a dropped `NSItemProvider` becomes a file URL, and
// which half of a Finder drag is a file rather than a folder. Both windows import through
// `BoardStore.importAttachments`, so a second copy of either rule would be a second folder-refusal
// semantics able to disagree with the board's about what a package is.
//
// Nothing here knows about a board's geometry, its lanes, or its drag session — which is what made
// the move mechanical. `FileDropTarget.Landing`, the one board-shaped type `land` still takes, stays
// in `BoardDrops.swift`: the card window passes `.attach(cardID:)` and the board resolves its own.

// MARK: - Loading what Finder dropped

/// The one place an `NSItemProvider` from an external drag is unwrapped into a file URL.
enum FileDropLoading {

    /// The file URL a dropped provider carries, or `nil` when it carries none.
    ///
    /// **`loadItem` on `.fileURL`, not `loadInPlaceFileRepresentation`.** A `public.file-url` item is
    /// what Finder actually puts on the dragging pasteboard, and the drag itself is what grants the
    /// sandbox the extension to read it — the caller opens the scope and copies. The in-place
    /// representation would hand back a URL valid only for the duration of its own completion block,
    /// forcing the copy to happen off the main actor inside a callback, for no benefit here.
    ///
    /// The completion fires on an arbitrary queue — never assume the main actor — so this is a plain
    /// continuation wrapper; a `CheckedContinuation` resumes from any queue whatever isolation the
    /// call site started from. Both shapes a `.fileURL` item arrives in are accepted: the `Data`
    /// encoding it normally takes, and a bare `URL`.
    ///
    /// `@MainActor` for a concurrency reason rather than a behavioural one: the provider comes off a
    /// `DropInfo` on the main actor and is not `Sendable`, so a nonisolated entry point would be
    /// *sending* it across domains. Staying on the actor it came from keeps the hand-off to the
    /// completion handler — which fires wherever AppKit likes — the only crossing there is.
    @MainActor
    static func url(from provider: NSItemProvider) async -> URL? {
        await withCheckedContinuation { continuation in
            provider.loadItem(forTypeIdentifier: UTType.fileURL.identifier, options: nil) { item, _ in
                if let data = item as? Data, let url = URL(dataRepresentation: data, relativeTo: nil) {
                    continuation.resume(returning: url)
                } else if let url = item as? URL {
                    continuation.resume(returning: url)
                } else {
                    continuation.resume(returning: nil)
                }
            }
        }
    }
}

// MARK: - Files in, folders out

/// What a Finder drag is carrying and where it lands — **the folder refusal, both halves**
/// (04-interactions.md ▸ Drag and drop: "Folders are refused at hover … the attachment model is flat
/// top-level files, and the importer refuses directories by design").
///
/// ### Why the payload is read twice
///
/// The rule is a *hover* rule, and hover has only the providers' declared types to go on: a file URL
/// is not resolved until the drop, and resolving one during hover is neither offered nor affordable.
/// So the drag is read twice, and the two reads have different jobs:
///
/// - **At hover, from `registeredTypeIdentifiers`** — Finder registers the concrete UTI beside
///   `public.file-url`, so a folder announces itself as `public.folder` before anything is loaded.
///   That is what makes a folders-only drag refuse *at the cursor*: no highlight, no shadows, the
///   incompatible-payload read (`BoardDropContext.acceptsFileDrop`).
/// - **At the drop, from the URLs themselves** — `FinderDrop.partition`, which is a filesystem fact
///   rather than a declaration and therefore the authority. It is what actually decides what gets
///   written.
///
/// **Unknown at hover is treated as a file**, deliberately: a provider that registers only
/// `public.file-url` and no concrete type — a synthetic drag, or an unusual source — cannot be
/// classified until its URL resolves, and the optimistic read means such a drag still engages, still
/// shows its shadows, and is sorted out authoritatively at the drop. The pessimistic read would make
/// an ordinary file drag silently dead, which is the far worse failure.
///
/// **A package is a directory.** Conformance to `public.directory` — not equality with
/// `public.folder` — is the test, so a `.app`, an `.rtfd`, or any other bundle is refused exactly as
/// a plain folder is: the flat top-level attachment model has no more room for one than for the
/// other, and `isDirectory(at:)` says the same thing at the drop.
enum FinderDrop {

    // MARK: The hover read — declared types

    /// Whether a provider's registered types describe a directory (folders and packages alike).
    ///
    /// An identifier the system does not know, and an empty list, are *not* directories: this is the
    /// optimistic side of the unknown-at-hover rule above.
    nonisolated static func isDirectory(typeIdentifiers: [String]) -> Bool {
        typeIdentifiers.contains { identifier in
            UTType(identifier)?.conforms(to: .directory) ?? false
        }
    }

    /// How many of these providers are importable — the file count every hover-time proposal is
    /// sized by, and `0` is the refusal that keeps a folders-only drag from ever engaging.
    ///
    /// `@MainActor` for the same reason `FileDropLoading.url(from:)` is: an `NSItemProvider` off a
    /// `DropInfo` is not `Sendable`, so it stays on the actor it arrived on.
    @MainActor
    static func importableCount(_ providers: [NSItemProvider]) -> Int {
        providers.filter { !isDirectory(typeIdentifiers: $0.registeredTypeIdentifiers) }.count
    }

    /// How many shadows the create path draws — **one nominal-height shadow per incoming file**
    /// (04-interactions.md ▸ Drag and drop, settled 2026-07-28: the multi-drag precedent), **floored
    /// at one**: "when macOS withholds item counts during hover the count floors at one shadow, the
    /// commit unaffected".
    ///
    /// The floor is a *hover* concession and nothing more. A drag whose providers cannot be counted
    /// still opens a slot the user can aim at, and the write is `FinderDrop.land`'s — resolved URLs,
    /// partitioned against the filesystem — so one shadow standing in for three files costs the drop
    /// nothing. It is read from the drag on every sample rather than captured at `dropEntered`: a
    /// delegate can be entered without this window ever having seen the enter callback (single-target
    /// dispatch hands the session to whichever region is deepest), and a count that was never set
    /// would draw the wrong number of shadows.
    @MainActor
    static func shadowCount(_ providers: [NSItemProvider]) -> Int {
        max(1, importableCount(providers))
    }

    // MARK: The drop read — the filesystem

    /// Whether `url` is a directory, as the filesystem answers it — the authoritative read.
    ///
    /// `resourceValues` first (the real answer, packages included), `fileExists` as the fallback for
    /// a URL whose resource values cannot be read, and the purely lexical `hasDirectoryPath` last,
    /// for a source that has already vanished between the drag and the drop.
    nonisolated static func isDirectory(at url: URL) -> Bool {
        if let flag = try? url.resourceValues(forKeys: [.isDirectoryKey]).isDirectory { return flag }
        var isDirectory: ObjCBool = false
        if FileManager.default.fileExists(atPath: url.path, isDirectory: &isDirectory) {
            return isDirectory.boolValue
        }
        return url.hasDirectoryPath
    }

    /// Splits a dropped set into what will be written and what will be named as skipped, preserving
    /// input order in both halves — the create path mints its cards in drop order, and the order the
    /// user dropped in is the only order there is.
    nonisolated static func partition(_ urls: [URL]) -> (files: [URL], folders: [URL]) {
        var files: [URL] = []
        var folders: [URL] = []
        for url in urls {
            if isDirectory(at: url) { folders.append(url) } else { files.append(url) }
        }
        return (files, folders)
    }

    // MARK: The write

    /// The drop's **write half**: the files land where the highlight or the shadows showed, and the
    /// folders are named in a loss row rather than attempted.
    ///
    /// **No card is ever minted for an import that cannot succeed** (04-interactions.md): the folders
    /// are gone before `createCards` sees the list, so the create path only ever fires with files —
    /// "the mint-fail-remove dance is gone" for this reason, not because the store stopped doing it.
    /// `BoardStore.createCards` still removes a card whose import failed for a *genuine* reason (an
    /// unreadable source, a full disk), and `BoardWriter.importAttachments` still refuses a directory
    /// outright: that throw stays as the model layer's backstop for every other caller, and folders
    /// simply never reach it from here.
    ///
    /// **Zero files is a valid arrival, and writes nothing.** The hover refusal means a folders-only
    /// drag normally never gets here at all; a payload whose types were unknown at hover can, and the
    /// honest answer is the loss row alone — no write, no empty card, nothing to undo.
    ///
    /// A **loss row, not a failure one-shot** (02-architecture.md § the banner vocabulary): nothing
    /// failed here. The files the user dropped arrived; the folders were never things this app could
    /// take, and `postSkippedFolders` is silent at zero, so an all-files drop says nothing at all.
    @MainActor
    static func land(_ urls: [URL], landing: FileDropTarget.Landing, into store: BoardStore) {
        let (files, folders) = partition(urls)
        if !files.isEmpty {
            switch landing {
            case let .attach(cardID):
                store.importAttachments(files, toCard: cardID)
            case let .create(laneID, index):
                store.createCards(fromFiles: files, inLane: laneID, at: index)
            }
        }
        store.banners.postSkippedFolders(count: folders.count)
    }
}