import AppKit
import Observation
import SwiftUI

// MARK: - The strip's half of a trash row's drag

/// What the strip lends the trash so a row can be dragged back onto the board (03-board-ui.md §
/// Trash ▸ Drag-to-restore).
///
/// `LaneHeaderDrag`'s sibling, and for its reason: the gesture lives on the row, but the one thing it
/// needs — *which lane is under the cursor* — is the strip's geometry, and it must be read at gesture
/// time rather than at body-evaluation time.
@MainActor
struct TrashRowDrag {
    /// The live lane under an x in strip coordinates, or `nil` when the point is not over one — a
    /// gap, the outer margin, or the trash itself (`LaneLayoutMath.laneIndex`).
    let laneUnder: (CGFloat) -> ItemID?
}

// MARK: - TrashDragSession

/// Window-local state for an in-flight drag out of the trash — `LaneReorderSession`'s sibling, and
/// deliberately as small.
///
/// It holds only what the pointer contributes: which card, and which lane is currently under it.
/// Everything else — the strip's geometry, the lanes themselves — is read fresh at render time, so a
/// foreign reload mid-drag cannot leave this holding a stale board (04-interactions.md ▸ Drag and
/// drop's re-grounding rule).
@MainActor
@Observable
final class TrashDragSession {

    /// The card being dragged out of the trash; `nil` when idle.
    private(set) var cardID: ItemID?

    /// The live lane the pointer is over, or `nil` when it is over nothing droppable. Observed: the
    /// lanes read it to draw the drop highlight, which is this milestone's whole visual feedback.
    private(set) var targetLaneID: ItemID?

    /// How far the pointer must travel before a click on a row becomes a drag — the same threshold
    /// the lane header uses, so the two gestures feel alike.
    static let threshold: CGFloat = 4

    var isActive: Bool { cardID != nil }

    func isDragging(_ id: ItemID) -> Bool { cardID == id }

    func isTarget(_ id: ItemID) -> Bool { targetLaneID == id }

    func begin(cardID: ItemID) {
        self.cardID = cardID
        targetLaneID = nil
    }

    func update(targetLaneID: ItemID?) {
        guard isActive else { return }
        self.targetLaneID = targetLaneID
    }

    /// Ends the drag, handing the caller nothing — the *commit* needs the current snapshot, which
    /// the view has and this session deliberately does not. Idempotent, because a gesture can end
    /// after the card it was carrying has already vanished.
    func end() {
        cardID = nil
        targetLaneID = nil
    }
}

// MARK: - TrashLaneView

/// The trash quasi-lane: the trailing, visually distinct column tombstoned items live in
/// (03-board-ui.md § Trash).
///
/// ### A pure view, and a quasi-lane
///
/// **Nothing here moves anything on disk.** Tombstoned items keep their `deleted:` key and stay
/// exactly where they are; this column is a rendering of `TrashModel.entries(of:)` and nothing more.
/// It is a *quasi*-lane because it wears a lane's shape while sharing none of a lane's machinery:
///
/// - it spans a **fixed one width unit** — no `width` frontmatter, no stepper, no resize handle, and
///   the edge drag never reaches it (`LaneLayoutMath.totalUnits(of:trashUnits:)` supplies the unit,
///   so there is no `Lane` value for any of those to act on);
/// - it is **not draggable and not reorderable** — the header carries no gesture, and it is absent
///   from the reorder proposal's `unitCounts` by construction, since `BoardView` builds that from
///   the snapshot's live lanes;
/// - it has **no new-card button**: nothing is created in the trash.
///
/// ### No editing in the trash
///
/// "Tombstoned cards don't open — double-click does nothing beyond selection; Put Back or drag out
/// first." There is deliberately no double-tap recogniser, no rename editor, and no Style… row: the
/// trash is for restoring or purging, not working.
///
/// ### What is still a later card's
///
/// The **search filter** ("shown, it participates in the filter like any lane") and the full
/// **keyboard grammar** — arrow walks into and out of the column, ⇧-ranges that stop at both the
/// liveness and the kind boundary, the rubber band, ⌘C copy-out — are m5's. So is the drag's replica:
/// what ships here is the drop, with the target lane highlighted and the source row dimmed in place.
struct TrashLaneView: View {

    let store: BoardStore

    /// The window's purge-alert host, threaded down rather than read from the focus system: a
    /// context menu's content is built in its own host, where a `@FocusedValue` is not reliably the
    /// board window's, and the row's Delete Immediately must raise the *same* alert the menu bar's
    /// does.
    let confirmations: TrashConfirmations

    /// The strip's drop resolution — see `TrashRowDrag`.
    let drag: TrashRowDrag

    /// The window's one drag-out session, owned by `BoardView` for the same lifetime the reorder
    /// session has.
    let dragSession: TrashDragSession

    /// The lane plate's corner radius — matched to `LaneView`'s so the column reads as a sibling of
    /// the lanes rather than as a different kind of object.
    private let cornerRadius: CGFloat = 10

    private let rowSpacing: CGFloat = 6

    var body: some View {
        VStack(alignment: .leading, spacing: 0) {
            header
            rows
        }
        .background(
            RoundedRectangle(cornerRadius: cornerRadius)
                .fill(.quaternary.opacity(0.35))
        )
    }

    /// The rows the column shows.
    ///
    // m5-search: the shown trash "participates in the filter like any lane", so the search predicate
    // narrows this collection exactly as it narrows `LaneView.renderedCards` — and the count badge
    // follows for free, because it reads this same value.
    private var entries: [TrashEntry] {
        TrashModel.entries(of: store.snapshot)
    }

    // MARK: - Header

    /// Dimmed and hatched, with the trash symbol, the stable "Trash" title and a count badge
    /// (03-board-ui.md § Trash ▸ Rendering).
    ///
    /// The hatching is what makes the column read as *not a lane* at a glance — the design asks for
    /// "visually distinct", and a lane's header is the surface this must not be mistaken for. It
    /// carries no gesture at all: no selection (the quasi-lane "is never selectable as a lane"), no
    /// reorder drag, no context menu.
    private var header: some View {
        HStack(alignment: .firstTextBaseline, spacing: 6) {
            Image(systemName: "trash")
                .foregroundStyle(.secondary)
                .imageScale(.medium)
            Text("Trash")
                .font(.headline)
                .foregroundStyle(.secondary)
                .lineLimit(1)
                .truncationMode(.tail)
            countBadge
            Spacer(minLength: 0)
        }
        .padding(.horizontal, 10)
        .padding(.vertical, 8)
        .background {
            UnevenRoundedRectangle(topLeadingRadius: cornerRadius, topTrailingRadius: cornerRadius)
                .fill(.quaternary.opacity(0.5))
                .overlay {
                    DiagonalHatch()
                        .stroke(.quaternary, lineWidth: 1)
                        .clipShape(UnevenRoundedRectangle(
                            topLeadingRadius: cornerRadius,
                            topTrailingRadius: cornerRadius
                        ))
                }
        }
        .accessibilityElement(children: .combine)
    }

    /// The entry count — the same collection the body renders, so the badge cannot disagree with
    /// what is on screen (`LaneView.countBadge`'s rule, and it is why m5's filter needs no second
    /// change here).
    private var countBadge: some View {
        Text("\(entries.count)")
            .font(.caption)
            .monospacedDigit()
            .foregroundStyle(.secondary)
            .padding(.horizontal, 6)
            .padding(.vertical, 1)
            .background(Capsule().fill(.quaternary))
    }

    // MARK: - Rows

    private var rows: some View {
        ScrollView(.vertical) {
            LazyVStack(alignment: .leading, spacing: rowSpacing) {
                ForEach(entries) { entry in
                    TrashEntryRow(
                        store: store,
                        entry: entry,
                        confirmations: confirmations,
                        drag: drag,
                        dragSession: dragSession
                    )
                }
            }
            .frame(maxWidth: .infinity, alignment: .topLeading)
            .padding(6)
        }
    }
}

// MARK: - The hatch

/// Diagonal hatching for the trash header — the "dimmed/hatched" treatment 03-board-ui.md asks for,
/// drawn rather than imaged so it takes whatever width the division gives the column.
///
/// The lines start a full header-height to the left of the leading edge so the first stroke reaches
/// the top-left corner instead of beginning partway across.
private struct DiagonalHatch: Shape {
    var spacing: CGFloat = 7

    func path(in rect: CGRect) -> Path {
        var path = Path()
        guard spacing > 0, rect.height > 0 else { return path }
        var x = rect.minX - rect.height
        while x < rect.maxX {
            path.move(to: CGPoint(x: x, y: rect.maxY))
            path.addLine(to: CGPoint(x: x + rect.height, y: rect.minY))
            x += spacing
        }
        return path
    }
}

// MARK: - Rows

/// One trash row: a compact, dimmed plate carrying the item's symbol and title — and, for a lane
/// entry, the count of cards Put Back would return with it.
///
/// **Face-like but not a card face.** It shares the plate, the symbol and the title, and it
/// deliberately shares none of the face's *editing* affordances: no rename editor, no Open, no
/// Style…, no attachment carousel. That is 03-board-ui.md's no-editing-in-the-trash rule expressed
/// as an absence rather than as a pile of `disabled` modifiers.
private struct TrashEntryRow: View {

    let store: BoardStore
    let entry: TrashEntry
    let confirmations: TrashConfirmations
    let drag: TrashRowDrag
    let dragSession: TrashDragSession

    private let cornerRadius: CGFloat = 6

    var body: some View {
        HStack(alignment: .firstTextBaseline, spacing: 6) {
            Image(systemName: ItemSymbol.name(entry.icon, fallback: symbolFallback))
                .foregroundStyle(.secondary)
                .imageScale(.small)
            VStack(alignment: .leading, spacing: 2) {
                Text(entry.title ?? "Untitled")
                    .font(.callout)
                    .foregroundStyle(.secondary)
                    .lineLimit(2)
                if case let .lane(_, returning) = entry {
                    // "N cards" — what Put Back brings back with the lane, not how many folders sit
                    // inside it (`TrashModel.entries`' returning-count rule).
                    Text("\(returning) card\(returning == 1 ? "" : "s")")
                        .font(.caption)
                        .foregroundStyle(.tertiary)
                }
            }
            .frame(maxWidth: .infinity, alignment: .leading)
        }
        .padding(.horizontal, 8)
        .padding(.vertical, 6)
        .background(RoundedRectangle(cornerRadius: cornerRadius).fill(.background.secondary.opacity(0.6)))
        .overlay(
            RoundedRectangle(cornerRadius: cornerRadius)
                .strokeBorder(isSelected ? AnyShapeStyle(Color.accentColor) : AnyShapeStyle(.clear), lineWidth: 1.5)
        )
        // The row being dragged out dims further, so the gesture reads even without a replica.
        .opacity(dragSession.isDragging(entry.id) ? 0.45 : 1)
        .contentShape(Rectangle())
        .gesture(rowGesture)
        .contextMenu { menu }
    }

    private var symbolFallback: String {
        entry.isLaneEntry ? ItemSymbol.lane : ItemSymbol.card
    }

    // MARK: - Selection

    private var isSelected: Bool {
        store.selection.liveness == .trashed && store.selection.ids.contains(entry.id)
    }

    /// A click replaces the selection with this one row, on the **trashed** side.
    ///
    /// Replace-only is what enforces both invariants at once here: a selection that is always exactly
    /// one row can never mix live with tombstoned, nor card entries with lane entries
    /// (04-interactions.md ▸ The trash). The extension grammar — ⌘-click, ⇧-ranges that go inert at
    /// both boundaries, the rubber band that stays on the side it started on — is **m5's
    /// selection-model card**, and nothing here should pre-empt it.
    ///
    /// **A double click is two of these and nothing more**: no editor, no card window, no timer.
    private func select() {
        store.select([entry.id], liveness: .trashed)
    }

    // MARK: - Drag out

    /// One gesture recognising the same click-versus-drag split the lane header uses: a plain click
    /// selects, and only movement past the threshold begins a drag out of the trash.
    ///
    /// **Lane entries are not draggable** (03-board-ui.md § Trash: "a lane entry is not draggable —
    /// its entry is a compact row, not the lane; its move-out is Put Back"), so the drag half is
    /// simply absent for them and the release still selects.
    ///
    /// The coordinate space is the strip's, because what the release needs is a *position* over the
    /// board, not a translation.
    private var rowGesture: some Gesture {
        DragGesture(minimumDistance: 0, coordinateSpace: .named(BoardView.stripSpace))
            .onChanged { value in
                guard isDraggable, !store.isReadOnly, !store.isEditingInline else { return }
                if !dragSession.isDragging(entry.id) {
                    let travelled = max(abs(value.translation.width), abs(value.translation.height))
                    guard travelled > TrashDragSession.threshold else { return }
                    dragSession.begin(cardID: entry.id)
                }
                dragSession.update(targetLaneID: drag.laneUnder(value.location.x))
            }
            .onEnded { value in
                guard dragSession.isDragging(entry.id) else {
                    select()
                    return
                }
                dragSession.end()
                // A drop over anything but a live lane — the trash itself, a gap, the outer margin —
                // writes nothing. There is no replica to snap back; the row never left.
                guard let lane = drag.laneUnder(value.location.x) else { return }
                store.restoreByDrag(cardID: entry.id, intoLane: lane)
            }
    }

    private var isDraggable: Bool { !entry.isLaneEntry }

    // MARK: - The trash entry's context menu

    /// Put Back, Delete Immediately, Reveal in Finder — the three rows 11-command-nexus.md gives a
    /// trash entry, and no others.
    ///
    /// Reveal in Finder is the odd one out and deliberately so: it is "not edit-shaped and stays
    /// enabled on tombstoned selections", read-only lock included — inspecting a folder before a
    /// purge is exactly the errand it exists for.
    @ViewBuilder
    private var menu: some View {
        Button("Put Back") {
            store.putBack(targetIDs)
        }
        .disabled(!store.acceptsBoardMutations)

        Button("Delete Immediately") {
            confirmations.requestPurge(of: targetIDs, in: store)
        }
        .disabled(!store.acceptsBoardMutations)

        Divider()

        Button("Reveal in Finder") {
            NSWorkspace.shared.activateFileViewerSelecting(targetFolders)
        }
    }

    /// What this row's menu acts on: the whole selection when this row is part of it, else this row
    /// alone — standard macOS context-menu targeting, and the same rule the card face and the lane
    /// header apply to Style…. Right-clicking something outside the selection acts on what was
    /// clicked, which is also what keeps a cross-kind menu from ever acting on a mixed set.
    private var targetIDs: Set<ItemID> {
        guard store.selection.liveness == .trashed, store.selection.ids.contains(entry.id) else {
            return [entry.id]
        }
        return store.selection.ids
    }

    private var targetFolders: [URL] {
        TrashModel.paths(of: targetIDs, on: .trashed, in: store.snapshot)
            .map { $0.folder(under: store.rootURL) }
    }
}