lanework/Kanban/UI/Board/BoardDrops.swift

import AppKit
import SwiftUI
import UniformTypeIdentifiers

// MARK: - What each lane draws, as the drag reads it

/// Where each lane's card grid is drawn and how tall its cards are — the measured half of the card
/// masonry's drop geometry, one registry per board window.
///
/// **Deliberately not `@Observable`.** Nothing renders off it: it exists so a drop delegate and the
/// autoscroll driver can ask, at *event* time, where the grid is and what the resting row extents
/// are. Observing it would invalidate the strip on every layout pass, which is the animation
/// feedback loop this whole model exists to avoid.
///
/// ### What is measured here, and why that is animation-proof
///
/// 03-board-ui.md § Motion forbids reading *mid-flight* measurements. Two things are read here and
/// neither is one:
///
/// - **The grid's frame.** A lane's card area does not move while a card session is in flight: the
///   masonry reflows *inside* it, and the strip only reflows for a lane session, which reads none of
///   this.
/// - **Each card's height.** A card's height is content-driven — the column width is fixed by the
///   lane's unit count — so it does not animate under the reflow; only positions do. The positions
///   are never measured: they are replayed analytically from these heights through
///   `MasonryPlacement.frames(heights:)`, which is the very function `MasonryLayout` places with
///   (DRAG-REORDER.md § The card masonry).
///
/// The one input that *is* frozen at drag start is the **dragged** cards' own heights, which live on
/// `DragSession`: the pickup transition scales the replica and corrupts its last measured frame.
@MainActor
final class LaneDropRegistry {

    /// One lane's masonry, as drawn.
    struct Grid: Equatable, Sendable {
        /// The card area's frame in the window's SwiftUI global space — the space the physical
        /// cursor is converted into (`BoardDropContext.globalCursor`).
        var frame: CGRect
        /// Interior masonry columns — the lane's width units.
        var columns: Int
        /// Spacing between columns and between stacked cards.
        var spacing: CGFloat
    }

    /// The height a card with no registered measurement is assumed to have — a lane whose faces have
    /// not laid out yet. Nominal rather than zero, so the resting rows still tile.
    static let nominalCardHeight: CGFloat = 44

    /// The board strip's own frame in the window's SwiftUI global space — the origin the strip
    /// coordinates `DropSlotMath.laneExtents` is written in are measured from. It lives here rather
    /// than in the view's `@State` so a delegate reads the *current* rectangle at event time.
    var stripFrame: CGRect = .zero

    private(set) var grids: [ItemID: Grid] = [:]
    private(set) var heights: [ItemID: CGFloat] = [:]

    func update(_ grid: Grid, for laneID: ItemID) { grids[laneID] = grid }
    func removeGrid(_ laneID: ItemID) { grids.removeValue(forKey: laneID) }

    func update(height: CGFloat, for cardID: ItemID) { heights[cardID] = height }
    func removeHeight(_ cardID: ItemID) { heights.removeValue(forKey: cardID) }
}

// MARK: - The board window's half of a drop

/// Everything a drop delegate — and the autoscroll driver — needs to answer "where would this land",
/// read at **event** time rather than captured at body-evaluation time.
///
/// The closures are the point: a captured snapshot of the strip's frame or its standard width goes
/// stale the moment the layout animates, and two delegates holding different snapshots would flap
/// the proposal between them.
@MainActor
struct BoardDropContext {

    let store: BoardStore
    let session: DragSession
    let registry: LaneDropRegistry

    /// The strip's inter-lane gap, which is also its outer margin.
    let gap: CGFloat

    /// The window hosting this board — the physical cursor is converted through it.
    let window: @MainActor () -> NSWindow?

    /// The strip's frame in the window's SwiftUI global space.
    let stripFrame: @MainActor () -> CGRect

    /// The strip's 1× lane width for the current drag context (`LaneLayoutMath.standardWidth`).
    let standard: @MainActor () -> CGFloat

    // MARK: The cursor

    /// The physical cursor in the window's SwiftUI global space.
    ///
    /// **`NSEvent.mouseLocation`, never `DropInfo.location`** (DRAG-REORDER.md § Animation-proof
    /// inputs): the drop callback's location is expressed in the target view's space, and that view
    /// may itself be mid-reflow. This is also what `LaneResizeSession` and `MarqueeSession` already
    /// read.
    func globalCursor() -> CGPoint? {
        guard let window = window(), let content = window.contentView else { return nil }
        let inWindow = window.convertPoint(fromScreen: NSEvent.mouseLocation)
        let inContent = content.convert(inWindow, from: nil)
        // SwiftUI's global space is top-left-origin; an unflipped `NSView` is bottom-left.
        let y = content.isFlipped ? inContent.y : content.bounds.height - inContent.y
        return CGPoint(x: inContent.x, y: y)
    }

    /// The cursor in strip coordinates — 0 at the strip's leading edge, outer margin included, which
    /// is the origin `DropSlotMath.laneExtents` assumes.
    func stripCursor() -> CGPoint? {
        guard let cursor = globalCursor() else { return nil }
        let frame = stripFrame()
        return CGPoint(x: cursor.x - frame.minX, y: cursor.y - frame.minY)
    }

    // MARK: Re-grounding

    /// **Rule 2 of the mid-drag re-grounding trio** (04-interactions.md ▸ Drag and drop): a proposal
    /// whose target lane was tombstoned or vanished in a reload is invalidated — tombstoned lanes are
    /// never drop targets — the shadow withdraws, and no proposal stands until the pointer reaches a
    /// live target.
    ///
    /// Run at the top of every callback *and* again at release, against the snapshot as it is then;
    /// the store's commits enforce the same rule independently, so the gesture and the write cannot
    /// disagree.
    func revalidateProposal() {
        guard let proposal = session.proposal,
              DragLocality.isSameBoard(proposal.boardRoot, store.rootURL),
              let laneID = proposal.laneID
        else { return }
        guard !store.snapshot.lanes.contains(where: { $0.id == laneID && !$0.isDeleted }) else { return }
        session.propose(nil)
    }

    // MARK: Retargeting — the one shared answer

    /// Where a **lane** session would land on this board's strip.
    ///
    /// The zones are analytic — `DropSlotMath.laneExtents` over the remaining lanes' unit counts and
    /// this strip's standard width — and the cursor is the physical mouse, so neither input is a
    /// measured frame (03-board-ui.md § Motion).
    ///
    /// **The terminal slot is clamped before the trash.** The quasi-lane consumes one unit while
    /// shown and is never a landing spot for anything (04-interactions.md ▸ The trash: "no move or
    /// paste ever targets the trash"), so it is absent from the slot list by construction and the end
    /// slot's uncapped reach past the last real lane lands *before* it.
    func retargetLanes() {
        guard session.isDraggingLanes, let cursor = stripCursor() else { return }
        let hidden = session.hiddenMembers(onBoardRooted: store.rootURL)
        let resting = store.snapshot.lanes.filter { !$0.isDeleted && !hidden.contains($0.id) }
        let restingUnits = resting.map { LaneLayoutMath.displayUnits(of: $0) }
        let slot = DropSlotMath.laneSlot(
            cursorX: cursor.x,
            restingUnits: restingUnits,
            draggedUnits: session.laneUnits,
            standard: standard(),
            gap: gap,
            current: session.stripProposal(onBoardRooted: store.rootURL)
        )
        guard let slot else { return }   // a dead region: hold the current proposal
        let index = min(max(0, slot), restingUnits.count)
        session.propose(DropTarget(boardRoot: store.rootURL, laneID: nil, index: index))
    }

    /// Where a **card** session would land in `laneID`'s masonry.
    ///
    /// The single answer the lane's own drop delegate, the strip's fall-through, and the autoscroll
    /// driver all go through — "the lane's drop delegate and the autoscroll driver must go through
    /// one shared retarget, so they can never disagree" (DRAG-REORDER.md § Edge autoscroll).
    func retargetCards(inLane laneID: ItemID) {
        guard session.isDraggingCards, let cursor = globalCursor() else { return }
        guard let lane = store.snapshot.lanes.first(where: { $0.id == laneID && !$0.isDeleted }) else {
            revalidateProposal()
            return
        }
        guard let grid = registry.grids[laneID] else { return }

        let hidden = session.hiddenMembers(onBoardRooted: store.rootURL)
        let rendered = lane.cards.filter { !$0.isDeleted && !hidden.contains($0.id) }
        let heights = rendered.map { registry.heights[$0.id] ?? LaneDropRegistry.nominalCardHeight }
        let placement = MasonryPlacement(
            columnCount: grid.columns,
            columnWidth: MasonryPlacement.columnWidth(
                totalWidth: grid.frame.width, columnCount: grid.columns, spacing: grid.spacing),
            spacing: grid.spacing,
            origin: grid.frame.origin
        )
        let slot = DropSlotMath.cardSlot(
            cursor: cursor,
            placement: placement,
            heights: heights,
            // The run's footprint at the landing spot: the first dragged card's frozen height, which
            // is the trigger rect the cursor is over (the rest stack below it).
            draggedHeight: session.cardHeights.first ?? LaneDropRegistry.nominalCardHeight,
            current: session.laneProposal(onBoardRooted: store.rootURL, laneID: laneID)
        )
        guard let slot else { return }   // a dead region: hold
        session.propose(DropTarget(boardRoot: store.rootURL, laneID: laneID, index: slot))
    }

    /// The strip's fall-through for card sessions: which lane is under the cursor, analytically.
    ///
    /// This is both the safety net for a lane whose own drop region goes dead (DRAG-REORDER.md §
    /// Single-target dispatch) and the live handler for the strip's own regions. A cursor over a gap,
    /// the outer margin, or the trash column is over no lane at all — `LaneLayoutMath.laneIndex`
    /// answers `nil` there — and the proposal simply **holds**, which is the hysteresis contract.
    func retargetCardsFromStrip() {
        guard session.isDraggingCards, let cursor = stripCursor() else { return }
        let lanes = store.snapshot.lanes.filter { !$0.isDeleted }
        let index = LaneLayoutMath.laneIndex(
            atX: cursor.x,
            unitCounts: lanes.map { LaneLayoutMath.displayUnits(of: $0) },
            standard: standard(),
            gap: gap
        )
        guard let index, lanes.indices.contains(index) else { return }
        retargetCards(inLane: lanes[index].id)
    }

    /// The retarget for whichever session type is in flight, from the strip's own surfaces.
    func retargetFromStrip() {
        revalidateProposal()
        if session.isDraggingLanes {
            retargetLanes()
        } else {
            retargetCardsFromStrip()
        }
    }

    // MARK: The drop proposal the badge tracks

    /// What `dropUpdated` answers: the effective operation while the shadows are on *this* board,
    /// `.cancel` otherwise.
    ///
    /// The operation is re-resolved here rather than at pickup, which is what makes the badge track
    /// live as the cursor crosses a board boundary (04-interactions.md ▸ Drag and drop).
    func dropProposal() -> DropProposal {
        guard let proposal = session.proposal,
              DragLocality.isSameBoard(proposal.boardRoot, store.rootURL)
        else { return DropProposal(operation: .cancel) }
        let operation = session.resolveOperation(destinationRoot: store.rootURL)
        return DropProposal(operation: operation == .copy ? .copy : .move)
    }

    // MARK: The commit

    /// Commits the current proposal — **the drop always lands exactly where the shadows show**
    /// (DRAG-REORDER.md § The pieces).
    ///
    /// The three re-grounding rules are applied here, against the snapshot as it is *now* rather than
    /// against whatever the last render believed:
    ///
    /// 1. the geometry was re-derived on every sample and the proposal is what it produced;
    /// 2. a proposal naming a vanished or tombstoned lane is invalidated, and **release with no valid
    ///    proposal cancels** — items return, nothing is written;
    /// 3. an emptied drag cancels itself, and a partly emptied one drops the survivors.
    ///
    /// The commit is the **destination** store's, one `performWrite` bracket per gesture whatever the
    /// set's size (DRAG-REORDER.md § The drop commits).
    func commitDrop() -> Bool {
        guard session.isActive, let kind = session.kind, let sourceRoot = session.sourceRoot else {
            return false
        }
        revalidateProposal()
        guard let target = session.proposal,
              DragLocality.isSameBoard(target.boardRoot, store.rootURL)
        else {
            cancelDrop()
            return false
        }
        let survivors = session.survivors
        guard !survivors.isEmpty else {
            cancelDrop()
            return false
        }
        let ids = survivors.map { session.members[$0] }
        let folders = survivors.map { session.folders[$0] }
        let within = DragLocality.isSameBoard(sourceRoot, store.rootURL)
        let operation = session.resolveOperation(destinationRoot: store.rootURL)

        switch kind {
        case .lanes:
            if within {
                store.moveLanes(Set(ids), toIndex: target.index)
            } else {
                store.receiveLanes(folders, operation: operation, at: target.index)
            }

        case .cards:
            guard let laneID = target.laneID else {
                cancelDrop()
                return false
            }
            switch (session.side, within) {
            case (.live, true):
                if operation == .copy {
                    store.copyCards(Set(ids), toLane: laneID, at: target.index)
                } else {
                    store.moveCards(Set(ids), toLane: laneID, at: target.index)
                }
            case (.live, false):
                store.receiveCards(folders, operation: operation, toLane: laneID, at: target.index)
            case (.trashed, true):
                // Drag-to-restore, and its ⌥ twin. "Dropping a tombstoned card into one of its own
                // board's lanes restores it at the drop position"; ⌥ is the copy-out instead — a
                // live copy lands and the tombstoned original stays (04-interactions.md ▸ The trash,
                // "⌘C, ⌥-drag … always yield live copies").
                if operation == .copy {
                    store.receiveRestoredCards(folders, operation: .copy, toLane: laneID, at: target.index)
                } else {
                    store.restoreByDrag(cardIDs: ids, intoLane: laneID, at: target.index)
                }
            case (.trashed, false):
                store.receiveRestoredCards(folders, operation: operation, toLane: laneID, at: target.index)
            }
        }

        // The committed-overlay hold: keep drawing the arrangement until this store's next snapshot.
        session.commit(into: store)
        return true
    }

    /// Release with nothing valid to write: the items return and nothing is written.
    func cancelDrop() {
        withAnimation(Motion.dragReflow(reduced: Motion.prefersReducedMotion)) { session.end() }
    }
}

// MARK: - Drop delegates

// **Single-target dispatch** (DRAG-REORDER.md, the constraint of the same name): SwiftUI/macOS
// delivers a drag session to the *deepest* drop region under the cursor with no fall-through, not
// even when that target's declared content types don't match the session's payload. So every
// delegate below accepts **both** board types and routes internally; a region whose topmost target
// understood only one of them would be a dead zone for the other — no hover callbacks, and a release
// there would snap back instead of committing.
//
// m5-finder-drops: the next card adds external Finder file sessions (`.fileURL`) to this same
// dispatch — files onto a card become attachments, files onto lane empty space become cards
// (04-interactions.md ▸ Drag and drop) — and the same dead-region rule applies to them, so the type
// list and the routing switch in each delegate below grow by one case rather than gaining a delegate
// of their own.

/// The board types every delegate declares. Spelled once so no target can accidentally accept fewer.
let boardDragTypes: [UTType] = [.laneworkCards, .laneworkLanes]

/// One lane's drop target, attached to the whole lane body.
///
/// **Card sessions** resolve against this lane's masonry zones. **Lane sessions** are forwarded to
/// the strip's logic (cursor converted to strip space by the shared context), so lane reordering
/// keeps working while the cursor crosses lane bodies.
struct LaneDropDelegate: DropDelegate {

    // m5-finder-drops: a file session resolves against these same masonry zones — onto a card it
    // becomes attachments, onto empty space a card per file (04-interactions.md ▸ Drag and drop).

    let context: BoardDropContext
    let laneID: ItemID

    func validateDrop(info: DropInfo) -> Bool {
        context.session.isActive && info.hasItemsConforming(to: boardDragTypes)
    }

    func dropEntered(info: DropInfo) { retarget() }

    func dropUpdated(info: DropInfo) -> DropProposal? {
        retarget()
        return context.dropProposal()
    }

    // No `dropExited`, deliberately: the proposal is meant to **hold** while the cursor leaves for
    // ambiguous territory — that is the hysteresis contract (DRAG-REORDER.md § Hysteresis).

    func performDrop(info: DropInfo) -> Bool {
        context.commitDrop()
    }

    private func retarget() {
        context.revalidateProposal()
        if context.session.isDraggingLanes {
            context.retargetLanes()
        } else {
            context.retargetCards(inLane: laneID)
        }
    }
}

/// The strip's drop target — the backdrop, the gaps, the outer margin, and the trash column's
/// footprint, which is never a landing spot of its own (04-interactions.md ▸ The trash) and so
/// simply falls through to here.
///
/// Lane sessions retarget against the strip's analytic zones; card sessions retarget through the
/// same shared function the lane delegates use, resolving the lane under the cursor analytically —
/// the safety net for a lane whose own drop region goes dead.
struct StripDropDelegate: DropDelegate {

    // m5-finder-drops: the strip is a file session's safety net too — the same dead-region
    // hit-testing bug can strand one, and without file support here it would have nowhere to land.

    let context: BoardDropContext

    func validateDrop(info: DropInfo) -> Bool {
        context.session.isActive && info.hasItemsConforming(to: boardDragTypes)
    }

    func dropEntered(info: DropInfo) { context.retargetFromStrip() }

    func dropUpdated(info: DropInfo) -> DropProposal? {
        context.retargetFromStrip()
        return context.dropProposal()
    }

    func performDrop(info: DropInfo) -> Bool {
        context.commitDrop()
    }
}

/// The window-level fallback, behind every specific target: a release over any in-window region they
/// do not cover (the banner strip, the window's edges) commits the current proposal rather than
/// leaking the session into a cancel-snapback. It retargets nothing — the drop lands where the
/// shadows already show, which is what the shadows promise.
struct BoardFallbackDropDelegate: DropDelegate {

    // m5-finder-drops: a file session released over uncovered window chrome commits whatever the
    // file target currently names, exactly as this commits the current card/lane proposal.

    let context: BoardDropContext

    func validateDrop(info: DropInfo) -> Bool {
        context.session.isActive && info.hasItemsConforming(to: boardDragTypes)
    }

    func dropUpdated(info: DropInfo) -> DropProposal? {
        context.dropProposal()
    }

    func performDrop(info: DropInfo) -> Bool {
        context.commitDrop()
    }
}