import AppKit
import SwiftUI

/// The lane strip — the board itself (03-board-ui.md § Layout — full visibility).
///
/// **Every lane is always on screen.** There is no horizontal scroll and no minimum lane width: the
/// window's width divides across the lanes' width units, a lane of n units taking n whole units of
/// that division, and resizing the window is the width control. A board with more units than the
/// window comfortably fits compresses every lane; that degenerate case is accepted, not floored
/// (the remedy is the user's — fewer units or a bigger window).
///
/// Two mechanisms change a lane's width and they are deliberately opposites (03-board-ui.md § Lane):
/// the **right-edge drag** grows or shrinks the *window* one standard width per snap so the other
/// lanes keep their exact pixels (`LaneResizeSession`), while the **stepper** — and its ⌥⌘→/⌥⌘←
/// keyboard face — re-divides the existing window width across the new unit total, compressing the
/// siblings and never touching the window (`BoardStore.setLaneWidth`).
///
/// ### What is deliberately not here yet
///
/// Selection, drag and drop, the trash quasi-lane, the toolbar, search, styling and the lane context
/// menu all belong to later milestone cards. This view is the layout and the resize interaction, and
/// the chrome inside `LaneView` is a placeholder those cards replace.
struct BoardView: View {

    let store: BoardStore

    /// How the resize session reaches the host window it grows and shrinks. Injected by
    /// `BoardWindowHost`, which owns the window controller; a closure because the window attaches
    /// after the first body evaluation.
    let window: @MainActor () -> NSWindow?

    /// One resize at a time, per window. `@State` so it lives exactly as long as this board window's
    /// view does, which is the interaction's whole lifetime.
    @State private var resize = LaneResizeSession()

    /// The inter-lane gap, and the strip's outer margin — one number, because the standard-width
    /// formula counts `units + 1` of them (03-board-ui.md § Layout; `LaneLayoutMath.standardWidth`).
    private let spacing: CGFloat = 12

    var body: some View {
        GeometryReader { viewport in
            let lanes = liveLanes
            // During a resize session the standard is FROZEN at its drag-start value: the window is
            // animating mid-resize, so deriving the standard from the live viewport width would feed
            // that animation back into every lane and pulse the whole strip. The window is sized on
            // each tick so this frozen value equals what the viewport formula yields once the
            // session ends — the handoff is seamless (see `LaneResizeSession`).
            let standard = resize.isActive
                ? resize.standard
                : LaneLayoutMath.standardWidth(
                    stripWidth: viewport.size.width,
                    totalUnits: LaneLayoutMath.totalUnits(of: lanes),
                    gap: spacing)
            HStack(alignment: .top, spacing: spacing) {
                ForEach(lanes) { lane in
                    laneSlot(lane, standard: standard)
                }
            }
            .padding(spacing)
            .frame(maxWidth: .infinity, maxHeight: .infinity, alignment: .topLeading)
        }
    }

    /// One lane's strip slot, plus its trailing grab strip.
    ///
    /// Normally a plain `LaneView` sized to its unit count's slot width (a wide lane swallows the
    /// interior gaps it spans). While THIS lane is being resized the slot holds two layers, kept
    /// structurally stable so the `LaneView` never loses identity — its scroll position, its
    /// masonry cache — across the drag:
    ///
    ///   • a shadow at the SNAPPED slot width, full strip height, behind the lane — the resting
    ///     footprint the siblings and the window are already aligned to;
    ///   • the live `LaneView` in front at `liveWidth`, which tracks the cursor and so overflows
    ///     (drawing over the right neighbour, hence the slot's `zIndex(1)`) or underfills the shadow
    ///     between ticks.
    ///
    /// The outer frame is always the snapped slot width, so the `HStack` lays the other lanes out
    /// off the tidy snapped layout regardless of the live overflow.
    @ViewBuilder
    private func laneSlot(_ lane: Lane, standard: CGFloat) -> some View {
        let resizing = resize.isResizing(lane.id)
        let units = resizing ? resize.units : LaneLayoutMath.displayUnits(of: lane)
        let slotWidth = LaneLayoutMath.slotWidth(units: units, standard: standard, gap: spacing)
        ZStack(alignment: .topLeading) {
            if resizing {
                LaneResizeShadow()
                    .frame(width: slotWidth)
                    .frame(maxHeight: .infinity)
                    .allowsHitTesting(false)
            }
            // Interior columns follow the SNAPPED unit count while this lane is being resized — a
            // column count is integral, so it tracks k (which ticks and animates), not the live
            // continuous width and not the not-yet-committed `lane.width`. The live width still
            // narrows and widens the columns continuously, so the cards reflow under the cursor
            // between ticks (free via `MasonryLayout`).
            LaneView(lane: lane, columns: units)
                .frame(width: resizing ? resize.liveWidth : slotWidth, alignment: .leading)
        }
        .frame(width: slotWidth, alignment: .topLeading)
        .zIndex(resizing ? 1 : 0)
        .overlay(alignment: .trailing) {
            LaneResizeHandle(
                store: store,
                session: resize,
                laneID: lane.id,
                committedUnits: LaneLayoutMath.displayUnits(of: lane),
                standard: standard,
                gap: spacing,
                window: window
            )
            // The read-only lock disables every mutating gesture, not just the menu items
            // (02-architecture.md § The lock's scope). It matters more here than elsewhere: a drag
            // resizes the *window* on the way, so a refused commit would leave the window grown
            // around a lane that snapped back — and the lock's row is already saying why nothing
            // can be written.
            .disabled(store.isReadOnly)
        }
    }

    /// The lanes the strip lays out, in snapshot order. **Tombstoned lanes render nowhere here** —
    /// 03-board-ui.md § Trash collapses each into a single restorable entry in the trash quasi-lane
    /// (a later card), and a lane that is not on the board consumes none of the window's width.
    private var liveLanes: [Lane] {
        store.snapshot.lanes.filter { !$0.isDeleted }
    }
}

// MARK: - Resize shadow

/// The resting footprint a lane snaps back to, drawn behind the live lane during a resize.
///
/// Minimal on purpose: 03-board-ui.md's placeholder/drag vocabulary lands with the drag milestone,
/// and this is the same shape that card and lane drops will want. Kept here rather than invented
/// twice.
private struct LaneResizeShadow: View {
    var body: some View {
        RoundedRectangle(cornerRadius: 10)
            .fill(.quaternary.opacity(0.5))
    }
}