import AppKit
import SwiftUI
import os

/// The template chooser — File ▸ New Board… (⌥⌘N), 09-templates.md's picker.
///
/// ### Pages' shape, and the templates are real board folders now
///
/// A grid of template cards, each showing a **mini per-lane preview** above its name, one selected
/// at a time, with Cancel and Choose at the bottom (03-board-ui.md § Welcome screen & templates: "a
/// Pages-style chooser with a mini per-lane preview per template"). The grid is filled by
/// `TemplateEngine.bundledTemplates()` — the app bundle's `Templates/` folder, each entry loaded
/// through the ordinary `BoardLoader` — so name, blurb, icon and preview all come off the template
/// board's own `index.md` rather than from a Swift catalog.
///
// m9-templates: the full chooser is its own card. What is still missing here is the *user* tier
// (`TemplateEngine.userStore`, listed after the bundled ones), the unloadable-template row, Reveal
// in Finder, and the in-progress row with Cancel that copy-shaped work is owed (02-architecture.md
// § The banner surface) — this window has no banner surface to host one yet. The engine already
// takes the cancellation seam (`TemplateEngine.instantiate(…, isCancelled:)`); this view runs the
// copy off the main actor so that row has something to spin over when it arrives.
///
/// ### Choosing is three steps, and the middle one is a save panel
///
/// Choose runs `NSSavePanel`, instantiates into the chosen location, and opens the result. The panel
/// is not ceremony: a sandboxed app cannot write anywhere the user has not pointed at, so the panel
/// *is* how a new board gets a location it is allowed to occupy (09 files this under "seed the save
/// panel's suggested name"). The chooser stays open if the panel is cancelled — a cancelled location
/// is not a cancelled choice.
///
/// ### Failure is an alert here, deliberately
///
/// Everywhere else in the app a failed write is a banner in the window that produced it
/// (02-architecture.md § Write-failure surfacing). A failed *create* has no such window: the board
/// that would host the banner is the one that did not get created, welcome may not be open (⌥⌘N
/// works from a board window), and filing it under `AppModel.launchFailures` would put it in a
/// recents-adjacent list belonging to boards the registry knows — which this one, having never
/// existed, is not. So it is an `NSAlert`, continuing the modal conversation the user is already in
/// with the save panel, in the same sentence the banner would have used
/// (`BannerCenter.headline(for:)`) so the app has one vocabulary for a failed write rather than two.
struct TemplateChooserView: View {

    @Environment(AppModel.self) private var appModel
    @Environment(\.dismiss) private var dismiss

    /// Discovered when the window appears, and not again: the bundle's `Templates/` folder cannot
    /// change under a running app, and discovery *loads every template board* — a default-value
    /// initializer would re-run it each time SwiftUI rebuilt this struct.
    @State private var templates: [BoardTemplate] = []

    @State private var selection: BoardTemplate.ID?

    private static let logger = Logger(subsystem: "dev.rzen.indie.Kanban", category: "templates")

    private var selected: BoardTemplate? {
        templates.first { $0.id == selection } ?? templates.first
    }

    var body: some View {
        VStack(spacing: 0) {
            header
            Divider()
            grid
            Divider()
            footer
        }
        .frame(width: 620, height: 460)
        .onAppear {
            guard templates.isEmpty else { return }
            templates = TemplateEngine.bundledTemplates()
        }
    }

    // MARK: Header

    private var header: some View {
        VStack(alignment: .leading, spacing: 2) {
            Text("Choose a Template")
                .font(.title3.weight(.semibold))
            Text("Every template is an ordinary board — lanes and cards you can change afterwards.")
                .font(.callout)
                .foregroundStyle(.secondary)
        }
        .frame(maxWidth: .infinity, alignment: .leading)
        .padding(20)
    }

    // MARK: Grid

    private var grid: some View {
        ScrollView {
            LazyVGrid(columns: [GridItem(.adaptive(minimum: 170), spacing: 20)], spacing: 20) {
                ForEach(templates) { template in
                    TemplateCard(template: template, isSelected: template.id == selected?.id)
                        .onTapGesture { selection = template.id }
                        // The list convention welcome's recents use, for the same reason: a
                        // double click is how a chooser is answered without reaching for a button.
                        .onTapGesture(count: 2) { choose() }
                        .accessibilityAddTraits(template.id == selected?.id ? [.isSelected] : [])
                }
            }
            .padding(20)
        }
        .frame(maxWidth: .infinity, maxHeight: .infinity, alignment: .topLeading)
        .background(Color(nsColor: .controlBackgroundColor))
    }

    // MARK: Footer

    private var footer: some View {
        HStack(alignment: .firstTextBaseline) {
            Text(selected?.blurb ?? "")
                .font(.callout)
                .foregroundStyle(.secondary)
                .lineLimit(2)

            Spacer(minLength: 16)

            Button("Cancel", role: .cancel) { dismiss() }
                .keyboardShortcut(.cancelAction)

            Button("Choose") { choose() }
                .keyboardShortcut(.defaultAction)
                .disabled(selected == nil)
        }
        .padding(20)
    }

    // MARK: - Choosing

    /// Panel, instantiate, open — and only then dismiss, so a cancelled panel leaves the chooser
    /// exactly as the user left it.
    ///
    /// The copy runs in a detached task, `DuplicateBoardCommand`'s reasoning at a smaller scale: a
    /// user template can be a real board with real attachments, and a main thread blocked inside a
    /// tree copy is a frozen window. Detached rather than a child task so its cancellation is only
    /// ever the one a Cancel affordance hands it, never something inherited.
    private func choose() {
        guard let template = selected, let url = Self.chooseLocation(for: template) else { return }
        let title = TemplateEngine.documentName(of: url)

        Task { @MainActor in
            let outcome = await Task.detached(priority: .userInitiated) {
                () -> Result<URL, TemplateEngine.Failure> in
                do throws(TemplateEngine.Failure) {
                    return .success(try TemplateEngine.instantiate(template: template, to: url, title: title))
                } catch {
                    return .failure(error)
                }
            }.value

            switch outcome {
            case .success:
                dismiss()
                // The ordinary open path, so the new board joins recents, gets its bookmark, and
                // closes welcome on the way in exactly like a board opened from a row.
                appModel.openBoard(at: url)
            case .failure(.cancelled):
                // Nothing was created and nothing failed, so nothing is said — the duplicate rule.
                Self.logger.notice("template instantiation cancelled — the partial board was removed")
            case let .failure(.failed(error)):
                Self.logger.error("template instantiation failed: \(error.description, privacy: .public)")
                Self.present(error)
            }
        }
    }

    /// The save panel — where the board goes and what it is called.
    ///
    /// **The suggestion is the template's own title** (`"Basic.kanban"`, `"Bug Tracker.kanban"`) —
    /// 09 ▸ Instantiation: "seed the save panel's suggested name from the template title". Whatever
    /// the user types instead becomes the new board's `title` as well as its folder name, so the two
    /// start out matching (01-storage-format.md § Board naming). The package extension is visible and
    /// editable, because an extension-less board folder is equally legal (§ Document packaging) and
    /// deleting the suffix should therefore work rather than be silently undone.
    ///
    /// A name that already exists gets the panel's own replace prompt; agreeing to it does not delete
    /// anything (the panel never does), so the engine's refusal to clobber an existing board is what
    /// the user sees — as an alert, naming the path. That is the honest outcome: this flow is a
    /// *create*, and quietly replacing a board with an empty one is not a thing it should be able to
    /// do.
    private static func chooseLocation(for template: BoardTemplate) -> URL? {
        let panel = NSSavePanel()
        panel.nameFieldStringValue = TemplateEngine.suggestedFileName(for: template)
        panel.canCreateDirectories = true
        panel.isExtensionHidden = false
        panel.allowsOtherFileTypes = true
        panel.prompt = "Create"
        panel.message = "Choose where to keep the new board."

        guard panel.runModal() == .OK, let url = panel.url else { return nil }
        return url
    }

    private static func present(_ error: BoardWriteError) {
        let alert = NSAlert()
        alert.alertStyle = .warning
        alert.messageText = BannerCenter.headline(for: error)
        alert.informativeText = error.path
        alert.addButton(withTitle: "OK")
        alert.runModal()
    }
}

// MARK: - Template card

/// One template in the grid: its mini per-lane preview, its name, and the selection ring.
private struct TemplateCard: View {

    let template: BoardTemplate
    let isSelected: Bool

    var body: some View {
        VStack(spacing: 8) {
            TemplatePreview(template: template)
                .frame(height: 96)
                .background(RoundedRectangle(cornerRadius: 8).fill(Color(nsColor: .textBackgroundColor)))
                .overlay(
                    RoundedRectangle(cornerRadius: 8)
                        .strokeBorder(isSelected ? Color.accentColor : Color(nsColor: .separatorColor),
                                      lineWidth: isSelected ? 3 : 1)
                )

            Label(template.name, systemImage: template.icon)
                .font(.callout)
                .labelStyle(.titleAndIcon)
        }
        .contentShape(Rectangle())
        .accessibilityElement(children: .combine)
        .accessibilityLabel(template.name)
        .accessibilityHint(template.blurb)
    }
}

/// The mini per-lane preview: one column per lane, each a title bar over a couple of card shapes.
///
/// Deliberately abstract — no text, because the point is the *shape* of the board and legible lane
/// names at this size are not available. It renders **from the template's loaded `BoardModel`**
/// (09-templates.md ▸ Why this format: "The picker's mini per-lane preview renders from a real
/// `BoardModel` via the normal loader"), taking the lane count and each lane's identity from it.
///
// m9-templates: the card shapes are still decoration — a lane's *real* starter cards (templates
// "may contain starter cards") should be what the column draws, once the chooser card gets to it.
private struct TemplatePreview: View {

    let template: BoardTemplate

    var body: some View {
        HStack(alignment: .top, spacing: 6) {
            ForEach(Array(template.lanes.enumerated()), id: \.element.id) { index, _ in
                VStack(spacing: 4) {
                    RoundedRectangle(cornerRadius: 2)
                        .fill(Color.accentColor.opacity(0.65))
                        .frame(height: 5)
                    // A descending number of cards, so the preview reads as work in flight rather
                    // than as three identical columns.
                    ForEach(0..<max(1, 3 - index), id: \.self) { _ in
                        RoundedRectangle(cornerRadius: 3)
                            .fill(.quaternary)
                            .frame(height: 14)
                    }
                    Spacer(minLength: 0)
                }
            }
        }
        .padding(10)
        .accessibilityHidden(true)
    }
}