import HapiClient
import HapiProtocol
import HapiUI
import SwiftUI

/// The chat screen: `ScrollView + LazyVStack` over the reduced
/// `VisibleChatBlock`s, newest at the bottom (M2f), plus the A-M3ab
/// interaction chrome — composer + queued bar (bottom inset), permission
/// action footers (via `\.chatInteractions`), the session config sheet
/// (toolbar gear), supersede renavigation, and toast notices.
///
/// Scrolling model (iOS 17 APIs, kept deliberately simple):
/// - `defaultScrollAnchor(.bottom)` opens the thread at the newest message
///   AND keeps the bottom edge pinned while the reader is at the bottom —
///   that is the auto-stick;
/// - `scrollPosition(id:anchor:.top)` is used for the two programmatic
///   jumps: the new-messages pill (scroll to the bottom sentinel) and
///   **older-page re-anchoring** — before `loadOlder()` the current top
///   block id is captured, and when `historyVersion` bumps (rows were
///   prepended above the viewport, which shifts a non-bottom-anchored
///   scroll view) the position is re-set to that id so the reader stays on
///   the block they were looking at. Lazy estimated layout makes this
///   re-anchor approximate to the row, which is the documented trade-off;
/// - at-bottom detection uses a 1 pt bottom sentinel's lazy realization
///   (`onAppear`/`onDisappear`) — slightly eager because of lazy prefetch,
///   which errs toward sticking, the harmless direction.
struct ChatView: View {
    @State private var model: ChatModel
    /// Scroll position binding (anchor `.top`, so it tracks/targets the
    /// top-visible block).
    @State private var positionID: String?
    @State private var isAtBottom = true
    /// Newest block id last seen while at the bottom (new-messages pill).
    @State private var newestSeenID: String?
    /// Top-visible block captured when an older page was requested.
    @State private var pendingAnchorID: String?
    /// Session config sheet (toolbar gear).
    @State private var configSheetOpen = false
    /// Files browser push (toolbar folder, A-M4a).
    @State private var filesOpen = false
    /// File viewer push for `hapi-file://` chat citations (A-M4a).
    @State private var viewerRoute: FileViewerRoute?
    /// Scratchlist sheet (toolbar note icon, A-M4b).
    @State private var scratchlistOpen = false

    /// Resume/reopen handed back a superseding session id — the host swaps
    /// its navigation entry (HomeView replaces the path element).
    private let onNavigateToSession: ((String) -> Void)?

    /// Kept for the files/viewer pushes (the model owns its own reference).
    private let session: HubSession
    private let sessionId: String

    private static let bottomSentinelID = "chat-bottom-sentinel"

    init(
        session: HubSession,
        sessionId: String,
        onNavigateToSession: ((String) -> Void)? = nil
    ) {
        _model = State(initialValue: ChatModel(session: session, sessionId: sessionId))
        self.session = session
        self.sessionId = sessionId
        self.onNavigateToSession = onNavigateToSession
    }

    var body: some View {
        Group {
            if model.isInitialLoading {
                initialLoading
            } else if model.loadFailed {
                loadFailedState
            } else if model.blocks.isEmpty {
                emptyState
            } else {
                blockList
            }
        }
        .safeAreaInset(edge: .top, spacing: 0) {
            banners
        }
        // Toast overlays the thread; applying it before the bottom inset
        // anchors it just above the composer instead of on top of it.
        .overlay(alignment: .bottom) {
            noticeToast
        }
        .safeAreaInset(edge: .bottom, spacing: 0) {
            VStack(spacing: 0) {
                QueuedMessagesBarView(interactor: model.interactor)
                ChatComposerView(interactor: model.interactor, dictation: model.dictation)
            }
        }
        .toolbar {
            ToolbarItem(placement: .principal) {
                headerTitle
            }
            // Two icons max (device feedback: a crowded trailing edge
            // squeezed the title out): gear for the frequent config
            // switches, everything else behind one menu.
            ToolbarItem(placement: .topBarTrailing) {
                Button {
                    configSheetOpen = true
                } label: {
                    Image(systemName: "gearshape")
                }
                .accessibilityLabel("Session settings")
            }
            ToolbarItem(placement: .topBarTrailing) {
                Menu {
                    Button {
                        filesOpen = true
                    } label: {
                        Label("Session files", systemImage: "folder")
                    }
                    Button {
                        scratchlistOpen = true
                    } label: {
                        let count = model.interactor.scratchlistCount
                        if count > 0 {
                            Label(
                                String(format: String(localized: "Scratchlist (%lld)"), Int64(count)),
                                systemImage: "note.text"
                            )
                        } else {
                            Label("Scratchlist", systemImage: "note.text")
                        }
                    }
                } label: {
                    Image(systemName: "ellipsis.circle")
                }
                .accessibilityLabel("More actions")
            }
        }
        .navigationBarTitleDisplayMode(.inline)
        .sheet(isPresented: $configSheetOpen) {
            SessionConfigView(interactor: model.interactor)
        }
        .navigationDestination(isPresented: $filesOpen) {
            FilesView(session: session, sessionId: sessionId)
        }
        .navigationDestination(item: $viewerRoute) { route in
            // A replaced route value must rebuild the screen's @State model
            // (HomeView's chat-push precedent).
            FileViewerView(session: session, route: route)
                .id(route)
        }
        // Chat markdown citations open the real viewer in full mode with the
        // cited line as a hint chip (replaces the root placeholder alert for
        // this subtree; the destinations above inherit the handler too).
        .handlesHapiLinks { link in
            viewerRoute = FileViewerRoute(
                sessionId: sessionId,
                path: link.path,
                mode: .file,
                line: link.line
            )
        }
        .sheet(isPresented: $scratchlistOpen) {
            ScratchlistView(
                store: model.scratchlist,
                sessionId: model.sessionId,
                attachments: model.scratchlistAttachments,
                interactor: model.interactor
            )
        }
        .environment(\.chatMedia, model.imageLoader)
        .environment(\.chatInteractions, model.interactor)
        .onChange(of: model.supersededSessionId) {
            if let superseding = model.supersededSessionId {
                onNavigateToSession?(superseding)
            }
        }
        .onAppear {
            model.start()
        }
        .onDisappear {
            // Closed, or a files/viewer push covered the chat — either way
            // the pipe parks; start() rebuilds fresh wiring at the saved
            // cursor when the screen returns.
            model.stop()
        }
    }

    /// Transient interaction notice (Android snackbar analogue).
    @ViewBuilder
    private var noticeToast: some View {
        if let notice = model.notice {
            Text(LocalizedNoticeMapper.map(notice))
                .font(.footnote)
                .lineLimit(3)
                .padding(.horizontal, 14)
                .padding(.vertical, 8)
                .background(.regularMaterial, in: RoundedRectangle(cornerRadius: 12, style: .continuous))
                .shadow(radius: 4, y: 2)
                .padding(.horizontal, 24)
                .padding(.bottom, 8)
                .transition(.move(edge: .bottom).combined(with: .opacity))
        }
    }

    // MARK: - Thread

    private var blockList: some View {
        ScrollView {
            LazyVStack(alignment: .leading, spacing: 10) {
                if model.hasMore || model.isLoadingOlder {
                    OlderHistoryRow(isLoading: model.isLoadingOlder)
                        .onAppear {
                            requestOlderPage()
                        }
                }
                ForEach(model.blocks, id: \.stableId) { block in
                    ChatBlockCard(block: block, basePath: model.basePath)
                }
                bottomSentinel
            }
            .scrollTargetLayout()
            .padding(.horizontal, 12)
            .padding(.top, 10)
        }
        .defaultScrollAnchor(.bottom)
        .scrollPosition(id: $positionID, anchor: .top)
        .scrollDismissesKeyboard(.interactively)
        .overlay(alignment: .bottom) {
            newMessagesPill
        }
        .onChange(of: model.historyVersion) {
            reanchorAfterPrepend()
        }
        .onChange(of: newestBlockID) {
            if isAtBottom {
                newestSeenID = newestBlockID
            }
        }
    }

    private var bottomSentinel: some View {
        Color.clear
            .frame(height: 1)
            .id(Self.bottomSentinelID)
            .onAppear {
                isAtBottom = true
                newestSeenID = newestBlockID
            }
            .onDisappear {
                isAtBottom = false
            }
    }

    private var newestBlockID: String? {
        model.blocks.last?.stableId
    }

    /// Capture the anchor BEFORE the prepend lands, then ask for the page.
    private func requestOlderPage() {
        if pendingAnchorID == nil {
            let id = positionID
            // The binding may currently point at chrome rows; fall back to
            // the oldest real block.
            let isBlock = id.map { candidate in
                model.blocks.contains { $0.stableId == candidate }
            } ?? false
            pendingAnchorID = isBlock ? id : model.blocks.first?.stableId
        }
        model.loadOlder()
    }

    /// `historyVersion` bumped: rows were prepended above the viewport.
    /// Re-set the position to the captured block on the next runloop turn
    /// (after SwiftUI laid the new rows out).
    private func reanchorAfterPrepend() {
        guard let anchor = pendingAnchorID else { return }
        pendingAnchorID = nil
        guard !isAtBottom else { return } // bottom-anchored: nothing shifted
        Task { @MainActor in
            positionID = anchor
        }
    }

    // MARK: - New-messages pill

    private var unseenCount: Int {
        guard !isAtBottom, let seen = newestSeenID else { return 0 }
        guard let index = model.blocks.lastIndex(where: { $0.stableId == seen }) else { return 0 }
        return model.blocks.count - 1 - index
    }

    @ViewBuilder
    private var newMessagesPill: some View {
        let count = unseenCount
        if count > 0 {
            Button {
                newestSeenID = newestBlockID
                positionID = Self.bottomSentinelID
            } label: {
                Text(count == 1
                    ? String(localized: "1 new message ↓")
                    : String(format: String(localized: "%lld new messages ↓"), Int64(count)))
                    .font(.footnote.weight(.medium))
                    .padding(.horizontal, 14)
                    .padding(.vertical, 7)
                    .background(.tint, in: Capsule())
                    .foregroundStyle(.white)
                    .shadow(radius: 3, y: 1)
            }
            .buttonStyle(.plain)
            .padding(.bottom, 12)
        }
    }

    // MARK: - Chrome

    private var headerTitle: some View {
        HStack(spacing: 8) {
            StatusDot(active: model.header.active, thinking: model.header.thinking)
            VStack(alignment: .leading, spacing: 0) {
                Text(model.header.title)
                    .font(.headline)
                    .lineLimit(1)
                if let subtitle = model.header.subtitle {
                    HStack(spacing: 4) {
                        if model.header.flavor != nil {
                            // Inherits .secondary like the meta text (web:
                            // currentColor under --app-hint); color variants
                            // ignore the tint.
                            AgentFlavorIconView(flavor: model.header.flavor, size: 12)
                        }
                        Text(subtitle)
                            .font(.caption2)
                            .lineLimit(1)
                    }
                    .foregroundStyle(.secondary)
                }
            }
        }
    }

    @ViewBuilder
    private var banners: some View {
        VStack(spacing: 0) {
            if let warning = model.warning {
                HStack(spacing: 8) {
                    Text(LocalizedNoticeMapper.map(warning))
                        .font(.footnote)
                        .lineLimit(2)
                    Spacer(minLength: 8)
                    Button("Retry") {
                        model.retry()
                    }
                    .font(.footnote.weight(.semibold))
                }
                .padding(.horizontal, 16)
                .padding(.vertical, 6)
                .frame(maxWidth: .infinity)
                .background(.red.opacity(0.14))
                .foregroundStyle(.red)
            }
            if case .backoff = model.connectionState {
                Text("Live updates interrupted — reconnecting…")
                    .font(.footnote)
                    .frame(maxWidth: .infinity)
                    .padding(.vertical, 5)
                    .background(.orange.opacity(0.15))
                    .foregroundStyle(.orange)
            }
        }
    }

    // MARK: - Empty / loading / error

    private var initialLoading: some View {
        VStack(spacing: 12) {
            ProgressView()
            Text("Loading messages…")
                .font(.subheadline)
                .foregroundStyle(.secondary)
        }
        .frame(maxWidth: .infinity, maxHeight: .infinity)
    }

    private var loadFailedState: some View {
        ContentUnavailableView {
            Label("Couldn't load this session", systemImage: "wifi.slash")
        } description: {
            Text("Check the connection to your hub and try again.")
        } actions: {
            Button("Retry") {
                model.retry()
            }
            .buttonStyle(.borderedProminent)
        }
    }

    private var emptyState: some View {
        ContentUnavailableView {
            Label("No messages yet", systemImage: "bubble.left.and.bubble.right")
        } description: {
            Text("Messages will appear here as the agent works.")
        }
    }
}

/// Centered "· · ·" / spinner row doubling as the load-older top sentinel.
private struct OlderHistoryRow: View {
    let isLoading: Bool

    var body: some View {
        HStack(spacing: 8) {
            if isLoading {
                ProgressView()
                    .controlSize(.small)
                Text("Loading older messages…")
                    .font(.caption)
                    .foregroundStyle(.secondary)
            } else {
                Text("· · ·")
                    .font(.caption)
                    .foregroundStyle(.secondary)
            }
        }
        .frame(maxWidth: .infinity)
        .padding(.vertical, 8)
    }
}