#include "ui_layer.h"

#include "scene/scene.h"
#include "core/log.h"
#include "core/settings_manager.h"
#include "core/hdri_manager.h"
#include "rendering/render_api.h"

#include <imgui.h>
#include <imgui_internal.h>     // DockBuilder*, DockSpace keep-alive, BeginViewportSideBar, ImGuiSettingsHandler
#include <ImGuizmo.h>
#include <glm/glm.hpp>
#include <glm/gtc/matrix_transform.hpp>
#include <algorithm>
#include <cfloat>
#include <cstdio>
#include <cstring>
#include <string>

namespace Donut
{
    // the workspaces and the panels, with stable keys. the keys salt the dock ids,
    // suffix the window names ("Black Hole###scene.black_hole") and name things in
    // imgui.ini, so a label can be reworded without losing anyone's saved layout.
    struct TabDesc   { const char* label; const char* key; View view; };
    struct PanelDesc { const char* label; const char* key; };

    static const TabDesc kTabs[kTabCount] = {
        { "Simulation",   "simulation", View::BlackHole },
        { "Scene Editor", "scene",      View::Scene     },
        { "Export",       "export",     View::BlackHole },
    };
    static const PanelDesc kPanels[kPanelCount] = {
        { "Outliner",    "outliner"   },
        { "Properties",  "properties" },
        { "Black Hole",  "black_hole" },
        { "Settings",    "settings"   },
        { "Export",      "export"     },
        { "Diagnostics", "stats"      },
    };

    // which panels each workspace can show and which open by default. columns
    // follow Panel. the sphere panels are allowed everywhere because the lensed
    // render draws the spheres too, so placing one while watching it is a thing.
    static const bool kAllowed[kTabCount][kPanelCount] = {
        //  outl   prop   bh     set    exp    stats
        { true,  true,  true,  true,  false, true  },   // simulation
        { true,  true,  false, true,  false, true  },   // scene editor
        { true,  true,  true,  true,  true,  true  },   // export
    };
    static const bool kDefaults[kTabCount][kPanelCount] = {
        { false, false, true,  false, false, true  },
        { true,  true,  false, false, false, true  },
        { false, false, true,  false, true,  true  },
    };

    // the type scale. two faces; ImGui 1.92 rasterises any size on demand at PushFont.
    static constexpr float kBodySize    = 15.0f;
    static constexpr float kCaptionSize = 13.0f;
    static constexpr float kHeadSize    = 18.0f;
    static constexpr float kBigSize     = 30.0f;

    // the palette: one warm accent (the accretion disk) on a cool near-black.
    static const ImVec4 kAmber   (0.98f, 0.62f, 0.20f, 1.00f);
    static const ImVec4 kAmberHi (1.00f, 0.73f, 0.36f, 1.00f);
    static const ImVec4 kInk     (0.09f, 0.07f, 0.03f, 1.00f);   // text sitting on amber
    static const ImVec4 kGreen   (0.55f, 0.85f, 0.45f, 1.00f);
    static const ImVec4 kRed     (0.95f, 0.45f, 0.35f, 1.00f);

    // what ImGui doesn't save on its own: the open workspace and each one's panel
    // set. lives outside UILayer so the ini handler can still write it during
    // ImGui's shutdown save, which runs after the Application has already torn
    // the UILayer down. the section reads
    //   [Donut][Workspaces]
    //   active=scene
    //   scene=outliner,properties,stats
    struct Persisted
    {
        int  active = 0;
        bool show[kTabCount][kPanelCount] = {};
        bool loaded = false;   // an entry came in from imgui.ini
    };
    static Persisted s_persisted;

    static auto settings_read_open(ImGuiContext*, ImGuiSettingsHandler*, const char* name) -> void*
    {
        return std::strcmp(name, "Workspaces") == 0 ? &s_persisted : nullptr;
    }

    static auto settings_read_line(ImGuiContext*, ImGuiSettingsHandler*, void*, const char* line) -> void
    {
        const char* eq = std::strchr(line, '=');
        if (!eq) return;
        const std::string lhs(line, eq);
        const char* rhs = eq + 1;

        if (lhs == "active")
        {
            for (int t = 0; t < kTabCount; ++t)
                if (std::strcmp(rhs, kTabs[t].key) == 0) { s_persisted.active = t; s_persisted.loaded = true; }
            return;
        }
        for (int t = 0; t < kTabCount; ++t)
        {
            if (lhs != kTabs[t].key) continue;
            s_persisted.loaded = true;
            for (bool& b : s_persisted.show[t]) b = false;
            for (const char* tok = rhs; *tok; )
            {
                const char*  end = std::strchr(tok, ',');
                const size_t len = end ? (size_t)(end - tok) : std::strlen(tok);
                for (int p = 0; p < kPanelCount; ++p)
                    if (std::strlen(kPanels[p].key) == len && std::strncmp(tok, kPanels[p].key, len) == 0)
                        s_persisted.show[t][p] = true;
                tok = end ? end + 1 : tok + len;
            }
        }
    }

    static auto settings_write_all(ImGuiContext*, ImGuiSettingsHandler* handler, ImGuiTextBuffer* out) -> void
    {
        out->appendf("[%s][Workspaces]\n", handler->TypeName);
        out->appendf("active=%s\n", kTabs[s_persisted.active].key);
        for (int t = 0; t < kTabCount; ++t)
        {
            out->appendf("%s=", kTabs[t].key);
            bool first = true;
            for (int p = 0; p < kPanelCount; ++p)
            {
                if (!s_persisted.show[t][p]) continue;
                if (!first) out->append(",");
                out->append(kPanels[p].key);
                first = false;
            }
            out->append("\n");
        }
        out->append("\n");
    }

    // the theme: soft-rounded cards on a near-black ground, 1px borders for edges,
    // and the accent reserved for "this is live / selected" — slider grabs, checks,
    // the active tab's underline, the one primary button per panel.
    static auto apply_donut_style() -> void
    {
        ImGuiStyle& s = ImGui::GetStyle();
        s.WindowRounding = 8.0f;  s.ChildRounding = 6.0f;  s.FrameRounding = 6.0f;
        s.GrabRounding   = 6.0f;  s.PopupRounding = 8.0f;  s.ScrollbarRounding = 8.0f; s.TabRounding = 6.0f;
        s.WindowPadding  = ImVec2(14.0f, 12.0f); s.FramePadding     = ImVec2(10.0f, 6.0f);
        s.ItemSpacing    = ImVec2(8.0f, 7.0f);   s.ItemInnerSpacing = ImVec2(8.0f, 5.0f);
        s.IndentSpacing  = 18.0f; s.ScrollbarSize = 10.0f; s.GrabMinSize = 10.0f;
        s.WindowBorderSize = 1.0f; s.FrameBorderSize = 0.0f; s.PopupBorderSize = 1.0f; s.ChildBorderSize = 1.0f;
        s.WindowTitleAlign = ImVec2(0.0f, 0.5f);
        s.TabBarOverlineSize   = 2.0f;
        s.DockingSeparatorSize = 2.0f;

        const ImVec4 bg0 (0.055f, 0.062f, 0.080f, 1.00f);   // ground
        const ImVec4 bg1 (0.095f, 0.105f, 0.130f, 1.00f);   // frames / cards
        const ImVec4 bg2 (0.140f, 0.155f, 0.190f, 1.00f);   // hover
        const ImVec4 bg3 (0.185f, 0.205f, 0.250f, 1.00f);   // active
        const ImVec4 line(0.170f, 0.190f, 0.230f, 1.00f);   // borders / separators
        const ImVec4 text(0.920f, 0.930f, 0.950f, 1.00f);
        const ImVec4 dim (0.550f, 0.580f, 0.640f, 1.00f);
        const ImVec4 amber_dim(0.98f, 0.62f, 0.20f, 0.35f);

        ImVec4* c = s.Colors;
        c[ImGuiCol_Text]                      = text;
        c[ImGuiCol_TextDisabled]              = dim;
        c[ImGuiCol_WindowBg]                  = bg0;
        c[ImGuiCol_ChildBg]                   = ImVec4(bg1.x, bg1.y, bg1.z, 0.60f);
        c[ImGuiCol_PopupBg]                   = ImVec4(bg0.x, bg0.y, bg0.z, 0.98f);
        c[ImGuiCol_Border]                    = line;
        c[ImGuiCol_BorderShadow]              = ImVec4(0.0f, 0.0f, 0.0f, 0.0f);
        c[ImGuiCol_FrameBg]                   = bg1;
        c[ImGuiCol_FrameBgHovered]            = bg2;
        c[ImGuiCol_FrameBgActive]             = bg3;
        c[ImGuiCol_TitleBg]                   = bg0;
        c[ImGuiCol_TitleBgActive]             = bg1;
        c[ImGuiCol_TitleBgCollapsed]          = bg0;
        c[ImGuiCol_MenuBarBg]                 = bg0;
        c[ImGuiCol_ScrollbarBg]               = ImVec4(0.0f, 0.0f, 0.0f, 0.0f);
        c[ImGuiCol_ScrollbarGrab]             = bg2;
        c[ImGuiCol_ScrollbarGrabHovered]      = bg3;
        c[ImGuiCol_ScrollbarGrabActive]       = amber_dim;
        c[ImGuiCol_CheckMark]                 = kAmberHi;
        c[ImGuiCol_SliderGrab]                = kAmber;
        c[ImGuiCol_SliderGrabActive]          = kAmberHi;
        c[ImGuiCol_Button]                    = bg2;
        c[ImGuiCol_ButtonHovered]             = bg3;
        c[ImGuiCol_ButtonActive]              = kAmber;
        c[ImGuiCol_Header]                    = ImVec4(0.110f, 0.120f, 0.150f, 1.00f);   // section bands
        c[ImGuiCol_HeaderHovered]             = bg2;
        c[ImGuiCol_HeaderActive]              = bg3;
        c[ImGuiCol_Separator]                 = line;
        c[ImGuiCol_SeparatorHovered]          = kAmber;
        c[ImGuiCol_SeparatorActive]           = kAmberHi;
        c[ImGuiCol_ResizeGrip]                = ImVec4(0.0f, 0.0f, 0.0f, 0.0f);
        c[ImGuiCol_ResizeGripHovered]         = amber_dim;
        c[ImGuiCol_ResizeGripActive]          = kAmber;
        c[ImGuiCol_Tab]                       = bg0;
        c[ImGuiCol_TabHovered]                = bg2;
        c[ImGuiCol_TabSelected]               = bg1;
        c[ImGuiCol_TabSelectedOverline]       = kAmber;      // the accent underline on the live tab
        c[ImGuiCol_TabDimmed]                 = bg0;
        c[ImGuiCol_TabDimmedSelected]         = bg1;
        c[ImGuiCol_TabDimmedSelectedOverline] = amber_dim;
        c[ImGuiCol_DockingPreview]            = ImVec4(kAmber.x, kAmber.y, kAmber.z, 0.50f);
        c[ImGuiCol_DockingEmptyBg]            = ImVec4(0.0f, 0.0f, 0.0f, 0.0f);   // pass-through
        c[ImGuiCol_PlotLines]                 = kAmber;
        c[ImGuiCol_PlotLinesHovered]          = kAmberHi;
        c[ImGuiCol_PlotHistogram]             = kAmber;
        c[ImGuiCol_PlotHistogramHovered]      = kAmberHi;
        c[ImGuiCol_TableHeaderBg]             = bg1;
        c[ImGuiCol_TableBorderStrong]         = line;
        c[ImGuiCol_TableBorderLight]          = ImVec4(line.x, line.y, line.z, 0.5f);
        c[ImGuiCol_TableRowBg]                = ImVec4(0.0f, 0.0f, 0.0f, 0.0f);
        c[ImGuiCol_TableRowBgAlt]             = ImVec4(1.0f, 1.0f, 1.0f, 0.025f);
        c[ImGuiCol_TextSelectedBg]            = amber_dim;
        c[ImGuiCol_DragDropTarget]            = kAmberHi;
    }

    // inspector row: label in a fixed left column, the control fills the rest.
    // pair with a "##hidden" widget label so ImGui doesn't draw its own on the right.
    static auto prop(const char* label, float column = 116.0f) -> void
    {
        ImGui::AlignTextToFramePadding();
        ImGui::TextUnformatted(label);
        ImGui::SameLine(column);
        ImGui::SetNextItemWidth(-FLT_MIN);
    }

    // a filled accent button: the one primary action in a panel.
    static auto accent_button(const char* label, const ImVec2& size = ImVec2(0.0f, 0.0f)) -> bool
    {
        ImGui::PushStyleColor(ImGuiCol_Button,        kAmber);
        ImGui::PushStyleColor(ImGuiCol_ButtonHovered, kAmberHi);
        ImGui::PushStyleColor(ImGuiCol_ButtonActive,  kAmberHi);
        ImGui::PushStyleColor(ImGuiCol_Text,          kInk);
        const bool pressed = ImGui::Button(label, size);
        ImGui::PopStyleColor(4);
        return pressed;
    }

    UILayer::UILayer()
    {
        for (int t = 0; t < kTabCount; ++t)
        {
            TabState& tab = m_tabs[t];
            tab.id      = static_cast<Tab>(t);
            tab.label   = kTabs[t].label;
            tab.key     = kTabs[t].key;
            tab.view    = kTabs[t].view;
            // ImHashStr is a plain CRC, no context needed. never derived from a window
            // name, and ImGui's own split nodes get small sequential ids, so no clashes.
            tab.dock_id = ImHashStr((std::string("donut.dock.") + tab.key).c_str());
            for (int p = 0; p < kPanelCount; ++p)
            {
                tab.name[p]     = std::string(kPanels[p].label) + "###" + tab.key + "." + kPanels[p].key;
                tab.allowed[p]  = kAllowed[t][p];
                tab.defaults[p] = kDefaults[t][p];
                tab.show[p]     = kDefaults[t][p];
            }
        }
    }

    auto UILayer::init() -> void
    {
        ImGuiIO& io = ImGui::GetIO();
        m_font_body    = io.Fonts->AddFontFromFileTTF("assets/fonts/inter/static/Inter_18pt-Regular.ttf",  kBodySize);
        m_font_heading = io.Fonts->AddFontFromFileTTF("assets/fonts/inter/static/Inter_18pt-SemiBold.ttf", kHeadSize);
        if (!m_font_body)    m_font_body    = io.Fonts->AddFontDefault();   // ship without the asset, not without a font
        if (!m_font_heading) m_font_heading = m_font_body;
        io.FontDefault = m_font_body;

        ImGuiSettingsHandler handler;
        handler.TypeName   = "Donut";
        handler.TypeHash   = ImHashStr("Donut");
        handler.ReadOpenFn = settings_read_open;
        handler.ReadLineFn = settings_read_line;
        handler.WriteAllFn = settings_write_all;
        ImGui::AddSettingsHandler(&handler);
    }

    // a collapsible heading in the heading face, sitting on a soft band.
    auto UILayer::section(const char* title, bool default_open) -> bool
    {
        ImGui::PushFont(m_font_heading, kHeadSize);
        ImGui::PushStyleVar(ImGuiStyleVar_FramePadding, ImVec2(8.0f, 6.0f));
        const bool open = ImGui::CollapsingHeader(title, default_open ? ImGuiTreeNodeFlags_DefaultOpen : 0);
        ImGui::PopStyleVar();
        ImGui::PopFont();
        if (open) ImGui::Spacing();
        return open;
    }

    // muted helper text, one step smaller than body.
    auto UILayer::caption(const char* text) -> void
    {
        ImGui::PushFont(m_font_body, kCaptionSize);
        ImGui::PushTextWrapPos(0.0f);
        ImGui::TextDisabled("%s", text);
        ImGui::PopTextWrapPos();
        ImGui::PopFont();
    }

    // a row of mutually-exclusive buttons; the active one wears the accent.
    auto UILayer::segmented(const char* id, const char* const* labels, int count, int& value) -> bool
    {
        bool changed = false;
        ImGui::PushID(id);
        const float gap   = ImGui::GetStyle().ItemInnerSpacing.x;
        const float width = (ImGui::GetContentRegionAvail().x - gap * (float)(count - 1)) / (float)count;
        for (int i = 0; i < count; ++i)
        {
            if (i > 0) ImGui::SameLine(0.0f, gap);
            const bool active = (value == i);
            if (active)
            {
                ImGui::PushStyleColor(ImGuiCol_Button,        kAmber);
                ImGui::PushStyleColor(ImGuiCol_ButtonHovered, kAmberHi);
                ImGui::PushStyleColor(ImGuiCol_Text,          kInk);
            }
            if (ImGui::Button(labels[i], ImVec2(width, 0.0f)) && !active) { value = i; changed = true; }
            if (active) ImGui::PopStyleColor(3);
        }
        ImGui::PopID();
        return changed;
    }

    auto UILayer::draw(const UIContext& ctx) -> View
    {
        static bool styled = false;
        if (!styled) { apply_donut_style(); styled = true; }

        ImGuizmo::BeginFrame();   // the Properties panel draws the scene gizmo

        // first frame: pick up what imgui.ini restored (ImGui applied it in NewFrame
        // and pruned any dock tree no saved window references) and seed the
        // workspaces that were never built. has to run before any DockSpace() call
        // this frame, since even a keep-alive ping creates the node and would hide
        // a fresh tab from the check.
        if (m_first_frame)
        {
            m_first_frame = false;
            if (s_persisted.loaded)
            {
                m_active_tab = std::clamp(s_persisted.active, 0, kTabCount - 1);
                for (int t = 0; t < kTabCount; ++t)
                    for (int p = 0; p < kPanelCount; ++p)
                        m_tabs[t].show[p] = m_tabs[t].allowed[p] && s_persisted.show[t][p];
            }
            for (TabState& tab : m_tabs)
            {
                if (ImGui::DockBuilderGetNode(tab.dock_id) != nullptr) continue;
                // no tree. fresh install or a new workspace: seed it. but if the ini
                // already knows this workspace's windows, the user floated every one
                // of them and ImGui pruned the empty tree: keep what they left.
                bool known = false;
                for (int p = 0; p < kPanelCount; ++p)
                    if (tab.allowed[p] && ImGui::FindWindowSettingsByID(ImHashStr(tab.name[p].c_str()))) known = true;
                tab.reset = !known;
            }
            m_tab_request = m_active_tab;   // the strip defaults to its first tab otherwise
            m_tabs[m_active_tab].appearing = true;
        }

        // seeding goes before the live dockspace so a fresh tree gets its host
        // window this frame, instead of its panels floating around for one.
        for (TabState& tab : m_tabs)
            if (tab.reset) { build_layout(tab); tab.reset = false; }

        draw_menu_bar(ctx);
        draw_tab_strip();         // the bars reserve their strips in call order, top to bottom
        draw_status_bar(ctx);

        // only the open workspace's tree is really submitted. the other two got a
        // keep-alive ping from the strip, which freezes them: ImGui neither lays
        // them out nor evicts their windows, and imgui.ini still gets every tree.
        TabState& tab = m_tabs[m_active_tab];
        ImGui::DockSpaceOverViewport(tab.dock_id, ImGui::GetMainViewport(), ImGuiDockNodeFlags_PassthruCentralNode);

        for (int p = 0; p < kPanelCount; ++p)
            if (tab.allowed[p] && tab.show[p]) draw_panel(ctx, tab, static_cast<Panel>(p));
        tab.appearing = false;

        sync_persisted();
        return tab.view;
    }

    auto UILayer::draw_menu_bar(const UIContext& /*ctx*/) -> void
    {
        if (!ImGui::BeginMainMenuBar()) return;

        // a small brand mark on the left.
        ImGui::PushFont(m_font_heading, kBodySize);
        ImGui::TextColored(kAmber, "Donut");
        ImGui::PopFont();
        ImGui::Separator();

        TabState& tab = m_tabs[m_active_tab];

        if (ImGui::BeginMenu("Workspace"))
        {
            const char* mod = ImGui::GetIO().ConfigMacOSXBehaviors ? "Cmd" : "Ctrl";
            for (int i = 0; i < kTabCount; ++i)
            {
                char shortcut[16];
                std::snprintf(shortcut, sizeof(shortcut), "%s+%d", mod, i + 1);
                if (ImGui::MenuItem(m_tabs[i].label, shortcut, m_active_tab == i)) request_tab(i);
            }
            ImGui::EndMenu();
        }

        if (ImGui::BeginMenu("Window"))
        {
            ImGui::TextDisabled("%s panels", tab.label);
            ImGui::Separator();
            for (int p = 0; p < kPanelCount; ++p)
                if (tab.allowed[p]) ImGui::MenuItem(kPanels[p].label, nullptr, &tab.show[p]);
            ImGui::EndMenu();
        }

        if (ImGui::BeginMenu("Layout"))
        {
            if (ImGui::MenuItem("Reset this workspace")) tab.reset = true;
            if (ImGui::MenuItem("Reset all workspaces")) for (TabState& t : m_tabs) t.reset = true;
            ImGui::EndMenu();
        }

        ImGui::EndMainMenuBar();
    }

    // the workspace tabs on their own strip under the main menu bar. the strip is
    // also where the two closed workspaces get their keep-alive ping.
    auto UILayer::draw_tab_strip() -> void
    {
        // Cmd+1/2/3 on macOS (ImGuiMod_Ctrl means Cmd there), Ctrl+1/2/3 elsewhere.
        for (int i = 0; i < kTabCount; ++i)
            if (ImGui::Shortcut(ImGuiMod_Ctrl | (ImGuiKey_1 + i), ImGuiInputFlags_RouteGlobal)) request_tab(i);

        ImGuiViewport* vp = ImGui::GetMainViewport();
        ImGui::PushStyleVar(ImGuiStyleVar_FramePadding,     ImVec2(18.0f, 6.0f));   // roomier tabs
        ImGui::PushStyleVar(ImGuiStyleVar_WindowPadding,    ImVec2(8.0f, 2.0f));
        ImGui::PushStyleVar(ImGuiStyleVar_WindowBorderSize, 0.0f);
        const float height = ImGui::GetFrameHeight() + 4.0f;   // the tab bar plus the window padding
        const ImGuiWindowFlags flags = ImGuiWindowFlags_NoScrollbar | ImGuiWindowFlags_NoScrollWithMouse
                                     | ImGuiWindowFlags_NoSavedSettings;
        if (ImGui::BeginViewportSideBar("##donut_tabs", vp, ImGuiDir_Up, height, flags))
        {
            ImGui::PushFont(m_font_heading, kBodySize);
            if (ImGui::BeginTabBar("##workspaces", ImGuiTabBarFlags_NoTooltip | ImGuiTabBarFlags_DrawSelectedOverline
                                                 | ImGuiTabBarFlags_NoTabListScrollingButtons))
            {
                // don't swap trees under a live window drag: the dragged window would
                // just stop being submitted mid-gesture.
                const bool can_switch = ImGui::GetCurrentContext()->MovingWindow == nullptr && !ImGui::IsDragDropActive();
                for (int i = 0; i < kTabCount; ++i)
                {
                    const ImGuiTabItemFlags item_flags = ImGuiTabItemFlags_NoReorder | ImGuiTabItemFlags_NoCloseWithMiddleMouseButton
                                                       | (m_tab_request == i ? ImGuiTabItemFlags_SetSelected : 0);
                    if (ImGui::BeginTabItem(m_tabs[i].label, nullptr, item_flags))
                    {
                        // on a request frame the strip's answer is stale (SetSelected lands
                        // next frame), and on its very first frame the tab bar just shows
                        // tab 0, so only trust a click when nothing programmatic is pending.
                        if (i != m_active_tab && can_switch && m_tab_request < 0) { m_active_tab = i; m_tabs[i].appearing = true; }
                        ImGui::EndTabItem();
                    }
                }
                ImGui::EndTabBar();
            }
            ImGui::PopFont();
        }
        // park the closed workspaces' trees. with KeepAliveOnly, DockSpace just
        // stamps the node's LastFrameAlive and returns: no host window, no layout,
        // and nothing docked in there gets undocked for being off screen.
        for (int i = 0; i < kTabCount; ++i)
            if (i != m_active_tab)
                ImGui::DockSpace(m_tabs[i].dock_id, ImVec2(0.0f, 0.0f), ImGuiDockNodeFlags_KeepAliveOnly);
        ImGui::End();
        ImGui::PopStyleVar(3);
        m_tab_request = -1;
    }

    // a thin bottom bar with the live facts. BeginViewportSideBar reserves its
    // strip in the viewport work area, so the dock space above shrinks to fit —
    // the same mechanism the main menu bar uses.
    auto UILayer::draw_status_bar(const UIContext& ctx) -> void
    {
        ImGuiViewport* vp = ImGui::GetMainViewport();
        const float height = ImGui::GetFrameHeight();
        const ImGuiWindowFlags flags = ImGuiWindowFlags_NoScrollbar | ImGuiWindowFlags_NoSavedSettings
                                     | ImGuiWindowFlags_MenuBar;
        if (ImGui::BeginViewportSideBar("##donut_status", vp, ImGuiDir_Down, height, flags))
        {
            if (ImGui::BeginMenuBar())
            {
                ImGuiIO& io = ImGui::GetIO();
                ImGui::PushFont(m_font_body, kCaptionSize);

                ImGui::TextDisabled("%s", ctx.device_name.empty() ? "GPU" : ctx.device_name.c_str());
                ImGui::Separator();
                ImGui::TextColored(kAmber, "%s", m_tabs[m_active_tab].label);
                ImGui::Separator();
                // DisplaySize is the window in points; the swapchain is points * scale (2x on retina)
                ImGui::TextDisabled("%.0f x %.0f", io.DisplaySize.x * io.DisplayFramebufferScale.x,
                                                  io.DisplaySize.y * io.DisplayFramebufferScale.y);

                // the Settings > Overlay toggles decide whether the timings show here.
                if (SettingsManager::get_show_fps())
                {
                    ImGui::Separator();
                    ImGui::Text("%.0f fps", io.Framerate);
                }
                if (SettingsManager::get_show_performance_metrics())
                {
                    ImGui::Separator();
                    ImGui::TextDisabled("%.2f ms", io.Framerate > 0.0f ? 1000.0f / io.Framerate : 0.0f);
                }

                // last export result, right-aligned.
                if (m_exp_reported)
                {
                    const char* msg = m_exp_written > 0 ? "export written" : "export failed";
                    const float tw = ImGui::CalcTextSize(msg).x;
                    ImGui::SameLine(ImGui::GetWindowWidth() - tw - 12.0f);
                    ImGui::TextColored(m_exp_written > 0 ? kGreen : kRed, "%s", msg);
                }

                ImGui::PopFont();
                ImGui::EndMenuBar();
            }
        }
        ImGui::End();
    }

    // build one workspace's default dock tree from scratch. every panel the
    // workspace owns gets a home here, shown or not, so toggling one on later
    // lands it somewhere sensible. a reset also restores the default panel set.
    auto UILayer::build_layout(TabState& tab) -> void
    {
        DONUT_INFO("seeding the {} workspace layout", tab.label);
        const ImGuiID root = tab.dock_id;
        ImGui::DockBuilderRemoveNode(root);
        // DockSpace is a private flag and PassthruCentralNode a public one, and
        // C++20 whines about OR-ing two different enum types, so cast to int and move on.
        ImGui::DockBuilderAddNode(root, (int)ImGuiDockNodeFlags_DockSpace | (int)ImGuiDockNodeFlags_PassthruCentralNode);
        // splits need a size to ratio against. the three bars can eat the whole work
        // area in a very short window and the builder asserts on zero, and the live
        // DockSpace resizes the root from the real work area next frame anyway.
        const ImGuiViewport* vp = ImGui::GetMainViewport();
        const ImVec2 seed_size = (vp->WorkSize.x > 0.0f && vp->WorkSize.y > 0.0f) ? vp->WorkSize : vp->Size;
        ImGui::DockBuilderSetNodeSize(root, seed_size);

        auto dock = [&](Panel p, ImGuiID node) { ImGui::DockBuilderDockWindow(tab.name[(int)p].c_str(), node); };
        ImGuiID centre = root;

        switch (tab.id)
        {
        case Tab::SceneEditor:
        {
            ImGuiID left = 0, left_bottom = 0, right = 0;
            ImGui::DockBuilderSplitNode(centre, ImGuiDir_Left,  0.24f, &left,  &centre);
            ImGui::DockBuilderSplitNode(left,   ImGuiDir_Down,  0.50f, &left_bottom, &left);
            ImGui::DockBuilderSplitNode(centre, ImGuiDir_Right, 0.24f, &right, &centre);
            dock(Panel::Outliner,   left);
            dock(Panel::Properties, left_bottom);
            dock(Panel::Stats,      right);
            dock(Panel::Settings,   right);
            break;
        }
        case Tab::Export:
        {
            ImGuiID right = 0, right_bottom = 0;
            ImGui::DockBuilderSplitNode(centre, ImGuiDir_Right, 0.28f, &right, &centre);
            ImGui::DockBuilderSplitNode(right,  ImGuiDir_Down,  0.50f, &right_bottom, &right);
            dock(Panel::Export,     right);
            dock(Panel::Settings,   right);
            dock(Panel::BlackHole,  right_bottom);
            dock(Panel::Stats,      right_bottom);
            dock(Panel::Outliner,   right_bottom);
            dock(Panel::Properties, right_bottom);
            break;
        }
        case Tab::Simulation:
        {
            ImGuiID right = 0, right_bottom = 0;
            ImGui::DockBuilderSplitNode(centre, ImGuiDir_Right, 0.24f, &right, &centre);
            ImGui::DockBuilderSplitNode(right,  ImGuiDir_Down,  0.34f, &right_bottom, &right);
            dock(Panel::BlackHole,  right);
            dock(Panel::Settings,   right);
            dock(Panel::Outliner,   right);
            dock(Panel::Properties, right);
            dock(Panel::Stats,      right_bottom);
            break;
        }
        }

        ImGui::DockBuilderFinish(root);
        std::copy(std::begin(tab.defaults), std::end(tab.defaults), tab.show);
    }

    // push the live workspace state into the ini snapshot and let ImGui know it's
    // worth a save, when anything actually changed.
    auto UILayer::sync_persisted() -> void
    {
        bool changed = !s_persisted.loaded || s_persisted.active != m_active_tab;
        for (int t = 0; t < kTabCount; ++t)
            for (int p = 0; p < kPanelCount; ++p)
                changed |= s_persisted.show[t][p] != m_tabs[t].show[p];
        if (!changed) return;

        s_persisted.active = m_active_tab;
        for (int t = 0; t < kTabCount; ++t)
            for (int p = 0; p < kPanelCount; ++p)
                s_persisted.show[t][p] = m_tabs[t].show[p];
        s_persisted.loaded = true;
        ImGui::MarkIniSettingsDirty();
    }

    // the one Begin() for panels, so the name DockBuilder docked against and the
    // name the window opens under can't drift apart.
    auto UILayer::begin_panel(TabState& tab, Panel p) -> bool
    {
        // the first frame back must not focus every re-appearing panel, or the last
        // Begin() in a node holding several would steal that node's selected tab.
        const ImGuiWindowFlags flags = tab.appearing ? ImGuiWindowFlags_NoFocusOnAppearing : 0;
        return ImGui::Begin(tab.name[(int)p].c_str(), &tab.show[(int)p], flags);
    }

    auto UILayer::draw_panel(const UIContext& ctx, TabState& tab, Panel p) -> void
    {
        switch (p)
        {
        case Panel::Outliner:   panel_outliner(ctx, tab);   break;
        case Panel::Properties: panel_properties(ctx, tab); break;
        case Panel::BlackHole:  panel_black_hole(ctx, tab); break;
        case Panel::Settings:   panel_settings(ctx, tab);   break;
        case Panel::Export:     panel_export(ctx, tab);     break;
        case Panel::Stats:      panel_stats(ctx, tab);      break;
        }
    }

    auto UILayer::panel_outliner(const UIContext& ctx, TabState& tab) -> void
    {
        if (begin_panel(tab, Panel::Outliner))
        {
            Scene& sc = ctx.scene;
            auto& objs = sc.objects;
            if (sc.selected_object >= (int)objs.size()) sc.selected_object = (int)objs.size() - 1;

            if (accent_button("+  Sphere") && objs.size() < 64)
            {
                SceneObject o;
                const int n = (int)objs.size();   // spread newcomers along a 5-wide grid
                o.position = glm::vec3((float)(n % 5) * 5.0f - 10.0f, 2.0f, (float)(n / 5) * 5.0f);
                o.radius   = 1.5f;
                o.color    = glm::vec3(0.35f + 0.12f * (float)(n % 5), 0.55f, 0.9f - 0.12f * (float)(n % 4));
                objs.push_back(o);
                sc.selected_object = (int)objs.size() - 1;
            }
            ImGui::SameLine();
            ImGui::BeginDisabled(objs.empty());
            if (ImGui::Button("Delete") && !objs.empty())
            {
                objs.erase(objs.begin() + sc.selected_object);
                if (sc.selected_object >= (int)objs.size()) sc.selected_object = (int)objs.size() - 1;
            }
            ImGui::EndDisabled();
            ImGui::SameLine();
            ImGui::PushFont(m_font_body, kCaptionSize);
            ImGui::TextDisabled("%d / 64", (int)objs.size());
            ImGui::PopFont();

            ImGui::Spacing();
            ImGui::BeginChild("obj_list");
            // each row carries a swatch of its colour, so the list reads like the scene.
            const float row_h  = ImGui::GetTextLineHeight();
            const float swatch = row_h * 0.72f;
            for (int i = 0; i < (int)objs.size(); ++i)
            {
                ImGui::PushID(i);
                const ImVec2 p = ImGui::GetCursorScreenPos();
                char label[32];
                std::snprintf(label, sizeof(label), "      Sphere %d", i);   // leading room for the swatch
                if (ImGui::Selectable(label, sc.selected_object == i)) sc.selected_object = i;
                const glm::vec3& col = objs[(size_t)i].color;
                const ImVec2 a(p.x + 6.0f, p.y + (row_h - swatch) * 0.5f);
                ImGui::GetWindowDrawList()->AddRectFilled(a, ImVec2(a.x + swatch, a.y + swatch),
                    ImGui::GetColorU32(ImVec4(col.r, col.g, col.b, 1.0f)), 3.0f);
                ImGui::PopID();
            }
            if (objs.empty()) caption("Nothing here yet. Add a sphere above.");
            ImGui::EndChild();
        }
        ImGui::End();
    }

    auto UILayer::panel_properties(const UIContext& ctx, TabState& tab) -> void
    {
        if (begin_panel(tab, Panel::Properties))
        {
            Scene& sc = ctx.scene;
            auto& objs = sc.objects;
            if (objs.empty() || sc.selected_object < 0 || sc.selected_object >= (int)objs.size())
            {
                caption("Select a sphere in the Outliner to edit it.");
            }
            else
            {
                SceneObject& o = objs[(size_t)sc.selected_object];
                ImGui::PushFont(m_font_heading, kHeadSize);
                ImGui::Text("Sphere %d", sc.selected_object);
                ImGui::PopFont();
                ImGui::Separator();
                ImGui::Spacing();

                prop("Position"); ImGui::DragFloat3("##pos", &o.position.x, 0.1f);
                prop("Radius");   ImGui::DragFloat("##rad", &o.radius, 0.05f, 0.1f, 20.0f);
                prop("Colour");   ImGui::ColorEdit3("##col", &o.color.x);

                ImGui::Spacing();
                ImGui::PushFont(m_font_body, kCaptionSize);
                ImGui::TextDisabled("GIZMO");
                ImGui::PopFont();
                static const char* const ops[] = { "Move", "Scale" };
                segmented("gizmo_op", ops, 2, m_gizmo_op);

                if (tab.view == View::Scene)
                {
                    // viewport gizmo for the selected sphere, using the scene camera
                    // (its projection was set for this frame by the Application). drawn
                    // into the background list so it sits over the render, under panels.
                    ImGuizmo::SetOrthographic(false);
                    ImGuizmo::SetDrawlist(ImGui::GetBackgroundDrawList());
                    ImGuiIO& io = ImGui::GetIO();
                    ImGuizmo::SetRect(0.0f, 0.0f, io.DisplaySize.x, io.DisplaySize.y);
                    glm::mat4 view = sc.scene_camera.get_view_matrix();
                    glm::mat4 proj = sc.scene_camera.get_projection_matrix();
                    glm::mat4 xform = glm::translate(glm::mat4(1.0f), o.position)
                                    * glm::scale(glm::mat4(1.0f), glm::vec3(o.radius));
                    ImGuizmo::OPERATION op = (m_gizmo_op == 1) ? ImGuizmo::SCALE : ImGuizmo::TRANSLATE;
                    if (ImGuizmo::Manipulate(&view[0][0], &proj[0][0], op, ImGuizmo::WORLD, &xform[0][0]))
                    {
                        float t[3], r[3], s[3];
                        ImGuizmo::DecomposeMatrixToComponents(&xform[0][0], t, r, s);
                        o.position = glm::vec3(t[0], t[1], t[2]);
                        o.radius   = std::max(0.05f, (s[0] + s[1] + s[2]) / 3.0f);
                    }
                    caption("Drag the handles in the viewport.");
                }
                else
                {
                    caption("The gizmo lives in the Scene Editor's viewport; the numbers work anywhere.");
                }
            }
        }
        ImGui::End();
    }

    auto UILayer::panel_black_hole(const UIContext& ctx, TabState& tab) -> void
    {
        if (begin_panel(tab, Panel::BlackHole))
        {
            Scene& sc = ctx.scene;
            BlackHoleParams& bh = sc.black_hole;

            if (section("Camera"))
            {
                static const char* const modes[] = { "Orbital", "Free-fly" };
                int mode = sc.is_free_fly() ? 1 : 0;
                if (segmented("cam_mode", modes, 2, mode)) sc.set_free_fly(mode == 1);
                if (ImGui::Button("Reset view", ImVec2(-FLT_MIN, 0.0f))) sc.reset_sim_camera();
                prop("Field of view"); ImGui::SliderFloat("##fov", &bh.fov_degrees, 20.0f, 90.0f, "%.0f\xc2\xb0");
                caption(sc.is_free_fly() ? "WASD to move  \xc2\xb7  Q/E down and up  \xc2\xb7  Shift to boost  \xc2\xb7  drag to look"
                                         : "Drag to orbit  \xc2\xb7  scroll to zoom");
            }

            if (section("Accretion disk"))
            {
                prop("Temperature");  ImGui::SliderFloat("##temp",   &bh.temperature,   2000.0f, 15000.0f, "%.0f K");
                prop("Brightness");   ImGui::SliderFloat("##bright", &bh.brightness,    0.0f,    3.0f,     "%.2f");
                prop("Inner radius"); ImGui::SliderFloat("##rin",    &bh.disk_inner_rs, 3.0f,    12.0f,    "%.1f r_s");
                if (bh.disk_outer_rs < bh.disk_inner_rs + 0.5f) bh.disk_outer_rs = bh.disk_inner_rs + 0.5f;
                prop("Outer radius"); ImGui::SliderFloat("##rout",   &bh.disk_outer_rs, bh.disk_inner_rs + 0.5f, 25.0f, "%.1f r_s");
                prop("Turbulence");   ImGui::SliderFloat("##turb",   &bh.turbulence,    0.0f,    2.0f,     "%.2f");
                caption("The inner edge sits at the ISCO (3 r_s). Novikov-Thorne temperature profile.");
            }

            if (section("Quality", false))
            {
                prop("Steps"); ImGui::SliderInt("##steps", &bh.quality_steps, 2000, 15000);
                caption("More steps resolve the disk at steep angles, at a cost. Settled frames are 4x supersampled.");
            }
        }
        ImGui::End();
    }

    auto UILayer::panel_settings(const UIContext& ctx, TabState& tab) -> void
    {
        if (begin_panel(tab, Panel::Settings))
        {
            auto& s = SettingsManager::get_settings();

            if (section("Display"))
            {
                const bool fs = ctx.actions.is_fullscreen();
                int cw = 0, ch = 0; ctx.actions.get_window_size(cw, ch);

                struct Res { const char* label; int w, h; };
                static const Res kRes[] = {
                    { "1280 x 720",  1280, 720 }, { "1600 x 900",  1600, 900 },
                    { "1920 x 1080", 1920, 1080 }, { "2560 x 1440", 2560, 1440 },
                };
                char cur_label[32]; std::snprintf(cur_label, sizeof(cur_label), "%d x %d", cw, ch);
                ImGui::BeginDisabled(fs);
                prop("Resolution");
                if (ImGui::BeginCombo("##res", cur_label))
                {
                    for (const auto& r : kRes)
                        if (ImGui::Selectable(r.label, cw == r.w && ch == r.h))
                            ctx.actions.set_resolution(r.w, r.h);
                    ImGui::EndCombo();
                }
                ImGui::EndDisabled();

                bool fs_v = fs;
                prop("Fullscreen"); if (ImGui::Checkbox("##fs", &fs_v)) ctx.actions.set_fullscreen(fs_v);

                prop("V-Sync");
                if (ImGui::Checkbox("##vsync", &s.graphics.v_sync_enabled))
                {
                    ctx.actions.set_vsync(s.graphics.v_sync_enabled);
                    SettingsManager::save_settings();
                }

                ImGui::BeginDisabled(s.graphics.v_sync_enabled);
                prop("Frame cap");
                if (ImGui::SliderInt("##cap", &s.simulation.target_fps, 30, 240, "%d fps"))
                    SettingsManager::save_settings();
                ImGui::EndDisabled();
                if (s.graphics.v_sync_enabled) caption("The frame cap only applies with V-Sync off.");

                prop("UI scale");
                if (ImGui::SliderFloat("##scale", &m_ui_scale, 0.8f, 1.6f, "%.2fx"))
                    ctx.actions.set_ui_scale(m_ui_scale);
            }

            if (section("Renderer"))
            {
                const char* apis[] = { "OpenGL", "Vulkan" };
                int cur = (s.graphics.render_api == "Vulkan") ? 1 : 0;
                prop("Graphics API");
                if (ImGui::Combo("##api", &cur, apis, 2))
                {
                    s.graphics.render_api = apis[cur];
                    SettingsManager::save_settings();
                }
                const char* running = (RendererAPI::get_api() == RendererAPI::API::Vulkan) ? "Vulkan" : "OpenGL";
                if (s.graphics.render_api != running)
                    ImGui::TextColored(kAmber, "Restart to apply  (%s is running)", running);
                else
                {
                    char note[160];
                    std::snprintf(note, sizeof(note), "Running %s on %s.", running, ctx.device_name.c_str());
                    caption(note);
                }
            }

            if (section("Overlay"))
            {
                bool dirty = false;
                prop("Show fps");        dirty |= ImGui::Checkbox("##showfps", &s.graphics.show_fps);
                prop("Show frame time"); dirty |= ImGui::Checkbox("##showms",  &s.graphics.show_performance_metrics);
                if (dirty) SettingsManager::save_settings();
                caption("Shown in the status bar along the bottom.");
            }

            if (section("Environment"))
            {
                auto& hdri = HDRIManager::get();
                std::string preview = hdri.get_hdri_name(ctx.scene.hdri_path);
                prop("Starfield");
                if (ImGui::BeginCombo("##hdri", preview.c_str()))
                {
                    for (const auto& path : hdri.get_available_hdri())
                    {
                        bool selected = (path == ctx.scene.hdri_path);
                        if (ImGui::Selectable(hdri.get_hdri_name(path).c_str(), selected))
                            ctx.scene.hdri_path = path;   // RenderPath rebuilds the cubemap next frame
                        if (selected) ImGui::SetItemDefaultFocus();
                    }
                    ImGui::EndCombo();
                }
                caption("Switching rebuilds the cubemap (a brief pause).");
            }
        }
        ImGui::End();
    }

    auto UILayer::panel_export(const UIContext& ctx, TabState& tab) -> void
    {
        if (begin_panel(tab, Panel::Export))
        {
            caption("Render the black hole and its observables to disk.");
            ImGui::Spacing();

            if (section("Channels"))
            {
                ImGui::Checkbox("Colour (tone-mapped)", &m_exp_color);
                ImGui::Checkbox("Redshift  g",          &m_exp_redshift);
                ImGui::Checkbox("Emission temperature", &m_exp_temperature);
                ImGui::Checkbox("Impact parameter",     &m_exp_impact);
                caption("Redshift and temperature are disk-only; impact covers the whole frame.");
            }

            if (section("Output"))
            {
                prop("Resolution"); ImGui::Combo("##res", &m_exp_resolution, "1280 x 720\0" "1920 x 1080\0" "2560 x 1440\0" "3840 x 2160\0");
                prop("Format");     ImGui::Combo("##fmt", &m_exp_format,     "Display image (PNG)\0" "Raw float image (PFM)\0" "Raw values (CSV)\0");
                caption(m_exp_format == 0 ? "Tone-mapped colour, or false-coloured observables."
                                          : "Actual physical values: g, temperature (K), impact (r_s).");
            }

            ImGui::Spacing();
            const bool any = m_exp_color || m_exp_redshift || m_exp_temperature || m_exp_impact;
            ImGui::BeginDisabled(!any);
            if (accent_button("Export frame", ImVec2(-FLT_MIN, 0.0f)))
            {
                static const int dims[][2] = { {1280,720}, {1920,1080}, {2560,1440}, {3840,2160} };
                ExportConfig cfg;
                cfg.color = m_exp_color; cfg.redshift = m_exp_redshift;
                cfg.temperature = m_exp_temperature; cfg.impact = m_exp_impact;
                cfg.width = dims[m_exp_resolution][0]; cfg.height = dims[m_exp_resolution][1];
                cfg.format = static_cast<ExportFormat>(m_exp_format);
                m_exp_written = ctx.actions.export_frame(cfg);
                m_exp_reported = true;
            }
            ImGui::EndDisabled();

            if (m_exp_reported)
            {
                ImGui::Spacing();
                if (m_exp_written > 0)
                    ImGui::TextColored(kGreen, "Wrote %d file(s) to exports/", m_exp_written);
                else
                    ImGui::TextColored(kRed, "Export failed (see log)");
            }
        }
        ImGui::End();
    }

    auto UILayer::panel_stats(const UIContext& ctx, TabState& tab) -> void
    {
        if (begin_panel(tab, Panel::Stats))
        {
            ImGuiIO& io = ImGui::GetIO();
            const float ms = io.DeltaTime * 1000.0f;
            m_frame_hist[m_frame_hist_i] = ms;
            m_frame_hist_i = (m_frame_hist_i + 1) % kFrameHist;

            // the headline number.
            ImGui::PushFont(m_font_heading, kBigSize);
            ImGui::Text("%.0f", io.Framerate);
            ImGui::PopFont();
            ImGui::SameLine(0.0f, 8.0f);
            ImGui::PushFont(m_font_body, kCaptionSize);
            ImGui::TextDisabled("fps   \xc2\xb7   %.2f ms", ms);
            ImGui::PopFont();

            // frame-time graph, autoscaled to the worst recent frame.
            float worst = 0.0f;
            for (float v : m_frame_hist) worst = std::max(worst, v);
            ImGui::PlotLines("##frametime", m_frame_hist, kFrameHist, m_frame_hist_i, nullptr,
                             0.0f, std::max(worst * 1.15f, 4.0f), ImVec2(-FLT_MIN, 64.0f));
            caption("Frame time over the last 120 frames.");
            ImGui::Spacing();

            // the facts, as an aligned two-column table: a fixed label column and a
            // stretch value column. (SizingStretchProp would derive the weights from
            // measured content widths, which on the first frame is 0/0 = NaN and
            // poisons the window's content size — UBSan caught it.)
            if (ImGui::BeginTable("facts", 2, ImGuiTableFlags_RowBg))
            {
                ImGui::TableSetupColumn("key",   ImGuiTableColumnFlags_WidthFixed, 118.0f);
                ImGui::TableSetupColumn("value", ImGuiTableColumnFlags_WidthStretch);
                auto row = [](const char* key, const char* value)
                {
                    ImGui::TableNextRow();
                    ImGui::TableSetColumnIndex(0); ImGui::TextDisabled("%s", key);
                    ImGui::TableSetColumnIndex(1); ImGui::TextUnformatted(value);
                };
                char buf[64];
                row("Device",    ctx.device_name.empty() ? "GPU" : ctx.device_name.c_str());
                row("API",       RendererAPI::get_api() == RendererAPI::API::Vulkan ? "Vulkan (MoltenVK)" : "OpenGL 4.1");
                row("Workspace", tab.label);
                std::snprintf(buf, sizeof(buf), "%.0f x %.0f", io.DisplaySize.x * io.DisplayFramebufferScale.x,
                                                                io.DisplaySize.y * io.DisplayFramebufferScale.y);
                row("Framebuffer", buf);
                std::snprintf(buf, sizeof(buf), "%d",  ctx.scene.black_hole.quality_steps);          row("Integration steps", buf);
                std::snprintf(buf, sizeof(buf), "%.0f K", ctx.scene.black_hole.temperature);         row("Disk temperature", buf);
                std::snprintf(buf, sizeof(buf), "%d",  (int)ctx.scene.objects.size());                row("Scene objects", buf);
                ImGui::EndTable();
            }
        }
        ImGui::End();
    }
}