using System;
using UnityEngine;
using UnityEngine.UI;
using UnityEngine.EventSystems;
#if ENABLE_INPUT_SYSTEM
using UnityEngine.InputSystem.UI;
#endif

/// <summary>
/// In-game paint UI. Shows its panels only while the active <see cref="GameMode"/> is
/// <see cref="PaintMode"/> (it follows the manager's ModeChanged event) and binds its
/// widgets directly to the live <see cref="MeshPainter"/> fields, so edits apply immediately.
///
/// The layout is a transparent root holding one independently positioned sub-panel per channel
/// (Albedo, Metallic+Smoothness, Occlusion, Emission) plus a global Brush card, each with a title
/// bar and draggable from any of its non-widget surface. The layout comes from one of two sources:
///  - an authored prefab assigned to <see cref="layoutPrefab"/> (instantiated, then bound), or
///  - generated in code (the fallback when no prefab is set).
/// Either way the *bindings* (listeners, picker gradient sprites, painter values) live in code
/// and are (re)applied at runtime by <see cref="WirePanel"/>. A prefab stores only the visual
/// layout, never the behaviour, so both paths run the exact same wiring against the same stable
/// widget names - there is no separate "dead" prefab path.
///
/// Widgets: channel toggles, an HSV colour picker for albedo, metallic / smoothness / occlusion
/// sliders, an HSV picker + intensity for emission, and brush size. Attach to an empty GameObject.
/// </summary>
[DisallowMultipleComponent]
public partial class PaintHud : MonoBehaviour
{
    [Tooltip("Mode manager driving visibility. Empty = found automatically.")]
    public GameModeManager modeManager;

    [Tooltip("Painter the HUD edits. Empty = the manager's painter, else found automatically.")]
    public MeshPainter painter;

    [Tooltip("Authored panel layout. If set it is instantiated and bound; empty = build the panel in code.")]
    public GameObject layoutPrefab;

    // Stable widget names shared by the code-gen builder, the baked prefab and the binder.
    private const string PanelSpray = "Panel_Spray";
    private const string PanelAlbedo = "Panel_Albedo";
    private const string PanelMetallicSmoothness = "Panel_MetallicSmoothness";
    private const string PanelOcclusion = "Panel_Occlusion";
    private const string PanelEmission = "Panel_Emission";
    private const string PanelBrushSize = "Panel_BrushSize";
    private const string ToggleAlbedo = "Toggle_Albedo";
    private const string ToggleMetallicSmoothness = "Toggle_MetallicSmoothness";
    private const string ToggleOcclusion = "Toggle_Occlusion";
    private const string ToggleEmission = "Toggle_Emission";
    private const string ToggleSpray = "Toggle_Spray";
    private const string TogglePipette = "Toggle_Pipette";
    private const string PickerAlbedo = "Picker_Albedo";
    private const string PickerEmission = "Picker_Emission";
    private const string SliderMetallic = "Slider_Metallic";
    private const string SliderSmoothness = "Slider_Smoothness";
    private const string SliderMetallicSmoothnessAlpha = "Slider_MetallicSmoothnessAlpha";
    private const string SliderOcclusion = "Slider_Occlusion";
    private const string SliderOcclusionAlpha = "Slider_OcclusionAlpha";
    private const string SliderEmissionIntensity = "Slider_EmissionIntensity";
    private const string SliderBrushSize = "Slider_BrushSize";
    private const string SliderSprayDots = "Slider_SprayDots";
    private const string SliderSprayDotSizeMin = "Slider_SprayDotSizeMin";
    private const string SliderSprayDotSizeMax = "Slider_SprayDotSizeMax";
    private const string SliderSprayDotAlphaMin = "Slider_SprayDotAlphaMin";
    private const string SliderSprayDotAlphaMax = "Slider_SprayDotAlphaMax";
    private const string MaskSelector = "MaskSelector";
    private const string MaskSwatchPrefix = "MaskSwatch_";

    // Panels made draggable at runtime - grabbable from any non-widget surface (both prefab and code-gen paths).
    private static readonly string[] DraggablePanels =
    {
        PanelAlbedo,
        PanelMetallicSmoothness,
        PanelOcclusion,
        PanelEmission,
        PanelBrushSize,
        PanelSpray
    };

    private GameObject _panel;
    private Font _font;

    private void Awake()
    {
        if (!modeManager) modeManager = FindFirstObjectByType<GameModeManager>();
    }

    private void OnEnable()
    {
        if (modeManager)
        {
            modeManager.ModeChanged += OnModeChanged;
            modeManager.PainterChanged += OnPainterChanged;
        }
    }

    private void OnDisable()
    {
        if (modeManager)
        {
            modeManager.ModeChanged -= OnModeChanged;
            modeManager.PainterChanged -= OnPainterChanged;
        }
    }

    private void Start()
    {
        _font = LoadFont();

        EnsureEventSystem();
        EnsureCanvas();

        if (layoutPrefab)
        {
            var go = Instantiate(layoutPrefab, transform);
            go.name = layoutPrefab.name; // strip the "(Clone)" suffix
            _panel = go;
        }

        // The paint library now lives inside the authored layout prefab; bind its widgets like the brush panel.
        BindLibrary(_panel ? _panel.transform : null);

        // Bind to a painter as soon as one exists. With networked spawn-on-connect there is none
        // at scene start; BindLocalPlayer then raises PainterChanged when the local avatar spawns
        // and OnPainterChanged wires the widgets. In a non-networked scene the painter is already
        // present and binds here immediately.
        var p = painter;
        if (!p) p = modeManager ? modeManager.Painter : null;
        if (!p) p = FindFirstObjectByType<MeshPainter>();
        if (p) BindPainter(p);

        // Set initial visibility (the manager's first SwitchTo may have fired before this Start).
        OnModeChanged(modeManager ? modeManager.ActiveMode : null);
    }

    private void OnPainterChanged(MeshPainter newPainter)
    {
        if (newPainter) BindPainter(newPainter);
    }

    /// <summary>
    /// (Re)binds the HUD's widgets to a live painter. Called from Start when one already exists
    /// (non-networked scene) or from the manager's PainterChanged event when the local networked
    /// avatar spawns. Binds exactly once per painter: re-binding the same painter is a no-op, so
    /// the deferred network path and the immediate scene path never double-add widget listeners.
    /// </summary>
    private void BindPainter(MeshPainter newPainter)
    {
        if (!newPainter || newPainter == painter || !_panel) return;

        if (painter) painter.Picked -= SyncFromPainter;
        painter = newPainter;

        WirePanel(_panel.transform);

        // Re-sync every widget when the pipette adopts a sampled material (painter -> HUD).
        painter.Picked += SyncFromPainter;

        // Restore the saved panel positions + widget values now that the painter exists.
        RestoreHudState();

        UpdateButtonStates(); // enable Save now that a painter is bound
    }

    private void OnDestroy()
    {
        if (painter) painter.Picked -= SyncFromPainter;
        SaveHudOnTeardown();
    }

    private void OnModeChanged(GameMode mode)
    {
        // Require a bound painter too: pre-spawn the manager sits in a player-less Paint mode (no
        // PlayerControl without an avatar), but there is nothing to paint yet - keep the HUD hidden until
        // the local avatar spawns and a painter is bound.
        bool paint = mode is PaintMode && painter;
        if (_panel) _panel.SetActive(paint); // toggles the brush panels and the library panel together
        if (paint)
        {
            if (_libraryBound) RefreshList(); // pick up slots saved/changed since last shown
        }
        else HideDialogs();
    }

    private static Font LoadFont()
    {
        var f = Resources.GetBuiltinResource<Font>("LegacyRuntime.ttf");
        if (!f) f = Resources.GetBuiltinResource<Font>("Arial.ttf");
        return f;
    }

    // --- Canvas ---

    private void EnsureCanvas()
    {
        var canvas = GetComponent<Canvas>();
        if (!canvas) canvas = gameObject.AddComponent<Canvas>();
        canvas.renderMode = RenderMode.ScreenSpaceOverlay;
        canvas.sortingOrder = 100;

        var scaler = GetComponent<CanvasScaler>();
        if (!scaler) scaler = gameObject.AddComponent<CanvasScaler>();
        scaler.uiScaleMode = CanvasScaler.ScaleMode.ScaleWithScreenSize;
        scaler.referenceResolution = new Vector2(1920f, 1080f);
        scaler.screenMatchMode = CanvasScaler.ScreenMatchMode.Expand;

        if (!GetComponent<GraphicRaycaster>()) gameObject.AddComponent<GraphicRaycaster>();
    }

    private void EnsureEventSystem()
    {
        if (FindFirstObjectByType<EventSystem>()) return;

        var es = new GameObject("EventSystem", typeof(EventSystem));
#if ENABLE_INPUT_SYSTEM
        var module = es.AddComponent<InputSystemUIInputModule>();
        if (!module.actionsAsset) module.AssignDefaultActions();
#else
        es.AddComponent<StandaloneInputModule>();
#endif
    }

    // --- Wiring (behaviour; shared by the code-gen and prefab paths) ---

    private void WirePanel(Transform panel)
    {
        EnsureFonts(panel); // builtin font references may not survive prefab serialization

        WireToggle(panel, ToggleAlbedo, painter && painter.paintAlbedo, v => { if (painter) painter.paintAlbedo = v; });
        WireToggle(panel, ToggleMetallicSmoothness, painter && painter.paintMetallicSmoothness, v => { if (painter) painter.paintMetallicSmoothness = v; });
        WireToggle(panel, ToggleOcclusion, painter && painter.paintOcclusion, v => { if (painter) painter.paintOcclusion = v; });
        WireToggle(panel, ToggleEmission, painter && painter.paintEmission, v => { if (painter) painter.paintEmission = v; });
        WireToggle(panel, ToggleSpray, painter && painter.spray, v => { if (painter) painter.spray = v; });
        WireToggle(panel, TogglePipette, painter && painter.pipette, v => { if (painter) painter.pipette = v; });

        // Dim a channel panel to 0.5 alpha while its toggle is off, full alpha while on.
        WirePanelFade(panel, PanelAlbedo, ToggleAlbedo);
        WirePanelFade(panel, PanelMetallicSmoothness, ToggleMetallicSmoothness);
        WirePanelFade(panel, PanelOcclusion, ToggleOcclusion);
        WirePanelFade(panel, PanelEmission, ToggleEmission);
        WirePanelFade(panel, PanelSpray, ToggleSpray);

        WirePicker(panel, PickerAlbedo, painter ? painter.brushColor : Color.red,
            c => { if (painter) painter.brushColor = c; });

        WireSlider(panel, SliderMetallic, painter ? painter.metallic : 0f, v => { if (painter) painter.metallic = v; });
        WireSlider(panel, SliderSmoothness, painter ? painter.smoothness : 0.5f, v => { if (painter) painter.smoothness = v; });
        WireSlider(panel, SliderMetallicSmoothnessAlpha, painter ? painter.metallicSmoothnessOpacity : 1f, v => { if (painter) painter.metallicSmoothnessOpacity = v; });
        WireSlider(panel, SliderOcclusion, painter ? painter.occlusion : 0.5f, v => { if (painter) painter.occlusion = v; });
        WireSlider(panel, SliderOcclusionAlpha, painter ? painter.occlusionOpacity : 1f, v => { if (painter) painter.occlusionOpacity = v; });

        WirePicker(panel, PickerEmission, painter ? painter.emissionColor : Color.black,
            c => { if (painter) painter.emissionColor = c; });
        WireSlider(panel, SliderEmissionIntensity, painter ? painter.emissionIntensity : 1f, v => { if (painter) painter.emissionIntensity = v; });

        WireSlider(panel, SliderBrushSize, painter ? painter.brushSize : 16f, v => { if (painter) painter.brushSize = Mathf.RoundToInt(v); });

        WireMaskSelector(panel);

        WireSlider(panel, SliderSprayDots, painter ? painter.sprayDots : 24f, v => { if (painter) painter.sprayDots = Mathf.RoundToInt(v); });
        WireSlider(panel, SliderSprayDotSizeMin, painter ? painter.sprayDotSizeMin : .05f, v => { if (painter) painter.sprayDotSizeMin = v; });
        WireSlider(panel, SliderSprayDotSizeMax, painter ? painter.sprayDotSizeMax : .18f, v => { if (painter) painter.sprayDotSizeMax = v; });
        WireSlider(panel, SliderSprayDotAlphaMin, painter ? painter.sprayDotAlphaMin : .25f, v => { if (painter) painter.sprayDotAlphaMin = v; });
        WireSlider(panel, SliderSprayDotAlphaMax, painter ? painter.sprayDotAlphaMax : 1f, v => { if (painter) painter.sprayDotAlphaMax = v; });

        foreach (var name in DraggablePanels)
            MakeDraggable(panel, name);
    }

    /// <summary>
    /// Pushes the painter's current values back into every widget (painter -> HUD), e.g. after a
    /// pipette pick changed them outside the UI. No listeners are added here: setting a toggle/slider
    /// value fires its existing change listener, which keeps the painter, value labels and panel-fade
    /// alphas in sync; the colour pickers update silently via SetColor (it does not raise Changed).
    /// </summary>
    private void SyncFromPainter()
    {
        if (!_panel || !painter) return;
        var panel = _panel.transform;

        SetToggle(panel, ToggleAlbedo, painter.paintAlbedo);
        SetToggle(panel, ToggleMetallicSmoothness, painter.paintMetallicSmoothness);
        SetToggle(panel, ToggleOcclusion, painter.paintOcclusion);
        SetToggle(panel, ToggleEmission, painter.paintEmission);
        SetToggle(panel, ToggleSpray, painter.spray);
        SetToggle(panel, TogglePipette, painter.pipette);

        SetPicker(panel, PickerAlbedo, painter.brushColor);
        SetPicker(panel, PickerEmission, painter.emissionColor);

        SetSlider(panel, SliderMetallic, painter.metallic);
        SetSlider(panel, SliderSmoothness, painter.smoothness);
        SetSlider(panel, SliderMetallicSmoothnessAlpha, painter.metallicSmoothnessOpacity);
        SetSlider(panel, SliderOcclusion, painter.occlusion);
        SetSlider(panel, SliderOcclusionAlpha, painter.occlusionOpacity);
        SetSlider(panel, SliderEmissionIntensity, painter.emissionIntensity);
        SetSlider(panel, SliderBrushSize, painter.brushSize);

        SetSlider(panel, SliderSprayDots, painter.sprayDots);
        SetSlider(panel, SliderSprayDotSizeMin, painter.sprayDotSizeMin);
        SetSlider(panel, SliderSprayDotSizeMax, painter.sprayDotSizeMax);
        SetSlider(panel, SliderSprayDotAlphaMin, painter.sprayDotAlphaMin);
        SetSlider(panel, SliderSprayDotAlphaMax, painter.sprayDotAlphaMax);

        RefreshMaskHighlights();
    }

    private static void SetToggle(Transform panel, string name, bool value)
    {
        var toggle = Find<Toggle>(panel, name);
        if (toggle) toggle.isOn = value; // only fires onValueChanged when it actually changes
    }

    private static void SetSlider(Transform panel, string name, float value)
    {
        var cell = FindDeep(panel, name);
        var slider = cell ? cell.GetComponentInChildren<Slider>() : null;
        if (slider) slider.value = value; // listener refreshes the value label + painter
    }

    private static void SetPicker(Transform panel, string name, Color color)
    {
        var picker = Find<HsvColorPicker>(panel, name);
        if (picker) picker.SetColor(color);
    }

    // Active = accent frame, inactive = subtle frame. Selection lives on the painter (activeBrushIndex);
    // the swatch click only sets it - the painter resolves the new mask on its next Update.
    private static readonly Color MaskFrameSelected = new Color(0.30f, 0.62f, 1f, 1f);
    private static readonly Color MaskFrameUnselected = new Color(0.18f, 0.18f, 0.21f, 1f);

    /// <summary>
    /// Binds the baked mask swatches (one per <c>MaskSwatch_i</c>) to the painter's brush masks:
    /// each swatch previews <c>painter.brushMasks[i]</c> and selects it on click. A swatch with no
    /// matching mask is hidden, so the prefab can carry more swatches than the painter has masks.
    /// </summary>
    private void WireMaskSelector(Transform panel)
    {
        var selector = FindDeep(panel, MaskSelector);
        if (!selector) return;
        var masks = painter ? painter.brushMasks : null;

        for (int i = 0; ; i++)
        {
            var swatch = FindDeep(selector, MaskSwatchPrefix + i);
            if (!swatch) break;

            bool has = masks != null && i < masks.Count && masks[i];
            swatch.gameObject.SetActive(has);
            if (!has) continue;

            var preview = swatch.Find("Preview");
            var previewImg = preview ? preview.GetComponent<Image>() : null;
            if (previewImg) previewImg.sprite = MaskSprite(masks[i]);

            var btn = swatch.GetComponent<Button>();
            if (btn)
            {
                int idx = i; // capture per swatch
                btn.onClick.RemoveAllListeners();
                btn.onClick.AddListener(() => SelectMask(idx));
            }
        }

        RefreshMaskHighlights();
    }

    private void SelectMask(int index)
    {
        if (painter) painter.activeBrushIndex = index;
        RefreshMaskHighlights();
    }

    private void RefreshMaskHighlights()
    {
        if (!_panel || !painter) return;
        var selector = FindDeep(_panel.transform, MaskSelector);
        if (!selector) return;

        for (int i = 0; ; i++)
        {
            var swatch = FindDeep(selector, MaskSwatchPrefix + i);
            if (!swatch) break;
            var frame = swatch.GetComponent<Image>();
            if (frame) frame.color = i == painter.activeBrushIndex ? MaskFrameSelected : MaskFrameUnselected;
        }
    }

    // One sprite per mask texture, reused across rewires (display only - no CPU readback needed).
    private static readonly System.Collections.Generic.Dictionary<Texture2D, Sprite> _maskSprites
        = new System.Collections.Generic.Dictionary<Texture2D, Sprite>();

    private static Sprite MaskSprite(Texture2D tex)
    {
        if (!tex) return null;
        if (_maskSprites.TryGetValue(tex, out var cached) && cached) return cached;
        var s = Sprite.Create(tex, new Rect(0f, 0f, tex.width, tex.height), new Vector2(0.5f, 0.5f), 100f);
        s.hideFlags = HideFlags.HideAndDontSave;
        _maskSprites[tex] = s;
        return s;
    }

    /// <summary>
    /// Attaches (idempotently) a <see cref="DraggablePanel"/> to the whole panel so it can be dragged
    /// from any of its non-interactive surface (header, padding, labels, background); the component
    /// ignores presses that start on a widget (toggle, slider, colour picker).
    /// </summary>
    private static void MakeDraggable(Transform root, string panelName)
    {
        var panel = FindDeep(root, panelName) as RectTransform;
        if (!panel) return;
        var drag = panel.GetComponent<DraggablePanel>();
        if (!drag) drag = panel.gameObject.AddComponent<DraggablePanel>();
        drag.target = panel;
    }

    private void EnsureFonts(Transform panel)
    {
        foreach (var t in panel.GetComponentsInChildren<Text>(true))
            if (!t.font) t.font = _font;
    }

    private static void WireToggle(Transform panel, string name, bool value, Action<bool> onChanged)
    {
        var toggle = Find<Toggle>(panel, name);
        if (!toggle) return;
        toggle.isOn = value;
        toggle.onValueChanged.AddListener(v => onChanged(v));
    }

    /// <summary>
    /// Ensures a <see cref="CanvasGroup"/> on the channel panel and drives its alpha from the toggle:
    /// 1 while the channel is enabled, 0.5 while disabled. Call after <see cref="WireToggle"/> so the
    /// initial alpha reflects the toggle's painter-synced state. The group stays interactive (default
    /// interactable/blocksRaycasts), so a dimmed panel can still be clicked to re-enable its channel.
    /// </summary>
    private static void WirePanelFade(Transform root, string panelName, string toggleName)
    {
        var panel = FindDeep(root, panelName);
        if (!panel) return;
        var group = panel.GetComponent<CanvasGroup>();
        if (!group) group = panel.gameObject.AddComponent<CanvasGroup>();

        var toggle = Find<Toggle>(panel, toggleName);
        if (!toggle) { group.alpha = 1f; return; }

        group.alpha = toggle.isOn ? 1f : 0.5f;
        toggle.onValueChanged.AddListener(v => group.alpha = v ? 1f : 0.5f);
    }

    private static void WireSlider(Transform panel, string name, float value, Action<float> onChanged)
    {
        var cell = FindDeep(panel, name);
        if (!cell) return;
        var slider = cell.GetComponentInChildren<Slider>();
        if (!slider) return;
        var valueRt = cell.Find("Value");
        var valueText = valueRt ? valueRt.GetComponent<TMPro.TMP_Text>() : null;

        bool whole = slider.wholeNumbers;
        slider.value = value;

        Action<float> setText = v =>
        {
            if (valueText) valueText.text = whole ? Mathf.RoundToInt(v).ToString() : v.ToString("0.00");
        };
        setText(slider.value); // show the actual (clamped) value, not the unclamped input
        slider.onValueChanged.AddListener(v => { setText(v); onChanged(v); });
    }

    private void WirePicker(Transform panel, string name, Color color, Action<Color> onChanged)
    {
        var picker = Find<HsvColorPicker>(panel, name);
        if (!picker) return;
        picker.Rebind(); // re-find children, re-assign runtime gradient sprites, re-hook drag relays
        picker.SetColor(color);
        picker.Changed += onChanged;
    }

    private static T Find<T>(Transform panel, string name) where T : Component
    {
        var child = FindDeep(panel, name);
        return child ? child.GetComponent<T>() : null;
    }

    /// <summary>Depth-first search by name (widgets now live nested inside per-channel sub-panels).</summary>
    private static Transform FindDeep(Transform root, string name)
    {
        if (root.name == name) return root;
        for (int i = 0; i < root.childCount; i++)
        {
            var found = FindDeep(root.GetChild(i), name);
            if (found) return found;
        }
        return null;
    }

    // --- Static UI factory (shared with HsvColorPicker) ---

    public static RectTransform NewRect(string name, Transform parent)
    {
        var go = new GameObject(name, typeof(RectTransform));
        go.transform.SetParent(parent, false);
        return (RectTransform)go.transform;
    }

    public static Image AddImage(GameObject go, Color color)
    {
        var img = go.AddComponent<Image>();
        img.color = color;
        return img;
    }

    /// <summary>Adds a full-rect, non-raycasting overlay image (sprite assigned by the caller).</summary>
    public static Image AddOverlay(RectTransform parent, string name)
    {
        var rt = NewRect(name, parent);
        rt.anchorMin = Vector2.zero;
        rt.anchorMax = Vector2.one;
        rt.offsetMin = Vector2.zero;
        rt.offsetMax = Vector2.zero;
        var img = rt.gameObject.AddComponent<Image>();
        img.type = Image.Type.Simple;
        img.raycastTarget = false;
        return img;
    }

    // Cached gradient sprites for the colour picker.
    private static Sprite _satSprite, _valSprite, _hueSprite, _alphaSprite;

    /// <summary>Horizontal white-out: opaque white (left) -> transparent (right) = saturation.</summary>
    public static Sprite SaturationSprite()
    {
        if (_satSprite) return _satSprite;
        const int n = 64;
        var tex = NewGradientTex(n, 1);
        for (int x = 0; x < n; x++)
        {
            byte a = (byte)Mathf.RoundToInt(255f * (1f - x / (float)(n - 1)));
            tex.SetPixel(x, 0, new Color32(255, 255, 255, a));
        }
        tex.Apply(false);
        _satSprite = ToSprite(tex);
        return _satSprite;
    }

    /// <summary>Vertical black-out: opaque black (bottom) -> transparent (top) = value.</summary>
    public static Sprite ValueSprite()
    {
        if (_valSprite) return _valSprite;
        const int n = 64;
        var tex = NewGradientTex(1, n);
        for (int y = 0; y < n; y++)
        {
            byte a = (byte)Mathf.RoundToInt(255f * (1f - y / (float)(n - 1)));
            tex.SetPixel(0, y, new Color32(0, 0, 0, a));
        }
        tex.Apply(false);
        _valSprite = ToSprite(tex);
        return _valSprite;
    }

    /// <summary>Horizontal white transparency ramp: transparent (left) -> opaque (right) = alpha.
    /// Tinted to the current colour by the picker so it reads as a colour-over-backdrop fade.</summary>
    public static Sprite AlphaSprite()
    {
        if (_alphaSprite) return _alphaSprite;
        const int n = 64;
        var tex = NewGradientTex(n, 1);
        for (int x = 0; x < n; x++)
        {
            byte a = (byte)Mathf.RoundToInt(255f * (x / (float)(n - 1)));
            tex.SetPixel(x, 0, new Color32(255, 255, 255, a));
        }
        tex.Apply(false);
        _alphaSprite = ToSprite(tex);
        return _alphaSprite;
    }

    /// <summary>Horizontal full-saturation hue ramp 0..1.</summary>
    public static Sprite HueSprite()
    {
        if (_hueSprite) return _hueSprite;
        const int n = 64;
        var tex = NewGradientTex(n, 1);
        for (int x = 0; x < n; x++)
            tex.SetPixel(x, 0, Color.HSVToRGB(x / (float)(n - 1), 1f, 1f));
        tex.Apply(false);
        _hueSprite = ToSprite(tex);
        return _hueSprite;
    }

    private static Texture2D NewGradientTex(int w, int h)
    {
        return new Texture2D(w, h, TextureFormat.RGBA32, false, false) // sRGB sampling for correct colours
        {
            name = "PaintHud_Gradient",
            wrapMode = TextureWrapMode.Clamp,
            filterMode = FilterMode.Bilinear,
            hideFlags = HideFlags.HideAndDontSave
        };
    }

    private static Sprite ToSprite(Texture2D tex)
    {
        var s = Sprite.Create(tex, new Rect(0f, 0f, tex.width, tex.height), new Vector2(0.5f, 0.5f), 100f);
        s.hideFlags = HideFlags.HideAndDontSave;
        return s;
    }
}