cursor-lang/CursorLang.Agent/Windows/NativePopupWindow.cs

using System.Drawing;
using System.Runtime.InteropServices;
using CursorLang.Agent.Interop;
using CursorLang.Agent.Services;
using CursorLang.Core.Interop;
using CursorLang.Core.Models;
using CursorLang.Core.Services;

namespace CursorLang.Agent.Windows;

/// <summary>
/// The popup with the short name of the layout, drawn by Win32 alone.
/// </summary>
/// <remarks>
/// A like-for-like replacement of the WPF popup this once was: a rounded rectangle of
/// radius 4 with 10×4 padding, the fill and the text colour from the settings, the whole
/// thing at the opacity from the settings, the name in Segoe UI SemiBold at the size
/// from the settings.
///
/// The picture is drawn into an off-screen bitmap and handed to the window whole, by
/// <c>UpdateLayeredWindow</c>. Painting on demand instead — a <c>WM_PAINT</c> after the
/// window is shown — is what the first version did, and it had the popup appear holding
/// the picture of the previous show: hiding a window does not throw its content away,
/// and the content is always the other layout. Here there is nothing to be stale,
/// because the window is never shown before its picture is in place.
///
/// It also does away with two devices the painted version needed: the corners came from
/// a window region, which cuts without antialiasing, and the opacity from
/// <c>SetLayeredWindowAttributes</c>. Both are now just pixels in the bitmap.
///
/// Responsible only for showing the popup, its size and its place on screen: when to
/// take it down is decided by <see cref="LayoutPopupService"/>.
/// </remarks>
internal sealed class NativePopupWindow : NativeWindow, ILayoutPopupWindow
{
    private const string ClassName = "CursorLang.Agent.Popup";

    // The numbers of the XAML: Border CornerRadius="4" Padding="10,4", all in WPF units
    private const double CornerRadius = 4;
    private const double PaddingX = 10;
    private const double PaddingY = 4;

    private readonly AppSettings _settings;

    private IntPtr _font;
    private double _fontSize;
    private double _fontScale;

    internal NativePopupWindow(AppSettings settings)
        : base(
            ClassName,
            "CursorLang popup",
            WindowNative.WS_POPUP,
            WindowNative.WS_EX_LAYERED | WindowNative.WS_EX_TOOLWINDOW |
            WindowNative.WS_EX_NOACTIVATE | WindowNative.WS_EX_TRANSPARENT |
            WindowNative.WS_EX_TOPMOST)
    {
        _settings = settings;
    }

    /// <summary>
    /// Shows the popup with the given text at the place set by the settings.
    /// </summary>
    public void ShowPopup(string text)
    {
        if (Handle == IntPtr.Zero)
        {
            return;
        }

        PopupWindowNative.Rect? anchor = TryGetAnchor();
        PopupModeSettings mode = ModeFor(anchor);

        PopupWindowNative.Rect work = default;
        double scale;

        if (anchor is { } at)
        {
            scale = PopupWindowNative.GetScaleAt(new PopupWindowNative.Point { X = at.Left, Y = at.Top });
        }
        else
        {
            (work, scale) = PopupWindowNative.GetActiveMonitorWorkArea();
        }

        EnsureFont(mode, scale);

        Size measured = MeasureText(text);
        int width = measured.Width + (2 * PopupLayout.ToPixels(PaddingX, scale));
        int height = measured.Height + (2 * PopupLayout.ToPixels(PaddingY, scale));

        PopupWindowNative.Point position = anchor is { } near
            ? PopupLayout.NearAnchor(
                near,
                SideForMode(),
                PopupLayout.ToPixels(mode.Offset, scale),
                width,
                height)
            : PopupLayout.OnScreen(
                work,
                _settings.FixedPoint.Position,
                PopupLayout.ToPixels(mode.Offset, scale),
                width,
                height);

        if (!Draw(mode, text, position, width, height, PopupLayout.ToPixels(CornerRadius, scale)))
        {
            return;
        }

        if (!WindowNative.IsWindowVisible(Handle))
        {
            WindowNative.ShowWindow(Handle, WindowNative.SW_SHOWNOACTIVATE);
        }
    }

    public void Hide()
    {
        if (Handle != IntPtr.Zero)
        {
            WindowNative.ShowWindow(Handle, WindowNative.SW_HIDE);
        }
    }

    /// <summary>Destroys the window. The agent only does this on the way out.</summary>
    public void Close() => Dispose();

    public override void Dispose()
    {
        ReleaseFont();
        base.Dispose();
    }

    /// <summary>
    /// Draws the popup off screen and hands the finished picture to the window.
    /// </summary>
    /// <remarks>
    /// The bitmap is thrown away afterwards rather than kept: it is a few tens of
    /// kilobytes for the length of one call, the popup is shown rarely, and a cached one
    /// would have to be rebuilt on every change of size, colour or scale anyway.
    /// </remarks>
    private bool Draw(
        PopupModeSettings mode, string text, PopupWindowNative.Point at, int width, int height, int radius)
    {
        IntPtr screen = GdiNative.GetDC(IntPtr.Zero);
        if (screen == IntPtr.Zero)
        {
            return false;
        }

        IntPtr memory = IntPtr.Zero;
        IntPtr surface = IntPtr.Zero;

        try
        {
            memory = GdiNative.CreateCompatibleDC(screen);
            if (memory == IntPtr.Zero)
            {
                return false;
            }

            surface = GdiNative.CreateSurface(memory, width, height, out IntPtr bits);
            if (surface == IntPtr.Zero)
            {
                return false;
            }

            GdiNative.SelectObject(memory, surface);

            Fill(mode, bits, width, height);
            DrawText(mode, memory, text, width, height);

            // GDI writes nothing into the alpha channel, so the letters it just drew are
            // sitting at zero alpha and would come out invisible. The inside of the
            // popup is opaque anyway, so the whole surface is simply declared so — and
            // the corners are rounded off afterwards, which is the only place alpha
            // varies
            MakeOpaque(bits, width, height);
            RoundTheCorners(bits, width, height, radius);

            var size = new WindowNative.Size { Width = width, Height = height };
            var alpha = (byte)Math.Clamp(Math.Round(mode.Opacity * 255), 0, 255);

            return WindowNative.SetContent(Handle, at, size, memory, alpha);
        }
        finally
        {
            // The context goes first: a bitmap still selected into one cannot be
            // deleted, and this way that holds however the method was left
            if (memory != IntPtr.Zero)
            {
                GdiNative.DeleteDC(memory);
            }

            if (surface != IntPtr.Zero)
            {
                GdiNative.DeleteObject(surface);
            }

            GdiNative.ReleaseDC(IntPtr.Zero, screen);
        }
    }

    private static void Fill(PopupModeSettings mode, IntPtr bits, int width, int height)
    {
        Color background = mode.BackgroundColor;

        // Straight into the bitmap rather than through a brush: the pixels have to be
        // written anyway to carry an alpha channel GDI would not touch
        int packed = (255 << 24) | (background.R << 16) | (background.G << 8) | background.B;
        var row = new int[width];
        Array.Fill(row, packed);

        for (var y = 0; y < height; y++)
        {
            Marshal.Copy(row, 0, bits + (y * width * 4), width);
        }
    }

    private void DrawText(
        PopupModeSettings mode, IntPtr deviceContext, string text, int width, int height)
    {
        if (_font == IntPtr.Zero || text.Length == 0)
        {
            return;
        }

        var bounds = new PopupWindowNative.Rect { Left = 0, Top = 0, Right = width, Bottom = height };

        IntPtr previousFont = GdiNative.SelectObject(deviceContext, _font);
        GdiNative.SetBkMode(deviceContext, GdiNative.TRANSPARENT);
        GdiNative.SetTextColor(deviceContext, GdiNative.ToColorRef(mode.ForegroundColor));

        GdiNative.DrawText(deviceContext, text, text.Length, ref bounds,
            GdiNative.DT_SINGLELINE | GdiNative.DT_CENTER | GdiNative.DT_VCENTER |
            GdiNative.DT_NOPREFIX | GdiNative.DT_NOCLIP);

        GdiNative.SelectObject(deviceContext, previousFont);
    }

    private static void MakeOpaque(IntPtr bits, int width, int height)
    {
        var row = new int[width];

        for (var y = 0; y < height; y++)
        {
            IntPtr line = bits + (y * width * 4);
            Marshal.Copy(line, row, 0, width);

            for (var x = 0; x < width; x++)
            {
                row[x] = (int)((uint)row[x] | 0xFF000000);
            }

            Marshal.Copy(row, 0, line, width);
        }
    }

    /// <summary>
    /// Cuts the four corners to a radius, fading the edge rather than stepping it.
    /// </summary>
    /// <remarks>
    /// The painted version cut them with a window region, which is a yes-or-no mask and
    /// left a visible staircase at 200% scale. Here the corner pixels carry a partial
    /// alpha worked out from how far the pixel centre is past the arc, which is what
    /// antialiasing amounts to. The colours are premultiplied to match, as
    /// <c>UpdateLayeredWindow</c> expects.
    /// </remarks>
    private static void RoundTheCorners(IntPtr bits, int width, int height, int radius)
    {
        if (radius <= 0)
        {
            return;
        }

        radius = Math.Min(radius, Math.Min(width, height) / 2);

        var row = new int[width];

        for (var y = 0; y < height; y++)
        {
            bool nearTop = y < radius;
            bool nearBottom = y >= height - radius;
            if (!nearTop && !nearBottom)
            {
                continue;
            }

            IntPtr line = bits + (y * width * 4);
            Marshal.Copy(line, row, 0, width);

            double centreY = nearTop ? radius - 0.5 : height - radius - 0.5;

            for (var x = 0; x < width; x++)
            {
                bool nearLeft = x < radius;
                bool nearRight = x >= width - radius;
                if (!nearLeft && !nearRight)
                {
                    continue;
                }

                double centreX = nearLeft ? radius - 0.5 : width - radius - 0.5;
                double distance = Math.Sqrt(
                    ((x - centreX) * (x - centreX)) + ((y - centreY) * (y - centreY)));

                // One pixel of softness across the arc: fully inside, fully outside,
                // and a ramp in between
                double coverage = Math.Clamp(radius - distance + 0.5, 0, 1);
                if (coverage >= 1)
                {
                    continue;
                }

                row[x] = Premultiply(row[x], coverage);
            }

            Marshal.Copy(row, 0, line, width);
        }
    }

    private static int Premultiply(int pixel, double coverage)
    {
        var value = (uint)pixel;
        var alpha = (uint)Math.Round(((value >> 24) & 0xFF) * coverage);

        uint red = (uint)Math.Round(((value >> 16) & 0xFF) * coverage);
        uint green = (uint)Math.Round(((value >> 8) & 0xFF) * coverage);
        uint blue = (uint)Math.Round((value & 0xFF) * coverage);

        return (int)((alpha << 24) | (red << 16) | (green << 8) | blue);
    }

    /// <summary>
    /// What the popup is placed next to, or <c>null</c> when there is nothing: the fixed
    /// point mode, and the caret mode where the application reports no caret.
    /// </summary>
    /// <remarks>
    /// The cursor is a rectangle of zero size, so the corner arithmetic is shared by it
    /// and the caret.
    /// </remarks>
    private PopupWindowNative.Rect? TryGetAnchor() => _settings.PlacementMode switch
    {
        PopupPlacementMode.FixedPoint => null,
        PopupPlacementMode.AtCaret => CaretNative.TryGetCaretRect(),
        _ => PopupLayout.AsAnchor(PopupWindowNative.GetCursorPosition()),
    };

    /// <summary>
    /// The settings the popup is shown with: those of the mode chosen, or those of the
    /// fixed point when there is no anchor to stand next to.
    /// </summary>
    /// <remarks>
    /// The caret mode falls back to the fixed point rather than to the mouse cursor:
    /// the cursor is wherever it was last left — off to a side, on another monitor, or
    /// over the very text being typed — and a popup that lands there while the eyes are
    /// on the caret is one that is looked for and not found. The fixed point is always
    /// in the same place, so it is known where to look.
    ///
    /// The look comes from the fixed point mode too, not just the place. The two are set
    /// up together for a reason: the popup by the caret is small and quiet because it
    /// sits inside a text being read, while the one in the corner of the monitor is
    /// looked for on purpose and is set larger. Keeping the caret look at the corner
    /// would put a popup meant to go unnoticed where nothing else draws the eye.
    /// </remarks>
    private PopupModeSettings ModeFor(PopupWindowNative.Rect? anchor) =>
        anchor is null ? _settings.FixedPoint : _settings.Current;

    // The caret has two sides to choose from and the cursor has six, so each mode names
    // its own side in its own terms
    private AnchorSide SideForMode() => _settings.PlacementMode == PopupPlacementMode.AtCaret
        ? _settings.AtCaret.Anchor
        : _settings.AtCursor.Side;

    private Size MeasureText(string text)
    {
        IntPtr deviceContext = GdiNative.GetDC(Handle);
        if (deviceContext == IntPtr.Zero)
        {
            return Size.Empty;
        }

        try
        {
            IntPtr previousFont = GdiNative.SelectObject(deviceContext, _font);
            Size measured = GdiNative.MeasureText(deviceContext, text);
            GdiNative.SelectObject(deviceContext, previousFont);

            return measured;
        }
        finally
        {
            GdiNative.ReleaseDC(Handle, deviceContext);
        }
    }

    // The font is rebuilt only when the size in the settings or the monitor scale
    // changes: it is the one expensive thing a show does. A switch of the placement
    // mode counts as a change of the size, since the size belongs to the mode
    private void EnsureFont(PopupModeSettings mode, double scale)
    {
        if (_font != IntPtr.Zero &&
            Math.Abs(_fontSize - mode.FontSize) < 0.01 &&
            Math.Abs(_fontScale - scale) < 0.01)
        {
            return;
        }

        ReleaseFont();

        _fontSize = mode.FontSize;
        _fontScale = scale;
        _font = GdiNative.CreateFont(_fontSize, scale);
    }

    private void ReleaseFont()
    {
        if (_font != IntPtr.Zero)
        {
            GdiNative.DeleteObject(_font);
            _font = IntPtr.Zero;
        }
    }
}