using System;
using System.Numerics;

namespace AcDream.App.UI;

/// <summary>
/// Generic scrollbar. Ports retail <c>UIElement_Scrollbar</c>
/// (RegisterElementClass(0xb) @ acclient_2013_pseudo_c.txt:124137);
/// thumb size = trackLen * ThumbRatio (min 8px); step ±1 line.
/// </summary>
/// <remarks>
/// Dat element ids (chat LayoutDesc 0x21000006): track 0x10000012 (X=474 Y=6 W=16 H=68),
/// thumb 0x1000048C. The track is instanced from base layout 0x2100003E which contains
/// the full scrollbar widget with distinct up/down button children:
///   Up button   element 0x10000071 — Y=0,  16×16, Normal sprite 0x06004C69.
///   Down button element 0x10000072 — Y=32, 16×16, Normal sprite 0x06004C6C.
/// Track body sprite: 0x06004C5F (48px tall in the base template; stretched to H=68 in chat).
/// Thumb is a 3-slice: top cap 0x06004C60, middle 0x06004C63, bottom cap 0x06004C66.
/// For Task H wiring: up/down regions occupy the top and bottom ButtonH (16px) of the
/// rendered scrollbar's height; the widget responds to those regions directly via hit
/// comparison in OnEvent without requiring separate child elements.
/// </remarks>
public sealed class UiScrollbar : UiElement
{
    /// <summary>The scroll model this bar reflects + drives (shared with the transcript).</summary>
    public UiScrollable? Model { get; set; }

    /// <summary>RenderSurface id → (GL tex, w, h). 0 id = skip.</summary>
    public Func<uint, (uint tex, int w, int h)>? SpriteResolve { get; set; }

    /// <summary>Track background sprite id (0x06004C5F from layout 0x2100003E element 0x10000455).</summary>
    public uint TrackSprite { get; set; }

    /// <summary>Thumb 3-slice MIDDLE tile sprite id (0x06004C63), tiled between the caps.</summary>
    public uint ThumbSprite { get; set; }

    /// <summary>Thumb 3-slice TOP cap sprite id (0x06004C60, 3px tall).</summary>
    public uint ThumbTopSprite { get; set; }

    /// <summary>Thumb 3-slice BOTTOM cap sprite id (0x06004C66, 3px tall).</summary>
    public uint ThumbBotSprite { get; set; }

    /// <summary>Up-arrow button sprite id (0x06004C69 Normal state, element 0x10000071).</summary>
    public uint UpSprite { get; set; }

    /// <summary>Down-arrow button sprite id (0x06004C6C Normal state, element 0x10000072).</summary>
    public uint DownSprite { get; set; }

    /// <summary>Retail attribute 0x89 floor: minimum thumb height in pixels.</summary>
    private const float MinThumb = 8f;

    /// <summary>Thumb cap height (native sprite height from base layout 0x2100003E).</summary>
    private const float CapH = 3f;

    /// <summary>Up/down button height in pixels. Matches element height 16px from
    /// the up/down button children in base layout 0x2100003E.</summary>
    private const float ButtonH = 16f;

    private bool _draggingThumb;
    private float _dragOffsetY;

    public UiScrollbar() { CapturesPointerDrag = true; }

    /// <summary>
    /// Computes the thumb rectangle (local y origin and height) within the track area
    /// between the two end buttons. Ports retail <c>UIElement_Scrollbar::UpdateLayout
    /// @0x4710d0</c>: thumb height = max(MinThumb, trackLen * ThumbRatio); thumb top
    /// offset = trackTop + (trackLen - thumbH) * PositionRatio.
    /// </summary>
    /// <param name="m">The scroll model.</param>
    /// <param name="trackTop">Y of the top of the usable track area (below up-button).</param>
    /// <param name="trackLen">Pixel length of the usable track area (between up and down buttons).</param>
    /// <returns>Local Y of the thumb's top edge, and its pixel height.</returns>
    public static (float y, float h) ThumbRect(UiScrollable m, float trackTop, float trackLen)
    {
        float h = MathF.Max(MinThumb, trackLen * m.ThumbRatio);
        float travel = trackLen - h;
        float y = trackTop + travel * m.PositionRatio;
        return (y, h);
    }

    protected override void OnDraw(UiRenderContext ctx)
    {
        if (Model is not { } m || SpriteResolve is not { } resolve) return;

        // Track background — TILED vertically (retail DrawMode=Normal). The native track
        // sprite (~16×32) repeats to fill the element height instead of stretch-distorting.
        DrawTiled(ctx, resolve, TrackSprite, 0f, 0f, Width, Height);

        // Up button — top ButtonH rows. UpSprite (0x06004C6C) is the up-arrow art, drawn 1:1.
        DrawSprite(ctx, resolve, UpSprite, 0f, 0f, Width, ButtonH);

        // Down button — bottom ButtonH rows. DownSprite (0x06004C69) is the down-arrow art.
        DrawSprite(ctx, resolve, DownSprite, 0f, Height - ButtonH, Width, ButtonH);

        // Thumb — only when content overflows the view. Retail 3-slice: top cap +
        // tiled middle + bottom cap (base layout 0x2100003E thumb sub-elements
        // 0x10000364/65/66). Falls back to a single tiled middle if the caps are unset
        // or the thumb is too short to hold both caps.
        if (m.HasOverflow)
        {
            float trackTop = ButtonH;
            float trackLen = Height - 2f * ButtonH;
            var (ty, th) = ThumbRect(m, trackTop, trackLen);
            if (ThumbTopSprite != 0 && ThumbBotSprite != 0 && th >= 2f * CapH)
            {
                DrawSprite(ctx, resolve, ThumbTopSprite, 0f, ty, Width, CapH);
                DrawTiled(ctx, resolve, ThumbSprite, 0f, ty + CapH, Width, th - 2f * CapH);
                DrawSprite(ctx, resolve, ThumbBotSprite, 0f, ty + th - CapH, Width, CapH);
            }
            else
            {
                DrawTiled(ctx, resolve, ThumbSprite, 0f, ty, Width, th);
            }
        }
    }

    /// <summary>Draw a sprite stretched 1:1 to the dest rect.</summary>
    private void DrawSprite(UiRenderContext ctx, Func<uint, (uint tex, int w, int h)> resolve,
        uint id, float x, float y, float w, float h)
    {
        if (id == 0 || w <= 0f || h <= 0f) return;
        var (tex, _, _) = resolve(id);
        if (tex == 0) return;
        ctx.DrawSprite(tex, x, y, w, h, 0f, 0f, 1f, 1f, Vector4.One);
    }

    /// <summary>Draw a sprite 1:1 but vertically FLIPPED (V0/V1 swapped) — used to point
    /// the top scroll button's (down-art) arrow upward.</summary>
    private void DrawSpriteFlipV(UiRenderContext ctx, Func<uint, (uint tex, int w, int h)> resolve,
        uint id, float x, float y, float w, float h)
    {
        if (id == 0 || w <= 0f || h <= 0f) return;
        var (tex, _, _) = resolve(id);
        if (tex == 0) return;
        ctx.DrawSprite(tex, x, y, w, h, 0f, 1f, 1f, 0f, Vector4.One);
    }

    /// <summary>Draw a sprite TILED to fill the dest rect (UV-repeat at native size on
    /// both axes — the UI texture is GL_REPEAT-wrapped). A native-width axis gives 1:1.</summary>
    private void DrawTiled(UiRenderContext ctx, Func<uint, (uint tex, int w, int h)> resolve,
        uint id, float x, float y, float w, float h)
    {
        if (id == 0 || w <= 0f || h <= 0f) return;
        var (tex, tw, th) = resolve(id);
        if (tex == 0 || tw == 0 || th == 0) return;
        ctx.DrawSprite(tex, x, y, w, h, 0f, 0f, w / tw, h / th, Vector4.One);
    }

    public override bool OnEvent(in UiEvent e)
    {
        if (Model is not { } m) return false;

        switch (e.Type)
        {
            case UiEventType.MouseDown:
            {
                // e.Data1 = local X, e.Data2 = local Y (int pixel coords, see UiRoot hit dispatch).
                float ly = e.Data2;

                // Up-button region: top ButtonH rows.
                if (ly <= ButtonH) { m.ScrollByLines(-1); return true; }

                // Down-button region: bottom ButtonH rows.
                if (ly >= Height - ButtonH) { m.ScrollByLines(1); return true; }

                // Track interior: start a thumb drag or page-scroll.
                float trackTop = ButtonH;
                float trackLen = Height - 2f * ButtonH;
                var (ty, th) = ThumbRect(m, trackTop, trackLen);

                if (ly >= ty && ly <= ty + th)
                {
                    // Clicked inside the thumb — begin drag with offset from thumb top.
                    _draggingThumb = true;
                    _dragOffsetY = ly - ty;
                }
                else
                {
                    // Clicked above or below thumb — page scroll (HandleButtonClick page case).
                    m.ScrollByPage(ly < ty ? -1 : 1);
                }
                return true;
            }

            case UiEventType.MouseMove when _draggingThumb:
            {
                // Map current local Y (minus drag offset from thumb top) back to a
                // position ratio across the available travel distance.
                float trackTop = ButtonH;
                float trackLen = Height - 2f * ButtonH;
                float thumbH = MathF.Max(MinThumb, trackLen * m.ThumbRatio);
                float travel = MathF.Max(1f, trackLen - thumbH);
                float newRatio = ((float)e.Data2 - _dragOffsetY - trackTop) / travel;
                m.SetPositionRatio(newRatio);
                return true;
            }

            case UiEventType.MouseUp:
                _draggingThumb = false;
                return true;
        }

        return false;
    }
}