using System;
using System.Collections.Generic;
using System.Numerics;
using System.Text;
using AcDream.App.Rendering;
using AcDream.App.UI.Layout;

namespace AcDream.App.UI;

/// <summary>
/// Scrollable text view for retail UIElement_Text elements
/// (<c>RegisterElementClass(0xc) @ acclient_2013_pseudo_c.txt:115655</c>).
/// Renders the lines from <see cref="LinesProvider"/> bottom-pinned (newest at the bottom,
/// like retail) with mouse-wheel scrollback. Whole-line vertical clipping keeps
/// text inside the window.
///
/// <para>
/// When retail Selectable property `0x27` is enabled, left-click-drag selects
/// characters, Ctrl+C copies the selected span, and Ctrl+A selects everything.
/// Display-only text remains click-through and cannot steal focus or window drag.
/// </para>
/// </summary>
public sealed class UiText : UiElement, IUiDatStateful
{
    /// <summary>Optional base-element click notice used by authored text tabs.</summary>
    public Action? OnClick { get; set; }
    public override bool HandlesClick => OnClick is not null || base.HandlesClick;
    /// <summary>Dat element id for imported UIElement_Text widgets. 0 for synthesized text.</summary>
    public uint ElementId { get; set; }

    /// <summary>One display line: pre-formatted text + its colour.</summary>
    public readonly record struct Line(string Text, Vector4 Color);

    /// <summary>A caret position: a line index into the cached line list plus a
    /// character index (0..line.Text.Length, i.e. a caret slot between glyphs).</summary>
    public readonly record struct Pos(int Line, int Col);

    /// <summary>Provider of the lines to show, oldest-first. Polled each frame.</summary>
    public Func<IReadOnlyList<Line>> LinesProvider { get; set; } = static () => Array.Empty<Line>();

    /// <summary>Font for the transcript; falls back to the context default.</summary>
    public BitmapFont? Font { get; set; }

    /// <summary>Retail dat font (0x40000000) for the transcript. When set, glyphs
    /// render via the two-pass dat-font blit and measure/hit-test use the dat glyph
    /// advance; when null, the debug BitmapFont path is used. Set by the controller.</summary>
    public UiDatFont? DatFont { get; set; }

    /// <summary>Keyboard device for clipboard (Ctrl+C) + modifier state. Wired by
    /// the host from <see cref="UiHost.Keyboard"/>.</summary>
    public Silk.NET.Input.IKeyboard? Keyboard { get; set; }

    /// <summary>
    /// Default line color used by controllers when they do not supply a per-line
    /// <see cref="Vector4"/> color explicitly.  Set by <c>DatWidgetFactory.BuildText</c>
    /// from <c>ElementInfo.FontColor</c> when the dat carries a 0x1B ColorBaseProperty;
    /// otherwise white (<see cref="Vector4.One"/>).
    ///
    /// <para>Controllers that supply a per-line color via <see cref="LinesProvider"/>
    /// (e.g. <c>new UiText.Line(text, explicitColor)</c>) are unaffected — they always
    /// win over this default.  This property is only a convenience starting point for
    /// controllers that want to read the dat color rather than hard-code it.</para>
    /// </summary>
    public Vector4 DefaultColor { get; set; } = Vector4.One;

    /// <summary>Backing fill behind the text. Defaults to transparent so an unbound
    /// UiText (no controller) draws nothing. Set to the retail translucent value by
    /// the controller (e.g. <c>ChatWindowController</c>).</summary>
    public Vector4 BackgroundColor { get; set; } = new(0f, 0f, 0f, 0f);

    /// <summary>Optional dat state-sprite background (the element's own media), drawn
    /// UNDER the text. Set by DatWidgetFactory.BuildText from the ElementInfo. 0 = none.</summary>
    public uint BackgroundSprite { get; set; }

    /// <summary>Resolves a dat RenderSurface id to (GL tex handle, pixel width, pixel height).
    /// Required when <see cref="BackgroundSprite"/> is non-zero.</summary>
    public Func<uint, (uint tex, int w, int h)>? SpriteResolve { get; set; }

    /// <summary>Highlight colour painted behind a selected character span.</summary>
    public Vector4 SelectionColor { get; set; } = new(0.25f, 0.45f, 0.85f, 0.5f);

    /// <summary>
    /// Uniform inner text inset in pixels. The retail default is zero:
    /// <c>UIElement_Text::UIElement_Text @ 0x00468570</c> clears all four margins
    /// before applying any authored margin properties.
    /// </summary>
    public float Padding { get; set; }

    /// <summary>Retail property 0x20. Independent of horizontal/vertical
    /// justification; false permits the normal multi-line layout path.</summary>
    public bool OneLine { get; set; }

    private bool _selectable;

    /// <summary>
    /// Retail property 0x27. A display-only text element does not claim the mouse,
    /// focus, or pointer drag. Selection and clipboard behavior are enabled only
    /// when this capability is true.
    /// </summary>
    public bool Selectable
    {
        get => _selectable;
        set
        {
            if (_selectable == value) return;
            _selectable = value;
            ClickThrough = !value;
            AcceptsFocus = value;
            IsEditControl = value;
            CapturesPointerDrag = value;
            if (!value)
            {
                _selecting = false;
                _selAnchor = null;
                _selCaret = null;
            }
        }
    }

    /// <summary>Static centered single-line mode (retail <c>UIElement_Text</c> center
    /// justification): draws the FIRST line centered horizontally AND vertically in the
    /// element rect, with NO scroll/selection machinery. Used for static labels such as
    /// the vitals cur/max numbers. The centering formula is IDENTICAL to
    /// <see cref="UiMeter"/>'s former number overlay so those numbers stay pixel-identical
    /// after the rewire. Pair with <c>ClickThrough = true</c> for non-interactive labels.</summary>
    public bool Centered { get; set; }

    /// <summary>Static right-aligned single-line mode: draws the FIRST line right-justified
    /// within the element rect, vertically centered, with NO scroll/selection machinery.
    /// Used for value labels in attribute/skill rows where the number must hug the right edge.
    /// Mutually exclusive with <see cref="Centered"/> — if both are true, Centered takes
    /// precedence. Pair with <c>ClickThrough = true</c> for non-interactive labels.</summary>
    public bool RightAligned { get; set; }

    /// <summary>
    /// Vertical position of the text within the element rect in single-line mode
    /// (<see cref="Centered"/> or <see cref="RightAligned"/>).
    /// <list type="bullet">
    ///   <item><description><b>Center</b> (default) — vertically centered, matching the original
    ///     behavior of the centered/right-aligned paths.</description></item>
    ///   <item><description><b>Top</b> — text is placed at <c>y = Padding</c> (top of the content
    ///     area), so the text sits at the top of the element rather than centering in it.
    ///     Used for footer title elements whose dat box is the full footer height (55 px) but
    ///     the text should render near the top.</description></item>
    ///   <item><description><b>Bottom</b> — text is placed at <c>y = Height - lineHeight - Padding</c>.</description></item>
    /// </list>
    /// Only meaningful when <see cref="Centered"/> or <see cref="RightAligned"/> is true.
    /// Has no effect on the scrollable multi-line path.
    /// </summary>
    public VJustify VerticalJustify { get; set; } = VJustify.Center;

    /// <summary>The scroll model — also read by the linked UiScrollbar.</summary>
    public UiScrollable Scroll { get; } = new();

    private const float WheelLines = 1f;   // lines advanced per wheel notch (retail = 1 line per notch)

    // ── Cached layout from the last OnDraw, so OnEvent hit-tests the SAME geometry ──
    private IReadOnlyList<Line> _lastLines = Array.Empty<Line>();
    private BitmapFont? _lastFont;
    private UiDatFont? _lastDatFont;
    private float _lastLineHeight = 16f;
    private float _lastBaseY;          // top Y of line 0 in local space
    private float _lastPadding;

    private ElementInfo? _datInfo;
    private uint _activeRetailStateId = UiStateInfo.DirectStateId;
    private string _activeDatStateName = "";

    // ── Selection state ──────────────────────────────────────────────────
    private Pos? _selAnchor;   // where the drag started
    private Pos? _selCaret;    // where the drag currently is
    private bool _selecting;

    public UiText()
    {
        // UIElement_Text starts display-only (m_bitField = DIRTY|CURSOR_VISIBLE).
        ClickThrough = true;
        AcceptsFocus = false;
        IsEditControl = false;
        CapturesPointerDrag = false;
    }

    /// <summary>
    /// Text widgets normally reproduce their own scroll/edit chrome. Retail tab text,
    /// however, uses <c>PassToChildren</c> states whose child elements are the authored
    /// Open/Closed cap sprites. Those children must remain in the retained tree so
    /// <see cref="TrySetRetailState"/> can propagate the state exactly as retail does.
    /// </summary>
    public override bool ConsumesDatChildren
    {
        get
        {
            if (_datInfo is null) return true;
            foreach (UiStateInfo state in _datInfo.States.Values)
                if (state.PassToChildren)
                    return false;
            return true;
        }
    }

    public uint ActiveRetailStateId => _activeRetailStateId;

    public override string ActiveCursorStateName => _activeDatStateName;

    internal void ConfigureDatState(ElementInfo info)
    {
        _datInfo = info;
        _activeRetailStateId = info.EffectiveDefaultStateId();
        ApplyDatState(_activeRetailStateId, propagate: false);
    }

    public bool TrySetRetailState(uint stateId)
        => ApplyDatState(stateId, propagate: true);

    private bool ApplyDatState(uint stateId, bool propagate)
    {
        if (_datInfo is null) return false;

        UiStateInfo? state = null;
        string stateName;
        if (stateId == UiStateInfo.DirectStateId)
        {
            if (!_datInfo.States.TryGetValue(stateId, out state)
                && !_datInfo.StateMedia.ContainsKey(""))
                return false;
            stateName = "";
        }
        else if (_datInfo.States.TryGetValue(stateId, out state))
        {
            stateName = state.Name;
        }
        else
        {
            stateName = UiButtonStateMachine.StateName(stateId);
            if (string.IsNullOrEmpty(stateName))
                stateName = RetailUiStateIds.StateName(stateId);
            if (string.IsNullOrEmpty(stateName)
                || !_datInfo.StateMedia.ContainsKey(stateName))
                return false;
        }

        _activeRetailStateId = stateId;
        _activeDatStateName = stateName;
        BackgroundSprite = _datInfo.StateMedia.TryGetValue(stateName, out var media)
            ? media.File
            : _datInfo.StateMedia.TryGetValue("", out var direct) ? direct.File : 0u;

        if (_datInfo.TryGetEffectiveProperty(0x1Bu, out UiPropertyValue color, stateId)
            && TryColor(color, out Vector4 resolvedColor))
            DefaultColor = resolvedColor;

        if (propagate && state?.PassToChildren == true)
        {
            foreach (UiElement child in Children)
                if (child is IUiDatStateful stateful)
                    stateful.TrySetRetailState(stateId);
        }

        return true;
    }

    private static bool TryColor(UiPropertyValue property, out Vector4 color)
    {
        UiPropertyValue? value = property.Kind == UiPropertyKind.Color
            ? property
            : property.Kind == UiPropertyKind.Array
                && property.ArrayValue.Count > 0
                && property.ArrayValue[0].Kind == UiPropertyKind.Color
                    ? property.ArrayValue[0]
                    : null;
        if (value is null)
        {
            color = default;
            return false;
        }

        UiColorValue c = value.ColorValue;
        float alpha = c.Alpha == 0 ? 1f : c.Alpha / 255f;
        color = new Vector4(c.Red / 255f, c.Green / 255f, c.Blue / 255f, alpha);
        return true;
    }

    /// <summary>
    /// Clamp a scroll offset to [0, max] where max = content-height - view-height
    /// (never negative — when everything fits, scroll is pinned to 0). Exposed for tests.
    /// </summary>
    public static float ClampScroll(float scroll, float contentHeight, float viewHeight)
    {
        float max = Math.Max(0f, contentHeight - viewHeight);
        if (scroll < 0f) return 0f;
        return scroll > max ? max : scroll;
    }

    protected override void OnDraw(UiRenderContext ctx)
    {
        // Optional dat state-sprite background drawn UNDER everything else.
        if (BackgroundSprite != 0 && SpriteResolve is { } sr)
        {
            var (tex, tw, th) = sr(BackgroundSprite);
            if (tex != 0 && tw != 0 && th != 0)
                ctx.DrawSprite(tex, 0, 0, Width, Height, 0, 0, Width / tw, Height / th, Vector4.One);
        }

        // Background must draw UNDER the transcript text. DrawStringDat emits into the
        // sprite bucket which flushes BEFORE rects, so a DrawRect background would wash
        // over the text. DrawFill routes the background through the sprite bucket too,
        // submitted first → text on top.
        ctx.DrawFill(0, 0, Width, Height, BackgroundColor);

        // Static centered single-line mode (vitals cur/max numbers etc.): draw the first
        // line centered H+V (or H+Top/Bottom per VerticalJustify) with the SAME formula
        // UIElement_Meter used for its label, then skip the scroll/selection machinery entirely.
        if (OneLine && Centered)
        {
            var cLines = LinesProvider();
            if (cLines.Count == 0) return;
            var line0 = cLines[0];
            if (DatFont is { } cdf)
            {
                float cx = (Width - cdf.MeasureWidth(line0.Text)) * 0.5f;
                float cy = VOffset(Height, cdf.LineHeight, Padding, VerticalJustify);
                ctx.DrawStringDat(cdf, line0.Text, cx, cy, line0.Color);
            }
            else if ((Font ?? ctx.DefaultFont) is { } cbf)
            {
                float cx = (Width - cbf.MeasureWidth(line0.Text)) * 0.5f;
                float cy = VOffset(Height, cbf.LineHeight, Padding, VerticalJustify);
                ctx.DrawString(line0.Text, cx, cy, line0.Color, cbf);
            }
            return;
        }

        // Static right-aligned single-line mode: draw the first line flush with the right
        // edge, vertical position per VerticalJustify, then skip the scroll/selection machinery.
        if (OneLine && RightAligned)
        {
            var rLines = LinesProvider();
            if (rLines.Count == 0) return;
            var line0 = rLines[0];
            if (DatFont is { } rdf)
            {
                float rx = Width - rdf.MeasureWidth(line0.Text) - Padding;
                float ry = VOffset(Height, rdf.LineHeight, Padding, VerticalJustify);
                ctx.DrawStringDat(rdf, line0.Text, rx, ry, line0.Color);
            }
            else if ((Font ?? ctx.DefaultFont) is { } rbf)
            {
                float rx = Width - rbf.MeasureWidth(line0.Text) - Padding;
                float ry = VOffset(Height, rbf.LineHeight, Padding, VerticalJustify);
                ctx.DrawString(line0.Text, rx, ry, line0.Color, rbf);
            }
            return;
        }

        // One-line is an independent text-layout property. Left justification uses
        // the same vertical policy without turning alignment into a line-count flag.
        if (OneLine)
        {
            var singleLines = LinesProvider();
            if (singleLines.Count == 0) return;
            var line0 = singleLines[0];
            if (DatFont is { } datSingle)
            {
                float y = VOffset(Height, datSingle.LineHeight, Padding, VerticalJustify);
                ctx.DrawStringDat(datSingle, line0.Text, Padding, y, line0.Color);
            }
            else if ((Font ?? ctx.DefaultFont) is { } bitmapSingle)
            {
                float y = VOffset(Height, bitmapSingle.LineHeight, Padding, VerticalJustify);
                ctx.DrawString(line0.Text, Padding, y, line0.Color, bitmapSingle);
            }
            return;
        }

        // Prefer the retail dat font when set; fall back to BitmapFont.
        var datFont = DatFont;
        var bitmapFont = datFont is null ? (Font ?? ctx.DefaultFont) : null;
        if (datFont is null && bitmapFont is null) return;

        var lines = LinesProvider();

        // Cache the geometry OnEvent will hit-test against. Even when there are no
        // lines we record the font/padding so a stray hit-test is harmless.
        _lastLines = lines;
        _lastDatFont = datFont;
        _lastFont = bitmapFont;
        _lastLineHeight = datFont is not null ? datFont.LineHeight : bitmapFont!.LineHeight;
        _lastPadding = Padding;

        if (lines.Count == 0) return;

        float lh = _lastLineHeight;
        float top = Padding, bottom = Height - Padding;
        float innerH = bottom - top;
        float contentH = lines.Count * lh;

        // Drive the shared scroll model with the current geometry.
        Scroll.LineHeight = (int)MathF.Round(lh);
        Scroll.SetExtents(
            (int)MathF.Ceiling(contentH),
            (int)MathF.Floor(innerH),
            preserveEnd: true);

        // UiScrollable: ScrollY=0 is TOP/oldest, ScrollY=MaxScroll is BOTTOM/newest.
        // Visual layout: newest at bottom → baseY = bottom - contentH (ScrollY at max).
        // Invert: baseY = bottom - contentH + (MaxScroll - ScrollY).
        // With _pinBottom: ScrollY=MaxScroll → baseY=bottom-contentH → last line ends at bottom. ✓
        // Scrolled to top:  ScrollY=0 → baseY=bottom-contentH+MaxScroll=bottom-innerH=top. ✓
        float baseY = bottom - contentH + (Scroll.MaxScroll - Scroll.ScrollY);
        _lastBaseY = baseY;

        // Normalised selection span (start <= end), if any.
        bool hasSel = TryGetOrderedSelection(out Pos selStart, out Pos selEnd);

        for (int i = 0; i < lines.Count; i++)
        {
            float y = baseY + i * lh;
            if (y < top || y + lh > bottom) continue;   // whole-line vertical clip (no scissor yet)

            string text = lines[i].Text;
            float lineX = HorizontalOffset(text, datFont, bitmapFont);

            // Selection highlight behind this line's selected character span.
            if (hasSel && i >= selStart.Line && i <= selEnd.Line)
            {
                int c0 = i == selStart.Line ? selStart.Col : 0;
                int c1 = i == selEnd.Line ? selEnd.Col : text.Length;
                c0 = Math.Clamp(c0, 0, text.Length);
                c1 = Math.Clamp(c1, 0, text.Length);
                if (c1 > c0)
                {
                    float hx, hw;
                    if (datFont is not null)
                    {
                        hx = lineX + datFont.MeasureWidth(text.Substring(0, c0));
                        hw = datFont.MeasureWidth(text.Substring(c0, c1 - c0));
                    }
                    else
                    {
                        hx = lineX + bitmapFont!.MeasureWidth(text.Substring(0, c0));
                        hw = bitmapFont.MeasureWidth(text.Substring(c0, c1 - c0));
                    }
                    // Highlight sits BEHIND the line's text → sprite bucket, submitted
                    // before this line's DrawStringDat.
                    ctx.DrawFill(hx, y, hw, lh, SelectionColor);
                }
            }

            if (datFont is not null)
                ctx.DrawStringDat(datFont, text, lineX, y, lines[i].Color);
            else
                ctx.DrawString(text, lineX, y, lines[i].Color, bitmapFont);
        }
    }

    private float HorizontalOffset(string text, UiDatFont? datFont, BitmapFont? bitmapFont)
    {
        float width = datFont is not null
            ? datFont.MeasureWidth(text)
            : bitmapFont?.MeasureWidth(text) ?? 0f;
        if (Centered)
            return Math.Max(Padding, (Width - width) * 0.5f);
        if (RightAligned)
            return Math.Max(Padding, Width - Padding - width);
        return Padding;
    }

    public override bool OnEvent(in UiEvent e)
    {
        if (e.Type == UiEventType.Click && OnClick is not null)
        {
            OnClick();
            return true;
        }
        switch (e.Type)
        {
            case UiEventType.Scroll:
            {
                if (!Selectable) return false;
                // Silk wheel +Y = scroll up = reveal older = toward the TOP = decrease ScrollY.
                // ScrollByLines sign: +down/newer, -up/older.
                // e.Data0 > 0 → wheel up → want older → ScrollByLines with negative lines.
                Scroll.ScrollByLines((int)(-e.Data0 * WheelLines));
                return true;
            }

            case UiEventType.MouseDown:
            {
                if (!Selectable) return false;
                // Data1/Data2 = local-to-target coords (UiRoot.OnMouseDown).
                var p = HitChar(e.Data1, e.Data2);
                _selAnchor = p;
                _selCaret = p;
                _selecting = true;
                return true;
            }

            case UiEventType.MouseMove:
            {
                if (!Selectable) return false;
                if (_selecting)
                {
                    // Data1/Data2 = local-to-target coords (DispatchMouseMove).
                    _selCaret = HitChar(e.Data1, e.Data2);
                    return true;
                }
                return false;
            }

            case UiEventType.MouseUp:
            {
                if (!Selectable) return false;
                _selecting = false;
                return true;
            }

            case UiEventType.KeyDown:
            {
                if (!Selectable) return false;
                var key = (Silk.NET.Input.Key)e.Data0;
                bool ctrl = Keyboard is not null
                    && (Keyboard.IsKeyPressed(Silk.NET.Input.Key.ControlLeft)
                        || Keyboard.IsKeyPressed(Silk.NET.Input.Key.ControlRight));
                if (ctrl && key == Silk.NET.Input.Key.C)
                {
                    // Only touch the clipboard when there's a selection — an empty
                    // copy must NOT clobber what the user previously copied.
                    if (Keyboard is not null)
                    {
                        string sel = SelectedText();
                        if (sel.Length > 0) Keyboard.ClipboardText = sel;
                    }
                    return true;
                }
                if (ctrl && key == Silk.NET.Input.Key.A)
                {
                    SelectAll();
                    return true;
                }
                return false;
            }
        }
        return false;
    }

    // ── Selection helpers ────────────────────────────────────────────────

    /// <summary>Select the entire cached transcript (Ctrl+A).</summary>
    private void SelectAll()
    {
        var lines = _lastLines;
        if (lines.Count == 0)
        {
            _selAnchor = _selCaret = null;
            return;
        }
        int last = lines.Count - 1;
        _selAnchor = new Pos(0, 0);
        _selCaret = new Pos(last, lines[last].Text.Length);
    }

    /// <summary>Normalise (anchor, caret) into ordered (start, end). False if no
    /// selection or it is empty (anchor == caret).</summary>
    private bool TryGetOrderedSelection(out Pos start, out Pos end)
    {
        start = default; end = default;
        if (_selAnchor is not { } a || _selCaret is not { } c) return false;
        (start, end) = Order(a, c);
        return !(start.Line == end.Line && start.Col == end.Col);
    }

    /// <summary>The currently-selected text against the cached lines. Empty when
    /// nothing is selected.</summary>
    public string SelectedText()
    {
        if (!TryGetOrderedSelection(out var start, out var end)) return string.Empty;
        return SelectedText(_lastLines, start, end);
    }

    // ── Pure, testable logic (no GL / no font texture) ───────────────────

    /// <summary>
    /// Compute the Y offset (local space) for a single line in the Centered/RightAligned
    /// single-line path, given the element height, font line-height, padding, and
    /// vertical justification.
    /// </summary>
    /// <param name="height">Element height in pixels.</param>
    /// <param name="lineHeight">Font line height in pixels.</param>
    /// <param name="padding">Content padding.</param>
    /// <param name="vj">Vertical justification.</param>
    public static float VOffset(float height, float lineHeight, float padding, VJustify vj)
        => vj switch
        {
            VJustify.Top    => padding,
            VJustify.Bottom => height - lineHeight - padding,
            _               => (height - lineHeight) * 0.5f,   // Center (default)
        };

    /// <summary>Order two caret positions so the first is <= the second (by line,
    /// then column).</summary>
    public static (Pos start, Pos end) Order(Pos a, Pos b)
    {
        if (a.Line < b.Line || (a.Line == b.Line && a.Col <= b.Col)) return (a, b);
        return (b, a);
    }

    /// <summary>
    /// Assemble the selected substring spanning <paramref name="start"/> ..
    /// <paramref name="end"/> (inclusive of start.Col, exclusive of end.Col) from
    /// <paramref name="lines"/>. Multi-line selections are joined with "\n":
    /// the first line from start.Col to its end, whole middle lines, and the last
    /// line up to end.Col. Pure — unit-testable without GL.
    /// </summary>
    public static string SelectedText(IReadOnlyList<Line> lines, Pos start, Pos end)
    {
        if (lines.Count == 0) return string.Empty;
        (start, end) = Order(start, end);

        int sl = Math.Clamp(start.Line, 0, lines.Count - 1);
        int el = Math.Clamp(end.Line, 0, lines.Count - 1);

        if (sl == el)
        {
            string t = lines[sl].Text;
            int c0 = Math.Clamp(start.Col, 0, t.Length);
            int c1 = Math.Clamp(end.Col, 0, t.Length);
            if (c1 <= c0) return string.Empty;
            return t.Substring(c0, c1 - c0);
        }

        var sb = new StringBuilder();

        // First line: from start.Col to its end.
        {
            string t = lines[sl].Text;
            int c0 = Math.Clamp(start.Col, 0, t.Length);
            sb.Append(t.AsSpan(c0));
        }

        // Whole middle lines.
        for (int i = sl + 1; i < el; i++)
        {
            sb.Append('\n');
            sb.Append(lines[i].Text);
        }

        // Last line: up to end.Col.
        {
            sb.Append('\n');
            string t = lines[el].Text;
            int c1 = Math.Clamp(end.Col, 0, t.Length);
            sb.Append(t.AsSpan(0, c1));
        }

        return sb.ToString();
    }

    /// <summary>
    /// Convert a local-space point to a caret <see cref="Pos"/> against the cached
    /// layout from the last draw. line = floor((localY - baseY)/lineHeight) clamped
    /// to the line range; col via <see cref="CharIndexAt"/>.
    /// </summary>
    private Pos HitChar(float localX, float localY)
    {
        var lines = _lastLines;
        if (lines.Count == 0) return new Pos(0, 0);

        float lh = _lastLineHeight <= 0f ? 16f : _lastLineHeight;
        int line = (int)MathF.Floor((localY - _lastBaseY) / lh);
        line = Math.Clamp(line, 0, lines.Count - 1);

        string text = lines[line].Text;
        float lineX = HorizontalOffset(text, _lastDatFont, _lastFont);
        int col = _lastDatFont is { } df
            ? CharIndexAt(text, ch => df.TryGetGlyph(ch, out var g) ? UiDatFont.GlyphAdvance(g) : 0f,
                          localX - lineX)
            : (_lastFont is { } bf
                ? CharIndexAt(text, ch => bf.TryGetGlyph(ch, out var bg) ? bg.Advance : 0f,
                              localX - lineX)
                : 0);
        return new Pos(line, col);
    }

    /// <summary>Word-wrap text to a measured pixel width, preserving explicit newlines.</summary>
    public static IReadOnlyList<string> WrapWords(
        string text,
        Func<string, float> measureWidth,
        float maximumWidth)
    {
        ArgumentNullException.ThrowIfNull(text);
        ArgumentNullException.ThrowIfNull(measureWidth);
        if (maximumWidth <= 0f) throw new ArgumentOutOfRangeException(nameof(maximumWidth));

        var result = new List<string>();
        string[] paragraphs = text.Replace("\r", string.Empty).Split('\n');
        foreach (string paragraph in paragraphs)
        {
            if (paragraph.Length == 0)
            {
                result.Add(string.Empty);
                continue;
            }

            string[] words = paragraph.Split(' ', StringSplitOptions.RemoveEmptyEntries);
            var line = new StringBuilder();
            foreach (string word in words)
            {
                string candidate = line.Length == 0 ? word : $"{line} {word}";
                if (measureWidth(candidate) <= maximumWidth)
                {
                    if (line.Length != 0) line.Append(' ');
                    line.Append(word);
                    continue;
                }

                if (line.Length != 0 && measureWidth(word) <= maximumWidth)
                {
                    result.Add(line.ToString());
                    line.Clear();
                    line.Append(word);
                    continue;
                }

                // Retail GlyphList wrapping can split an over-width glyph run.
                // Pack as much of the long token as possible onto the current
                // line, then continue at character boundaries without hyphens.
                for (int i = 0; i < word.Length; i++)
                {
                    string prefix = i == 0 && line.Length != 0 ? " " : string.Empty;
                    if (line.Length != 0
                        && measureWidth(line + prefix + word[i]) > maximumWidth)
                    {
                        result.Add(line.ToString());
                        line.Clear();
                        prefix = string.Empty;
                    }
                    line.Append(prefix).Append(word[i]);
                }
            }

            result.Add(line.ToString());
        }

        return result;
    }

    /// <summary>
    /// The caret column for a horizontal position <paramref name="x"/> (already
    /// adjusted for the left padding, so x=0 is the start of the text). Walks the
    /// string accumulating each glyph's advance and snaps the caret to whichever
    /// side of the glyph midpoint <paramref name="x"/> falls on — natural
    /// Windows-like caret placement. Pure — unit-testable with a synthetic advance.
    /// </summary>
    /// <param name="text">The line text.</param>
    /// <param name="advanceOf">Per-character advance (pixels) lookup.</param>
    /// <param name="x">Horizontal position relative to the text's left edge.</param>
    public static int CharIndexAt(string text, Func<char, float> advanceOf, float x)
    {
        if (string.IsNullOrEmpty(text) || x <= 0f) return 0;

        float cursor = 0f;
        for (int i = 0; i < text.Length; i++)
        {
            float adv = advanceOf(text[i]);
            float mid = cursor + adv * 0.5f;
            if (x < mid) return i;            // caret sits before this glyph
            cursor += adv;
        }
        return text.Length;                   // past the last glyph → end caret
    }
}