using System;

namespace AcDream.App.UI;

/// <summary>
/// Pixel-based vertical scroll model. Port of retail <c>UIElement_Scrollable</c>:
/// the scroll offset is an integer pixel value (<c>m_iScrollableY</c>) clamped to
/// [0, ContentHeight - ViewHeight]; the thumb ratio is view/content; the position
/// ratio is scroll/(content-view). Pure (no GL) so it is fully unit-tested and
/// shared by the transcript (UiText) and the scrollbar (UiScrollbar).
/// Decomp anchors: SetScrollableXY @0x4740c0, UpdateScrollbarSize_ @0x4741a0,
/// UpdateScrollbarPosition_ @0x473f20, UIElement_Text::InqScrollDelta @0x4689b0.
/// </summary>
public sealed class UiScrollable
{
    /// <summary>Total wrapped content height in px (m_iScrollableHeight).</summary>
    public int ContentHeight { get; set; }
    /// <summary>Visible viewport height in px.</summary>
    public int ViewHeight { get; set; }
    /// <summary>Pixels per text line (scroll quantum). InqScrollDelta line case.</summary>
    public int LineHeight { get; set; } = 16;

    private int _scrollY;
    /// <summary>Current scroll offset in px from the top of the content.</summary>
    public int ScrollY => _scrollY;

    /// <summary>Max scroll = max(0, content - view).</summary>
    public int MaxScroll => Math.Max(0, ContentHeight - ViewHeight);

    /// <summary>True when content exceeds the view (a scrollbar is warranted).</summary>
    public bool HasOverflow => ContentHeight > ViewHeight;

    /// <summary>True when the offset is at (or past) the bottom — used for bottom-pin.</summary>
    public bool AtEnd => _scrollY >= MaxScroll;

    /// <summary>
    /// Update the content/view extents and clamp the current offset. When
    /// <paramref name="preserveEnd"/> is true, a view that was at the end before
    /// the geometry change remains at the end afterward. User-driven scrollbar
    /// changes move <see cref="ScrollY"/> off the end, so later layout passes
    /// preserve that selected position instead of snapping it back.
    /// </summary>
    public void SetExtents(int contentHeight, int viewHeight, bool preserveEnd = false)
    {
        bool wasAtEnd = AtEnd;
        ContentHeight = Math.Max(0, contentHeight);
        ViewHeight = Math.Max(0, viewHeight);

        if (preserveEnd && wasAtEnd)
            ScrollToEnd();
        else
            SetScrollY(_scrollY);
    }

    /// <summary>Set the offset, clamped to [0, MaxScroll] (SetScrollableXY clamp).</summary>
    public void SetScrollY(int y) => _scrollY = Math.Clamp(y, 0, MaxScroll);

    /// <summary>Pin to the bottom (newest content visible).</summary>
    public void ScrollToEnd() => _scrollY = MaxScroll;

    /// <summary>Thumb size ratio = view/content, clamped to 1 (UpdateScrollbarSize_).</summary>
    public float ThumbRatio => ContentHeight <= 0 ? 1f : Math.Min(1f, (float)ViewHeight / ContentHeight);

    /// <summary>Position ratio = scroll/(content-view) in [0,1] (UpdateScrollbarPosition_).</summary>
    public float PositionRatio => MaxScroll <= 0 ? 0f : (float)_scrollY / MaxScroll;

    /// <summary>Inverse of PositionRatio — used when the user drags the thumb.</summary>
    public void SetPositionRatio(float ratio)
        => SetScrollY((int)MathF.Round(Math.Clamp(ratio, 0f, 1f) * MaxScroll));

    /// <summary>Scroll by whole lines (sign: +down/newer, -up/older).</summary>
    public void ScrollByLines(int lines) => SetScrollY(_scrollY + lines * LineHeight);

    /// <summary>Scroll by a page = one view height (InqScrollDelta page case).</summary>
    public void ScrollByPage(int pages) => SetScrollY(_scrollY + pages * ViewHeight);
}