using System;
using System.Collections;

namespace ytLive.Helpers;

/// <summary>
/// The arithmetic behind dragging a row in the layer list, kept out of the window's code-behind so it
/// can be tested without a mouse. It used to live inline in the preview-mouse handlers, and the only
/// test that covered it injected REAL cursor movement (<c>SetCursorPos</c> + <c>mouse_event</c>) — which
/// took over whatever machine ran the tests and failed whenever the desktop was busy. 2026-11-05: the
/// arithmetic moved here (pure, tested), the injected-input test was deleted.
/// </summary>
public static class DragReorderMath
{
    /// <summary>True once the pointer has travelled far enough to mean "drag", not "click". WPF's
    /// own thresholds are the caller's (<c>SystemParameters.MinimumHorizontal/VerticalDragDistance</c>).</summary>
    public static bool ExceededThreshold(double dx, double dy, double horizontal, double vertical)
        => Math.Abs(dx) >= horizontal || Math.Abs(dy) >= vertical;

    /// <summary>Moves one item to the position currently occupied by another. Returns the index the
    /// item actually landed on (the caller keeps dragging from there), or -1 when the move is a no-op
    /// or the indices are out of range.
    /// <para>Order matters: the item is removed first, so a downward move inserts at the target's old
    /// index and lands exactly under the pointer — this is the behaviour the mouse test used to pin.</para>
    /// </summary>
    public static int TryMove(IList items, int from, int to)
    {
        if (items == null || from < 0 || to < 0 || from >= items.Count || to >= items.Count) return -1;
        if (from == to) return -1;

        var moved = items[from]!;
        items.RemoveAt(from);
        items.Insert(to, moved);
        return to;
    }
}