// This code is based on http://objectlistview.sourceforge.net (GPLv3 tree/list controls 
// by phillip.piper@gmail.com). Phillip has explicitly granted permission for his design
// and code to be used in this library under the MIT license.

using System.Collections.ObjectModel;

namespace Terminal.Gui;

/// <summary>
///     Interface for all non-generic members of <see cref="TreeView{T}"/>.
///     <a href="../docs/treeview.md">See TreeView Deep Dive for more information</a>.
/// </summary>
public interface ITreeView
{
    /// <summary>Contains options for changing how the tree is rendered.</summary>
    TreeStyle Style { get; set; }

    /// <summary>Removes all objects from the tree and clears selection.</summary>
    void ClearObjects ();

    /// <summary>Sets a flag indicating this view needs to be redisplayed because its state has changed.</summary>
    void SetNeedsDisplay ();
}

/// <summary>
///     Convenience implementation of generic <see cref="TreeView{T}"/> for any tree were all nodes implement
///     <see cref="ITreeNode"/>. <a href="../docs/treeview.md">See TreeView Deep Dive for more information</a>.
/// </summary>
public class TreeView : TreeView<ITreeNode>
{
    /// <summary>
    ///     Creates a new instance of the tree control with absolute positioning and initialises
    ///     <see cref="TreeBuilder{T}"/> with default <see cref="ITreeNode"/> based builder.
    /// </summary>
    public TreeView ()
    {
        TreeBuilder = new TreeNodeBuilder ();
        AspectGetter = o => o is null ? "Null" : o.Text ?? o?.ToString () ?? "Unamed Node";
    }
}

/// <summary>
///     Hierarchical tree view with expandable branches. Branch objects are dynamically determined when expanded using
///     a user defined <see cref="ITreeBuilder{T}"/>.
///     <a href="../docs/treeview.md">See TreeView Deep Dive for more information</a>.
/// </summary>
public class TreeView<T> : View, ITreeView where T : class
{
    /// <summary>
    ///     Error message to display when the control is not properly initialized at draw time (nodes added but no tree
    ///     builder set).
    /// </summary>
    public static string NoBuilderError = "ERROR: TreeBuilder Not Set";

    /// <summary>
    ///     Interface for filtering which lines of the tree are displayed e.g. to provide text searching.  Defaults to
    ///     <see langword="null"/> (no filtering).
    /// </summary>
    public ITreeViewFilter<T> Filter = null;

    /// <summary>Secondary selected regions of tree when <see cref="MultiSelect"/> is true.</summary>
    private readonly Stack<TreeSelection<T>> multiSelectedRegions = new ();

    /// <summary>Cached result of <see cref="BuildLineMap"/></summary>
    private IReadOnlyCollection<Branch<T>> cachedLineMap;

    private KeyCode objectActivationKey = KeyCode.Enter;
    private int scrollOffsetHorizontal;
    private int scrollOffsetVertical;

    /// <summary>private variable for <see cref="SelectedObject"/></summary>
    private T selectedObject;

    /// <summary>
    ///     Creates a new tree view with absolute positioning. Use <see cref="AddObjects(IEnumerable{T})"/> to set set
    ///     root objects for the tree. Children will not be rendered until you set <see cref="TreeBuilder"/>.
    /// </summary>
    public TreeView ()
    {
        CanFocus = true;

        // Things this view knows how to do
        AddCommand (
                    Command.PageUp,
                    () =>
                    {
                        MovePageUp ();

                        return true;
                    }
                   );

        AddCommand (
                    Command.PageDown,
                    () =>
                    {
                        MovePageDown ();

                        return true;
                    }
                   );

        AddCommand (
                    Command.PageUpExtend,
                    () =>
                    {
                        MovePageUp (true);

                        return true;
                    }
                   );

        AddCommand (
                    Command.PageDownExtend,
                    () =>
                    {
                        MovePageDown (true);

                        return true;
                    }
                   );

        AddCommand (
                    Command.Expand,
                    () =>
                    {
                        Expand ();

                        return true;
                    }
                   );

        AddCommand (
                    Command.ExpandAll,
                    () =>
                    {
                        ExpandAll (SelectedObject);

                        return true;
                    }
                   );

        AddCommand (
                    Command.Collapse,
                    () =>
                    {
                        CursorLeft (false);

                        return true;
                    }
                   );

        AddCommand (
                    Command.CollapseAll,
                    () =>
                    {
                        CursorLeft (true);

                        return true;
                    }
                   );

        AddCommand (
                    Command.LineUp,
                    () =>
                    {
                        AdjustSelection (-1);

                        return true;
                    }
                   );

        AddCommand (
                    Command.LineUpExtend,
                    () =>
                    {
                        AdjustSelection (-1, true);

                        return true;
                    }
                   );

        AddCommand (
                    Command.LineUpToFirstBranch,
                    () =>
                    {
                        AdjustSelectionToBranchStart ();

                        return true;
                    }
                   );

        AddCommand (
                    Command.LineDown,
                    () =>
                    {
                        AdjustSelection (1);

                        return true;
                    }
                   );

        AddCommand (
                    Command.LineDownExtend,
                    () =>
                    {
                        AdjustSelection (1, true);

                        return true;
                    }
                   );

        AddCommand (
                    Command.LineDownToLastBranch,
                    () =>
                    {
                        AdjustSelectionToBranchEnd ();

                        return true;
                    }
                   );

        AddCommand (
                    Command.TopHome,
                    () =>
                    {
                        GoToFirst ();

                        return true;
                    }
                   );

        AddCommand (
                    Command.BottomEnd,
                    () =>
                    {
                        GoToEnd ();

                        return true;
                    }
                   );

        AddCommand (
                    Command.SelectAll,
                    () =>
                    {
                        SelectAll ();

                        return true;
                    }
                   );

        AddCommand (
                    Command.ScrollUp,
                    () =>
                    {
                        ScrollUp ();

                        return true;
                    }
                   );

        AddCommand (
                    Command.ScrollDown,
                    () =>
                    {
                        ScrollDown ();

                        return true;
                    }
                   );

        AddCommand (Command.Select, ActivateSelectedObjectIfAny);
        AddCommand (Command.Accept, ActivateSelectedObjectIfAny);

        // Default keybindings for this view
        KeyBindings.Add (Key.PageUp, Command.PageUp);
        KeyBindings.Add (Key.PageDown, Command.PageDown);
        KeyBindings.Add (Key.PageUp.WithShift, Command.PageUpExtend);
        KeyBindings.Add (Key.PageDown.WithShift, Command.PageDownExtend);
        KeyBindings.Add (Key.CursorRight, Command.Expand);
        KeyBindings.Add (Key.CursorRight.WithCtrl, Command.ExpandAll);
        KeyBindings.Add (Key.CursorLeft, Command.Collapse);
        KeyBindings.Add (Key.CursorLeft.WithCtrl, Command.CollapseAll);

        KeyBindings.Add (Key.CursorUp, Command.LineUp);
        KeyBindings.Add (Key.CursorUp.WithShift, Command.LineUpExtend);
        KeyBindings.Add (Key.CursorUp.WithCtrl, Command.LineUpToFirstBranch);

        KeyBindings.Add (Key.CursorDown, Command.LineDown);
        KeyBindings.Add (Key.CursorDown.WithShift, Command.LineDownExtend);
        KeyBindings.Add (Key.CursorDown.WithCtrl, Command.LineDownToLastBranch);

        KeyBindings.Add (Key.Home, Command.TopHome);
        KeyBindings.Add (Key.End, Command.BottomEnd);
        KeyBindings.Add (Key.A.WithCtrl, Command.SelectAll);
        KeyBindings.Add (ObjectActivationKey, Command.Select);
    }

    /// <summary>
    ///     Initialises <see cref="TreeBuilder"/>.Creates a new tree view with absolute positioning. Use
    ///     <see cref="AddObjects(IEnumerable{T})"/> to set set root objects for the tree.
    /// </summary>
    public TreeView (ITreeBuilder<T> builder) : this () { TreeBuilder = builder; }

    /// <summary>True makes a letter key press navigate to the next visible branch that begins with that letter/digit.</summary>
    /// <value></value>
    public bool AllowLetterBasedNavigation { get; set; } = true;

    /// <summary>
    ///     Returns the string representation of model objects hosted in the tree. Default implementation is to call
    ///     <see cref="object.ToString"/>.
    /// </summary>
    /// <value></value>
    public AspectGetterDelegate<T> AspectGetter { get; set; } = o => o.ToString () ?? "";

    /// <summary>
    ///     Delegate for multi colored tree views. Return the <see cref="ColorScheme"/> to use for each passed object or
    ///     null to use the default.
    /// </summary>
    public Func<T, ColorScheme> ColorGetter { get; set; }

    /// <summary>The current number of rows in the tree (ignoring the controls bounds).</summary>
    public int ContentHeight => BuildLineMap ().Count ();

    /// <summary>
    ///     Gets the <see cref="CollectionNavigator"/> that searches the <see cref="Objects"/> collection as the user
    ///     types.
    /// </summary>
    public CollectionNavigator KeystrokeNavigator { get; } = new ();

    /// <summary>Maximum number of nodes that can be expanded in any given branch.</summary>
    public int MaxDepth { get; set; } = 100;

    /// <summary>True to allow multiple objects to be selected at once.</summary>
    /// <value></value>
    public bool MultiSelect { get; set; } = true;

    /// <summary>
    ///     Mouse event to trigger <see cref="TreeView{T}.ObjectActivated"/>. Defaults to double click (
    ///     <see cref="MouseFlags.Button1DoubleClicked"/>). Set to null to disable this feature.
    /// </summary>
    /// <value></value>
    public MouseFlags? ObjectActivationButton { get; set; } = MouseFlags.Button1DoubleClicked;

    // TODO: Update to use Key instead of KeyCode
    /// <summary>Key which when pressed triggers <see cref="TreeView{T}.ObjectActivated"/>. Defaults to Enter.</summary>
    public KeyCode ObjectActivationKey
    {
        get => objectActivationKey;
        set
        {
            if (objectActivationKey != value)
            {
                KeyBindings.Replace (ObjectActivationKey, value);
                objectActivationKey = value;
            }
        }
    }

    /// <summary>The root objects in the tree, note that this collection is of root objects only.</summary>
    public IEnumerable<T> Objects => roots.Keys;

    /// <summary>The amount of tree view that has been scrolled to the right (horizontally).</summary>
    /// <remarks>
    ///     Setting a value of less than 0 will result in a offset of 0. To see changes in the UI call
    ///     <see cref="View.SetNeedsDisplay()"/>.
    /// </remarks>
    public int ScrollOffsetHorizontal
    {
        get => scrollOffsetHorizontal;
        set => scrollOffsetHorizontal = Math.Max (0, value);
    }

    /// <summary>The amount of tree view that has been scrolled off the top of the screen (by the user scrolling down).</summary>
    /// <remarks>
    ///     Setting a value of less than 0 will result in a offset of 0. To see changes in the UI call
    ///     <see cref="View.SetNeedsDisplay()"/>.
    /// </remarks>
    public int ScrollOffsetVertical
    {
        get => scrollOffsetVertical;
        set => scrollOffsetVertical = Math.Max (0, value);
    }

    /// <summary>
    ///     The currently selected object in the tree. When <see cref="MultiSelect"/> is true this is the object at which
    ///     the cursor is at.
    /// </summary>
    public T SelectedObject
    {
        get => selectedObject;
        set
        {
            T oldValue = selectedObject;
            selectedObject = value;

            if (!ReferenceEquals (oldValue, value))
            {
                OnSelectionChanged (new SelectionChangedEventArgs<T> (this, oldValue, value));
            }
        }
    }

    /// <summary>Determines how sub branches of the tree are dynamically built at runtime as the user expands root nodes.</summary>
    /// <value></value>
    public ITreeBuilder<T> TreeBuilder { get; set; }

    /// <summary>
    ///     Map of root objects to the branches under them. All objects have a <see cref="Branch{T}"/> even if that branch
    ///     has no children.
    /// </summary>
    internal Dictionary<T, Branch<T>> roots { get; set; } = new ();

    /// <summary>Contains options for changing how the tree is rendered.</summary>
    public TreeStyle Style { get; set; } = new ();

    /// <summary>Removes all objects from the tree and clears <see cref="SelectedObject"/>.</summary>
    public void ClearObjects ()
    {
        SelectedObject = default (T);
        multiSelectedRegions.Clear ();
        roots = new Dictionary<T, Branch<T>> ();
        InvalidateLineMap ();
        SetNeedsDisplay ();
    }

    /// <summary>
    ///     <para>Triggers the <see cref="ObjectActivated"/> event with the <see cref="SelectedObject"/>.</para>
    ///     <para>This method also ensures that the selected object is visible.</para>
    /// </summary>
    /// <returns><see langword="true"/> if <see cref="ObjectActivated"/> was fired.</returns>
    public bool? ActivateSelectedObjectIfAny ()
    {
        // By default, Command.Accept calls OnAccept, so we need to call it here to ensure that the event is fired.
        if (OnAccept () == true)
        {
            return true;
        }

        T o = SelectedObject;

        if (o is { })
        {
            // TODO: Should this be cancelable?
            ObjectActivatedEventArgs<T> e = new (this, o);
            OnObjectActivated (e);
            return true;
        }
        return false;
    }

    /// <summary>Adds a new root level object unless it is already a root of the tree.</summary>
    /// <param name="o"></param>
    public void AddObject (T o)
    {
        if (!roots.ContainsKey (o))
        {
            roots.Add (o, new Branch<T> (this, null, o));
            InvalidateLineMap ();
            SetNeedsDisplay ();
        }
    }

    /// <summary>Adds many new root level objects. Objects that are already root objects are ignored.</summary>
    /// <param name="collection">Objects to add as new root level objects.</param>
    /// .\
    public void AddObjects (IEnumerable<T> collection)
    {
        var objectsAdded = false;

        foreach (T o in collection)
        {
            if (!roots.ContainsKey (o))
            {
                roots.Add (o, new Branch<T> (this, null, o));
                objectsAdded = true;
            }
        }

        if (objectsAdded)
        {
            InvalidateLineMap ();
            SetNeedsDisplay ();
        }
    }

    /// <summary>
    ///     The number of screen lines to move the currently selected object by. Supports negative values.
    ///     <paramref name="offset"/>. Each branch occupies 1 line on screen.
    /// </summary>
    /// <remarks>
    ///     If nothing is currently selected or the selected object is no longer in the tree then the first object in the
    ///     tree is selected instead.
    /// </remarks>
    /// <param name="offset">Positive to move the selection down the screen, negative to move it up</param>
    /// <param name="expandSelection">
    ///     True to expand the selection (assuming <see cref="MultiSelect"/> is enabled). False to
    ///     replace.
    /// </param>
    public void AdjustSelection (int offset, bool expandSelection = false)
    {
        // if it is not a shift click or we don't allow multi select
        if (!expandSelection || !MultiSelect)
        {
            multiSelectedRegions.Clear ();
        }

        if (SelectedObject is null)
        {
            SelectedObject = roots.Keys.FirstOrDefault ();
        }
        else
        {
            IReadOnlyCollection<Branch<T>> map = BuildLineMap ();

            int idx = map.IndexOf (b => b.Model.Equals (SelectedObject));

            if (idx == -1)
            {
                // The current selection has disapeared!
                SelectedObject = roots.Keys.FirstOrDefault ();
            }
            else
            {
                int newIdx = Math.Min (Math.Max (0, idx + offset), map.Count - 1);

                Branch<T> newBranch = map.ElementAt (newIdx);

                // If it is a multi selection
                if (expandSelection && MultiSelect)
                {
                    if (multiSelectedRegions.Any ())
                    {
                        // expand the existing head selection
                        TreeSelection<T> head = multiSelectedRegions.Pop ();
                        multiSelectedRegions.Push (new TreeSelection<T> (head.Origin, newIdx, map));
                    }
                    else
                    {
                        // or start a new multi selection region
                        multiSelectedRegions.Push (new TreeSelection<T> (map.ElementAt (idx), newIdx, map));
                    }
                }

                SelectedObject = newBranch.Model;

                EnsureVisible (SelectedObject);
            }
        }

        SetNeedsDisplay ();
    }

    /// <summary>Moves the selection to the last child in the currently selected level.</summary>
    public void AdjustSelectionToBranchEnd ()
    {
        T o = SelectedObject;

        if (o is null)
        {
            return;
        }

        IReadOnlyCollection<Branch<T>> map = BuildLineMap ();

        int currentIdx = map.IndexOf (b => Equals (b.Model, o));

        if (currentIdx == -1)
        {
            return;
        }

        Branch<T> currentBranch = map.ElementAt (currentIdx);
        Branch<T> next = currentBranch;

        for (; currentIdx < map.Count; currentIdx++)
        {
            //if it is the end of the current depth of branch
            if (currentBranch.Depth != next.Depth)
            {
                SelectedObject = currentBranch.Model;
                EnsureVisible (currentBranch.Model);
                SetNeedsDisplay ();

                return;
            }

            // look at next branch for consideration
            currentBranch = next;
            next = map.ElementAt (currentIdx);
        }

        GoToEnd ();
    }

    /// <summary>Moves the selection to the first child in the currently selected level.</summary>
    public void AdjustSelectionToBranchStart ()
    {
        T o = SelectedObject;

        if (o is null)
        {
            return;
        }

        IReadOnlyCollection<Branch<T>> map = BuildLineMap ();

        int currentIdx = map.IndexOf (b => Equals (b.Model, o));

        if (currentIdx == -1)
        {
            return;
        }

        Branch<T> currentBranch = map.ElementAt (currentIdx);
        Branch<T> next = currentBranch;

        for (; currentIdx >= 0; currentIdx--)
        {
            //if it is the beginning of the current depth of branch
            if (currentBranch.Depth != next.Depth)
            {
                SelectedObject = currentBranch.Model;
                EnsureVisible (currentBranch.Model);
                SetNeedsDisplay ();

                return;
            }

            // look at next branch up for consideration
            currentBranch = next;
            next = map.ElementAt (currentIdx);
        }

        // We ran all the way to top of tree
        GoToFirst ();
    }

    /// <summary>
    ///     <para>Moves the <see cref="SelectedObject"/> to the next item that begins with <paramref name="character"/>.</para>
    ///     <para>This method will loop back to the start of the tree if reaching the end without finding a match.</para>
    /// </summary>
    /// <param name="character">The first character of the next item you want selected.</param>
    /// <param name="caseSensitivity">Case sensitivity of the search.</param>
    public void AdjustSelectionToNextItemBeginningWith (
        char character,
        StringComparison caseSensitivity = StringComparison.CurrentCultureIgnoreCase
    )
    {
        // search for next branch that begins with that letter
        var characterAsStr = character.ToString ();
        AdjustSelectionToNext (b => AspectGetter (b.Model).StartsWith (characterAsStr, caseSensitivity));
    }

    /// <summary>
    ///     Returns true if the given object <paramref name="o"/> is exposed in the tree and can be expanded otherwise
    ///     false.
    /// </summary>
    /// <param name="o"></param>
    /// <returns></returns>
    public bool CanExpand (T o) { return ObjectToBranch (o)?.CanExpand () ?? false; }

    /// <summary>Collapses the <see cref="SelectedObject"/></summary>
    public void Collapse () { Collapse (selectedObject); }

    /// <summary>Collapses the supplied object if it is currently expanded .</summary>
    /// <param name="toCollapse">The object to collapse.</param>
    public void Collapse (T toCollapse) { CollapseImpl (toCollapse, false); }

    /// <summary>
    ///     Collapses the supplied object if it is currently expanded. Also collapses all children branches (this will
    ///     only become apparent when/if the user expands it again).
    /// </summary>
    /// <param name="toCollapse">The object to collapse.</param>
    public void CollapseAll (T toCollapse) { CollapseImpl (toCollapse, true); }

    /// <summary>Collapses all root nodes in the tree.</summary>
    public void CollapseAll ()
    {
        foreach (KeyValuePair<T, Branch<T>> item in roots)
        {
            item.Value.Collapse ();
        }

        InvalidateLineMap ();
        SetNeedsDisplay ();
    }

    /// <summary>
    ///     Called once for each visible row during rendering.  Can be used to make last minute changes to color or text
    ///     rendered
    /// </summary>
    public event EventHandler<DrawTreeViewLineEventArgs<T>> DrawLine;

    /// <summary>
    ///     Adjusts the <see cref="ScrollOffsetVertical"/> to ensure the given <paramref name="model"/> is visible. Has no
    ///     effect if already visible.
    /// </summary>
    public void EnsureVisible (T model)
    {
        IReadOnlyCollection<Branch<T>> map = BuildLineMap ();

        int idx = map.IndexOf (b => Equals (b.Model, model));

        if (idx == -1)
        {
            return;
        }

        /*this -1 allows for possible horizontal scroll bar in the last row of the control*/
        int leaveSpace = Style.LeaveLastRow ? 1 : 0;

        if (idx < ScrollOffsetVertical)
        {
            //if user has scrolled up too far to see their selection
            ScrollOffsetVertical = idx;
        }
        else if (idx >= ScrollOffsetVertical + Viewport.Height - leaveSpace)
        {
            //if user has scrolled off bottom of visible tree
            ScrollOffsetVertical = Math.Max (0, idx + 1 - (Viewport.Height - leaveSpace));
        }
    }

    /// <summary>Expands the currently <see cref="SelectedObject"/>.</summary>
    public void Expand () { Expand (SelectedObject); }

    /// <summary>
    ///     Expands the supplied object if it is contained in the tree (either as a root object or as an exposed branch
    ///     object).
    /// </summary>
    /// <param name="toExpand">The object to expand.</param>
    public void Expand (T toExpand)
    {
        if (toExpand is null)
        {
            return;
        }

        ObjectToBranch (toExpand)?.Expand ();
        InvalidateLineMap ();
        SetNeedsDisplay ();
    }

    /// <summary>Expands the supplied object and all child objects.</summary>
    /// <param name="toExpand">The object to expand.</param>
    public void ExpandAll (T toExpand)
    {
        if (toExpand is null)
        {
            return;
        }

        ObjectToBranch (toExpand)?.ExpandAll ();
        InvalidateLineMap ();
        SetNeedsDisplay ();
    }

    /// <summary>
    ///     Fully expands all nodes in the tree, if the tree is very big and built dynamically this may take a while (e.g.
    ///     for file system).
    /// </summary>
    public void ExpandAll ()
    {
        foreach (KeyValuePair<T, Branch<T>> item in roots)
        {
            item.Value.ExpandAll ();
        }

        InvalidateLineMap ();
        SetNeedsDisplay ();
    }

    /// <summary>
    ///     Returns <see cref="SelectedObject"/> (if not null) and all multi selected objects if <see cref="MultiSelect"/>
    ///     is true
    /// </summary>
    /// <returns></returns>
    public IEnumerable<T> GetAllSelectedObjects ()
    {
        IReadOnlyCollection<Branch<T>> map = BuildLineMap ();

        // To determine multi selected objects, start with the line map, that avoids yielding 
        // hidden nodes that were selected then the parent collapsed e.g. programmatically or
        // with mouse click
        if (MultiSelect)
        {
            foreach (T m in map.Select (b => b.Model).Where (IsSelected))
            {
                yield return m;
            }
        }
        else
        {
            if (SelectedObject is { })
            {
                yield return SelectedObject;
            }
        }
    }

    /// <summary>
    ///     Returns the currently expanded children of the passed object. Returns an empty collection if the branch is not
    ///     exposed or not expanded.
    /// </summary>
    /// <param name="o">An object in the tree.</param>
    /// <returns></returns>
    public IEnumerable<T> GetChildren (T o)
    {
        Branch<T> branch = ObjectToBranch (o);

        if (branch is null || !branch.IsExpanded)
        {
            return new T [0];
        }

        return branch.ChildBranches?.Values?.Select (b => b.Model)?.ToArray () ?? new T [0];
    }

    /// <summary>Returns the maximum width line in the tree including prefix and expansion symbols.</summary>
    /// <param name="visible">
    ///     True to consider only rows currently visible (based on window bounds and
    ///     <see cref="ScrollOffsetVertical"/>. False to calculate the width of every exposed branch in the tree.
    /// </param>
    /// <returns></returns>
    public int GetContentWidth (bool visible)
    {
        IReadOnlyCollection<Branch<T>> map = BuildLineMap ();

        if (map.Count == 0)
        {
            return 0;
        }

        if (visible)
        {
            //Somehow we managed to scroll off the end of the control
            if (ScrollOffsetVertical >= map.Count)
            {
                return 0;
            }

            // If control has no height to it then there is no visible area for content
            if (Viewport.Height == 0)
            {
                return 0;
            }

            return map.Skip (ScrollOffsetVertical).Take (Viewport.Height).Max (b => b.GetWidth (Driver));
        }

        return map.Max (b => b.GetWidth (Driver));
    }

    /// <summary>
    ///     Returns the object in the tree list that is currently visible. at the provided row. Returns null if no object
    ///     is at that location.
    ///     <remarks></remarks>
    ///     If you have screen coordinates then use <see cref="View.ScreenToFrame"/> to translate these into the client area of
    ///     the <see cref="TreeView{T}"/>.
    /// </summary>
    /// <param name="row">The row of the <see cref="View.Viewport"/> of the <see cref="TreeView{T}"/>.</param>
    /// <returns>The object currently displayed on this row or null.</returns>
    public T GetObjectOnRow (int row) { return HitTest (row)?.Model; }

    /// <summary>
    ///     <para>
    ///         Returns the Y coordinate within the <see cref="View.Viewport"/> of the tree at which <paramref name="toFind"/>
    ///         would be displayed or null if it is not currently exposed (e.g. its parent is collapsed).
    ///     </para>
    ///     <para>
    ///         Note that the returned value can be negative if the TreeView is scrolled down and the
    ///         <paramref name="toFind"/> object is off the top of the view.
    ///     </para>
    /// </summary>
    /// <param name="toFind"></param>
    /// <returns></returns>
    public int? GetObjectRow (T toFind)
    {
        int idx = BuildLineMap ().IndexOf (o => o.Model.Equals (toFind));

        if (idx == -1)
        {
            return null;
        }

        return idx - ScrollOffsetVertical;
    }

    /// <summary>
    ///     Returns the parent object of <paramref name="o"/> in the tree. Returns null if the object is not exposed in
    ///     the tree.
    /// </summary>
    /// <param name="o">An object in the tree.</param>
    /// <returns></returns>
    public T GetParent (T o) { return ObjectToBranch (o)?.Parent?.Model; }

    /// <summary>
    ///     Returns the index of the object <paramref name="o"/> if it is currently exposed (it's parent(s) have been
    ///     expanded). This can be used with <see cref="ScrollOffsetVertical"/> and <see cref="View.SetNeedsDisplay()"/> to
    ///     scroll to a specific object.
    /// </summary>
    /// <remarks>Uses the Equals method and returns the first index at which the object is found or -1 if it is not found.</remarks>
    /// <param name="o">An object that appears in your tree and is currently exposed.</param>
    /// <returns>The index the object was found at or -1 if it is not currently revealed or not in the tree at all.</returns>
    public int GetScrollOffsetOf (T o)
    {
        IReadOnlyCollection<Branch<T>> map = BuildLineMap ();

        for (var i = 0; i < map.Count; i++)
        {
            if (map.ElementAt (i).Model.Equals (o))
            {
                return i;
            }
        }

        //object not found
        return -1;
    }

    /// <summary>
    ///     Changes the <see cref="SelectedObject"/> to <paramref name="toSelect"/> and scrolls to ensure it is visible.
    ///     Has no effect if <paramref name="toSelect"/> is not exposed in the tree (e.g. its parents are collapsed).
    /// </summary>
    /// <param name="toSelect"></param>
    public void GoTo (T toSelect)
    {
        if (ObjectToBranch (toSelect) is null)
        {
            return;
        }

        SelectedObject = toSelect;
        EnsureVisible (toSelect);
        SetNeedsDisplay ();
    }

    /// <summary>Changes the <see cref="SelectedObject"/> to the last object in the tree and scrolls so that it is visible.</summary>
    public void GoToEnd ()
    {
        IReadOnlyCollection<Branch<T>> map = BuildLineMap ();
        ScrollOffsetVertical = Math.Max (0, map.Count - Viewport.Height + 1);
        SelectedObject = map.LastOrDefault ()?.Model;

        SetNeedsDisplay ();
    }

    /// <summary>
    ///     Changes the <see cref="SelectedObject"/> to the first root object and resets the
    ///     <see cref="ScrollOffsetVertical"/> to 0.
    /// </summary>
    public void GoToFirst ()
    {
        ScrollOffsetVertical = 0;
        SelectedObject = roots.Keys.FirstOrDefault ();

        SetNeedsDisplay ();
    }

    /// <summary>Clears any cached results of the tree state.</summary>
    public void InvalidateLineMap () { cachedLineMap = null; }

    /// <summary>Returns true if the given object <paramref name="o"/> is exposed in the tree and expanded otherwise false.</summary>
    /// <param name="o"></param>
    /// <returns></returns>
    public bool IsExpanded (T o) { return ObjectToBranch (o)?.IsExpanded ?? false; }

    /// <summary>
    ///     Returns true if the <paramref name="model"/> is either the <see cref="SelectedObject"/> or part of a
    ///     <see cref="MultiSelect"/>.
    /// </summary>
    /// <param name="model"></param>
    /// <returns></returns>
    public bool IsSelected (T model) { return Equals (SelectedObject, model) || (MultiSelect && multiSelectedRegions.Any (s => s.Contains (model))); }

    ///<inheritdoc/>
    protected internal override bool OnMouseEvent (MouseEvent me)
    {
        // If it is not an event we care about
        if (!me.Flags.HasFlag (MouseFlags.Button1Clicked)
            && !me.Flags.HasFlag (ObjectActivationButton ?? MouseFlags.Button1DoubleClicked)
            && !me.Flags.HasFlag (MouseFlags.WheeledDown)
            && !me.Flags.HasFlag (MouseFlags.WheeledUp)
            && !me.Flags.HasFlag (MouseFlags.WheeledRight)
            && !me.Flags.HasFlag (MouseFlags.WheeledLeft))
        {
            // do nothing
            return base.OnMouseEvent (me);
        }

        if (!HasFocus && CanFocus)
        {
            SetFocus ();
        }

        if (me.Flags == MouseFlags.WheeledDown)
        {
            ScrollDown ();

            return true;
        }

        if (me.Flags == MouseFlags.WheeledUp)
        {
            ScrollUp ();

            return true;
        }

        if (me.Flags == MouseFlags.WheeledRight)
        {
            ScrollOffsetHorizontal++;
            SetNeedsDisplay ();

            return true;
        }

        if (me.Flags == MouseFlags.WheeledLeft)
        {
            ScrollOffsetHorizontal--;
            SetNeedsDisplay ();

            return true;
        }

        if (me.Flags.HasFlag (MouseFlags.Button1Clicked))
        {
            // The line they clicked on a branch
            Branch<T> clickedBranch = HitTest (me.Position.Y);

            if (clickedBranch is null)
            {
                return false;
            }

            bool isExpandToggleAttempt = clickedBranch.IsHitOnExpandableSymbol (Driver, me.Position.X);

            // If we are already selected (double click)
            if (Equals (SelectedObject, clickedBranch.Model))
            {
                isExpandToggleAttempt = true;
            }

            // if they clicked on the +/- expansion symbol
            if (isExpandToggleAttempt)
            {
                if (clickedBranch.IsExpanded)
                {
                    clickedBranch.Collapse ();
                    InvalidateLineMap ();
                }
                else if (clickedBranch.CanExpand ())
                {
                    clickedBranch.Expand ();
                    InvalidateLineMap ();
                }
                else
                {
                    SelectedObject = clickedBranch.Model; // It is a leaf node
                    multiSelectedRegions.Clear ();
                }
            }
            else
            {
                // It is a first click somewhere in the current line that doesn't look like an expansion/collapse attempt
                SelectedObject = clickedBranch.Model;
                multiSelectedRegions.Clear ();
            }

            SetNeedsDisplay ();

            return true;
        }

        // If it is activation via mouse (e.g. double click)
        if (ObjectActivationButton.HasValue && me.Flags.HasFlag (ObjectActivationButton.Value))
        {
            // The line they clicked on a branch
            Branch<T> clickedBranch = HitTest (me.Position.Y);

            if (clickedBranch is null)
            {
                return false;
            }

            // Double click changes the selection to the clicked node as well as triggering
            // activation otherwise it feels wierd
            SelectedObject = clickedBranch.Model;
            SetNeedsDisplay ();

            // trigger activation event				
            OnObjectActivated (new ObjectActivatedEventArgs<T> (this, clickedBranch.Model));

            // mouse event is handled.
            return true;
        }

        return false;
    }

    /// <summary>Moves the selection down by the height of the control (1 page).</summary>
    /// <param name="expandSelection">True if the navigation should add the covered nodes to the selected current selection.</param>
    /// <exception cref="NotImplementedException"></exception>
    public void MovePageDown (bool expandSelection = false) { AdjustSelection (Viewport.Height, expandSelection); }

    /// <summary>Moves the selection up by the height of the control (1 page).</summary>
    /// <param name="expandSelection">True if the navigation should add the covered nodes to the selected current selection.</param>
    /// <exception cref="NotImplementedException"></exception>
    public void MovePageUp (bool expandSelection = false) { AdjustSelection (-Viewport.Height, expandSelection); }

    /// <summary>
    ///     This event is raised when an object is activated e.g. by double clicking or pressing
    ///     <see cref="ObjectActivationKey"/>.
    /// </summary>
    public event EventHandler<ObjectActivatedEventArgs<T>> ObjectActivated;

    ///<inheritdoc/>
    public override void OnDrawContent (Rectangle viewport)
    {
        if (roots is null)
        {
            return;
        }

        if (TreeBuilder is null)
        {
            Move (0, 0);
            Driver.AddStr (NoBuilderError);

            return;
        }

        IReadOnlyCollection<Branch<T>> map = BuildLineMap ();

        for (var line = 0; line < Viewport.Height; line++)
        {
            int idxToRender = ScrollOffsetVertical + line;

            // Is there part of the tree view to render?
            if (idxToRender < map.Count)
            {
                // Render the line
                map.ElementAt (idxToRender).Draw (Driver, ColorScheme, line, Viewport.Width);
            }
            else
            {
                // Else clear the line to prevent stale symbols due to scrolling etc
                Move (0, line);
                Driver.SetAttribute (GetNormalColor ());
                Driver.AddStr (new string (' ', Viewport.Width));
            }
        }
    }

    ///<inheritdoc/>
    public override bool OnEnter (View view)
    {
        if (SelectedObject is null && Objects.Any ())
        {
            SelectedObject = Objects.First ();
        }

        return base.OnEnter (view);
    }

    /// <inheritdoc/>
    public override bool OnProcessKeyDown (Key keyEvent)
    {
        if (!Enabled)
        {
            return false;
        }

        // BUGBUG: this should move to OnInvokingKeyBindings
        // If not a keybinding, is the key a searchable key press?
        if (CollectionNavigatorBase.IsCompatibleKey (keyEvent) && AllowLetterBasedNavigation)
        {
            IReadOnlyCollection<Branch<T>> map;

            // If there has been a call to InvalidateMap since the last time
            // we need a new one to reflect the new exposed tree state
            map = BuildLineMap ();

            // Find the current selected object within the tree
            int current = map.IndexOf (b => b.Model == SelectedObject);
            int? newIndex = KeystrokeNavigator?.GetNextMatchingItem (current, (char)keyEvent);

            if (newIndex is int && newIndex != -1)
            {
                SelectedObject = map.ElementAt ((int)newIndex).Model;
                EnsureVisible (selectedObject);
                SetNeedsDisplay ();

                return true;
            }
        }

        return false;
    }

    /// <summary>Positions the cursor at the start of the selected objects line (if visible).</summary>
    public override Point? PositionCursor ()
    {
        if (CanFocus && HasFocus && Visible && SelectedObject is { })
        {
            IReadOnlyCollection<Branch<T>> map = BuildLineMap ();
            int idx = map.IndexOf (b => b.Model.Equals (SelectedObject));

            // if currently selected line is visible
            if (idx - ScrollOffsetVertical >= 0 && idx - ScrollOffsetVertical < Viewport.Height)
            {
                Move (0, idx - ScrollOffsetVertical);

                return MultiSelect ? new (0, idx - ScrollOffsetVertical) : null ;
            }
        }
        return base.PositionCursor ();
    }

    /// <summary>
    ///     Rebuilds the tree structure for all exposed objects starting with the root objects. Call this method when you
    ///     know there are changes to the tree but don't know which objects have changed (otherwise use
    ///     <see cref="RefreshObject(T, bool)"/>).
    /// </summary>
    public void RebuildTree ()
    {
        foreach (Branch<T> branch in roots.Values)
        {
            branch.Rebuild ();
        }

        InvalidateLineMap ();
        SetNeedsDisplay ();
    }

    /// <summary>
    ///     Refreshes the state of the object <paramref name="o"/> in the tree. This will recompute children, string
    ///     representation etc.
    /// </summary>
    /// <remarks>This has no effect if the object is not exposed in the tree.</remarks>
    /// <param name="o"></param>
    /// <param name="startAtTop">
    ///     True to also refresh all ancestors of the objects branch (starting with the root). False to
    ///     refresh only the passed node.
    /// </param>
    public void RefreshObject (T o, bool startAtTop = false)
    {
        Branch<T> branch = ObjectToBranch (o);

        if (branch is { })
        {
            branch.Refresh (startAtTop);
            InvalidateLineMap ();
            SetNeedsDisplay ();
        }
    }

    /// <summary>Removes the given root object from the tree</summary>
    /// <remarks>If <paramref name="o"/> is the currently <see cref="SelectedObject"/> then the selection is cleared</remarks>
    /// .
    /// <param name="o"></param>
    public void Remove (T o)
    {
        if (roots.ContainsKey (o))
        {
            roots.Remove (o);
            InvalidateLineMap ();
            SetNeedsDisplay ();

            if (Equals (SelectedObject, o))
            {
                SelectedObject = default (T);
            }
        }
    }

    /// <summary>Scrolls the view area down a single line without changing the current selection.</summary>
    public void ScrollDown ()
    {
        if (ScrollOffsetVertical <= ContentHeight - 2)
        {
            ScrollOffsetVertical++;
            SetNeedsDisplay ();
        }
    }

    /// <summary>Scrolls the view area up a single line without changing the current selection.</summary>
    public void ScrollUp ()
    {
        if (scrollOffsetVertical > 0)
        {
            ScrollOffsetVertical--;
            SetNeedsDisplay ();
        }
    }

    /// <summary>Selects all objects in the tree when <see cref="MultiSelect"/> is enabled otherwise does nothing.</summary>
    public void SelectAll ()
    {
        if (!MultiSelect)
        {
            return;
        }

        multiSelectedRegions.Clear ();

        IReadOnlyCollection<Branch<T>> map = BuildLineMap ();

        if (map.Count == 0)
        {
            return;
        }

        multiSelectedRegions.Push (new TreeSelection<T> (map.ElementAt (0), map.Count, map));
        SetNeedsDisplay ();

        OnSelectionChanged (new SelectionChangedEventArgs<T> (this, SelectedObject, SelectedObject));
    }

    /// <summary>Called when the <see cref="SelectedObject"/> changes.</summary>
    public event EventHandler<SelectionChangedEventArgs<T>> SelectionChanged;

    /// <summary>
    ///     Implementation of <see cref="Collapse(T)"/> and <see cref="CollapseAll(T)"/>. Performs operation and updates
    ///     selection if disapeared.
    /// </summary>
    /// <param name="toCollapse"></param>
    /// <param name="all"></param>
    protected void CollapseImpl (T toCollapse, bool all)
    {
        if (toCollapse is null)
        {
            return;
        }

        Branch<T> branch = ObjectToBranch (toCollapse);

        // Nothing to collapse
        if (branch is null)
        {
            return;
        }

        if (all)
        {
            branch.CollapseAll ();
        }
        else
        {
            branch.Collapse ();
        }

        if (SelectedObject is { } && ObjectToBranch (SelectedObject) is null)
        {
            // If the old selection suddenly became invalid then clear it
            SelectedObject = null;
        }

        InvalidateLineMap ();
        SetNeedsDisplay ();
    }

    /// <summary>
    ///     Determines systems behaviour when the left arrow key is pressed. Default behaviour is to collapse the current
    ///     tree node if possible otherwise changes selection to current branches parent.
    /// </summary>
    protected virtual void CursorLeft (bool ctrl)
    {
        if (IsExpanded (SelectedObject))
        {
            if (ctrl)
            {
                CollapseAll (SelectedObject);
            }
            else
            {
                Collapse (SelectedObject);
            }
        }
        else
        {
            T parent = GetParent (SelectedObject);

            if (parent is { })
            {
                SelectedObject = parent;
                AdjustSelection (0);
                SetNeedsDisplay ();
            }
        }
    }

    /// <inheritdoc/>
    protected override void Dispose (bool disposing)
    {
        base.Dispose (disposing);

        ColorGetter = null;
    }

    /// <summary>Raises the <see cref="ObjectActivated"/> event.</summary>
    /// <param name="e"></param>
    protected virtual void OnObjectActivated (ObjectActivatedEventArgs<T> e) { ObjectActivated?.Invoke (this, e); }

    /// <summary>Raises the SelectionChanged event.</summary>
    /// <param name="e"></param>
    protected virtual void OnSelectionChanged (SelectionChangedEventArgs<T> e) { SelectionChanged?.Invoke (this, e); }

    /// <summary>
    ///     Calculates all currently visible/expanded branches (including leafs) and outputs them by index from the top of
    ///     the screen.
    /// </summary>
    /// <remarks>
    ///     Index 0 of the returned array is the first item that should be visible in the top of the control, index 1 is
    ///     the next etc.
    /// </remarks>
    /// <returns></returns>
    internal IReadOnlyCollection<Branch<T>> BuildLineMap ()
    {
        if (cachedLineMap is { })
        {
            return cachedLineMap;
        }

        List<Branch<T>> toReturn = new ();

        foreach (Branch<T> root in roots.Values)
        {
            IEnumerable<Branch<T>> toAdd = AddToLineMap (root, false, out bool isMatch);

            if (isMatch)
            {
                toReturn.AddRange (toAdd);
            }
        }

        cachedLineMap = new ReadOnlyCollection<Branch<T>> (toReturn);

        // Update the collection used for search-typing
        KeystrokeNavigator.Collection = cachedLineMap.Select (b => AspectGetter (b.Model)).ToArray ();

        return cachedLineMap;
    }

    /// <summary>Raises the DrawLine event</summary>
    /// <param name="e"></param>
    internal void OnDrawLine (DrawTreeViewLineEventArgs<T> e) { DrawLine?.Invoke (this, e); }

    private IEnumerable<Branch<T>> AddToLineMap (Branch<T> currentBranch, bool parentMatches, out bool match)
    {
        bool weMatch = IsFilterMatch (currentBranch);
        var anyChildMatches = false;

        List<Branch<T>> toReturn = new ();
        List<Branch<T>> children = new ();

        if (currentBranch.IsExpanded)
        {
            foreach (Branch<T> subBranch in currentBranch.ChildBranches.Values)
            {
                foreach (Branch<T> sub in AddToLineMap (subBranch, weMatch, out bool childMatch))
                {
                    if (childMatch)
                    {
                        children.Add (sub);
                        anyChildMatches = true;
                    }
                }
            }
        }

        if (parentMatches || weMatch || anyChildMatches)
        {
            match = true;
            toReturn.Add (currentBranch);
        }
        else
        {
            match = false;
        }

        toReturn.AddRange (children);

        return toReturn;
    }

    /// <summary>Sets the selection to the next branch that matches the <paramref name="predicate"/>.</summary>
    /// <param name="predicate"></param>
    private void AdjustSelectionToNext (Func<Branch<T>, bool> predicate)
    {
        IReadOnlyCollection<Branch<T>> map = BuildLineMap ();

        // empty map means we can't select anything anyway
        if (map.Count == 0)
        {
            return;
        }

        // Start searching from the first element in the map
        var idxStart = 0;

        // or the current selected branch
        if (SelectedObject is { })
        {
            idxStart = map.IndexOf (b => Equals (b.Model, SelectedObject));
        }

        // if currently selected object mysteriously vanished, search from beginning
        if (idxStart == -1)
        {
            idxStart = 0;
        }

        // loop around all indexes and back to first index
        for (int idxCur = (idxStart + 1) % map.Count; idxCur != idxStart; idxCur = (idxCur + 1) % map.Count)
        {
            if (predicate (map.ElementAt (idxCur)))
            {
                SelectedObject = map.ElementAt (idxCur).Model;
                EnsureVisible (map.ElementAt (idxCur).Model);
                SetNeedsDisplay ();

                return;
            }
        }
    }

    /// <summary>Returns the branch at the given <paramref name="y"/> client coordinate e.g. following a click event.</summary>
    /// <param name="y">Client Y position in the controls bounds.</param>
    /// <returns>The clicked branch or null if outside of tree region.</returns>
    private Branch<T> HitTest (int y)
    {
        IReadOnlyCollection<Branch<T>> map = BuildLineMap ();

        int idx = y + ScrollOffsetVertical;

        // click is outside any visible nodes
        if (idx < 0 || idx >= map.Count)
        {
            return null;
        }

        // The line they clicked on
        return map.ElementAt (idx);
    }

    private bool IsFilterMatch (Branch<T> branch) { return Filter?.IsMatch (branch.Model) ?? true; }

    /// <summary>
    ///     Returns the corresponding <see cref="Branch{T}"/> in the tree for <paramref name="toFind"/>. This will not
    ///     work for objects hidden by their parent being collapsed.
    /// </summary>
    /// <param name="toFind"></param>
    /// <returns>The branch for <paramref name="toFind"/> or null if it is not currently exposed in the tree.</returns>
    private Branch<T> ObjectToBranch (T toFind) { return BuildLineMap ().FirstOrDefault (o => o.Model.Equals (toFind)); }
}

internal class TreeSelection<T> where T : class
{
    private readonly HashSet<T> included = new ();

    /// <summary>Creates a new selection between two branches in the tree</summary>
    /// <param name="from"></param>
    /// <param name="toIndex"></param>
    /// <param name="map"></param>
    public TreeSelection (Branch<T> from, int toIndex, IReadOnlyCollection<Branch<T>> map)
    {
        Origin = from;
        included.Add (Origin.Model);

        int oldIdx = map.IndexOf (from);

        int lowIndex = Math.Min (oldIdx, toIndex);
        int highIndex = Math.Max (oldIdx, toIndex);

        // Select everything between the old and new indexes
        foreach (Branch<T> alsoInclude in map.Skip (lowIndex).Take (highIndex - lowIndex))
        {
            included.Add (alsoInclude.Model);
        }
    }

    public Branch<T> Origin { get; }
    public bool Contains (T model) { return included.Contains (model); }
}
