// This code is based on http://objectlistview.sourceforge.net (GPLv3 tree/list controls by phillip.piper@gmail.com)

using System;
using System.Collections.Generic;
using System.Linq;

namespace Terminal.Gui {

	/// <summary>
	/// Hierarchical tree view with expandable branches.  Branch objects are dynamically determined when expanded using a user defined <see cref="ChildrenGetterDelegate"/>
	/// </summary>
	public class TreeView : View
	{   
		/// <summary>
		/// Default implementation of a <see cref="ChildrenGetterDelegate"/>, returns an empty collection (i.e. no children)
		/// </summary>
		static ChildrenGetterDelegate DefaultChildrenGetter = (s)=>{return new object[0];};

		/// <summary>
		/// This is the delegate that will be used to fetch the children of a model object
		/// </summary>
		public ChildrenGetterDelegate ChildrenGetter {
			get { return childrenGetter ?? DefaultChildrenGetter; }
			set { childrenGetter = value; }
		}
	
		private ChildrenGetterDelegate childrenGetter;
		private CanExpandGetterDelegate canExpandGetter;

		/// <summary>
		/// Optional delegate where <see cref="ChildrenGetter"/> is expensive.  This should quickly return true/false for whether an object is expandable.  (e.g. indicating to a user that all folders can be expanded because they are folders without having to calculate contents)
		/// </summary>
		/// <remarks>When this is null <see cref="ChildrenGetter"/> is used directly to determine if a node should be expandable</remarks>
		public CanExpandGetterDelegate CanExpandGetter {
			get { return canExpandGetter; }
			set { canExpandGetter = value; }
		}

		/// <summary>
		/// The currently selected object in the tree
		/// </summary>
		public object SelectedObject {get;set;}

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

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

		/// <summary>
		/// The amount of tree view that has been scrolled off the top of the screen (by the user scrolling down)
		/// </summary>
		public int ScrollOffset {get; private set;}

		/// <summary>
		/// Creates a new tree view with absolute positioning.  Use <see cref="AddObjects(IEnumerable{object})"/> to set set root objects for the tree
		/// </summary>
		public TreeView ():base()
		{
			CanFocus = true;
		}

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

		/// <summary>
		/// Removes all objects from the tree and clears <see cref="SelectedObject"/>
		/// </summary>
		public void ClearObjects()
		{
			SelectedObject = null;
			roots = new Dictionary<object, Branch>();
			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(object o)
		{
			if(roots.ContainsKey(o)) {
				roots.Remove(o);
				SetNeedsDisplay();

				if(Equals(SelectedObject,o))
					SelectedObject = null;
			}
		}
		
		/// <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<object> collection)
		{
			bool objectsAdded = false;

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

		/// <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 AspectGetter {get;set;} = (o)=>o.ToString();

		///<inheritdoc/>
		public override void Redraw (Rect bounds)
		{
			if(roots == null)
				return;

			var map = BuildLineMap();

			for(int line = 0 ; line < bounds.Height; line++){

				var idxToRender = ScrollOffset + line;

				// Is there part of the tree view to render?
				if(idxToRender < map.Length) {
					// Render the line
					map[idxToRender].Draw(Driver,ColorScheme,line,bounds.Width);
				} else {

					// Else clear the line to prevent stale symbols due to scrolling etc
					Move(0,line);
					Driver.SetAttribute(ColorScheme.Normal);
					Driver.AddStr(new string(' ',bounds.Width));
				}
					
			}
		}

		/// <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>
		private Branch[] BuildLineMap()
		{
			List<Branch> toReturn = new List<Branch>();

			foreach(var root in roots.Values) {
				toReturn.AddRange(AddToLineMap(root));
			}

			return toReturn.ToArray();
		}

		private IEnumerable<Branch> AddToLineMap (Branch currentBranch)
		{
			yield return currentBranch;

			if(currentBranch.IsExpanded){

				foreach(var subBranch in currentBranch.ChildBranches.Values){
					foreach(var sub in AddToLineMap(subBranch)) {
						yield return sub;
					}
				}
			}
		}

		/// <summary>
		/// Symbol to use for expanded branch nodes to indicate to the user that they can be collapsed.  Defaults to '-'
		/// </summary>
		public char ExpandedSymbol {get;set;} = '-';

		/// <summary>
		/// Symbol to use for branch nodes that can be expanded to indicate this to the user.  Defaults to '+'
		/// </summary>
		public char ExpandableSymbol {get;set;} = '+';

		/// <summary>
		/// Symbol to use for branch nodes that cannot be expanded (as they have no children).  Defaults to space ' '
		/// </summary>
		public char LeafSymbol {get;set;} = ' ';

		/// <inheritdoc/>
		public override bool ProcessKey (KeyEvent keyEvent)
		{
			switch (keyEvent.Key) {
				case Key.CursorRight:
					Expand(SelectedObject);
				break;
				case Key.CursorLeft:
					Collapse(SelectedObject);
				break;
			
				case Key.CursorUp:
					AdjustSelection(-1);
				break;
				case Key.CursorDown:
					AdjustSelection(1);
				break;
				case Key.PageUp:
					AdjustSelection(-Bounds.Height);
				break;
				
				case Key.PageDown:
					AdjustSelection(Bounds.Height);
				break;
				case Key.Home:
					GoToFirst();
				break;
				case Key.End:
					GoToEnd();
				break;

				default:
					// we don't care about this keystroke
					return false;
			}

			PositionCursor ();
			return true;
		}

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

			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 ()
		{
			var map = BuildLineMap();
			ScrollOffset = Math.Max(0,map.Length - Bounds.Height +1);
			SelectedObject = map.Last().Model;
						
			SetNeedsDisplay();
		}

		/// <summary>
		/// Changes the selected object by a number of screen lines
		/// </summary>
		/// <remarks>If nothing is currently selected the first root is selected.  If the selected object is no longer in the tree the first object is selected</remarks>
		/// <param name="offset"></param>
		private void AdjustSelection (int offset)
		{
			if(SelectedObject == null){
				SelectedObject = roots.Keys.FirstOrDefault();
			}
			else {
				var map = BuildLineMap();

				var idx = Array.FindIndex(map,b=>b.Model.Equals(SelectedObject));

				if(idx == -1) {

					// The current selection has disapeared!
					SelectedObject = roots.Keys.FirstOrDefault();
				}
				else {
					var newIdx = Math.Min(Math.Max(0,idx+offset),map.Length-1);
					SelectedObject = map[newIdx].Model;

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

					}
				}

			}
						
			SetNeedsDisplay();
		}

		/// <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(object toExpand)
		{
			if(toExpand == null)
				return;
			
			ObjectToBranch(toExpand)?.Expand();
			SetNeedsDisplay();
		}

		/// <summary>
		/// Collapses the supplied object if it is currently expanded 
		/// </summary>
		/// <param name="toCollapse">The object to collapse</param>
		public void Collapse(object toCollapse)
		{
			if(toCollapse == null)
				return;

			ObjectToBranch(toCollapse)?.Collapse();
			SetNeedsDisplay();
		}

		/// <summary>
		/// Returns the corresponding <see cref="Branch"/> 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 ObjectToBranch(object toFind)
		{
			return BuildLineMap().FirstOrDefault(o=>o.Model.Equals(toFind));
		}
	}

	class Branch
	{
		/// <summary>
		/// True if the branch is expanded to reveal child branches
		/// </summary>
		public bool IsExpanded {get;set;}

		/// <summary>
		/// The users object that is being displayed by this branch of the tree
		/// </summary>
		public object Model {get;set;}
		
		/// <summary>
		/// The depth of the current branch.  Depth of 0 indicates root level branches
		/// </summary>
		public int Depth {get;set;} = 0;

		/// <summary>
		/// The children of the current branch.  This is null until the first call to <see cref="FetchChildren"/> to avoid enumerating the entire underlying hierarchy
		/// </summary>
		public Dictionary<object,Branch> ChildBranches {get;set;}


		private TreeView tree;

		/// <summary>
		/// Declares a new branch of <paramref name="tree"/> in which the users object <paramref name="model"/> is presented
		/// </summary>
		/// <param name="tree">The UI control in which the branch resides</param>
		/// <param name="parentBranchIfAny">Pass null for root level branches, otherwise pass the parent</param>
		/// <param name="model">The user's object that should be displayed</param>
		public Branch(TreeView tree,Branch parentBranchIfAny,object model)
		{
			this.tree  = tree;
			this.Model = model;
			
			if(parentBranchIfAny != null) {
				Depth = parentBranchIfAny.Depth +1;
			}
		}


		/// <summary>
		/// Fetch the children of this branch. This method populates <see cref="ChildBranches"/>
		/// </summary>
		public virtual void FetchChildren()
		{
			if (tree.ChildrenGetter == null)
				return;

			this.ChildBranches = tree.ChildrenGetter(this.Model).ToDictionary(k=>k,val=>new Branch(tree,this,val));
		}

		/// <summary>
		/// Renders the current <see cref="Model"/> on the specified line <paramref name="y"/>
		/// </summary>
		/// <param name="driver"></param>
		/// <param name="colorScheme"></param>
		/// <param name="y"></param>
		/// <param name="availableWidth"></param>
		public virtual void Draw(ConsoleDriver driver,ColorScheme colorScheme, int y, int availableWidth)
		{
			string representation = new string(' ',Depth) + GetExpandableIcon() + tree.AspectGetter(Model);
            
			tree.Move(0,y);

			driver.SetAttribute(tree.SelectedObject == Model ?
				colorScheme.HotFocus :
				colorScheme.Normal);

			driver.AddStr(representation.PadRight(availableWidth));
		}

		/// <summary>
		/// Returns an appropriate symbol for displaying next to the string representation of the <see cref="Model"/> object to indicate whether it <see cref="IsExpanded"/> or not (or it is a leaf)
		/// </summary>
		/// <returns></returns>
		public char GetExpandableIcon()
		{
			if(IsExpanded)
				return tree.ExpandedSymbol;

			if(ChildBranches == null) {
			
				//if there is a rapid method for determining whether there are children
				if(tree.CanExpandGetter != null) {
					return tree.CanExpandGetter(Model) ? tree.ExpandableSymbol : tree.LeafSymbol;
				}
				
				//there is no way of knowing whether we can expand without fetching the children
				FetchChildren();
			}

			//we fetched or already know the children, so return whether we are a leaf or a expandable branch
			return ChildBranches.Any() ? tree.ExpandableSymbol : tree.LeafSymbol;
		}

		/// <summary>
		/// Expands the current branch if possible
		/// </summary>
		public void Expand()
		{
			if(ChildBranches == null) {
				FetchChildren();
			}

			if (ChildBranches.Any ()) {
				IsExpanded = true;
			}
		}

		internal void Collapse ()
		{
			IsExpanded = false;
		}
	}
   
	/// <summary>
	/// Delegates of this type are used to fetch the children of the given model object
	/// </summary>
	/// <param name="model">The parent whose children should be fetched</param>
	/// <returns>An enumerable over the children</returns>
	public delegate IEnumerable<object> ChildrenGetterDelegate(object model);

	/// <summary>
	/// Delegates of this type are used to fetch string representations of user's model objects
	/// </summary>
	/// <param name="model"></param>
	/// <returns></returns>
	public delegate string AspectGetterDelegate(object model);

	/// <summary>
	/// Delegates of this type are used to quickly display to the user whether a given user object can be expanded when fetching it's children is expensive (e.g. indicating to a user that all 1000 folders can be expanded because they are folders without having to calculate contents)
	/// </summary>
	/// <param name="model"></param>
	/// <returns></returns>
	public delegate bool CanExpandGetterDelegate(object model);
}