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Using AvalonEdit (WPF Text Editor)

, 1 Apr 2013
AvalonEdit is an extensible Open-Source text editor with support for syntax highlighting and folding.
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// Copyright (c) AlphaSierraPapa for the SharpDevelop Team (for details please see \doc\copyright.txt)
// This code is distributed under the GNU LGPL (for details please see \doc\license.txt)

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Text;

using ICSharpCode.AvalonEdit.Utils;

namespace ICSharpCode.AvalonEdit.Document
{
	/// <summary>
	/// A tree of TextAnchorNodes.
	/// </summary>
	sealed class TextAnchorTree
	{
		// The text anchor tree has difficult requirements:
		// - it must QUICKLY update the offset in all anchors whenever there is a document change
		// - it must not reference text anchors directly, using weak references instead
		
		// Clearly, we cannot afford updating an Offset property on all anchors (that would be O(N)).
		// So instead, the anchors need to be able to calculate their offset from a data structure
		// that can be efficiently updated.
		
		// This implementation is built using an augmented red-black-tree.
		// There is a 'TextAnchorNode' for each text anchor.
		// Such a node represents a section of text (just the length is stored) with a (weakly referenced) text anchor at the end.
		
		// Basically, you can imagine the list of text anchors as a sorted list of text anchors, where each anchor
		// just stores the distance to the previous anchor.
		// (next node = TextAnchorNode.Successor, distance = TextAnchorNode.length)
		// Distances are never negative, so this representation means anchors are always sorted by offset
		// (the order of anchors at the same offset is undefined)
		
		// Of course, a linked list of anchors would be way too slow (one would need to traverse the whole list
		// every time the offset of an anchor is being looked up).
		// Instead, we use a red-black-tree. We aren't actually using the tree for sorting - it's just a binary tree
		// as storage format for what's conceptually a list, the red-black properties are used to keep the tree balanced.
		// Other balanced binary trees would work, too.
		
		// What makes the tree-form efficient is that is augments the data by a 'totalLength'. Where 'length'
		// represents the distance to the previous node, 'totalLength' is the sum of all 'length' values in the subtree
		// under that node.
		// This allows computing the Offset from an anchor by walking up the list of parent nodes instead of going
		// through all predecessor nodes. So computing the Offset runs in O(log N).
		
		readonly TextDocument document;
		readonly List<TextAnchorNode> nodesToDelete = new List<TextAnchorNode>();
		TextAnchorNode root;
		
		public TextAnchorTree(TextDocument document)
		{
			this.document = document;
		}
		
		[Conditional("DEBUG")]
		static void Log(string text)
		{
			Debug.WriteLine("TextAnchorTree: " + text);
		}
		
		#region Insert Text
		void InsertText(int offset, int length, bool defaultAnchorMovementIsBeforeInsertion)
		{
			if (length == 0 || root == null || offset > root.totalLength)
				return;
			
			// find the range of nodes that are placed exactly at offset
			// beginNode is inclusive, endNode is exclusive
			if (offset == root.totalLength) {
				PerformInsertText(FindActualBeginNode(root.RightMost), null, length, defaultAnchorMovementIsBeforeInsertion);
			} else {
				TextAnchorNode endNode = FindNode(ref offset);
				Debug.Assert(endNode.length > 0);
				
				if (offset > 0) {
					// there are no nodes exactly at offset
					endNode.length += length;
					UpdateAugmentedData(endNode);
				} else {
					PerformInsertText(FindActualBeginNode(endNode.Predecessor), endNode, length, defaultAnchorMovementIsBeforeInsertion);
				}
			}
			DeleteMarkedNodes();
		}
		
		TextAnchorNode FindActualBeginNode(TextAnchorNode node)
		{
			// now find the actual beginNode
			while (node != null && node.length == 0)
				node = node.Predecessor;
			if (node == null) {
				// no predecessor = beginNode is first node in tree
				node = root.LeftMost;
			}
			return node;
		}
		
		// Sorts the nodes in the range [beginNode, endNode) by MovementType
		// and inserts the length between the BeforeInsertion and the AfterInsertion nodes.
		void PerformInsertText(TextAnchorNode beginNode, TextAnchorNode endNode, int length, bool defaultAnchorMovementIsBeforeInsertion)
		{
			Debug.Assert(beginNode != null);
			// endNode may be null at the end of the anchor tree
			
			// now we need to sort the nodes in the range [beginNode, endNode); putting those with
			// MovementType.BeforeInsertion in front of those with MovementType.AfterInsertion
			List<TextAnchorNode> beforeInsert = new List<TextAnchorNode>();
			//List<TextAnchorNode> afterInsert = new List<TextAnchorNode>();
			TextAnchorNode temp = beginNode;
			while (temp != endNode) {
				TextAnchor anchor = (TextAnchor)temp.Target;
				if (anchor == null) {
					// afterInsert.Add(temp);
					MarkNodeForDelete(temp);
				} else if (defaultAnchorMovementIsBeforeInsertion
				           ? anchor.MovementType != AnchorMovementType.AfterInsertion
				           : anchor.MovementType == AnchorMovementType.BeforeInsertion)
				{
					beforeInsert.Add(temp);
//				} else {
//					afterInsert.Add(temp);
				}
				temp = temp.Successor;
			}
			// now again go through the range and swap the nodes with those in the beforeInsert list
			temp = beginNode;
			foreach (TextAnchorNode node in beforeInsert) {
				SwapAnchors(node, temp);
				temp = temp.Successor;
			}
			// now temp is pointing to the first node that is afterInsert,
			// or to endNode, if there is no afterInsert node at the offset
			// So add the length to temp
			if (temp == null) {
				// temp might be null if endNode==null and no afterInserts
				Debug.Assert(endNode == null);
			} else {
				temp.length += length;
				UpdateAugmentedData(temp);
			}
		}
		
		/// <summary>
		/// Swaps the anchors stored in the two nodes.
		/// </summary>
		void SwapAnchors(TextAnchorNode n1, TextAnchorNode n2)
		{
			if (n1 != n2) {
				TextAnchor anchor1 = (TextAnchor)n1.Target;
				TextAnchor anchor2 = (TextAnchor)n2.Target;
				if (anchor1 == null && anchor2 == null) {
					// -> no swap required
					return;
				}
				n1.Target = anchor2;
				n2.Target = anchor1;
				if (anchor1 == null) {
					// unmark n1 from deletion, mark n2 for deletion
					nodesToDelete.Remove(n1);
					MarkNodeForDelete(n2);
					anchor2.node = n1;
				} else if (anchor2 == null) {
					// unmark n2 from deletion, mark n1 for deletion
					nodesToDelete.Remove(n2);
					MarkNodeForDelete(n1);
					anchor1.node = n2;
				} else {
					anchor1.node = n2;
					anchor2.node = n1;
				}
			}
		}
		#endregion
		
		#region Remove or Replace text
		public void HandleTextChange(OffsetChangeMapEntry entry, DelayedEvents delayedEvents)
		{
			//Log("HandleTextChange(" + entry + ")");
			if (entry.RemovalLength == 0) {
				// This is a pure insertion.
				// Unlike a replace with removal, a pure insertion can result in nodes at the same location
				// to split depending on their MovementType.
				// Thus, we handle this case on a separate code path
				// (the code below looks like it does something similar, but it can only split
				// the set of deletion survivors, not all nodes at an offset)
				InsertText(entry.Offset, entry.InsertionLength, entry.DefaultAnchorMovementIsBeforeInsertion);
				return;
			}
			// When handling a replacing text change, we need to:
			// - find all anchors in the deleted segment and delete them / move them to the appropriate
			//   surviving side.
			// - adjust the segment size between the left and right side
			
			int offset = entry.Offset;
			int remainingRemovalLength = entry.RemovalLength;
			// if the text change is happening after the last anchor, we don't have to do anything
			if (root == null || offset >= root.totalLength)
				return;
			TextAnchorNode node = FindNode(ref offset);
			TextAnchorNode firstDeletionSurvivor = null;
			// go forward through the tree and delete all nodes in the removal segment
			while (node != null && offset + remainingRemovalLength > node.length) {
				TextAnchor anchor = (TextAnchor)node.Target;
				if (anchor != null && (anchor.SurviveDeletion || entry.RemovalNeverCausesAnchorDeletion)) {
					if (firstDeletionSurvivor == null)
						firstDeletionSurvivor = node;
					// This node should be deleted, but it wants to survive.
					// We'll just remove the deleted length segment, so the node will be positioned
					// in front of the removed segment.
					remainingRemovalLength -= node.length - offset;
					node.length = offset;
					offset = 0;
					UpdateAugmentedData(node);
					node = node.Successor;
				} else {
					// delete node
					TextAnchorNode s = node.Successor;
					remainingRemovalLength -= node.length;
					RemoveNode(node);
					// we already deleted the node, don't delete it twice
					nodesToDelete.Remove(node);
					if (anchor != null)
						anchor.OnDeleted(delayedEvents);
					node = s;
				}
			}
			// 'node' now is the first anchor after the deleted segment.
			// If there are no anchors after the deleted segment, 'node' is null.
			
			// firstDeletionSurvivor was set to the first node surviving deletion.
			// Because all non-surviving nodes up to 'node' were deleted, the node range
			// [firstDeletionSurvivor, node) now refers to the set of all deletion survivors.
			
			// do the remaining job of the removal
			if (node != null) {
				node.length -= remainingRemovalLength;
				Debug.Assert(node.length >= 0);
			}
			if (entry.InsertionLength > 0) {
				// we are performing a replacement
				if (firstDeletionSurvivor != null) {
					// We got deletion survivors which need to be split into BeforeInsertion
					// and AfterInsertion groups.
					// Take care that we don't regroup everything at offset, but only the deletion
					// survivors - from firstDeletionSurvivor (inclusive) to node (exclusive).
					// This ensures that nodes immediately before or after the replaced segment
					// stay where they are (independent from their MovementType)
					PerformInsertText(firstDeletionSurvivor, node, entry.InsertionLength, entry.DefaultAnchorMovementIsBeforeInsertion);
				} else if (node != null) {
					// No deletion survivors:
					// just perform the insertion
					node.length += entry.InsertionLength;
				}
			}
			if (node != null) {
				UpdateAugmentedData(node);
			}
			DeleteMarkedNodes();
		}
		#endregion
		
		#region Node removal when TextAnchor was GC'ed
		void MarkNodeForDelete(TextAnchorNode node)
		{
			if (!nodesToDelete.Contains(node))
				nodesToDelete.Add(node);
		}
		
		void DeleteMarkedNodes()
		{
			CheckProperties();
			while (nodesToDelete.Count > 0) {
				int pos = nodesToDelete.Count - 1;
				TextAnchorNode n = nodesToDelete[pos];
				// combine section of n with the following section
				TextAnchorNode s = n.Successor;
				if (s != null) {
					s.length += n.length;
				}
				RemoveNode(n);
				if (s != null) {
					UpdateAugmentedData(s);
				}
				nodesToDelete.RemoveAt(pos);
				CheckProperties();
			}
			CheckProperties();
		}
		#endregion
		
		#region FindNode
		/// <summary>
		/// Finds the node at the specified offset.
		/// After the method has run, offset is relative to the beginning of the returned node.
		/// </summary>
		TextAnchorNode FindNode(ref int offset)
		{
			TextAnchorNode n = root;
			while (true) {
				if (n.left != null) {
					if (offset < n.left.totalLength) {
						n = n.left; // descend into left subtree
						continue;
					} else {
						offset -= n.left.totalLength; // skip left subtree
					}
				}
				if (!n.IsAlive)
					MarkNodeForDelete(n);
				if (offset < n.length) {
					return n; // found correct node
				} else {
					offset -= n.length; // skip this node
				}
				if (n.right != null) {
					n = n.right; // descend into right subtree
				} else {
					// didn't find any node containing the offset
					return null;
				}
			}
		}
		#endregion
		
		#region UpdateAugmentedData
		void UpdateAugmentedData(TextAnchorNode n)
		{
			if (!n.IsAlive)
				MarkNodeForDelete(n);
			
			int totalLength = n.length;
			if (n.left != null)
				totalLength += n.left.totalLength;
			if (n.right != null)
				totalLength += n.right.totalLength;
			if (n.totalLength != totalLength) {
				n.totalLength = totalLength;
				if (n.parent != null)
					UpdateAugmentedData(n.parent);
			}
		}
		#endregion
		
		#region CreateAnchor
		public TextAnchor CreateAnchor(int offset)
		{
			Log("CreateAnchor(" + offset + ")");
			TextAnchor anchor = new TextAnchor(document);
			anchor.node = new TextAnchorNode(anchor);
			if (root == null) {
				// creating the first text anchor
				root = anchor.node;
				root.totalLength = root.length = offset;
			} else if (offset >= root.totalLength) {
				// append anchor at end of tree
				anchor.node.totalLength = anchor.node.length = offset - root.totalLength;
				InsertAsRight(root.RightMost, anchor.node);
			} else {
				// insert anchor in middle of tree
				TextAnchorNode n = FindNode(ref offset);
				Debug.Assert(offset < n.length);
				// split segment 'n' at offset
				anchor.node.totalLength = anchor.node.length = offset;
				n.length -= offset;
				InsertBefore(n, anchor.node);
			}
			DeleteMarkedNodes();
			return anchor;
		}
		
		void InsertBefore(TextAnchorNode node, TextAnchorNode newNode)
		{
			if (node.left == null) {
				InsertAsLeft(node, newNode);
			} else {
				InsertAsRight(node.left.RightMost, newNode);
			}
		}
		#endregion
		
		#region Red/Black Tree
		internal const bool RED = true;
		internal const bool BLACK = false;
		
		void InsertAsLeft(TextAnchorNode parentNode, TextAnchorNode newNode)
		{
			Debug.Assert(parentNode.left == null);
			parentNode.left = newNode;
			newNode.parent = parentNode;
			newNode.color = RED;
			UpdateAugmentedData(parentNode);
			FixTreeOnInsert(newNode);
		}
		
		void InsertAsRight(TextAnchorNode parentNode, TextAnchorNode newNode)
		{
			Debug.Assert(parentNode.right == null);
			parentNode.right = newNode;
			newNode.parent = parentNode;
			newNode.color = RED;
			UpdateAugmentedData(parentNode);
			FixTreeOnInsert(newNode);
		}
		
		void FixTreeOnInsert(TextAnchorNode node)
		{
			Debug.Assert(node != null);
			Debug.Assert(node.color == RED);
			Debug.Assert(node.left == null || node.left.color == BLACK);
			Debug.Assert(node.right == null || node.right.color == BLACK);
			
			TextAnchorNode parentNode = node.parent;
			if (parentNode == null) {
				// we inserted in the root -> the node must be black
				// since this is a root node, making the node black increments the number of black nodes
				// on all paths by one, so it is still the same for all paths.
				node.color = BLACK;
				return;
			}
			if (parentNode.color == BLACK) {
				// if the parent node where we inserted was black, our red node is placed correctly.
				// since we inserted a red node, the number of black nodes on each path is unchanged
				// -> the tree is still balanced
				return;
			}
			// parentNode is red, so there is a conflict here!
			
			// because the root is black, parentNode is not the root -> there is a grandparent node
			TextAnchorNode grandparentNode = parentNode.parent;
			TextAnchorNode uncleNode = Sibling(parentNode);
			if (uncleNode != null && uncleNode.color == RED) {
				parentNode.color = BLACK;
				uncleNode.color = BLACK;
				grandparentNode.color = RED;
				FixTreeOnInsert(grandparentNode);
				return;
			}
			// now we know: parent is red but uncle is black
			// First rotation:
			if (node == parentNode.right && parentNode == grandparentNode.left) {
				RotateLeft(parentNode);
				node = node.left;
			} else if (node == parentNode.left && parentNode == grandparentNode.right) {
				RotateRight(parentNode);
				node = node.right;
			}
			// because node might have changed, reassign variables:
			parentNode = node.parent;
			grandparentNode = parentNode.parent;
			
			// Now recolor a bit:
			parentNode.color = BLACK;
			grandparentNode.color = RED;
			// Second rotation:
			if (node == parentNode.left && parentNode == grandparentNode.left) {
				RotateRight(grandparentNode);
			} else {
				// because of the first rotation, this is guaranteed:
				Debug.Assert(node == parentNode.right && parentNode == grandparentNode.right);
				RotateLeft(grandparentNode);
			}
		}
		
		void RemoveNode(TextAnchorNode removedNode)
		{
			if (removedNode.left != null && removedNode.right != null) {
				// replace removedNode with it's in-order successor
				
				TextAnchorNode leftMost = removedNode.right.LeftMost;
				RemoveNode(leftMost); // remove leftMost from its current location
				
				// and overwrite the removedNode with it
				ReplaceNode(removedNode, leftMost);
				leftMost.left = removedNode.left;
				if (leftMost.left != null) leftMost.left.parent = leftMost;
				leftMost.right = removedNode.right;
				if (leftMost.right != null) leftMost.right.parent = leftMost;
				leftMost.color = removedNode.color;
				
				UpdateAugmentedData(leftMost);
				if (leftMost.parent != null) UpdateAugmentedData(leftMost.parent);
				return;
			}
			
			// now either removedNode.left or removedNode.right is null
			// get the remaining child
			TextAnchorNode parentNode = removedNode.parent;
			TextAnchorNode childNode = removedNode.left ?? removedNode.right;
			ReplaceNode(removedNode, childNode);
			if (parentNode != null) UpdateAugmentedData(parentNode);
			if (removedNode.color == BLACK) {
				if (childNode != null && childNode.color == RED) {
					childNode.color = BLACK;
				} else {
					FixTreeOnDelete(childNode, parentNode);
				}
			}
		}
		
		void FixTreeOnDelete(TextAnchorNode node, TextAnchorNode parentNode)
		{
			Debug.Assert(node == null || node.parent == parentNode);
			if (parentNode == null)
				return;
			
			// warning: node may be null
			TextAnchorNode sibling = Sibling(node, parentNode);
			if (sibling.color == RED) {
				parentNode.color = RED;
				sibling.color = BLACK;
				if (node == parentNode.left) {
					RotateLeft(parentNode);
				} else {
					RotateRight(parentNode);
				}
				
				sibling = Sibling(node, parentNode); // update value of sibling after rotation
			}
			
			if (parentNode.color == BLACK
			    && sibling.color == BLACK
			    && GetColor(sibling.left) == BLACK
			    && GetColor(sibling.right) == BLACK)
			{
				sibling.color = RED;
				FixTreeOnDelete(parentNode, parentNode.parent);
				return;
			}
			
			if (parentNode.color == RED
			    && sibling.color == BLACK
			    && GetColor(sibling.left) == BLACK
			    && GetColor(sibling.right) == BLACK)
			{
				sibling.color = RED;
				parentNode.color = BLACK;
				return;
			}
			
			if (node == parentNode.left &&
			    sibling.color == BLACK &&
			    GetColor(sibling.left) == RED &&
			    GetColor(sibling.right) == BLACK)
			{
				sibling.color = RED;
				sibling.left.color = BLACK;
				RotateRight(sibling);
			}
			else if (node == parentNode.right &&
			         sibling.color == BLACK &&
			         GetColor(sibling.right) == RED &&
			         GetColor(sibling.left) == BLACK)
			{
				sibling.color = RED;
				sibling.right.color = BLACK;
				RotateLeft(sibling);
			}
			sibling = Sibling(node, parentNode); // update value of sibling after rotation
			
			sibling.color = parentNode.color;
			parentNode.color = BLACK;
			if (node == parentNode.left) {
				if (sibling.right != null) {
					Debug.Assert(sibling.right.color == RED);
					sibling.right.color = BLACK;
				}
				RotateLeft(parentNode);
			} else {
				if (sibling.left != null) {
					Debug.Assert(sibling.left.color == RED);
					sibling.left.color = BLACK;
				}
				RotateRight(parentNode);
			}
		}
		
		void ReplaceNode(TextAnchorNode replacedNode, TextAnchorNode newNode)
		{
			if (replacedNode.parent == null) {
				Debug.Assert(replacedNode == root);
				root = newNode;
			} else {
				if (replacedNode.parent.left == replacedNode)
					replacedNode.parent.left = newNode;
				else
					replacedNode.parent.right = newNode;
			}
			if (newNode != null) {
				newNode.parent = replacedNode.parent;
			}
			replacedNode.parent = null;
		}
		
		void RotateLeft(TextAnchorNode p)
		{
			// let q be p's right child
			TextAnchorNode q = p.right;
			Debug.Assert(q != null);
			Debug.Assert(q.parent == p);
			// set q to be the new root
			ReplaceNode(p, q);
			
			// set p's right child to be q's left child
			p.right = q.left;
			if (p.right != null) p.right.parent = p;
			// set q's left child to be p
			q.left = p;
			p.parent = q;
			UpdateAugmentedData(p);
			UpdateAugmentedData(q);
		}
		
		void RotateRight(TextAnchorNode p)
		{
			// let q be p's left child
			TextAnchorNode q = p.left;
			Debug.Assert(q != null);
			Debug.Assert(q.parent == p);
			// set q to be the new root
			ReplaceNode(p, q);
			
			// set p's left child to be q's right child
			p.left = q.right;
			if (p.left != null) p.left.parent = p;
			// set q's right child to be p
			q.right = p;
			p.parent = q;
			UpdateAugmentedData(p);
			UpdateAugmentedData(q);
		}
		
		static TextAnchorNode Sibling(TextAnchorNode node)
		{
			if (node == node.parent.left)
				return node.parent.right;
			else
				return node.parent.left;
		}
		
		static TextAnchorNode Sibling(TextAnchorNode node, TextAnchorNode parentNode)
		{
			Debug.Assert(node == null || node.parent == parentNode);
			if (node == parentNode.left)
				return parentNode.right;
			else
				return parentNode.left;
		}
		
		static bool GetColor(TextAnchorNode node)
		{
			return node != null ? node.color : BLACK;
		}
		#endregion
		
		#region CheckProperties
		[Conditional("DATACONSISTENCYTEST")]
		internal void CheckProperties()
		{
			#if DEBUG
			if (root != null) {
				CheckProperties(root);
				
				// check red-black property:
				int blackCount = -1;
				CheckNodeProperties(root, null, RED, 0, ref blackCount);
			}
			#endif
		}
		
		#if DEBUG
		void CheckProperties(TextAnchorNode node)
		{
			int totalLength = node.length;
			if (node.left != null) {
				CheckProperties(node.left);
				totalLength += node.left.totalLength;
			}
			if (node.right != null) {
				CheckProperties(node.right);
				totalLength += node.right.totalLength;
			}
			Debug.Assert(node.totalLength == totalLength);
		}
		
		/*
		1. A node is either red or black.
		2. The root is black.
		3. All leaves are black. (The leaves are the NIL children.)
		4. Both children of every red node are black. (So every red node must have a black parent.)
		5. Every simple path from a node to a descendant leaf contains the same number of black nodes. (Not counting the leaf node.)
		 */
		void CheckNodeProperties(TextAnchorNode node, TextAnchorNode parentNode, bool parentColor, int blackCount, ref int expectedBlackCount)
		{
			if (node == null) return;
			
			Debug.Assert(node.parent == parentNode);
			
			if (parentColor == RED) {
				Debug.Assert(node.color == BLACK);
			}
			if (node.color == BLACK) {
				blackCount++;
			}
			if (node.left == null && node.right == null) {
				// node is a leaf node:
				if (expectedBlackCount == -1)
					expectedBlackCount = blackCount;
				else
					Debug.Assert(expectedBlackCount == blackCount);
			}
			CheckNodeProperties(node.left, node, node.color, blackCount, ref expectedBlackCount);
			CheckNodeProperties(node.right, node, node.color, blackCount, ref expectedBlackCount);
		}
		#endif
		#endregion
		
		#region GetTreeAsString
		#if DEBUG
		[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Performance", "CA1811:AvoidUncalledPrivateCode")]
		public string GetTreeAsString()
		{
			if (root == null)
				return "<empty tree>";
			StringBuilder b = new StringBuilder();
			AppendTreeToString(root, b, 0);
			return b.ToString();
		}
		
		static void AppendTreeToString(TextAnchorNode node, StringBuilder b, int indent)
		{
			if (node.color == RED)
				b.Append("RED   ");
			else
				b.Append("BLACK ");
			b.AppendLine(node.ToString());
			indent += 2;
			if (node.left != null) {
				b.Append(' ', indent);
				b.Append("L: ");
				AppendTreeToString(node.left, b, indent);
			}
			if (node.right != null) {
				b.Append(' ', indent);
				b.Append("R: ");
				AppendTreeToString(node.right, b, indent);
			}
		}
		#endif
		#endregion
	}
}

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Daniel Grunwald

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I am the lead developer on the SharpDevelop open source project.
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