using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Xml;
using System.Xml.Linq;
using UnrealBuildTool;
using AutomationTool;
using System.Reflection;
using System.Diagnostics;
using System.Xml.Schema;

namespace AutomationTool
{
	/// <summary>
	/// Implementation of XmlDocument which preserves line numbers for its elements
	/// </summary>
	class ScriptDocument : XmlDocument
	{
		/// <summary>
		/// The file being read
		/// </summary>
		FileReference File;

		/// <summary>
		/// Interface to the LineInfo on the active XmlReader
		/// </summary>
		IXmlLineInfo LineInfo;

		/// <summary>
		/// Set to true if the reader encounters an error
		/// </summary>
		bool bHasErrors;

		/// <summary>
		/// Private constructor. Use ScriptDocument.Load to read an XML document.
		/// </summary>
		ScriptDocument(FileReference InFile)
		{
			File = InFile;
		}

		/// <summary>
		/// Overrides XmlDocument.CreateElement() to construct ScriptElements rather than XmlElements
		/// </summary>
		public override XmlElement CreateElement(string Prefix, string LocalName, string NamespaceUri)
		{
			return new ScriptElement(File, LineInfo.LineNumber, Prefix, LocalName, NamespaceUri, this);
		}

		/// <summary>
		/// Loads a script document from the given file
		/// </summary>
		/// <param name="File">The file to load</param>
		/// <param name="Schema">The schema to validate against</param>
		/// <param name="OutDocument">If successful, the document that was read</param>
		/// <returns>True if the document could be read, false otherwise</returns>
		public static bool TryRead(FileReference File, ScriptSchema Schema, out ScriptDocument OutDocument)
		{
			ScriptDocument Document = new ScriptDocument(File);

            XmlReaderSettings Settings = new XmlReaderSettings();
            Settings.Schemas.Add(Schema.CompiledSchema);
            Settings.ValidationType = ValidationType.Schema;
			Settings.ValidationEventHandler += Document.ValidationEvent;
	
			using(XmlReader Reader = XmlReader.Create(File.FullName, Settings))
			{
				// Read the document
				Document.LineInfo = (IXmlLineInfo)Reader;
				Document.Load(Reader);

				// If we hit any errors while parsing
				if(Document.bHasErrors)
				{
					OutDocument = null;
					return false;
				}

				// Check that the root element is valid. If not, we didn't actually validate against the schema.
				if(Document.DocumentElement.Name != ScriptSchema.RootElementName)
				{
					CommandUtils.LogError("Script does not have a root element called '{0}'", ScriptSchema.RootElementName);
					OutDocument = null;
					return false;
				}
				if(Document.DocumentElement.NamespaceURI != ScriptSchema.NamespaceURI)
				{
					CommandUtils.LogError("Script root element is not in the '{0}' namespace (add the xmlns=\"{0}\" attribute)", ScriptSchema.NamespaceURI);
					OutDocument = null;
					return false;
				}
			}

			OutDocument = Document;
			return true;
		}

		/// <summary>
		/// Callback for validation errors in the document
		/// </summary>
		/// <param name="Sender">Standard argument for ValidationEventHandler</param>
		/// <param name="Args">Standard argument for ValidationEventHandler</param>
		void ValidationEvent(object Sender, ValidationEventArgs Args)
		{
			if(Args.Severity == XmlSeverityType.Warning)
			{
				CommandUtils.LogWarning("{0}({1}): {2}", File.FullName, Args.Exception.LineNumber, Args.Message);
			}
			else
			{
				CommandUtils.LogError("{0}({1}): {2}", File.FullName, Args.Exception.LineNumber, Args.Message);
				bHasErrors = true;
			}
		}
	}

	/// <summary>
	/// Implementation of XmlElement which preserves line numbers
	/// </summary>
	class ScriptElement : XmlElement
	{
		/// <summary>
		/// The file containing this element
		/// </summary>
		public readonly FileReference File;

		/// <summary>
		/// The line number containing this element
		/// </summary>
		public readonly int LineNumber;

		/// <summary>
		/// Constructor
		/// </summary>
		public ScriptElement(FileReference InFile, int InLineNumber, string Prefix, string LocalName, string NamespaceUri, ScriptDocument Document) 
			: base(Prefix, LocalName, NamespaceUri, Document)
		{
			File = InFile;
			LineNumber = InLineNumber;
		}
	}

	/// <summary>
	/// Reader for build graph definitions. Instanced to contain temporary state; public interface is through ScriptReader.TryRead().
	/// </summary>
	class ScriptReader
	{
		/// <summary>
		/// The current graph
		/// </summary>
		Graph Graph = new Graph();

		/// <summary>
		/// Mapping from name to group
		/// </summary>
		Dictionary<string, AgentGroup> NameToGroup = new Dictionary<string,AgentGroup>(StringComparer.InvariantCultureIgnoreCase);

		/// <summary>
		/// Mapping of global property name to values.
		/// </summary>
		Dictionary<string, string> GlobalProperties = new Dictionary<string,string>();

		/// <summary>
		/// List of property name to value lookups. Modifications to properties are scoped to nodes and groups. EnterScope() pushes an empty dictionary onto the end of this list, and LeaveScope() removes one. 
		/// ExpandProperties() searches from last to first lookup when trying to resolve a property name, and takes the first it finds.
		/// </summary>
		List<Dictionary<string, string>> ScopedProperties = new List<Dictionary<string,string>>();

		/// <summary>
		/// Schema for the script
		/// </summary>
		ScriptSchema Schema;

		/// <summary>
		/// The number of errors encountered during processing so far
		/// </summary>
		int NumErrors;

		/// <summary>
		/// Private constructor. Use ScriptReader.TryRead() to read a script file.
		/// </summary>
		/// <param name="Properties">Predefined property name to value mapping</param>
		/// <param name="InSchema">Schema for the script</param>
		private ScriptReader(IDictionary<string, string> Properties, ScriptSchema InSchema)
		{
			GlobalProperties = new Dictionary<string,string>(Properties, StringComparer.InvariantCultureIgnoreCase);
			ScopedProperties.Add(GlobalProperties);
			Schema = InSchema;
		}

		/// <summary>
		/// Try to read a script file from the given file.
		/// </summary>
		/// <param name="File">File to read from</param>
		/// <param name="DefaultProperties">Manually defined properties to parse the graph with</param>
		/// <param name="InSchema">Schema for the script</param>
		/// <param name="Graph">If successful, the graph constructed from the given script</param>
		/// <returns>True if the graph was read, false if there were errors</returns>
		public static bool TryRead(FileReference File, IDictionary<string, string> DefaultProperties, ScriptSchema Schema, out Graph Graph)
		{
			// Check the file exists before doing anything.
			if(!File.Exists())
			{
				CommandUtils.LogError("Cannot open '{0}'", File.FullName);
				Graph = null;
				return false;
			}

			// Read the file and build the graph
			ScriptReader Reader = new ScriptReader(DefaultProperties, Schema);
			if(Reader.TryRead(File) && Reader.NumErrors == 0)
			{
				Graph = Reader.Graph;
				return true;
			}
			else
			{
				Graph = null;
				return false;
			}
		}

		/// <summary>
		/// Read the script from the given file
		/// </summary>
		/// <param name="File">File to read from</param>
		bool TryRead(FileReference File)
		{
			// Read the document and validate it against the schema
			ScriptDocument Document;
			if(!ScriptDocument.TryRead(File, Schema, out Document))
			{
				NumErrors++;
				return false;
			}

			// Read the root BuildGraph element
			EnterScope();
			foreach(ScriptElement Element in Document.DocumentElement.ChildNodes.OfType<ScriptElement>())
			{
				switch(Element.Name)
				{
					case "Include":
						ReadInclude(Element, File.Directory);
						break;
					case "Property":
						ReadProperty(Element);
						break;
					case "Local":
						ReadLocalProperty(Element);
						break;
					case "Group":
						ReadGroup(Element, null);
						break;
					case "Aggregate":
						ReadAggregate(Element);
						break;
					case "Notify":
						ReadNotifier(Element);
						break;
					case "Trigger":
						ReadTrigger(Element);
						break;
					default:
						LogError(Element, "Invalid element '{0}'", Element.Name);
						break;
				}
			}
			LeaveScope();
			return true;
		}

		/// <summary>
		/// Handles validation messages from validating the document against its schema
		/// </summary>
		/// <param name="Sender">The source of the event</param>
		/// <param name="Args">Event arguments</param>
		void ValidationHandler(object Sender, ValidationEventArgs Args)
		{
			if(Args.Severity == XmlSeverityType.Warning)
			{
				CommandUtils.LogWarning("Script: {0}", Args.Message);
			}
			else
			{
				CommandUtils.LogError("Script: {0}", Args.Message);
				NumErrors++;
			}
		}

		/// <summary>
		/// Push a new property scope onto the stack
		/// </summary>
		void EnterScope()
		{
			ScopedProperties.Add(new Dictionary<string,string>(StringComparer.InvariantCultureIgnoreCase));
		}

		/// <summary>
		/// Pop a property scope from the stack
		/// </summary>
		void LeaveScope()
		{
			ScopedProperties.RemoveAt(ScopedProperties.Count - 1);
		}

		/// <summary>
		/// Reads the definition for an agent group.
		/// </summary>
		/// <param name="Element">Xml element to read the definition from</param>
		void ReadTrigger(ScriptElement Element)
		{
			string[] QualifiedName;
			if(EvaluateCondition(Element) && TryReadQualifiedObjectName(Element, out QualifiedName))
			{
				// Validate all the parent triggers
				ManualTrigger ParentTrigger = null;
				for(int Idx = 0; Idx < QualifiedName.Length - 1; Idx++)
				{
					ManualTrigger NextTrigger;
					if(!Graph.NameToTrigger.TryGetValue(QualifiedName[Idx], out NextTrigger))
					{
						LogError(Element, "Unknown trigger '{0}'", QualifiedName[Idx]);
						return;
					}
					if(NextTrigger.Parent != ParentTrigger)
					{
						LogError(Element, "Qualified name of trigger '{0}' is '{1}'", NextTrigger.Name, NextTrigger.QualifiedName);
						return;
					}
					ParentTrigger = NextTrigger;
				}

				// Get the name of the new trigger
				string Name = QualifiedName[QualifiedName.Length - 1];

				// Create the new trigger
				ManualTrigger Trigger;
				if(!Graph.NameToTrigger.TryGetValue(Name, out Trigger))
				{
					Trigger = new ManualTrigger(ParentTrigger, Name);
					Graph.NameToTrigger.Add(Name, Trigger);
				}
				else if(Trigger.Parent != ParentTrigger)
				{
					LogError(Element, "Conflicting parent for '{0}' - previously declared as '{1}', now '{2}'", Name, Trigger.QualifiedName, new ManualTrigger(ParentTrigger, Name).QualifiedName);
					return;
				}

				// Read the root BuildGraph element
				EnterScope();
				foreach(ScriptElement ChildElement in Element.ChildNodes.OfType<ScriptElement>())
				{
					switch(ChildElement.Name)
					{
						case "Property":
							ReadProperty(ChildElement);
							break;
						case "Local":
							ReadLocalProperty(ChildElement);
							break;
						case "Group":
							ReadGroup(ChildElement, Trigger);
							break;
						case "Aggregate":
							ReadAggregate(ChildElement);
							break;
						case "Notifier":
							ReadNotifier(ChildElement);
							break;
						default:
							LogError(ChildElement, "Invalid element '{0}'", ChildElement.Name);
							break;
					}
				}
				LeaveScope();
			}
		}

		/// <summary>
		/// Read an include directive, and the contents of the target file
		/// </summary>
		/// <param name="Element">Xml element to read the definition from</param>
		/// <param name="BaseDir">Base directory to resolve relative include paths from </param>
		void ReadInclude(ScriptElement Element, DirectoryReference BaseDir)
		{
			if(EvaluateCondition(Element))
			{
				FileReference Script = FileReference.Combine(BaseDir, Element.GetAttribute("Script"));
				if(Script.Exists())
				{
					TryRead(Script);
				}
				else
				{
					LogError(Element, "Cannot find included script '{0}'", Script.FullName);
				}
			}
		}

		/// <summary>
		/// Reads a property assignment.
		/// </summary>
		/// <param name="Element">Xml element to read the definition from</param>
		void ReadProperty(ScriptElement Element)
		{
			if(EvaluateCondition(Element))
			{
				string Name = Element.GetAttribute("Name");
				GlobalProperties[Name] = ReadAttribute(Element, "Value");
			}
		}

		/// <summary>
		/// Reads a local property assignment.
		/// </summary>
		/// <param name="Element">Xml element to read the definition from</param>
		void ReadLocalProperty(ScriptElement Element)
		{
			if(EvaluateCondition(Element))
			{
				string Name = Element.GetAttribute("Name");
				ScopedProperties[ScopedProperties.Count - 1][Name] = ReadAttribute(Element, "Value");
			}
		}

		/// <summary>
		/// Reads the definition for an agent group.
		/// </summary>
		/// <param name="Element">Xml element to read the definition from</param>
		/// <param name="Trigger">The controlling trigger for nodes in this group</param>
		void ReadGroup(ScriptElement Element, ManualTrigger Trigger)
		{
			string Name;
			if(EvaluateCondition(Element) && TryReadObjectName(Element, out Name))
			{
				// Create the group object, or continue an existing one
				AgentGroup Group;
				if(NameToGroup.TryGetValue(Name, out Group))
				{
					if(Element.HasAttribute("Agent"))
					{
						LogError(Element, "Agent may only be specified for first definition of a group");
					}
				}
				else
				{
					string[] AgentTypes = ReadListAttribute(Element, "Agent");
					if(AgentTypes.Length == 0)
					{
						LogError(Element, "Missing agent type for group '{0}'", Name);
					}

					Group = new AgentGroup(Name, AgentTypes);
					NameToGroup.Add(Name, Group);
					Graph.Groups.Add(Group);
				}

				// Process all the child elements.
				EnterScope();
				foreach(ScriptElement ChildElement in Element.ChildNodes.OfType<ScriptElement>())
				{
					switch(ChildElement.Name)
					{
						case "Property":
							ReadProperty(ChildElement);
							break;
						case "Local":
							ReadLocalProperty(ChildElement);
							break;
						case "Node":
							ReadNode(ChildElement, Group, Trigger);
							break;
						case "Aggregate":
							ReadAggregate(ChildElement);
							break;
						default:
							LogError(ChildElement, "Unexpected element type '{0}'", ChildElement.Name);
							break;
					}
				}
				LeaveScope();
			}
		}

		/// <summary>
		/// Reads the definition for an aggregate, and adds it to the given agent group
		/// </summary>
		/// <param name="Element">Xml element to read the definition from</param>
		void ReadAggregate(ScriptElement Element)
		{
			string Name;
			if(EvaluateCondition(Element) && TryReadObjectName(Element, out Name) && CheckNameIsUnique(Element, Name))
			{
				string[] RequiredNames = ReadTagListAttribute(Element, "Requires");
				Graph.AggregateNameToNodes.Add(Name, ResolveReferences(Element, RequiredNames).ToArray());
			}
		}

		/// <summary>
		/// Reads the definition for a node, and adds it to the given agent group
		/// </summary>
		/// <param name="Element">Xml element to read the definition from</param>
		/// <param name="Group">Group for the node to be added to</param>
		/// <param name="ControllingTrigger">The controlling trigger for this node</param>
		void ReadNode(ScriptElement Element, AgentGroup Group, ManualTrigger ControllingTrigger)
		{
			string Name;
			if(EvaluateCondition(Element) && TryReadObjectName(Element, out Name))
			{
				string[] InputNames = ReadTagListAttribute(Element, "Requires");
				string[] OutputNames = ReadTagListAttribute(Element, "Produces");
				string[] AfterNames = ReadTagListAttribute(Element, "After");

				// Add all the tasks
				EnterScope();
				List<CustomTask> Tasks = new List<CustomTask>();
				foreach(ScriptElement ChildElement in Element.ChildNodes.OfType<ScriptElement>())
				{
					switch(ChildElement.Name)
					{
						case "Property":
							ReadProperty(ChildElement);
							break;
						case "Local":
							ReadLocalProperty(ChildElement);
							break;
						default:
							ReadTask(ChildElement, Tasks);
							break;
					}
				}
				LeaveScope();

				// Expand every node name in the list of inputs to include all of that node's inputs, recursively, plus its named outputs.
				HashSet<string> ExpandedInputNames = new HashSet<string>(InputNames, StringComparer.InvariantCultureIgnoreCase);
				foreach(string InputName in InputNames)
				{
					Node InputNode;
					if(Graph.NameToNode.TryGetValue(InputName, out InputNode))
					{
						ExpandedInputNames.UnionWith(InputNode.InputNames);
						ExpandedInputNames.UnionWith(InputNode.OutputNames);
					}
				}
				InputNames = ExpandedInputNames.ToArray();

				// Add the name of the node itself to the list of outputs.
				OutputNames = OutputNames.Union(new string[]{ Name }, StringComparer.InvariantCultureIgnoreCase).ToArray();

				// Gather up all the input dependencies
				HashSet<Node> InputDependencies = ResolveReferences(Element, InputNames);

				// Check they're all upstream of the current node
				foreach(Node InputDependency in InputDependencies)
				{
					if(InputDependency.ControllingTrigger != null && InputDependency.ControllingTrigger != ControllingTrigger && !InputDependency.ControllingTrigger.IsUpstreamFrom(ControllingTrigger))
					{
						LogError(Element, "'{0}' is dependent on '{1}', which is behind a different controlling trigger ({2})", Name, InputDependency.Name, InputDependency.ControllingTrigger.QualifiedName);
					}
				}

				// Recursively include all their dependencies too
				foreach(Node InputDependency in InputDependencies.ToArray())
				{
					InputDependencies.UnionWith(InputDependency.InputDependencies);
				}

				// Gather up all the order dependencies
				HashSet<Node> OrderDependencies = ResolveReferences(Element, AfterNames);
				OrderDependencies.UnionWith(InputDependencies);

				// Construct and register the node
				if(CheckNameIsUnique(Element, Name))
				{
					// Add it to the node lookup
					Node NewNode = new Node(Name, InputNames, OutputNames, InputDependencies.ToArray(), OrderDependencies.ToArray(), ControllingTrigger, Tasks);
					Graph.NameToNode.Add(Name, NewNode);

					// Register each of the outputs as a reference to this node
					foreach(string OutputName in OutputNames)
					{
						if(String.Compare(OutputName, Name, StringComparison.InvariantCultureIgnoreCase) == 0 || CheckNameIsUnique(Element, OutputName))
						{
							Graph.OutputNameToNode.Add(OutputName, NewNode);
						}
					}

					// Add it to the current agent group
					Group.Nodes.Add(NewNode);
				}
			}
		}

		/// <summary>
		/// Reads a task definition from the given element, and add it to the given list
		/// </summary>
		/// <param name="Element">Xml element to read the definition from</param>
		/// <param name="Tasks">List of tasks to add to</param>
		void ReadTask(ScriptElement Element, List<CustomTask> Tasks)
		{
			// Get the reflection info for this element
			ScriptTask Task;
			if(!Schema.TryGetTask(Element.Name, out Task))
			{
				LogError(Element, "Unknown task '{0}'", Element.Name);
				return;
			}

			// Check all the required parameters are present
			bool bHasRequiredAttributes = true;
			foreach(ScriptTaskParameter Parameter in Task.NameToParameter.Values)
			{
				if(!Parameter.bOptional && !Element.HasAttribute(Parameter.Name))
				{
					LogError(Element, "Missing required attribute - {0}", Parameter.Name);
					bHasRequiredAttributes = false;
				}
			}

			// Read all the attributes into a parameters object for this task
			object ParametersObject = Activator.CreateInstance(Task.ParametersClass);
			foreach(XmlAttribute Attribute in Element.Attributes)
			{
				// Get the field that this attribute should be written to in the parameters object
				ScriptTaskParameter Parameter;
				if(!Task.NameToParameter.TryGetValue(Attribute.Name, out Parameter))
				{
					LogError(Element, "Unknown attribute '{0}'", Attribute.Name);
					continue;
				}

				// Expand variables in the value
				string ExpandedValue = ExpandProperties(Attribute.Value);

				// Parse it and assign it to the parameters object
				object Value;
				if(Parameter.FieldInfo.FieldType.IsEnum)
				{
					Value = Enum.Parse(Parameter.FieldInfo.FieldType, ExpandedValue);
				}
				else if(Parameter.FieldInfo.FieldType == typeof(Boolean))
				{
					Value = Condition.Evaluate(ExpandedValue);
				}
				else
				{
					Value = Convert.ChangeType(ExpandedValue, Parameter.FieldInfo.FieldType);
				}
				Parameter.FieldInfo.SetValue(ParametersObject, Value);
			}

			// Construct the task
			if(bHasRequiredAttributes)
			{
				Tasks.Add((CustomTask)Activator.CreateInstance(Task.TaskClass, ParametersObject));
			}
		}

		/// <summary>
		/// Reads the definition for an email notifier
		/// </summary>
		/// <param name="Element">Xml element to read the definition from</param>
		void ReadNotifier(ScriptElement Element)
		{
			if(EvaluateCondition(Element))
			{
				string[] TargetNames = ReadTagListAttribute(Element, "Targets");
				string[] ExceptNames = ReadTagListAttribute(Element, "Except");
				string[] IndividualNodeNames = ReadTagListAttribute(Element, "Nodes");
				string[] TriggerNames = ReadTagListAttribute(Element, "Triggers");
				string[] Users = ReadListAttribute(Element, "Users");
				string[] Submitters = ReadListAttribute(Element, "Submitters");
				bool? bWarnings = Element.HasAttribute("Warnings")? (bool?)ReadBooleanAttribute(Element, "Warnings", true) : null;

				// Find the list of targets which are included, and recurse through all their dependencies
				HashSet<Node> Nodes = new HashSet<Node>();
				if(TargetNames != null)
				{
					HashSet<Node> TargetNodes = ResolveReferences(Element, TargetNames);
					foreach(Node Node in TargetNodes)
					{
						Nodes.Add(Node);
						Nodes.UnionWith(Node.InputDependencies);
					}
				}

				// Add all the individually referenced nodes
				if(IndividualNodeNames != null)
				{
					HashSet<Node> IndividualNodes = ResolveReferences(Element, IndividualNodeNames);
					Nodes.UnionWith(IndividualNodes);
				}

				// Exclude all the exceptions
				if(ExceptNames != null)
				{
					HashSet<Node> ExceptNodes = ResolveReferences(Element, ExceptNames);
					Nodes.ExceptWith(ExceptNodes);
				}

				// Update all the referenced nodes with the settings
				foreach(Node Node in Nodes)
				{
					if(Users != null)
					{
						Node.NotifyUsers.UnionWith(Users);
					}
					if(Submitters != null)
					{
						Node.NotifySubmitters.UnionWith(Submitters);
					}
					if(bWarnings.HasValue)
					{
						Node.bNotifyOnWarnings = bWarnings.Value;
					}
				}

				// Add the users to the list of triggers
				if(TriggerNames != null)
				{
					foreach(string TriggerName in TriggerNames)
					{
						ManualTrigger Trigger;
						if(Graph.NameToTrigger.TryGetValue(TriggerName, out Trigger))
						{
							Trigger.NotifyUsers.UnionWith(Users);
						}
						else
						{
							LogError(Element, "Trigger '{0}' has not been defined", TriggerName);
						}
					}
				}
			}
		}

		/// <summary>
		/// Checks that the given name does not already used to refer to a node, and print an error if it is.
		/// </summary>
		/// <param name="Element">Xml element to read from</param>
		/// <param name="Name">Name of the alias</param>
		/// <param name="Nodes">Array of nodes that this name should resolve to</param>
		/// <returns>True if the name was registered correctly, false otherwise.</returns>
		bool CheckNameIsUnique(ScriptElement Element, string Name)
		{
			// Get the nodes that it maps to
			Node[] OtherNodes;
			if(Graph.TryResolveReference(Name, out OtherNodes))
			{
				LogError(Element, "'{0}' is already defined; cannot add a second time", Name);
				return false;
			}
			return true;
		}

		/// <summary>
		/// Resolve a list of references to a set of nodes
		/// </summary>
		/// <param name="Element">Element used to locate any errors</param>
		/// <param name="ReferencedNames">Sequence of names to look up</param>
		/// <returns>Hashset of all the nodes included by the given names</returns>
		HashSet<Node> ResolveReferences(ScriptElement Element, IEnumerable<string> ReferencedNames)
		{
			HashSet<Node> Nodes = new HashSet<Node>();
			foreach(string ReferencedName in ReferencedNames)
			{
				Node[] OtherNodes;
				if(Graph.TryResolveReference(ReferencedName, out OtherNodes))
				{
					Nodes.UnionWith(OtherNodes);
				}
				else
				{
					LogError(Element, "Reference to '{0}' cannot be resolved; check it has been defined.", ReferencedName);
				}
			}
			return Nodes;
		}

		/// <summary>
		/// Reads an object name from its defining element. Outputs an error if the name is missing.
		/// </summary>
		/// <param name="Element">Element to read the name for</param>
		/// <param name="Name">Output variable to receive the name of the object</param>
		/// <returns>True if the object had a valid name (assigned to the Name variable), false if the name was invalid or missing.</returns>
		bool TryReadObjectName(ScriptElement Element, out string Name)
		{
			// Check the name attribute is present
			if(!Element.HasAttribute("Name"))
			{
				LogError(Element, "Missing 'Name' attribute");
				Name = null;
				return false;
			}

			// Get the value of it, strip any leading or trailing whitespace, and make sure it's not empty
			string Value = ReadAttribute(Element, "Name");
			if(!ValidateName(Element, Value))
			{
				Name = null;
				return false;
			}

			// Return it
			Name = Value;
			return true;
		}

		/// <summary>
		/// Reads an qualified object name from its defining element. Outputs an error if the name is missing.
		/// </summary>
		/// <param name="Element">Element to read the name for</param>
		/// <param name="QualifiedName">Output variable to receive the name of the object</param>
		/// <returns>True if the object had a valid name (assigned to the Name variable), false if the name was invalid or missing.</returns>
		bool TryReadQualifiedObjectName(ScriptElement Element, out string[] QualifiedName)
		{
			// Check the name attribute is present
			if(!Element.HasAttribute("Name"))
			{
				LogError(Element, "Missing 'Name' attribute");
				QualifiedName = null;
				return false;
			}

			// Get the value of it, strip any leading or trailing whitespace, and make sure it's not empty
			string[] Values = ReadAttribute(Element, "Name").Split('.');
			foreach(string Value in Values)
			{
				if(!ValidateName(Element, Value))
				{
					QualifiedName = null;
					return false;
				}
			}

			// Return it
			QualifiedName = Values;
			return true;
		}

		/// <summary>
		/// Checks that the given name is valid syntax
		/// </summary>
		/// <param name="Element">The element that contains the name</param>
		/// <param name="Name">The name to check</param>
		/// <returns>True if the name is valid</returns>
		bool ValidateName(ScriptElement Element, string Name)
		{
			// Check it's not empty
			if(Name.Length == 0)
			{
				LogError(Element, "Name is empty");
				return false;
			}

			// Check there are no invalid characters
			for(int Idx = 0; Idx < Name.Length; Idx++)
			{
				if(Idx > 0 && Name[Idx] == ' ' && Name[Idx - 1] == ' ')
				{
					LogError(Element, "Consecutive spaces in object name");
					return false;
				}
				if(!Char.IsLetterOrDigit(Name[Idx]) && Name[Idx] != '_' && Name[Idx] != ' ')
				{
					LogError(Element, "Invalid character in object name - '{0}'", Name[Idx]);
					return false;
				}
			}
			return true;
		}

		/// <summary>
		/// Expands any properties and reads an attribute.
		/// </summary>
		/// <param name="Element">Element to read the attribute from</param>
		/// <param name="Name">Name of the attribute</param>
		/// <returns>Array of names, with all leading and trailing whitespace removed</returns>
		string ReadAttribute(ScriptElement Element, string Name)
		{
			return ExpandProperties(Element.GetAttribute(Name));
		}

		/// <summary>
		/// Expands any properties and reads a list of strings from an attribute, separated by semi-colon characters
		/// </summary>
		/// <param name="Element"></param>
		/// <param name="Name"></param>
		/// <returns>Array of names, with all leading and trailing whitespace removed</returns>
		string[] ReadListAttribute(ScriptElement Element, string Name)
		{
			string Value = ReadAttribute(Element, Name);
			return Value.Split(new char[]{ ';' }).Select(x => x.Trim()).Where(x => x.Length > 0).ToArray();
		}

		/// <summary>
		/// Expands any properties and reads a list of tag names from an attribute, separated by semi-colon characters. Strips leading '#' characters from each name.
		/// </summary>
		/// <param name="Element">Element to read the attribute from</param>
		/// <param name="Name">Name of the attribute</param>
		/// <returns>Array of tag names, with all leading and trailing whitespace, and any initial '#' character removed</returns>
		string[] ReadTagListAttribute(ScriptElement Element, string Name)
		{
			string[] Values = ReadListAttribute(Element, Name);
			return Values.Select(x => x.StartsWith("#")? x.Substring(1) : x).ToArray();
		}

		/// <summary>
		/// Reads an attribute from the given XML element, expands any properties in it, and parses it as a boolean.
		/// </summary>
		/// <param name="Element">Element to read the attribute from</param>
		/// <param name="Name">Name of the attribute</param>
		/// <param name="DefaultValue">Default value if the attribute is missing</param>
		/// <returns>The value of the attribute field</returns>
		bool ReadBooleanAttribute(ScriptElement Element, string Name, bool bDefaultValue)
		{
			bool bResult = bDefaultValue;
			if(Element.HasAttribute(Name))
			{
				string Value = ReadAttribute(Element, Name).Trim();
				if(Value.Equals("true", StringComparison.InvariantCultureIgnoreCase))
				{
					bResult = true;
				}
				else if(Value.Equals("false", StringComparison.InvariantCultureIgnoreCase))
				{
					bResult = false;
				}
				else
				{
					LogError(Element, "Invalid boolean value '{0}' - expected 'true' or 'false'", Value);
				}
			}
			return bResult;
		}

		/// <summary>
		/// Reads an attribute from the given XML element, expands any properties in it, and parses it as an enum of the given type.
		/// </summary>
		/// <typeparam name="T">The enum type to parse the attribute as</typeparam>
		/// <param name="Element">Element to read the attribute from</param>
		/// <param name="Name">Name of the attribute</param>
		/// <param name="DefaultValue">Default value for the enum, if the attribute is missing</param>
		/// <returns>The value of the attribute field</returns>
		T ReadEnumAttribute<T>(ScriptElement Element, string Name, T DefaultValue) where T : struct
		{
			T Result = DefaultValue;
			if(Element.HasAttribute(Name))
			{
				string Value = ReadAttribute(Element, Name).Trim();

				T EnumValue;
				if(Enum.TryParse(Value, true, out EnumValue))
				{
					Result = EnumValue;
				}
				else
				{
					LogError(Element, "Invalid value '{0}' - expected {1}", Value, String.Join("/", Enum.GetNames(typeof(T))));
				}
			}
			return Result;
		}

		/// <summary>
		/// Outputs an error message to the log and increments the number of errors, referencing the file and line number of the element that caused it.
		/// </summary>
		/// <param name="Element">The script element causing the error</param>
		/// <param name="Format">Standard String.Format()-style format string</param>
		/// <param name="Args">Optional arguments</param>
		void LogError(ScriptElement Element, string Format, params object[] Args)
		{
			CommandUtils.LogError("{0}({1}): {2}", Element.File.FullName, Element.LineNumber, String.Format(Format, Args));
			NumErrors++;
		}

		/// <summary>
		/// Evaluates the (optional) conditional expression on a given XML element via the If="..." attribute, and returns true if the element is enabled.
		/// </summary>
		/// <param name="Element">The element to check</param>
		/// <returns>True if the element's condition evaluates to true (or doesn't have a conditional expression), false otherwise</returns>
		bool EvaluateCondition(ScriptElement Element)
		{
			// Check if the element has a conditional attribute
			const string AttributeName = "If";
			if(!Element.HasAttribute(AttributeName))
			{
				return true;
			}

			// If it does, try to evaluate it.
			try
			{
				string Text = ExpandProperties(Element.GetAttribute("If"));
				return Condition.Evaluate(Text);
			}
			catch(ConditionException Ex)
			{
				LogError(Element, "Error in condition: {0}", Ex.Message);
				return false;
			}
		}

		/// <summary>
		/// Expand all the property references (of the form $(PropertyName)) in a string.
		/// </summary>
		/// <param name="Text">The input string to expand properties in</param>
		/// <returns>The expanded string</returns>
		string ExpandProperties(string Text)
		{
			string Result = Text;
			for (int Idx = Result.IndexOf("$("); Idx != -1; Idx = Result.IndexOf("$(", Idx))
			{
				// Find the end of the variable name
				int EndIdx = Result.IndexOf(')', Idx + 2);
				if (EndIdx == -1)
				{
					break;
				}

				// Extract the variable name from the string
				string Name = Result.Substring(Idx + 2, EndIdx - (Idx + 2));

				// Find the value for it, either from the dictionary or the environment block
				string Value = null;
				for(int ScopeIdx = ScopedProperties.Count - 1; ScopeIdx >= 0; ScopeIdx--)
				{
					if(ScopedProperties[ScopeIdx].TryGetValue(Name, out Value))
					{
						break;
					}
				}
				Value = Value ?? "";

				// Replace the variable, or skip past it
				Result = Result.Substring(0, Idx) + Value + Result.Substring(EndIdx + 1);

				// Make sure we skip over the expanded variable; we don't want to recurse on it.
				Idx += Value.Length;
			}
			return Result;
		}
	}
}