// Copyright (c) 2011 AlphaSierraPapa for the SharpDevelop Team
// 
// Permission is hereby granted, free of charge, to any person obtaining a copy of this
// software and associated documentation files (the "Software"), to deal in the Software
// without restriction, including without limitation the rights to use, copy, modify, merge,
// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
// to whom the Software is furnished to do so, subject to the following conditions:
// 
// The above copyright notice and this permission notice shall be included in all copies or
// substantial portions of the Software.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
// DEALINGS IN THE SOFTWARE.

using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using ICSharpCode.Decompiler.CSharp.Syntax;
using ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching;
using ICSharpCode.Decompiler.IL;
using ICSharpCode.Decompiler.Semantics;
using ICSharpCode.Decompiler.TypeSystem;
using ICSharpCode.Decompiler.Util;

namespace ICSharpCode.Decompiler.CSharp.Transforms
{
	/// <summary>
	/// Insert variable declarations.
	/// </summary>
	public class DeclareVariables : IAstTransform
	{
		/// <summary>
		/// Represents a position immediately before nextNode.
		/// nextNode is either an ExpressionStatement in a BlockStatement, or an initializer in a for-loop.
		/// </summary>
		[DebuggerDisplay("level = {level}, nextNode = {nextNode}")]
		struct InsertionPoint
		{
			/// <summary>
			/// The nesting level of `nextNode` within the AST.
			/// Used to speed up FindCommonParent().
			/// </summary>
			internal int level;
			internal AstNode nextNode;
			
			/// <summary>Go up one level</summary>
			internal InsertionPoint Up()
			{
				return new InsertionPoint {
					level = level - 1,
					nextNode = nextNode.Parent
				};
			}
			
			internal InsertionPoint UpTo(int targetLevel)
			{
				InsertionPoint result = this;
				while (result.level > targetLevel) {
					result.nextNode = result.nextNode.Parent;
					result.level -= 1;
				}
				return result;
			}
		}
		
		[DebuggerDisplay("VariableToDeclare(Name={Name})")]
		class VariableToDeclare
		{
			public readonly IType Type;
			public readonly string Name;

			/// <summary>
			/// Whether the variable needs to be default-initialized.
			/// </summary>
			public bool DefaultInitialization;
			
			/// <summary>
			/// Integer value that can be used to compare to VariableToDeclare instances
			/// to determine which variable was used first in the source code.
			/// 
			/// Assuming both insertion points are on the same level, the variable
			/// with the lower SourceOrder value has the insertion point that comes
			/// first in the source code.
			/// </summary>
			public int SourceOrder;
			
			public InsertionPoint InsertionPoint;

			public VariableToDeclare ReplacementDueToCollision;
			public bool RemovedDueToCollision => ReplacementDueToCollision != null;

			public VariableToDeclare(IType type, string name, bool defaultInitialization, InsertionPoint insertionPoint, int sourceOrder)
			{
				this.Type = type;
				this.Name = name;
				this.DefaultInitialization = defaultInitialization;
				this.InsertionPoint = insertionPoint;
				this.SourceOrder = sourceOrder;
			}
		}
		
		readonly Dictionary<ILVariable, VariableToDeclare> variableDict = new Dictionary<ILVariable, VariableToDeclare>();
		
		public void Run(AstNode rootNode, TransformContext context)
		{
			try {
				variableDict.Clear();
				EnsureExpressionStatementsAreValid(rootNode);
				FindInsertionPoints(rootNode, 0);
				ResolveCollisions();
				InsertVariableDeclarations(context);
			} finally {
				variableDict.Clear();
			}
		}

		/// <summary>
		/// Analyze the input AST (containing undeclared variables)
		/// for where those variables would be declared by this transform.
		/// Analysis does not modify the AST.
		/// </summary>
		public void Analyze(AstNode rootNode)
		{
			variableDict.Clear();
			FindInsertionPoints(rootNode, 0);
			ResolveCollisions();
		}

		/// <summary>
		/// Get the position where the declaration for the variable will be inserted.
		/// </summary>
		public AstNode GetDeclarationPoint(ILVariable variable)
		{
			VariableToDeclare v = variableDict[variable];
			while (v.ReplacementDueToCollision != null) {
				v = v.ReplacementDueToCollision;
			}
			return v.InsertionPoint.nextNode;
		}

		public void ClearAnalysisResults()
		{
			variableDict.Clear();
		}

		#region EnsureExpressionStatementsAreValid
		void EnsureExpressionStatementsAreValid(AstNode rootNode)
		{
			foreach (var stmt in rootNode.DescendantsAndSelf.OfType<ExpressionStatement>()) {
				if (!IsValidInStatementExpression(stmt.Expression)) {
					// assign result to dummy variable
					var v = new ILVariable(VariableKind.StackSlot, stmt.Expression.GetResolveResult().Type, 0);
					v.Name = "_";
					stmt.Expression = new AssignmentExpression(
						new IdentifierExpression(v.Name).WithRR(new ILVariableResolveResult(v, v.Type)),
						stmt.Expression.Detach());
				}
			}
		}

		private static bool IsValidInStatementExpression(Expression expr)
		{
			switch (expr) {
				case InvocationExpression ie:
				case ObjectCreateExpression oce:
				case AssignmentExpression ae:
				case ErrorExpression ee:
					return true;
				case UnaryOperatorExpression uoe:
					switch (uoe.Operator) {
						case UnaryOperatorType.PostIncrement:
						case UnaryOperatorType.PostDecrement:
						case UnaryOperatorType.Increment:
						case UnaryOperatorType.Decrement:
						case UnaryOperatorType.Await:
							return true;
						default:
							return false;
					}
				default:
					return false;
			}
		}
		#endregion

		#region FindInsertionPoints
		List<(InsertionPoint InsertionPoint, BlockContainer Loop)> loopTracking = new List<(InsertionPoint, BlockContainer)>();

		/// <summary>
		/// Finds insertion points for all variables used within `node`
		/// and adds them to the variableDict.
		/// 
		/// `level` == nesting depth of `node` within root node.
		/// </summary>
		/// <remarks>
		/// Insertion point for a variable = common parent of all uses of that variable
		/// = smallest possible scope that contains all the uses of the variable
		/// </remarks>
		void FindInsertionPoints(AstNode node, int nodeLevel)
		{
			BlockContainer loop = node.Annotation<BlockContainer>();
			if (loop != null) {
				loopTracking.Add((new InsertionPoint { level = nodeLevel, nextNode = node }, loop));
			}
			try {
				for (AstNode child = node.FirstChild; child != null; child = child.NextSibling) {
					FindInsertionPoints(child, nodeLevel + 1);
				}
				var identExpr = node as IdentifierExpression;
				if (identExpr != null) {
					var rr = identExpr.GetResolveResult() as ILVariableResolveResult;
					if (rr != null && VariableNeedsDeclaration(rr.Variable.Kind)) {
						var variable = rr.Variable;
						InsertionPoint newPoint;
						int startIndex = loopTracking.Count - 1;
						if (variable.CaptureScope != null && startIndex > -1 && variable.CaptureScope != loopTracking[startIndex].Loop) {
							while (startIndex > -1 && loopTracking[startIndex].Loop != variable.CaptureScope)
								startIndex--;
							newPoint = loopTracking[startIndex + 1].InsertionPoint;
						} else {
							newPoint = new InsertionPoint { level = nodeLevel, nextNode = identExpr };
						}
						VariableToDeclare v;
						if (variableDict.TryGetValue(rr.Variable, out v)) {
							v.InsertionPoint = FindCommonParent(v.InsertionPoint, newPoint);
						} else {
							v = new VariableToDeclare(
								rr.Variable.Type, rr.Variable.Name, rr.Variable.HasInitialValue,
								newPoint, sourceOrder: variableDict.Count);
							variableDict.Add(rr.Variable, v);
						}
					}
				}
			} finally {
				if (loop != null)
					loopTracking.RemoveAt(loopTracking.Count - 1);
			}
		}

		bool VariableNeedsDeclaration(VariableKind kind)
		{
			switch (kind) {
				case VariableKind.PinnedLocal:
				case VariableKind.Parameter:
				case VariableKind.Exception:
				case VariableKind.UsingLocal:
				case VariableKind.ForeachLocal:
					return false;
				default:
					return true;
			}
		}
		
		/// <summary>
		/// Finds an insertion point in a common parent instruction.
		/// </summary>
		InsertionPoint FindCommonParent(InsertionPoint oldPoint, InsertionPoint newPoint)
		{
			// First ensure we're looking at nodes on the same level:
			oldPoint = oldPoint.UpTo(newPoint.level);
			newPoint = newPoint.UpTo(oldPoint.level);
			Debug.Assert(newPoint.level == oldPoint.level);
			// Then go up the tree until both points share the same parent:
			while (oldPoint.nextNode.Parent != newPoint.nextNode.Parent) {
				oldPoint = oldPoint.Up();
				newPoint = newPoint.Up();
			}
			// return oldPoint as that one comes first in the source code
			return oldPoint;
		}
		#endregion
		
		/// <summary>
		/// Some variable declarations in C# are illegal (colliding),
		/// even though the variable live ranges are not overlapping.
		/// 
		/// Multiple declarations in same block:
		/// <code>
		/// int i = 1; use(1);
		/// int i = 2; use(2);
		/// </code>
		/// 
		/// "Hiding" declaration in nested block:
		/// <code>
		/// int i = 1; use(1);
		/// if (...) {
		///   int i = 2; use(2);
		/// }
		/// </code>
		/// 
		/// Nested blocks are illegal even if the parent block
		/// declares the variable later:
		/// <code>
		/// if (...) {
		///   int i = 1; use(i);
		/// }
		/// int i = 2; use(i);
		/// </code>
		/// 
		/// ResolveCollisions() detects all these cases, and combines the variable declarations
		/// to a single declaration that is usable for the combined scopes.
		/// </summary>
		void ResolveCollisions()
		{
			var multiDict = new MultiDictionary<string, VariableToDeclare>();
			foreach (var v in variableDict.Values) {
				// We can only insert variable declarations in blocks, but FindInsertionPoints() didn't
				// guarantee that it finds only blocks.
				// Fix that up now.
				while (!(v.InsertionPoint.nextNode.Parent is BlockStatement)) {
					if (v.InsertionPoint.nextNode.Parent is ForStatement f && v.InsertionPoint.nextNode == f.Initializers.FirstOrDefault() && IsMatchingAssignment(v, out _))
						break;
					if (v.InsertionPoint.nextNode.Parent is UsingStatement u && v.InsertionPoint.nextNode == u.ResourceAcquisition && IsMatchingAssignment(v, out _))
						break;
					v.InsertionPoint = v.InsertionPoint.Up();
				}
				
				// Go through all potentially colliding variables:
				foreach (var prev in multiDict[v.Name]) {
					if (prev.RemovedDueToCollision)
						continue;
					// Go up until both nodes are on the same level:
					InsertionPoint point1 = prev.InsertionPoint.UpTo(v.InsertionPoint.level);
					InsertionPoint point2 = v.InsertionPoint.UpTo(prev.InsertionPoint.level);
					Debug.Assert(point1.level == point2.level);
					if (point1.nextNode.Parent == point2.nextNode.Parent) {
						// We found a collision!
						prev.ReplacementDueToCollision = v;
						// Continue checking other entries in multiDict against the new position of `v`.
						if (prev.SourceOrder < v.SourceOrder) {
							// If we switch v's insertion point to prev's insertion point,
							// we also need to copy prev's SourceOrder value.
							v.InsertionPoint = point1;
							v.SourceOrder = prev.SourceOrder;
						} else {
							v.InsertionPoint = point2;
						}
						v.DefaultInitialization |= prev.DefaultInitialization;
						// I think we don't need to re-check the dict entries that we already checked earlier,
						// because the new v.InsertionPoint only collides with another point x if either
						// the old v.InsertionPoint or the old prev.InsertionPoint already collided with x.
					}
				}
				
				multiDict.Add(v.Name, v);
			}
		}

		bool IsMatchingAssignment(VariableToDeclare v, out AssignmentExpression assignment)
		{
			assignment = v.InsertionPoint.nextNode as AssignmentExpression ?? (v.InsertionPoint.nextNode as ExpressionStatement)?.Expression as AssignmentExpression;
			Expression expectedExpr = new IdentifierExpression(v.Name);
			if (v.Type.Kind == TypeKind.ByReference) {
				expectedExpr = new DirectionExpression(FieldDirection.Ref, expectedExpr);
			}
			if (assignment != null && assignment.Operator == AssignmentOperatorType.Assign && assignment.Left.IsMatch(expectedExpr))
				return true;
			return false;
		}

		void InsertVariableDeclarations(TransformContext context)
		{
			var replacements = new List<KeyValuePair<AstNode, AstNode>>();
			foreach (var p in variableDict) {
				var v = p.Value;
				if (v.RemovedDueToCollision)
					continue;
				
				if (IsMatchingAssignment(v, out AssignmentExpression assignment)) {
					AstType type;
					if (context.Settings.AnonymousTypes && v.Type.ContainsAnonymousType()) {
						type = new SimpleType("var");
					} else {
						type = context.TypeSystemAstBuilder.ConvertType(v.Type);
					}
					var vds = new VariableDeclarationStatement(type, v.Name, assignment.Right.Detach());
					var init = vds.Variables.Single();
					init.AddAnnotation(assignment.Left.GetResolveResult());
					foreach (object annotation in assignment.Left.Annotations.Concat(assignment.Annotations)) {
						if (!(annotation is ResolveResult)) {
							init.AddAnnotation(annotation);
						}
					}
					replacements.Add(new KeyValuePair<AstNode, AstNode>(v.InsertionPoint.nextNode, vds));
				} else {
					Expression initializer = null;
					AstType type = context.TypeSystemAstBuilder.ConvertType(v.Type);
					if (v.DefaultInitialization) {
						initializer = new DefaultValueExpression(type.Clone());
					}
					var vds = new VariableDeclarationStatement(type, v.Name, initializer);
					vds.Variables.Single().AddAnnotation(new ILVariableResolveResult(p.Key, p.Key.Type));
					Debug.Assert(v.InsertionPoint.nextNode.Role == BlockStatement.StatementRole);
					v.InsertionPoint.nextNode.Parent.InsertChildBefore(
						v.InsertionPoint.nextNode,
						vds,
						BlockStatement.StatementRole);
				}
			}
			// perform replacements at end, so that we don't replace a node while it is still referenced by a VariableToDeclare
			foreach (var pair in replacements) {
				pair.Key.ReplaceWith(pair.Value);
			}
		}
	}
}