// Copyright (c) AlphaSierraPapa for the SharpDevelop Team (for details please see \doc\copyright.txt)
// This code is distributed under MIT X11 license (for details please see \doc\license.txt)

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using Mono.Cecil;

namespace ICSharpCode.Decompiler.ILAst
{
	/// <summary>
	/// Performs inlining transformations.
	/// </summary>
	public class ILInlining
	{
		readonly ILBlock method;
		internal Dictionary<ILVariable, int> numStloc  = new Dictionary<ILVariable, int>();
		internal Dictionary<ILVariable, int> numLdloc  = new Dictionary<ILVariable, int>();
		internal Dictionary<ILVariable, int> numLdloca = new Dictionary<ILVariable, int>();
		
		public ILInlining(ILBlock method)
		{
			this.method = method;
			AnalyzeMethod();
		}
		
		void AnalyzeMethod()
		{
			numStloc.Clear();
			numLdloc.Clear();
			numLdloca.Clear();
			
			// Analyse the whole method
			foreach(ILExpression expr in method.GetSelfAndChildrenRecursive<ILExpression>()) {
				ILVariable locVar = expr.Operand as ILVariable;
				if (locVar != null) {
					if (expr.Code == ILCode.Stloc) {
						numStloc[locVar] = numStloc.GetOrDefault(locVar) + 1;
					} else if (expr.Code == ILCode.Ldloc) {
						numLdloc[locVar] = numLdloc.GetOrDefault(locVar) + 1;
					} else if (expr.Code == ILCode.Ldloca) {
						numLdloca[locVar] = numLdloca.GetOrDefault(locVar) + 1;
					} else {
						throw new NotSupportedException(expr.Code.ToString());
					}
				}
			}
		}
		
		public bool InlineAllVariables()
		{
			bool modified = false;
			ILInlining i = new ILInlining(method);
			foreach (ILBlock block in method.GetSelfAndChildrenRecursive<ILBlock>())
				modified |= i.InlineAllInBlock(block);
			return modified;
		}
		
		public bool InlineAllInBlock(ILBlock block)
		{
			bool modified = false;
			List<ILNode> body = block.Body;
			for(int i = 0; i < body.Count - 1;) {
				ILVariable locVar;
				ILExpression expr;
				if (body[i].Match(ILCode.Stloc, out locVar, out expr) && InlineOneIfPossible(block.Body, i, aggressive: false)) {
					modified = true;
					i = Math.Max(0, i - 1); // Go back one step
				} else {
					i++;
				}
			}
			foreach(ILBasicBlock bb in body.OfType<ILBasicBlock>()) {
				modified |= InlineAllInBasicBlock(bb);
		}
			return modified;
		}
		
		public bool InlineAllInBasicBlock(ILBasicBlock bb)
		{
			bool modified = false;
			List<ILNode> body = bb.Body;
			for(int i = 0; i < body.Count - 1;) {
				ILVariable locVar;
				ILExpression expr;
				if (body[i].Match(ILCode.Stloc, out locVar, out expr) && InlineOneIfPossible(bb.Body, i, aggressive: false)) {
					modified = true;
					i = Math.Max(0, i - 1); // Go back one step
				} else {
					i++;
				}
			}
			return modified;
		}
		
		/// <summary>
		/// Inlines instructions before pos into block.Body[pos].
		/// </summary>
		/// <returns>The number of instructions that were inlined.</returns>
		public int InlineInto(List<ILNode> body, int pos, bool aggressive)
		{
			if (pos >= body.Count)
				return 0;
			int count = 0;
			while (--pos >= 0) {
				ILExpression expr = body[pos] as ILExpression;
				if (expr == null || expr.Code != ILCode.Stloc)
					break;
				if (InlineOneIfPossible(body, pos, aggressive))
					count++;
				else
					break;
			}
			return count;
		}
		
		/// <summary>
		/// Aggressively inlines the stloc instruction at block.Body[pos] into the next instruction, if possible.
		/// If inlining was possible; we will continue to inline (non-aggressively) into the the combined instruction.
		/// </summary>
		/// <remarks>
		/// After the operation, pos will point to the new combined instruction.
		/// </remarks>
		public bool InlineIfPossible(List<ILNode> body, ref int pos)
		{
			if (InlineOneIfPossible(body, pos, true)) {
				pos -= InlineInto(body, pos, false);
				return true;
			}
			return false;
		}
		
		/// <summary>
		/// Inlines the stloc instruction at block.Body[pos] into the next instruction, if possible.
		/// </summary>
		public bool InlineOneIfPossible(List<ILNode> body, int pos, bool aggressive)
		{
			ILVariable v;
			ILExpression inlinedExpression;
			if (body[pos].Match(ILCode.Stloc, out v, out inlinedExpression) && !v.IsPinned) {
				if (InlineIfPossible(v, inlinedExpression, body.ElementAtOrDefault(pos+1), aggressive)) {
					// Assign the ranges of the stloc instruction:
					inlinedExpression.ILRanges.AddRange(((ILExpression)body[pos]).ILRanges);
					// Remove the stloc instruction:
					body.RemoveAt(pos);
					return true;
				} else if (numLdloc.GetOrDefault(v) == 0 && numLdloca.GetOrDefault(v) == 0) {
					// The variable is never loaded
					if (inlinedExpression.HasNoSideEffects()) {
						// Remove completely
						body.RemoveAt(pos);
						return true;
					} else if (inlinedExpression.CanBeExpressionStatement() && v.IsGenerated) {
						// Assign the ranges of the stloc instruction:
						inlinedExpression.ILRanges.AddRange(((ILExpression)body[pos]).ILRanges);
						// Remove the stloc, but keep the inner expression
						body[pos] = inlinedExpression;
						return true;
					}
				}
			}
			return false;
		}
		
		/// <summary>
		/// Inlines 'expr' into 'next', if possible.
		/// </summary>
		bool InlineIfPossible(ILVariable v, ILExpression inlinedExpression, ILNode next, bool aggressive)
		{
			// ensure the variable is accessed only a single time
			if (!(numStloc.GetOrDefault(v) == 1 && numLdloc.GetOrDefault(v) == 1 && numLdloca.GetOrDefault(v) == 0))
				return false;
			
			if (next is ILCondition)
				next = ((ILCondition)next).Condition;
			else if (next is ILWhileLoop)
				next = ((ILWhileLoop)next).Condition;
			
			ILExpression parent;
			int pos;
			if (FindLoadInNext(next as ILExpression, v, inlinedExpression, out parent, out pos) == true) {
				if (!aggressive && !v.IsGenerated && !NonAggressiveInlineInto((ILExpression)next, parent, inlinedExpression))
					return false;
				
				// Assign the ranges of the ldloc instruction:
				inlinedExpression.ILRanges.AddRange(parent.Arguments[pos].ILRanges);
				
				parent.Arguments[pos] = inlinedExpression;
				
				return true;
			}
			return false;
		}
		
		bool NonAggressiveInlineInto(ILExpression next, ILExpression parent, ILExpression inlinedExpression)
		{
			switch (inlinedExpression.Code) {
				case ILCode.InitArray:
				case ILCode.InitCollection:
				case ILCode.DefaultValue:
					return true;
			}
			
			switch (next.Code) {
				case ILCode.Ret:
					return parent.Code == ILCode.Ret;
				case ILCode.Brtrue:
					return parent.Code == ILCode.Brtrue;
				case ILCode.Switch:
					return parent.Code == ILCode.Switch || parent.Code == ILCode.Sub;
				default:
					return false;
			}
		}
		
		/// <summary>
		/// Gets whether 'expressionBeingMoved' can be inlined into 'expr'.
		/// </summary>
		public bool CanInlineInto(ILExpression expr, ILVariable v, ILExpression expressionBeingMoved)
		{
			ILExpression parent;
			int pos;
			return FindLoadInNext(expr, v, expressionBeingMoved, out parent, out pos) == true;
		}
		
		/// <summary>
		/// Finds the position to inline to.
		/// </summary>
		/// <returns>true = found; false = cannot continue search; null = not found</returns>
		bool? FindLoadInNext(ILExpression expr, ILVariable v, ILExpression expressionBeingMoved, out ILExpression parent, out int pos)
		{
			parent = null;
			pos = 0;
			if (expr == null)
				return false;
			for (int i = 0; i < expr.Arguments.Count; i++) {
				// Stop when seeing an opcode that does not guarantee that its operands will be evaluated.
				// Inlining in that case might result in the inlined expresion not being evaluted.
				if (i == 1 && (expr.Code == ILCode.LogicAnd || expr.Code == ILCode.LogicOr || expr.Code == ILCode.TernaryOp))
					return false;
				
				ILExpression arg = expr.Arguments[i];
				
				if (arg.Code == ILCode.Ldloc && arg.Operand == v) {
					parent = expr;
					pos = i;
					return true;
				}
				bool? r = FindLoadInNext(arg, v, expressionBeingMoved, out parent, out pos);
				if (r != null)
					return r;
			}
			if (IsSafeForInlineOver(expr, expressionBeingMoved))
				return null; // continue searching
			else
				return false; // abort, inlining not possible
		}
		
		/// <summary>
		/// Determines whether it is save to move 'expressionBeingMoved' past 'expr'
		/// </summary>
		bool IsSafeForInlineOver(ILExpression expr, ILExpression expressionBeingMoved)
		{
			switch (expr.Code) {
				case ILCode.Ldloc:
					ILVariable loadedVar = (ILVariable)expr.Operand;
					if (numLdloca.GetOrDefault(loadedVar) != 0) {
						// abort, inlining is not possible
						return false;
					}
					foreach (ILExpression potentialStore in expressionBeingMoved.GetSelfAndChildrenRecursive<ILExpression>()) {
						if (potentialStore.Code == ILCode.Stloc && potentialStore.Operand == loadedVar)
							return false;
					}
					// the expression is loading a non-forbidden variable
					return true;
				case ILCode.Ldloca:
				case ILCode.Ldflda:
				case ILCode.Ldsflda:
				case ILCode.Ldelema:
					// address-loading instructions are safe if their arguments are safe
					foreach (ILExpression arg in expr.Arguments) {
						if (!IsSafeForInlineOver(arg, expressionBeingMoved))
							return false;
					}
					return true;
				default:
					// abort, inlining is not possible
					return false;
			}
		}
		
		/// <summary>
		/// Runs a very simple form of copy propagation.
		/// Copy propagation is used in two cases:
		/// 1) assignments from arguments to local variables
		///    If the target variable is assigned to only once (so always is that argument) and the argument is never changed (no ldarga/starg),
		///    then we can replace the variable with the argument.
		/// 2) assignments of address-loading instructions to local variables
		/// </summary>
		public void CopyPropagation()
		{
			foreach (ILBlock block in method.GetSelfAndChildrenRecursive<ILBlock>()) {
				for (int i = 0; i < block.Body.Count; i++) {
					ILVariable v;
					ILExpression copiedExpr;
					if (block.Body[i].Match(ILCode.Stloc, out v, out copiedExpr)
					    && !v.IsParameter && numStloc.GetOrDefault(v) == 1 && numLdloca.GetOrDefault(v) == 0
					    && CanPerformCopyPropagation(copiedExpr, v))
					{
						// un-inline the arguments of the ldArg instruction
						ILVariable[] uninlinedArgs = new ILVariable[copiedExpr.Arguments.Count];
						for (int j = 0; j < uninlinedArgs.Length; j++) {
							uninlinedArgs[j] = new ILVariable { IsGenerated = true, Name = v.Name + "_cp_" + j };
							block.Body.Insert(i++, new ILExpression(ILCode.Stloc, uninlinedArgs[j], copiedExpr.Arguments[j]));
						}
						
						// perform copy propagation:
						foreach (var expr in method.GetSelfAndChildrenRecursive<ILExpression>()) {
							if (expr.Code == ILCode.Ldloc && expr.Operand == v) {
								expr.Code = copiedExpr.Code;
								expr.Operand = copiedExpr.Operand;
								for (int j = 0; j < uninlinedArgs.Length; j++) {
									expr.Arguments.Add(new ILExpression(ILCode.Ldloc, uninlinedArgs[j]));
								}
							}
						}
						
						block.Body.RemoveAt(i);
						if (uninlinedArgs.Length > 0) {
							// if we un-inlined stuff; we need to update the usage counters
							AnalyzeMethod();
						}
						InlineInto(block.Body, i, aggressive: false); // maybe inlining gets possible after the removal of block.Body[i]
						i -= uninlinedArgs.Length + 1;
					}
				}
			}
		}
		
		bool CanPerformCopyPropagation(ILExpression expr, ILVariable copyVariable)
		{
			switch (expr.Code) {
				case ILCode.Ldloca:
				case ILCode.Ldelema:
				case ILCode.Ldflda:
				case ILCode.Ldsflda:
					// All address-loading instructions always return the same value for a given operand/argument combination,
					// so they can be safely copied.
					return true;
				case ILCode.Ldloc:
					ILVariable v = (ILVariable)expr.Operand;
					if (v.IsParameter) {
						// Parameters can be copied only if they aren't assigned to (directly or indirectly via ldarga)
						return numLdloca.GetOrDefault(v) == 0 && numStloc.GetOrDefault(v) == 0;
					} else {
						// Variables are be copied only if both they and the target copy variable are generated,
						// and if the variable has only a single assignment
						return v.IsGenerated && copyVariable.IsGenerated && numLdloca.GetOrDefault(v) == 0 && numStloc.GetOrDefault(v) == 1;
					}
				default:
					return false;
			}
		}
	}
}