// Copyright (c) 2017 Siegfried Pammer
// 
// 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.Diagnostics;

namespace ICSharpCode.Decompiler.IL
{
	/// <summary>Null coalescing operator expression. <c>if.notnull(valueInst, fallbackInst)</c></summary>
	/// <remarks>
	/// This instruction can used in 3 different cases:
	/// Case 1: both ValueInst and FallbackInst are of reference type.
	///    Semantics: equivalent to "valueInst != null ? valueInst : fallbackInst",
	///               except that valueInst is evaluated only once.
	/// Case 2: both ValueInst and FallbackInst are of type Nullable{T}.
	///    Semantics: equivalent to "valueInst.HasValue ? valueInst : fallbackInst",
	///               except that valueInst is evaluated only once.
	/// Case 3: ValueInst is Nullable{T}, but FallbackInst is non-nullable value type.
	///    Semantics: equivalent to "valueInst.HasValue ? valueInst.Value : fallbackInst",
	///               except that valueInst is evaluated only once.
	/// </remarks>
	partial class NullCoalescingInstruction
	{
		public NullCoalescingInstruction(ILInstruction valueInst, ILInstruction fallbackInst) : base(OpCode.NullCoalescingInstruction)
		{
			this.ValueInst = valueInst;
			this.FallbackInst = fallbackInst;
		}

		internal override void CheckInvariant(ILPhase phase)
		{
			base.CheckInvariant(phase);
			Debug.Assert(valueInst.ResultType == StackType.O || valueInst.ResultType == fallbackInst.ResultType);
		}

		public override StackType ResultType {
			get {
				return fallbackInst.ResultType;
			}
		}

		public override InstructionFlags DirectFlags {
			get {
				return InstructionFlags.ControlFlow;
			}
		}

		protected override InstructionFlags ComputeFlags()
		{
			// valueInst is always executed; fallbackInst only sometimes
			return InstructionFlags.ControlFlow | valueInst.Flags
				| SemanticHelper.CombineBranches(InstructionFlags.None, fallbackInst.Flags);
		}

		public override void WriteTo(ITextOutput output)
		{
			output.Write(OpCode);
			output.Write("(");
			valueInst.WriteTo(output);
			output.Write(", ");
			fallbackInst.WriteTo(output);
			output.Write(")");
		}
	}
}