// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.
 
using System;
using System.Diagnostics;
using Microsoft.CodeAnalysis.CSharp.Symbols;
using Roslyn.Utilities;
 
namespace Microsoft.CodeAnalysis.CSharp
{
    internal partial class LocalDataFlowPass<TLocalState, TLocalFunctionState>
    {
        internal readonly struct VariableIdentifier : IEquatable<VariableIdentifier>
        {
            public readonly Symbol Symbol;
            /// <summary>
            /// Indicates whether this variable is nested inside another tracked variable.
            /// For instance, if a field `x` of a struct is a tracked variable, the symbol is not sufficient
            /// to uniquely determine which field is being tracked. The containing slot(s) would
            /// identify which tracked variable the field `x` is part of.
            /// </summary>
            public readonly int ContainingSlot;
 
            public VariableIdentifier(Symbol symbol, int containingSlot = 0)
            {
                Debug.Assert(containingSlot >= 0);
                Debug.Assert(symbol.Kind switch
                {
                    SymbolKind.Local => true,
                    SymbolKind.Parameter => true,
                    SymbolKind.Field => true,
                    SymbolKind.Property => true,
                    SymbolKind.Event => true,
                    SymbolKind.ErrorType => true,
                    SymbolKind.Method when symbol is MethodSymbol m && m.MethodKind == MethodKind.LocalFunction => true,
                    _ => false
                });
                Symbol = symbol;
                ContainingSlot = containingSlot;
            }
 
            public bool Exists
            {
                get { return (object)Symbol != null; }
            }
 
            public override int GetHashCode()
            {
                Debug.Assert(Exists);
 
                int currentKey = ContainingSlot;
                // MemberIndexOpt, if available, is a fast approach to comparing relative members,
                // and is necessary in cases such as anonymous types where OriginalDefinition will be distinct.
                int? thisIndex = Symbol.MemberIndexOpt;
                return thisIndex.HasValue ?
                    Hash.Combine(thisIndex.GetValueOrDefault(), currentKey) :
                    Hash.Combine(Symbol.OriginalDefinition, currentKey);
            }
 
            public bool Equals(VariableIdentifier other)
            {
                Debug.Assert(Exists);
                Debug.Assert(other.Exists);
 
                if (ContainingSlot != other.ContainingSlot)
                {
                    return false;
                }
 
                // MemberIndexOpt, if available, is a fast approach to comparing relative members,
                // and is necessary in cases such as anonymous types where OriginalDefinition will be distinct.
                int? thisIndex = Symbol.MemberIndexOpt;
                int? otherIndex = other.Symbol.MemberIndexOpt;
                if (thisIndex != otherIndex)
                {
                    return false;
                }
 
                if (thisIndex.HasValue)
                {
                    return true;
                }
 
                return Symbol.Equals(other.Symbol, TypeCompareKind.AllIgnoreOptions);
            }
 
            public override bool Equals(object? obj)
            {
                throw ExceptionUtilities.Unreachable();
            }
 
            [Obsolete]
            public static bool operator ==(VariableIdentifier left, VariableIdentifier right)
            {
                throw ExceptionUtilities.Unreachable();
            }
 
            [Obsolete]
            public static bool operator !=(VariableIdentifier left, VariableIdentifier right)
            {
                throw ExceptionUtilities.Unreachable();
            }
 
            public override string ToString()
            {
                return $"ContainingSlot={ContainingSlot}, Symbol={Symbol.GetDebuggerDisplay()}";
            }
        }
    }
}