// 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.Collections.Generic;
using System.Linq;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.Editing;
using Roslyn.Utilities;
 
namespace Microsoft.CodeAnalysis.CodeGeneration;
 
internal abstract class AbstractFlagsEnumGenerator : IComparer<(IFieldSymbol field, ulong value)>
{
    protected abstract SyntaxNode CreateExplicitlyCastedLiteralValue(INamedTypeSymbol enumType, SpecialType underlyingSpecialType, object constantValue);
    protected abstract bool IsValidName(INamedTypeSymbol enumType, string name);
 
    protected abstract SyntaxGeneratorInternal SyntaxGenerator { get; }
 
    public SyntaxNode CreateEnumConstantValue(INamedTypeSymbol enumType, object constantValue)
    {
        // Code copied from System.Enum.
        var isFlagsEnum = IsFlagsEnum(enumType);
        if (isFlagsEnum)
        {
            return CreateFlagsEnumConstantValue(enumType, constantValue);
        }
        else
        {
            // Try to see if its one of the enum values.  If so, add that.  Otherwise, just add
            // the literal value of the enum.
            return CreateNonFlagsEnumConstantValue(enumType, constantValue);
        }
    }
 
    private static bool IsFlagsEnum(INamedTypeSymbol typeSymbol)
    {
        if (typeSymbol.TypeKind != TypeKind.Enum)
        {
            return false;
        }
 
        foreach (var attribute in typeSymbol.GetAttributes())
        {
            var ctor = attribute.AttributeConstructor;
            if (ctor != null)
            {
                var type = ctor.ContainingType;
                if (!ctor.Parameters.Any() &&
                     type.Name == "FlagsAttribute" &&
                     type.ContainingSymbol is INamespaceSymbol { Name: "System", ContainingNamespace.IsGlobalNamespace: true })
                {
                    return true;
                }
            }
        }
 
        return false;
    }
 
    private SyntaxNode CreateFlagsEnumConstantValue(INamedTypeSymbol enumType, object constantValue)
    {
        // These values are sorted by value. Don't change this.
        var allFieldsAndValues = new List<(IFieldSymbol field, ulong value)>();
        GetSortedEnumFieldsAndValues(enumType, allFieldsAndValues);
 
        var usedFieldsAndValues = new List<(IFieldSymbol field, ulong value)>();
        return CreateFlagsEnumConstantValue(enumType, constantValue, allFieldsAndValues, usedFieldsAndValues);
    }
 
    private SyntaxNode CreateFlagsEnumConstantValue(
        INamedTypeSymbol enumType,
        object constantValue,
        List<(IFieldSymbol field, ulong value)> allFieldsAndValues,
        List<(IFieldSymbol field, ulong value)> usedFieldsAndValues)
    {
        Contract.ThrowIfNull(enumType.EnumUnderlyingType);
        var underlyingSpecialType = enumType.EnumUnderlyingType.SpecialType;
        var constantValueULong = underlyingSpecialType.ConvertUnderlyingValueToUInt64(constantValue);
 
        var result = constantValueULong;
 
        // We will not optimize this code further to keep it maintainable. There are some
        // boundary checks that can be applied to minimize the comparisons required. This code
        // works the same for the best/worst case. In general the number of items in an enum are
        // sufficiently small and not worth the optimization.
        for (var index = allFieldsAndValues.Count - 1; index >= 0 && result != 0; index--)
        {
            var fieldAndValue = allFieldsAndValues[index];
            var valueAtIndex = fieldAndValue.value;
 
            if (valueAtIndex != 0 && (result & valueAtIndex) == valueAtIndex)
            {
                result -= valueAtIndex;
                usedFieldsAndValues.Add(fieldAndValue);
            }
        }
 
        // We were able to represent this number as a bitwise OR of valid flags.
        if (result == 0 && usedFieldsAndValues.Count > 0)
        {
            // We want to emit the fields in lower to higher value.  So we walk backward.
            SyntaxNode? finalNode = null;
            for (var i = usedFieldsAndValues.Count - 1; i >= 0; i--)
            {
                var field = usedFieldsAndValues[i];
                var node = CreateMemberAccessExpression(field.field, enumType, underlyingSpecialType);
                if (finalNode == null)
                {
                    finalNode = node;
                }
                else
                {
                    finalNode = this.SyntaxGenerator.BitwiseOrExpression(finalNode, node);
                }
            }
 
            Contract.ThrowIfNull(finalNode);
            return finalNode;
        }
 
        // We couldn't find fields to OR together to make the value.
 
        // If we had 0 as the value, and there's an enum value equal to 0, then use that.
        var zeroField = GetZeroField(allFieldsAndValues);
        if (constantValueULong == 0 && zeroField != null)
        {
            return CreateMemberAccessExpression(zeroField, enumType, underlyingSpecialType);
        }
        else
        {
            // Add anything else in as a literal value.
            return CreateExplicitlyCastedLiteralValue(enumType, underlyingSpecialType, constantValue);
        }
    }
 
    private SyntaxNode CreateMemberAccessExpression(
        IFieldSymbol field, INamedTypeSymbol enumType, SpecialType underlyingSpecialType)
    {
        if (IsValidName(enumType, field.Name))
        {
            return this.SyntaxGenerator.MemberAccessExpression(
                this.SyntaxGenerator.TypeExpression(enumType),
                this.SyntaxGenerator.IdentifierName(field.Name));
        }
        else
        {
            Contract.ThrowIfNull(field.ConstantValue);
            return CreateExplicitlyCastedLiteralValue(enumType, underlyingSpecialType, field.ConstantValue);
        }
    }
 
    private static IFieldSymbol? GetZeroField(List<(IFieldSymbol field, ulong value)> allFieldsAndValues)
    {
        for (var i = allFieldsAndValues.Count - 1; i >= 0; i--)
        {
            var (field, value) = allFieldsAndValues[i];
            if (value == 0)
            {
                return field;
            }
        }
 
        return null;
    }
 
    private void GetSortedEnumFieldsAndValues(
        INamedTypeSymbol enumType,
        List<(IFieldSymbol field, ulong value)> allFieldsAndValues)
    {
        Contract.ThrowIfNull(enumType.EnumUnderlyingType);
        var underlyingSpecialType = enumType.EnumUnderlyingType.SpecialType;
        foreach (var field in enumType.GetMembers().OfType<IFieldSymbol>())
        {
            if (field is { HasConstantValue: true, ConstantValue: not null })
            {
                var value = underlyingSpecialType.ConvertUnderlyingValueToUInt64(field.ConstantValue);
                allFieldsAndValues.Add((field, value));
            }
        }
 
        allFieldsAndValues.Sort(this);
    }
 
    private SyntaxNode CreateNonFlagsEnumConstantValue(INamedTypeSymbol enumType, object constantValue)
    {
        Contract.ThrowIfNull(enumType.EnumUnderlyingType);
        var underlyingSpecialType = enumType.EnumUnderlyingType.SpecialType;
        var constantValueULong = underlyingSpecialType.ConvertUnderlyingValueToUInt64(constantValue);
 
        // See if there's a member with this value.  If so, then use that.
        foreach (var field in enumType.GetMembers().OfType<IFieldSymbol>())
        {
            if (field is { HasConstantValue: true, ConstantValue: not null })
            {
                var fieldValue = underlyingSpecialType.ConvertUnderlyingValueToUInt64(field.ConstantValue);
                if (constantValueULong == fieldValue)
                    return CreateMemberAccessExpression(field, enumType, underlyingSpecialType);
            }
        }
 
        // Otherwise, just add the enum as a literal.
        return CreateExplicitlyCastedLiteralValue(enumType, underlyingSpecialType, constantValue);
    }
 
    int IComparer<(IFieldSymbol field, ulong value)>.Compare((IFieldSymbol field, ulong value) x, (IFieldSymbol field, ulong value) y)
    {
        unchecked
        {
            return
                (long)x.value < (long)y.value
                    ? -1
                    : (long)x.value > (long)y.value
                        ? 1
                        : -x.field.Name.CompareTo(y.field.Name);
        }
    }
}