// 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.Collections.Immutable;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.FindSymbols;
using Microsoft.CodeAnalysis.Shared.Extensions;
using Roslyn.Utilities;

namespace Microsoft.CodeAnalysis.GenerateMember.GenerateParameterizedMember;

internal abstract partial class AbstractGenerateParameterizedMemberService<TService, TSimpleNameSyntax, TExpressionSyntax, TInvocationExpressionSyntax>
{
    private static async ValueTask<ITypeSymbol> ReplaceTypeParametersBasedOnTypeConstraintsAsync(
        Project project,
        ITypeSymbol type,
        Compilation compilation,
        ISet<string> availableTypeParameterNames,
        CancellationToken cancellationToken)
    {
        var visitor = new DetermineSubstitutionsVisitor(
            compilation, availableTypeParameterNames, project, cancellationToken);

        await visitor.Visit(type).ConfigureAwait(false);
        return type.SubstituteTypes(visitor.Substitutions, compilation);
    }

    private sealed class DetermineSubstitutionsVisitor(
        Compilation compilation, ISet<string> availableTypeParameterNames, Project project, CancellationToken cancellationToken) : AsyncSymbolVisitor
    {
        public readonly Dictionary<ITypeSymbol, ITypeSymbol> Substitutions = [];
        private readonly CancellationToken _cancellationToken = cancellationToken;
        private readonly Compilation _compilation = compilation;
        private readonly ISet<string> _availableTypeParameterNames = availableTypeParameterNames;
        private readonly Project _project = project;

        public override ValueTask VisitDynamicType(IDynamicTypeSymbol symbol)
            => default;

        public override ValueTask VisitArrayType(IArrayTypeSymbol symbol)
            => symbol.ElementType.Accept(this);

        public override async ValueTask VisitNamedType(INamedTypeSymbol symbol)
        {
            foreach (var typeArg in symbol.TypeArguments)
                await typeArg.Accept(this).ConfigureAwait(false);
        }

        public override ValueTask VisitPointerType(IPointerTypeSymbol symbol)
            => symbol.PointedAtType.Accept(this);

        public override async ValueTask VisitTypeParameter(ITypeParameterSymbol symbol)
        {
            if (_availableTypeParameterNames.Contains(symbol.Name))
                return;

            switch (symbol.ConstraintTypes.Length)
            {
                case 0:
                    // If there are no constraint then there is no replacement required.
                    return;

                case 1:
                    // If there is one constraint which is a INamedTypeSymbol then return the INamedTypeSymbol
                    // because the TypeParameter is expected to be of that type
                    // else return the original symbol
                    if (symbol.ConstraintTypes.ElementAt(0) is INamedTypeSymbol namedType)
                        Substitutions.Add(symbol, namedType);

                    return;
            }

            var commonDerivedType = await DetermineCommonDerivedTypeAsync(symbol).ConfigureAwait(false);
            if (commonDerivedType != null)
                Substitutions.Add(symbol, commonDerivedType);
        }

        private async ValueTask<ITypeSymbol?> DetermineCommonDerivedTypeAsync(ITypeParameterSymbol symbol)
        {
            if (!symbol.ConstraintTypes.All(t => t is INamedTypeSymbol))
                return null;

            var solution = _project.Solution;
            var projects = solution.Projects.ToImmutableHashSet();

            var commonTypes = await GetDerivedAndImplementedTypesAsync(
                (INamedTypeSymbol)symbol.ConstraintTypes[0], projects).ConfigureAwait(false);

            for (var i = 1; i < symbol.ConstraintTypes.Length; i++)
            {
                var currentTypes = await GetDerivedAndImplementedTypesAsync(
                    (INamedTypeSymbol)symbol.ConstraintTypes[i], projects).ConfigureAwait(false);
                commonTypes.IntersectWith(currentTypes);

                if (commonTypes.Count == 0)
                    return null;
            }

            // If there was any intersecting derived type among the constraint types then pick the first of the lot.
            if (commonTypes.Count == 0)
                return null;

            var commonType = commonTypes.First();

            // If the resultant intersecting type contains any Type arguments that could be replaced 
            // using the type constraints then recursively update the type until all constraints are appropriately handled
            var substitutedType = await ReplaceTypeParametersBasedOnTypeConstraintsAsync(
                _project, commonType, _compilation, _availableTypeParameterNames, _cancellationToken).ConfigureAwait(false);

            var similarTypes = SymbolFinder.FindSimilarSymbols(substitutedType, _compilation, _cancellationToken);
            if (similarTypes.Any())
                return similarTypes.First();

            similarTypes = SymbolFinder.FindSimilarSymbols(commonType, _compilation, _cancellationToken);
            return similarTypes.FirstOrDefault() ?? symbol;
        }

        private async Task<ISet<INamedTypeSymbol>> GetDerivedAndImplementedTypesAsync(
            INamedTypeSymbol constraintType, IImmutableSet<Project> projects)
        {
            var solution = _project.Solution;

            var symbol = constraintType;
            var derivedClasses = await SymbolFinder.FindDerivedClassesAsync(
                symbol, solution, transitive: true, projects, _cancellationToken).ConfigureAwait(false);

            var implementedTypes = await SymbolFinder.FindImplementationsAsync(
                symbol, solution, transitive: true, projects, _cancellationToken).ConfigureAwait(false);

            return derivedClasses.Concat(implementedTypes).ToSet();
        }
    }
}