// 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.Composition;
using System.Diagnostics;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.CodeAnalysis.CSharp.Syntax;
using Microsoft.CodeAnalysis.CSharp.Utilities;
using Microsoft.CodeAnalysis.Editing;
using Microsoft.CodeAnalysis.Formatting;
using Microsoft.CodeAnalysis.Host.Mef;
using Microsoft.CodeAnalysis.PooledObjects;
using Microsoft.CodeAnalysis.ReplaceDiscardDeclarationsWithAssignments;
using Microsoft.CodeAnalysis.Shared.Extensions;
using Roslyn.Utilities;
 
namespace Microsoft.CodeAnalysis.CSharp.ReplaceDiscardDeclarationsWithAssignments;
 
using static SyntaxFactory;
 
[ExportLanguageService(typeof(IReplaceDiscardDeclarationsWithAssignmentsService), LanguageNames.CSharp), Shared]
internal sealed class CSharpReplaceDiscardDeclarationsWithAssignmentsService : IReplaceDiscardDeclarationsWithAssignmentsService
{
    private const string DiscardVariableName = "_";
 
    [ImportingConstructor]
    [Obsolete(MefConstruction.ImportingConstructorMessage, error: true)]
    public CSharpReplaceDiscardDeclarationsWithAssignmentsService()
    {
    }
 
    public async Task<SyntaxNode> ReplaceAsync(
        Document document,
        SyntaxNode memberDeclaration,
        CancellationToken cancellationToken)
    {
        var semanticModel = await document.GetRequiredSemanticModelAsync(cancellationToken).ConfigureAwait(false);
        var editor = new SyntaxEditor(memberDeclaration, document.Project.Solution.Services);
        foreach (var child in memberDeclaration.DescendantNodes())
        {
            switch (child)
            {
                case LocalDeclarationStatementSyntax localDeclarationStatement:
                    if (localDeclarationStatement.Declaration.Variables.Any(IsDiscardDeclaration))
                    {
                        // Skip replacing discard declarations in "using var"
                        if (localDeclarationStatement.UsingKeyword != default)
                        {
                            continue;
                        }
 
                        RemoveDiscardHelper.ProcessDeclarationStatement(localDeclarationStatement, editor);
                    }
 
                    break;
 
                case CatchDeclarationSyntax catchDeclaration:
                    if (IsDiscardDeclaration(catchDeclaration))
                    {
                        // "catch (Exception _)" => "catch (Exception)"
                        editor.ReplaceNode(catchDeclaration, catchDeclaration.WithIdentifier(default));
                    }
 
                    break;
 
                case DeclarationExpressionSyntax declarationExpression:
                    if (declarationExpression.Designation is DiscardDesignationSyntax discardSyntax)
                    {
                        // "M(out var _)" => "M(out _)"
                        // "M(out int _)" => "M(out _)"
 
                        var discardToken = Identifier(
                            leading: declarationExpression.GetLeadingTrivia(),
                            contextualKind: SyntaxKind.UnderscoreToken,
                            text: discardSyntax.UnderscoreToken.Text,
                            valueText: discardSyntax.UnderscoreToken.ValueText,
                            trailing: declarationExpression.GetTrailingTrivia());
                        var replacementNode = IdentifierName(discardToken);
 
                        // Removing explicit type is possible only if there are no overloads of the method with same parameter.
                        // For example, if method "M" had overloads with signature "void M(int x)" and "void M(char x)",
                        // then the replacement "M(out int _)" => "M(out _)" will cause overload resolution error.
                        // Bail out if replacement changes semantics.
                        var speculationAnalyzer = new SpeculationAnalyzer(declarationExpression,
                            replacementNode, semanticModel, cancellationToken);
                        if (!speculationAnalyzer.ReplacementChangesSemantics())
                        {
                            editor.ReplaceNode(declarationExpression, replacementNode);
                        }
                    }
 
                    break;
 
                case DeclarationPatternSyntax declarationPattern:
                    if (declarationPattern.Designation is DiscardDesignationSyntax discardDesignationSyntax &&
                        declarationPattern.Parent is IsPatternExpressionSyntax isPatternExpression)
                    {
                        // "x is int _" => "x is int"
                        var replacementNode = BinaryExpression(
                            kind: SyntaxKind.IsExpression,
                            left: isPatternExpression.Expression,
                            operatorToken: isPatternExpression.IsKeyword,
                            right: declarationPattern.Type.WithTrailingTrivia(declarationPattern.GetTrailingTrivia()));
                        editor.ReplaceNode(isPatternExpression, replacementNode);
                    }
 
                    break;
            }
        }
 
        return editor.GetChangedRoot();
    }
 
    private static bool IsDiscardDeclaration(VariableDeclaratorSyntax variable)
        => variable.Identifier.Text == DiscardVariableName;
    private static bool IsDiscardDeclaration(CatchDeclarationSyntax catchDeclaration)
        => catchDeclaration.Identifier.Text == DiscardVariableName;
 
    private sealed class RemoveDiscardHelper : IDisposable
    {
        private readonly LocalDeclarationStatementSyntax _localDeclarationStatement;
        private readonly SyntaxEditor _editor;
        private readonly ArrayBuilder<StatementSyntax> _statementsBuilder;
        private SeparatedSyntaxList<VariableDeclaratorSyntax> _currentNonDiscardVariables = [];
 
        private RemoveDiscardHelper(LocalDeclarationStatementSyntax localDeclarationStatement, SyntaxEditor editor)
        {
            _localDeclarationStatement = localDeclarationStatement;
            _editor = editor;
 
            _statementsBuilder = ArrayBuilder<StatementSyntax>.GetInstance();
        }
 
        public static void ProcessDeclarationStatement(
            LocalDeclarationStatementSyntax localDeclarationStatement,
            SyntaxEditor editor)
        {
            using var helper = new RemoveDiscardHelper(localDeclarationStatement, editor);
            helper.ProcessDeclarationStatement();
        }
 
        public void Dispose() => _statementsBuilder.Free();
 
        private void ProcessDeclarationStatement()
        {
            // We will replace all discard variable declarations in this method with discard assignments,
            // For example,
            //  1. "int _ = M();" is replaced with "_ = M();"
            //  2. "int x = 1, _ = M(), y = 2;" is replaced with following statements:
            //          int x = 1;
            //          _ = M();
            //          int y = 2;
            // This is done to prevent compiler errors where the existing method has a discard
            // variable declaration at a line following the one we added a discard assignment in our fix.
 
            // Process all the declared variables in the given local declaration statement,
            // tracking the currently encountered non-discard variables. 
            foreach (var variable in _localDeclarationStatement.Declaration.Variables)
            {
                if (!IsDiscardDeclaration(variable))
                {
                    // Add to the list of currently encountered non-discard variables
                    _currentNonDiscardVariables = _currentNonDiscardVariables.Add(variable);
                }
                else
                {
                    // Process currently encountered non-discard variables to generate
                    // a local declaration statement with these variables.
                    GenerateDeclarationStatementForCurrentNonDiscardVariables();
 
                    // Process the discard variable declaration to replace it
                    // with an assignment to discard.
                    GenerateAssignmentForDiscardVariable(variable);
                }
            }
 
            // Process all the remaining variable declarators to generate
            // a local declaration statement with these variables.
            GenerateDeclarationStatementForCurrentNonDiscardVariables();
 
            // Now replace the original local declaration statement with
            // the replacement statement list tracked in _statementsBuilder.
 
            if (_statementsBuilder.Count == 0)
            {
                // Nothing to replace.
                return;
            }
 
            // Move the leading trivia from original local declaration statement
            // to the first statement of the replacement statement list.
            var leadingTrivia = _localDeclarationStatement.Declaration.Type.GetLeadingTrivia()
                .Concat(_localDeclarationStatement.Declaration.Type.GetTrailingTrivia());
            _statementsBuilder[0] = _statementsBuilder[0].WithLeadingTrivia(leadingTrivia);
 
            // Move the trailing trivia from original local declaration statement
            // to the last statement of the replacement statement list.
            var last = _statementsBuilder.Count - 1;
            var trailingTrivia = _localDeclarationStatement.SemicolonToken.GetAllTrivia();
            _statementsBuilder[last] = _statementsBuilder[last].WithTrailingTrivia(trailingTrivia);
 
            // Replace the original local declaration statement with new statement list
            // from _statementsBuilder.
            if (_localDeclarationStatement.Parent is BlockSyntax or SwitchSectionSyntax)
            {
                if (_statementsBuilder.Count > 1)
                {
                    _editor.InsertAfter(_localDeclarationStatement, _statementsBuilder.Skip(1));
                }
 
                _editor.ReplaceNode(_localDeclarationStatement, _statementsBuilder[0]);
            }
            else
            {
                _editor.ReplaceNode(_localDeclarationStatement, Block(_statementsBuilder));
            }
        }
 
        private void GenerateDeclarationStatementForCurrentNonDiscardVariables()
        {
            // Generate a variable declaration with all the currently tracked non-discard declarators.
            // For example, for a declaration "int x = 1, y = 2, _ = M(), z = 3;", we generate two variable declarations:
            //   1. "int x = 1, y = 2;" and
            //   2. "int z = 3;",
            // which are split by a single assignment statement "_ = M();"
            if (_currentNonDiscardVariables.Count > 0)
            {
                var statement = LocalDeclarationStatement(
                                    VariableDeclaration(_localDeclarationStatement.Declaration.Type, _currentNonDiscardVariables))
                                .WithAdditionalAnnotations(Formatter.Annotation);
                _statementsBuilder.Add(statement);
                _currentNonDiscardVariables = [];
            }
        }
 
        private void GenerateAssignmentForDiscardVariable(VariableDeclaratorSyntax variable)
        {
            Debug.Assert(IsDiscardDeclaration(variable));
 
            // Convert a discard declaration with initializer of the form "int _ = M();" into
            // a discard assignment "_ = M();"
            if (variable.Initializer != null)
            {
                _statementsBuilder.Add(
                    ExpressionStatement(
                        AssignmentExpression(
                            kind: SyntaxKind.SimpleAssignmentExpression,
                            left: IdentifierName(variable.Identifier),
                            operatorToken: variable.Initializer.EqualsToken,
                            right: variable.Initializer.Value))
                    .WithAdditionalAnnotations(Formatter.Annotation));
            }
        }
    }
}