// 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.Threading;
using Microsoft.CodeAnalysis.CSharp.Extensions;
using Microsoft.CodeAnalysis.CSharp.Extensions.ContextQuery;
using Microsoft.CodeAnalysis.CSharp.Syntax;
using Microsoft.CodeAnalysis.Shared.Extensions;

namespace Microsoft.CodeAnalysis.CSharp.Completion.KeywordRecommenders;

internal sealed class NamespaceKeywordRecommender() : AbstractSyntacticSingleKeywordRecommender(SyntaxKind.NamespaceKeyword)
{
    protected override bool IsValidContext(int position, CSharpSyntaxContext context, CancellationToken cancellationToken)
    {
        var syntaxTree = context.SyntaxTree;

        // namespaces are illegal in interactive code:
        if (syntaxTree.IsScript())
        {
            return false;
        }

        // cases:
        // root: |

        // root: n|

        // extern alias a;
        // |

        // extern alias a;
        // n|

        // using Goo;
        // |

        // using Goo;
        // n|

        // using Goo = Bar;
        // |

        // using Goo = Bar;
        // n|

        // namespace N {}
        // |

        // namespace N {}
        // n|

        // class C {}
        // |

        // class C {}
        // n|

        var leftToken = context.LeftToken;
        var token = context.TargetToken;

        // root: n|

        // ns Goo { n|

        // extern alias a;
        // n|

        // using Goo;
        // n|

        // using Goo = Bar;
        // n|

        // a namespace can't come before usings/externs
        // a child namespace can't come before usings/externs
        var nextToken = leftToken.GetNextToken(includeSkipped: true);
        if (nextToken.IsUsingOrExternKeyword() ||
            (nextToken.Kind() == SyntaxKind.GlobalKeyword && nextToken.GetAncestor<UsingDirectiveSyntax>()?.GlobalKeyword == nextToken))
        {
            return false;
        }

        // root: |
        if (token.Kind() == SyntaxKind.None)
        {
            // root namespace
            var root = (CompilationUnitSyntax)syntaxTree.GetRoot(cancellationToken);
            if (root.Externs.Count > 0 ||
                root.Usings.Count > 0)
            {
                return false;
            }

            return true;
        }

        if (token.Kind() == SyntaxKind.OpenBraceToken &&
            token.Parent.IsKind(SyntaxKind.NamespaceDeclaration))
        {
            return true;
        }

        // extern alias a;
        // |

        // using Goo;
        // |
        if (token.Kind() == SyntaxKind.SemicolonToken)
        {
            if (token.Parent is (kind: SyntaxKind.ExternAliasDirective or SyntaxKind.UsingDirective) &&
                !token.Parent.Parent.IsKind(SyntaxKind.FileScopedNamespaceDeclaration))
            {
                return true;
            }
        }

        // class C {}
        // |
        if (token.Kind() == SyntaxKind.CloseBraceToken)
        {
            if (token.Parent is TypeDeclarationSyntax &&
                token.Parent.Parent is not TypeDeclarationSyntax)
            {
                return true;
            }
            else if (token.Parent.IsKind(SyntaxKind.NamespaceDeclaration))
            {
                return true;
            }
        }

        // delegate void D();
        // |

        if (token.Kind() == SyntaxKind.SemicolonToken)
        {
            if (token.Parent.IsKind(SyntaxKind.DelegateDeclaration) &&
                token.Parent.Parent is not TypeDeclarationSyntax)
            {
                return true;
            }
        }

        // [assembly: goo]
        // |

        if (token.Kind() == SyntaxKind.CloseBracketToken &&
            token.Parent.IsKind(SyntaxKind.AttributeList) &&
            token.Parent.IsParentKind(SyntaxKind.CompilationUnit))
        {
            return true;
        }

        return false;
    }
}