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

namespace Microsoft.CodeAnalysis.CSharp.Extensions;

internal static partial class SyntaxTreeExtensions
{
    public static bool IsPrimaryFunctionExpressionContext(this SyntaxTree syntaxTree, int position, SyntaxToken tokenOnLeftOfPosition)
    {
        return
            syntaxTree.IsTypeOfExpressionContext(position, tokenOnLeftOfPosition) ||
            syntaxTree.IsDefaultExpressionContext(position, tokenOnLeftOfPosition) ||
            syntaxTree.IsSizeOfExpressionContext(position, tokenOnLeftOfPosition);
    }

    public static bool IsInNonUserCode(this SyntaxTree syntaxTree, int position, CancellationToken cancellationToken)
    {
        return
            syntaxTree.IsEntirelyWithinNonUserCodeComment(position, cancellationToken) ||
            syntaxTree.IsEntirelyWithinConflictMarker(position, cancellationToken) ||
            syntaxTree.IsEntirelyWithinStringOrCharLiteral(position, cancellationToken) ||
            syntaxTree.IsInInactiveRegion(position, cancellationToken);
    }

    public static bool IsInPartiallyWrittenGeneric(
        this SyntaxTree syntaxTree, int position, CancellationToken cancellationToken)
    {
        return syntaxTree.IsInPartiallyWrittenGeneric(position, cancellationToken, out _, out _);
    }

    public static bool IsInPartiallyWrittenGeneric(
        this SyntaxTree syntaxTree,
        int position,
        CancellationToken cancellationToken,
        out SyntaxToken genericIdentifier)
    {
        return syntaxTree.IsInPartiallyWrittenGeneric(position, cancellationToken, out genericIdentifier, out _);
    }

    public static bool IsInPartiallyWrittenGeneric(
        this SyntaxTree syntaxTree,
        int position,
        CancellationToken cancellationToken,
        out SyntaxToken genericIdentifier,
        out SyntaxToken lessThanToken)
    {
        genericIdentifier = default;
        lessThanToken = default;
        var index = 0;

        var token = syntaxTree.FindTokenOnLeftOfPosition(position, cancellationToken);
        if (token.Kind() == SyntaxKind.None)
        {
            return false;
        }

        // check whether we are under type or member decl
        if (token.GetAncestor<TypeParameterListSyntax>() != null)
        {
            return false;
        }

        var stack = 0;
        while (true)
        {
            switch (token.Kind())
            {
                case SyntaxKind.LessThanToken:
                    if (stack == 0)
                    {
                        // got here so we read successfully up to a < now we have to read the
                        // name before that and we're done!
                        lessThanToken = token;
                        token = token.GetPreviousToken(includeSkipped: true);
                        if (token.Kind() == SyntaxKind.None)
                        {
                            return false;
                        }

                        // ok
                        // so we've read something like:
                        // ~~~~~~~~~<a,b,...
                        // but we need to know the simple name that precedes the <
                        // it could be
                        // ~~~~~~goo<a,b,...
                        if (token.Kind() == SyntaxKind.IdentifierToken)
                        {
                            // okay now check whether it is actually partially written
                            if (IsFullyWrittenGeneric(token, lessThanToken))
                            {
                                return false;
                            }

                            genericIdentifier = token;
                            return true;
                        }

                        return false;
                    }
                    else
                    {
                        stack--;
                        break;
                    }

                // fall through
                case SyntaxKind.GreaterThanToken:
                case SyntaxKind.GreaterThanGreaterThanToken:
                case SyntaxKind.GreaterThanGreaterThanGreaterThanToken:

                    // FindTokenOnLeftOfPosition returns the token that we are contained in. This means in cases like
                    // G<G<G<int>>$$> the compiler might have parsed that final >> as a shift operator. We want to only count the
                    // number of >s to the left of where we actually are.
                    if (token.Span.End <= position)
                        stack += token.Span.Length;
                    else
                        stack += (position - token.Span.Start);

                    break;

                case SyntaxKind.AsteriskToken:      // for int*
                case SyntaxKind.QuestionToken:      // for int?
                case SyntaxKind.ColonToken:         // for global::  (so we don't dismiss help as you type the first :)
                case SyntaxKind.ColonColonToken:    // for global::
                case SyntaxKind.CloseBracketToken:
                case SyntaxKind.OpenBracketToken:
                case SyntaxKind.DotToken:
                case SyntaxKind.IdentifierToken:
                    break;

                case SyntaxKind.CommaToken:
                    if (stack == 0)
                    {
                        index++;
                    }

                    break;

                default:
                    // user might have typed "in" on the way to typing "int"
                    // don't want to disregard this genericname because of that
                    if (SyntaxFacts.IsKeywordKind(token.Kind()))
                    {
                        break;
                    }

                    // anything else and we're sunk.
                    return false;
            }

            // look backward one token, include skipped tokens, because the parser frequently
            // does skip them in cases like: "Func<A, B", which get parsed as: expression
            // statement "Func<A" with missing semicolon, expression statement "B" with missing
            // semicolon, and the "," is skipped.
            token = token.GetPreviousToken(includeSkipped: true);
            if (token.Kind() == SyntaxKind.None)
            {
                return false;
            }
        }
    }

    private static bool IsFullyWrittenGeneric(SyntaxToken token, SyntaxToken lessThanToken)
    {
        return token.Parent is GenericNameSyntax genericName && genericName.TypeArgumentList != null &&
               genericName.TypeArgumentList.LessThanToken == lessThanToken && !genericName.TypeArgumentList.GreaterThanToken.IsMissing;
    }
}