// 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.

#nullable disable

using System;
using System.Collections.Generic;
using System.Composition;
using System.Linq;
using System.Threading;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.Collections;
using Microsoft.CodeAnalysis.CSharp.Extensions;
using Microsoft.CodeAnalysis.CSharp.Syntax;
using Microsoft.CodeAnalysis.Highlighting;
using Microsoft.CodeAnalysis.Host.Mef;
using Microsoft.CodeAnalysis.Shared.Collections;
using Microsoft.CodeAnalysis.Text;

namespace Microsoft.CodeAnalysis.CSharp.KeywordHighlighting;

[ExportHighlighter(LanguageNames.CSharp), Shared]
[method: ImportingConstructor]
[method: Obsolete(MefConstruction.ImportingConstructorMessage, error: true)]
internal sealed class IfStatementHighlighter() : AbstractKeywordHighlighter<IfStatementSyntax>(findInsideTrivia: false)
{
    protected override bool ContainsHighlightableToken(ref TemporaryArray<SyntaxToken> tokens)
        => tokens.Any(static t => t.Kind() is SyntaxKind.IfKeyword or SyntaxKind.ElseKeyword);

    protected override void AddHighlights(
        IfStatementSyntax ifStatement, List<TextSpan> highlights, CancellationToken cancellationToken)
    {
        if (ifStatement.Parent.Kind() != SyntaxKind.ElseClause)
        {
            ComputeSpans(ifStatement, highlights);
        }
    }

    private static void ComputeSpans(
        IfStatementSyntax ifStatement, List<TextSpan> highlights)
    {
        highlights.Add(ifStatement.IfKeyword.Span);

        // Loop to get all the else if parts
        while (ifStatement != null && ifStatement.Else != null)
        {
            // Check for 'else if' scenario' (the statement in the else clause is an if statement)
            var elseKeyword = ifStatement.Else.ElseKeyword;

            if (ifStatement.Else.Statement is IfStatementSyntax elseIfStatement)
            {
                if (OnlySpacesBetween(elseKeyword, elseIfStatement.IfKeyword))
                {
                    // Highlight both else and if tokens if they are on the same line
                    highlights.Add(TextSpan.FromBounds(
                        elseKeyword.SpanStart,
                        elseIfStatement.IfKeyword.Span.End));
                }
                else
                {
                    // Highlight the else and if tokens separately
                    highlights.Add(elseKeyword.Span);
                    highlights.Add(elseIfStatement.IfKeyword.Span);
                }

                // Continue the enumeration looking for more else blocks
                ifStatement = elseIfStatement;
            }
            else
            {
                // Highlight just the else and we're done
                highlights.Add(elseKeyword.Span);
                break;
            }
        }
    }

    public static bool OnlySpacesBetween(SyntaxToken first, SyntaxToken second)
    {
        return first.TrailingTrivia.AsString().All(c => c == ' ') &&
               second.LeadingTrivia.AsString().All(c => c == ' ');
    }
}