using System.Text;

namespace LehrerApp.Core.Importing;

/// <summary>Erkennt den Notenlistenexport und erklärt dessen fehlende Teilnehmerdaten.</summary>
public sealed class MarksPerLessonCsvImportHandler : IImportHandler<ImportedStudent>
{
    private static readonly string[] SignatureHeaders =
        ["Datum", "Name", "Klasse", "Fach", "Prüfungsart", "Note", "Bemerkung", "Benutzer", "Schlüssel (extern)", "Gesamtnote"];
    private static readonly UTF8Encoding StrictUtf8 = new(false, true);

    public ImportFormatId FormatId => StudentImportFormats.MarksPerLessonCsv;
    public string DisplayName => "Unterrichts-Notenliste";
    public IReadOnlyCollection<string> SupportedExtensions { get; } = [".csv"];

    public ValueTask<ImportDetection> DetectAsync(ImportFile file, CancellationToken cancellationToken = default)
    {
        cancellationToken.ThrowIfCancellationRequested();
        string text;
        try { text = StrictUtf8.GetString(file.Content.Span); }
        catch (DecoderFallbackException) { return ValueTask.FromResult(ImportDetection.NoMatch()); }
        var end = text.IndexOfAny(['\r', '\n']);
        var firstLine = (end >= 0 ? text[..end] : text).TrimStart('\uFEFF');
        var headers = firstLine.Split('\t').Select(x => x.Trim()).ToHashSet(StringComparer.OrdinalIgnoreCase);
        return ValueTask.FromResult(SignatureHeaders.All(headers.Contains)
            ? ImportDetection.Match(file.Extension.Equals(".csv", StringComparison.OrdinalIgnoreCase) ? 100 : 95)
            : ImportDetection.NoMatch());
    }

    public ValueTask<ImportParseResult<ImportedStudent>> ParseAsync(ImportFile file, CancellationToken cancellationToken = default)
    {
        cancellationToken.ThrowIfCancellationRequested();
        IReadOnlyList<ImportMessage> messages =
        [
            new(ImportMessageSeverity.Error, "marks-per-lesson.no-participants",
                "Die Notenliste enthält keine verlässlichen Teilnehmerdaten. Bitte exportiere die echte Kurs-/Teilnehmerliste.")
        ];
        return ValueTask.FromResult(new ImportParseResult<ImportedStudent>([], messages));
    }
}