LehrerApp/LehrerApp.Core/Importing/ImportAbstractions.cs

using System.Collections.ObjectModel;

namespace LehrerApp.Core.Importing;

/// <summary>
/// Stabile, typsichere Kennung eines Importformats. Werte werden normalisiert, damit
/// Groß-/Kleinschreibung oder versehentliche Leerzeichen nicht zu Tippfehlern führen.
/// </summary>
public readonly record struct ImportFormatId
{
    public string Value { get; }

    public ImportFormatId(string value)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(value);
        Value = value.Trim().ToLowerInvariant();
    }

    public override string ToString() => Value ?? "";
}

/// <summary>Eine vollständig in den Speicher geladene Importdatei.</summary>
public sealed class ImportFile
{
    public string FileName { get; }
    public ReadOnlyMemory<byte> Content { get; }
    public string Extension => Path.GetExtension(FileName).ToLowerInvariant();

    public ImportFile(string fileName, ReadOnlyMemory<byte> content)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(fileName);
        FileName = Path.GetFileName(fileName);
        Content = content;
    }
}

public enum ImportMessageSeverity { Information, Warning, Error }

public sealed record ImportMessage(
    ImportMessageSeverity Severity,
    string Code,
    string Message,
    string? SourceReference = null);

/// <summary>
/// Ergebnis einer schnellen Formaterkennung. 0 bedeutet „kein Treffer“, 100 einen
/// eindeutigen Treffer. Die eigentliche vollständige Prüfung erfolgt erst beim Parsen.
/// </summary>
public sealed record ImportDetection
{
    public int Confidence { get; }
    public string? Reason { get; }
    public bool IsMatch => Confidence > 0;

    public ImportDetection(int confidence, string? reason = null)
    {
        if (confidence is < 0 or > 100)
            throw new ArgumentOutOfRangeException(nameof(confidence));
        Confidence = confidence;
        Reason = reason;
    }

    public static ImportDetection NoMatch(string? reason = null) => new(0, reason);
    public static ImportDetection Match(int confidence, string? reason = null) =>
        new(confidence, reason);
}

public sealed class ImportParseResult<T>
{
    public IReadOnlyList<T> Items { get; }
    public IReadOnlyList<ImportMessage> Messages { get; }
    public bool IsValid => Messages.All(message => message.Severity != ImportMessageSeverity.Error);

    public ImportParseResult(IEnumerable<T> items, IEnumerable<ImportMessage>? messages = null)
    {
        ArgumentNullException.ThrowIfNull(items);
        Items = ReadOnly(items);
        Messages = ReadOnly(messages ?? []);
    }

    private static IReadOnlyList<TItem> ReadOnly<TItem>(IEnumerable<TItem> values) =>
        new ReadOnlyCollection<TItem>(values.ToList());
}

/// <summary>
/// Ein registrierbarer, UI-unabhängiger Handler für genau ein Dateiformat und ein
/// normalisiertes fachliches Importmodell.
/// </summary>
public interface IImportHandler<T>
{
    ImportFormatId FormatId { get; }
    string DisplayName { get; }
    IReadOnlyCollection<string> SupportedExtensions { get; }

    ValueTask<ImportDetection> DetectAsync(
        ImportFile file,
        CancellationToken cancellationToken = default);

    ValueTask<ImportParseResult<T>> ParseAsync(
        ImportFile file,
        CancellationToken cancellationToken = default);
}

public sealed record DetectedImportHandler<T>(
    IImportHandler<T> Handler,
    ImportDetection Detection);

public abstract class ImportFormatException(string message) : Exception(message);

public sealed class UnsupportedImportFormatException(string fileName)
    : ImportFormatException($"Für die Datei „{fileName}“ wurde kein passendes Importformat gefunden.");

public sealed class AmbiguousImportFormatException(string fileName, IEnumerable<string> formats)
    : ImportFormatException(
        $"Die Datei „{fileName}“ passt zu mehreren Importformaten: {string.Join(", ", formats)}.");

/// <summary>
/// Verwaltet Handler eines fachlichen Importziels und wählt anhand des Dateiinhalts
/// den besten Treffer. Die Dateiendung dient bei gleicher Erkennungsqualität als Hinweis.
/// </summary>
public sealed class ImportHandlerCatalog<T>
{
    private readonly IReadOnlyList<IImportHandler<T>> _handlers;

    public ImportHandlerCatalog(IEnumerable<IImportHandler<T>> handlers)
    {
        ArgumentNullException.ThrowIfNull(handlers);
        var materialized = handlers.ToList();
        var duplicate = materialized
            .GroupBy(handler => handler.FormatId)
            .FirstOrDefault(group => group.Count() > 1);
        if (duplicate is not null)
            throw new ArgumentException(
                $"Das Importformat „{duplicate.Key}“ wurde mehrfach registriert.", nameof(handlers));
        _handlers = new ReadOnlyCollection<IImportHandler<T>>(materialized);
    }

    public IReadOnlyList<IImportHandler<T>> Handlers => _handlers;

    public IReadOnlyList<string> SupportedExtensions => _handlers
        .SelectMany(handler => handler.SupportedExtensions)
        .Select(NormalizeExtension)
        .Where(extension => extension.Length > 0)
        .Distinct(StringComparer.OrdinalIgnoreCase)
        .OrderBy(extension => extension, StringComparer.OrdinalIgnoreCase)
        .ToList()
        .AsReadOnly();

    public async ValueTask<DetectedImportHandler<T>> DetectAsync(
        ImportFile file,
        CancellationToken cancellationToken = default)
    {
        ArgumentNullException.ThrowIfNull(file);
        var matches = new List<Candidate>();
        foreach (var handler in _handlers)
        {
            cancellationToken.ThrowIfCancellationRequested();
            var detection = await handler.DetectAsync(file, cancellationToken).ConfigureAwait(false);
            if (detection.IsMatch)
                matches.Add(new Candidate(handler, detection, SupportsExtension(handler, file.Extension)));
        }

        if (matches.Count == 0)
            throw new UnsupportedImportFormatException(file.FileName);

        var highestConfidence = matches.Max(match => match.Detection.Confidence);
        var best = matches.Where(match => match.Detection.Confidence == highestConfidence).ToList();
        if (best.Count > 1 && best.Any(match => match.ExtensionMatches))
            best = best.Where(match => match.ExtensionMatches).ToList();

        if (best.Count != 1)
            throw new AmbiguousImportFormatException(
                file.FileName,
                best.Select(match => match.Handler.DisplayName));

        return new DetectedImportHandler<T>(best[0].Handler, best[0].Detection);
    }

    private static bool SupportsExtension(IImportHandler<T> handler, string extension) =>
        handler.SupportedExtensions
            .Select(NormalizeExtension)
            .Contains(extension, StringComparer.OrdinalIgnoreCase);

    private static string NormalizeExtension(string extension)
    {
        if (string.IsNullOrWhiteSpace(extension)) return "";
        var trimmed = extension.Trim().ToLowerInvariant();
        return trimmed.StartsWith('.') ? trimmed : $".{trimmed}";
    }

    private sealed record Candidate(
        IImportHandler<T> Handler,
        ImportDetection Detection,
        bool ExtensionMatches);
}

public sealed record ImportConflictOption(string Id, string Label, string? Description = null);

public sealed record ImportConflict(
    string Id,
    string Title,
    string Description,
    string ImportedValue,
    string? ExistingValue,
    IReadOnlyList<ImportConflictOption> Options,
    string DefaultOptionId,
    string? SourceReference = null);

/// <summary>Vom UI getroffene, für den Core rein datenförmige Konfliktentscheidung.</summary>
public sealed record ImportDecision(string ConflictId, string SelectedOptionId);