using System.Text.Json;

namespace Mr.BrickAdventures.Progression.Adapters;

/// <summary>
/// Reading and writing versioned save files. Shared by the Playthrough and Profile stores, which
/// differ only in what they hold and where they live.
/// </summary>
internal static class JsonFiles
{
    internal static readonly JsonSerializerOptions Options = new()
    {
        WriteIndented = true,
        PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
        PropertyNameCaseInsensitive = true,
    };

    /// <summary>
    /// Writes to a temporary file and then renames it over the destination. A crash mid-write leaves
    /// the previous file intact rather than a half-written one — which matters more once there are
    /// several slots to lose.
    /// </summary>
    internal static void WriteAtomic<T>(string path, int version, T data)
    {
        Directory.CreateDirectory(Path.GetDirectoryName(path)!);

        var temp = path + ".tmp";
        File.WriteAllText(temp, JsonSerializer.Serialize(new SaveEnvelope<T>(version, data), Options));
        File.Move(temp, path, overwrite: true);
    }

    /// <summary>
    /// Reads a save file. Absent when there is no file; Corrupt when there is one but it cannot be
    /// turned into <typeparamref name="TAggregate"/> — unreadable JSON, an unknown version, or a
    /// snapshot the domain rejects. Never conflates the two: that conflation is what let a version
    /// bump silently overwrite a player's save.
    /// </summary>
    internal static LoadResult<TAggregate> Read<TSnapshot, TAggregate>(
        string path,
        int currentVersion,
        Func<TSnapshot, TAggregate> restore)
        where TSnapshot : class
        where TAggregate : class
    {
        if (!File.Exists(path)) return LoadResult<TAggregate>.Absent();

        SaveEnvelope<TSnapshot>? envelope;
        try
        {
            envelope = JsonSerializer.Deserialize<SaveEnvelope<TSnapshot>>(File.ReadAllText(path), Options);
        }
        catch (JsonException e)
        {
            return LoadResult<TAggregate>.Corrupt($"unreadable json: {e.Message}");
        }

        if (envelope?.Data is null)
            return LoadResult<TAggregate>.Corrupt("file held no save data");

        if (envelope.Version > currentVersion)
            return LoadResult<TAggregate>.Corrupt(
                $"saved by a newer build (version {envelope.Version}, this build reads {currentVersion})");

        if (envelope.Version < currentVersion)
            return LoadResult<TAggregate>.Corrupt(
                $"version {envelope.Version} has no migration to {currentVersion}");

        try
        {
            return LoadResult<TAggregate>.Loaded(restore(envelope.Data));
        }
        catch (Exception e)
        {
            return LoadResult<TAggregate>.Corrupt($"snapshot rejected by the domain: {e.Message}");
        }
    }
}