using System.Globalization;
using System.Text;
using System.Text.Json;
using MoShiro.Backend.Application;
using MoShiro.Protocol;

namespace MoShiro.Backend.Storage.Sqlite;

/// <summary>Backend 到宿主机的文件信箱。状态卷只读挂载，业务进程改写不了验签和安装结果。</summary>
public sealed class BackendUpdateFiles : IBackendUpdateFiles
{
    private readonly string _requests;
    private readonly string _status;

    public BackendUpdateFiles(BackendOptions options)
    {
        _requests = Path.GetFullPath(options.BackendUpdateDirectory ?? Path.Combine(options.DataDirectory, "update"));
        _status = Path.GetFullPath(options.BackendUpdateStatusDirectory ?? Path.Combine(options.DataDirectory, "update-status"));
    }

    public async ValueTask WriteOfferAsync(string manifest, string signature, CancellationToken ct)
    {
        await WriteAsync("offer.manifest", manifest, ct);
        await WriteAsync("offer.sig", signature, ct);
    }

    public async ValueTask<UpdateStatus?> ReadStatusAsync(CancellationToken ct)
    {
        string? json = await ReadAsync(Path.Combine(_status, "status.json"), 4096, ct);
        if (json is null)
        {
            return null;
        }

        try
        {
            return JsonSerializer.Deserialize(json, MoShiroJsonContext.Default.UpdateStatus);
        }
        catch (JsonException)
        {
            return null;
        }
    }

    public async ValueTask<long?> ReadRequestAsync(CancellationToken ct)
    {
        string? request = await ReadAsync(Path.Combine(_requests, "apply.request"), 64, ct);
        return long.TryParse(request, NumberStyles.None, CultureInfo.InvariantCulture, out long seq) ? seq : null;
    }

    public ValueTask RequestAsync(long seq, CancellationToken ct) => WriteAsync("apply.request", seq.ToString(CultureInfo.InvariantCulture), ct);

    private async ValueTask WriteAsync(string name, string value, CancellationToken ct)
    {
        Directory.CreateDirectory(_requests);
        string path = Path.Combine(_requests, name);
        string temporary = Path.Combine(_requests, $".{name}.{Guid.NewGuid():N}");
        try
        {
            await File.WriteAllTextAsync(temporary, value, new UTF8Encoding(false), ct);
            File.Move(temporary, path, overwrite: true);
        }
        finally
        {
            if (File.Exists(temporary))
            {
                File.Delete(temporary);
            }
        }
    }

    private static async ValueTask<string?> ReadAsync(string path, int limit, CancellationToken ct)
    {
        try
        {
            var file = new FileInfo(path);
            if (!file.Exists || file.LinkTarget is not null || file.Length > limit)
            {
                return null;
            }

            return await File.ReadAllTextAsync(path, ct);
        }
        catch (IOException)
        {
            return null; // 更新器正做原子替换，下一次读取再试
        }
    }
}