using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.IO;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Xml;
using System.Xml.Serialization;
using Microsoft.Extensions.Logging;
using Schaad.Accounting.Interfaces;

namespace Schaad.Accounting.Repositories
{
    public abstract class BaseRepository
    {
        // One semaphore per absolute file path so concurrent SaveAsync+SaveAsync and
        // SaveAsync+LoadAsync are serialized and can't observe a half-written file.
        private static readonly ConcurrentDictionary<string, SemaphoreSlim> FileLocks =
            new(StringComparer.OrdinalIgnoreCase);

        protected readonly ISettingsService settingsService;
        protected readonly RepositoryCache cache;
        protected readonly ILogger logger;

        protected BaseRepository(ISettingsService settingsService, RepositoryCache cache, ILogger logger)
        {
            this.settingsService = settingsService;
            this.cache = cache;
            this.logger = logger;
        }

        /// <summary>
        /// Copies fileName from last year's directory to the current year's directory when the
        /// current year doesn't have it yet. Kept synchronous because it runs from repository
        /// constructors; File.Copy has no async equivalent in .NET 9 either way.
        /// </summary>
        protected void EnsureFileExists(string fileName)
        {
            string filePath = Path.Combine(settingsService.GetDbPath(), fileName);
            var semaphore = GetLock(filePath);
            semaphore.Wait();
            try
            {
                if (File.Exists(filePath) == false)
                {
                    var lastYearFile = Path.Combine(settingsService.GetLastYearDbPath(), fileName);
                    if (File.Exists(lastYearFile))
                    {
                        File.Copy(lastYearFile, filePath);
                    }
                }
            }
            finally
            {
                semaphore.Release();
            }
        }

        /// <summary>
        /// Load a list, cached per scope. First call reads the XML file; subsequent calls in
        /// the same scope return the cached list. <see cref="SaveAsync"/> invalidates the entry.
        /// </summary>
        protected Task<List<T>> LoadListAsync<T>(string fileName)
        {
            var filePath = Path.Combine(settingsService.GetDbPath(), fileName);
            return cache.GetOrLoadAsync(filePath, async () => await LoadAsync<List<T>>(fileName) ?? new List<T>());
        }

        /// <summary>
        /// Save an object to an xml file. Writes to a .tmp sibling and then atomically
        /// renames it, so a crash mid-write leaves the previous file intact. Invalidates
        /// the corresponding cache entry.
        /// </summary>
        protected async Task SaveAsync<T>(T obj, string fileName)
        {
            var filePath = Path.Combine(settingsService.GetDbPath(), fileName);
            var tmpPath = filePath + ".tmp";

            var semaphore = GetLock(filePath);
            await semaphore.WaitAsync();
            try
            {
                var settings = new XmlWriterSettings
                {
                    Encoding = Encoding.UTF8,
                    Indent = true
                };

                try
                {
                    // XmlSerializer is synchronous — serialize to memory, then write async.
                    using var memory = new MemoryStream();
                    using (var writer = XmlWriter.Create(memory, settings))
                    {
                        var serializer = new XmlSerializer(typeof(T));
                        serializer.Serialize(writer, obj);
                    }
                    await File.WriteAllBytesAsync(tmpPath, memory.ToArray());

                    // File.Move has no async form in .NET 9.
                    File.Move(tmpPath, filePath, overwrite: true);
                }
                catch (Exception ex)
                {
                    logger.LogError(ex, "Failed to save {FilePath}", filePath);
                    if (File.Exists(tmpPath))
                    {
                        try { File.Delete(tmpPath); } catch { /* best effort */ }
                    }
                    throw;
                }
            }
            finally
            {
                semaphore.Release();
            }

            cache.Invalidate(filePath);
        }

        /// <summary>
        /// Load an object from an xml file (uncached; used for one-off reads such as file
        /// existence bootstrapping). Prefer <see cref="LoadListAsync{T}"/> for repository reads.
        /// </summary>
        protected async Task<T> LoadAsync<T>(string fileName)
        {
            var filePath = Path.Combine(settingsService.GetDbPath(), fileName);

            var semaphore = GetLock(filePath);
            await semaphore.WaitAsync();
            try
            {
                if (File.Exists(filePath) == false)
                {
                    return default(T);
                }

                var bytes = await File.ReadAllBytesAsync(filePath);
                using var memory = new MemoryStream(bytes);
                var serializer = new XmlSerializer(typeof(T));
                return (T)serializer.Deserialize(memory);
            }
            finally
            {
                semaphore.Release();
            }
        }

        private static SemaphoreSlim GetLock(string filePath)
            => FileLocks.GetOrAdd(filePath, _ => new SemaphoreSlim(1, 1));
    }
}