using System.Net.Http.Json;
using System.Text.Json;
using EnvelopeGenerator.Application.Common.Dto;
using Microsoft.Extensions.Logging;

namespace EnvelopeGenerator.Server.Client.Services;

/// <summary>
/// Retrieves and persists user UI preferences via the server API.
/// The server stores preferences in the distributed cache (dbo.TBDD_CACHE).
///
/// Client-side memory cache:
///   Preferences are cached in-memory for <see cref="CacheDuration"/> after the first fetch.
///   All reads (GetAsync, GetCachedAsync) are served from cache after the first load.
///   Writes (SaveAsync) update both the cache and the server atomically.
///   Cache is invalidated explicitly via InvalidateCache().
/// </summary>
public class UserPreferencesService(
    IHttpClientFactory clientFactory,
    ILogger<UserPreferencesService> logger)
{
    private static readonly JsonSerializerOptions _jsonOptions = new(JsonSerializerDefaults.Web);
    private const string Endpoint = "/api/UserPreferences";

    // ── Client-side memory cache ──────────────────────────────────────────────
    private static readonly TimeSpan CacheDuration = TimeSpan.FromHours(1);
    private UserPreferencesDto? _cached;
    private DateTimeOffset _cachedAt = DateTimeOffset.MinValue;

    private bool IsCacheValid => _cached is not null
        && DateTimeOffset.UtcNow - _cachedAt < CacheDuration;

    /// <summary>
    /// Returns preferences from in-memory cache if valid; otherwise fetches from the server.
    /// On first call after login this issues one HTTP GET — all subsequent calls within
    /// <see cref="CacheDuration"/> are served from memory with zero network overhead.
    /// </summary>
    public async Task<UserPreferencesDto> GetAsync(CancellationToken ct = default)
    {
        if (IsCacheValid)
            return _cached!;

        try
        {
            var client = clientFactory.CreateClient("EnvelopeGenerator.Server");
            var response = await client.GetAsync(Endpoint, ct);

            // 404 means no preferences saved yet — return and cache defaults.
            if (response.StatusCode == System.Net.HttpStatusCode.NotFound)
            {
                SetCache(new UserPreferencesDto());
                return _cached!;
            }

            response.EnsureSuccessStatusCode();

            var dto = await response.Content.ReadFromJsonAsync<UserPreferencesDto>(_jsonOptions, ct)
                ?? new UserPreferencesDto();

            SetCache(dto);
            return _cached!;
        }
        catch (Exception ex) when (ex is not TaskCanceledException)
        {
            logger.LogError(ex, "Failed to load user preferences from server.");
            throw;
        }
    }

    /// <summary>
    /// Returns the cached preferences without hitting the network.
    /// Returns null if the cache has not been populated yet (before first GetAsync call).
    /// </summary>
    public UserPreferencesDto? GetCached() => IsCacheValid ? _cached : null;

    /// <summary>
    /// Persists the given preferences on the server and updates the in-memory cache.
    /// </summary>
    public async Task<UserPreferencesDto> SaveAsync(UserPreferencesDto preferences, CancellationToken ct = default)
    {
        try
        {
            var client = clientFactory.CreateClient("EnvelopeGenerator.Server");
            var response = await client.PutAsJsonAsync(Endpoint, preferences, _jsonOptions, ct);
            response.EnsureSuccessStatusCode();

            var saved = await response.Content.ReadFromJsonAsync<UserPreferencesDto>(_jsonOptions, ct)
                ?? preferences;

            SetCache(saved);
            return saved;
        }
        catch (Exception ex) when (ex is not TaskCanceledException)
        {
            logger.LogError(ex, "Failed to save user preferences to server.");
            throw;
        }
    }

    /// <summary>
    /// Updates a single grid layout key without a network read.
    /// Uses the in-memory cache for the current state, merges the new layout, then saves.
    /// If the cache is empty (cold start), fetches from server first.
    /// </summary>
    /// <param name="gridKey">Stable grid identifier, e.g. "sender.active-envelopes".</param>
    /// <param name="layoutJson">Opaque layout JSON produced by DxGrid.SaveLayoutAsync().</param>
    public async Task SaveGridLayoutAsync(string gridKey, string layoutJson, CancellationToken ct = default)
    {
        // Use cached copy — avoids a GET before every grid layout save.
        var prefs = GetCached() ?? await GetAsync(ct);
        prefs.GridLayouts[gridKey] = layoutJson;
        await SaveAsync(prefs, ct);
    }

    /// <summary>
    /// Invalidates the in-memory cache, forcing the next GetAsync to fetch from the server.
    /// Call this after logout or when a forced refresh is needed.
    /// </summary>
    public void InvalidateCache()
    {
        _cached = null;
        _cachedAt = DateTimeOffset.MinValue;
    }

    private void SetCache(UserPreferencesDto dto)
    {
        _cached = dto;
        _cachedAt = DateTimeOffset.UtcNow;
    }
}