using System;
using System.Collections.Generic;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Text;
using System.Text.Json;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.Extensions.Logging;

namespace Jellyfin.Plugin.RedditComments.Services;

/// <summary>
/// Minimal Reddit REST API client using the OAuth2 "client credentials" (app-only) flow.
/// Every request goes through the <see cref="RateLimiter"/>.
/// </summary>
public class RedditClient
{
    private const string TokenUrl = "https://www.reddit.com/api/v1/access_token";
    private const string OAuthBaseUrl = "https://oauth.reddit.com";

    private readonly ILogger<RedditClient> _logger;
    private readonly RateLimiter _rateLimiter;
    private readonly HttpClient _httpClient;
    private readonly SemaphoreSlim _tokenLock = new SemaphoreSlim(1, 1);

    private string? _accessToken;
    private DateTime _tokenExpiresAt = DateTime.MinValue;

    /// <summary>
    /// Initializes a new instance of the <see cref="RedditClient"/> class.
    /// </summary>
    /// <param name="logger">Logger.</param>
    /// <param name="rateLimiter">Rate limiter.</param>
    public RedditClient(ILogger<RedditClient> logger, RateLimiter rateLimiter)
    {
        _logger = logger;
        _rateLimiter = rateLimiter;
        _httpClient = new HttpClient();
    }

    private static string UserAgent
    {
        get
        {
            var ua = Plugin.Instance?.Configuration.RedditUserAgent;
            return string.IsNullOrWhiteSpace(ua)
                ? "linux:jellyfin-plugin-reddit-comments:v1.0.0 (by /u/unknown)"
                : ua;
        }
    }

    /// <summary>
    /// Gets a value indicating whether the user has configured Reddit API credentials.
    /// </summary>
    public static bool IsConfigured
    {
        get
        {
            var config = Plugin.Instance?.Configuration;
            return config is not null
                && !string.IsNullOrWhiteSpace(config.RedditClientId)
                && !string.IsNullOrWhiteSpace(config.RedditClientSecret);
        }
    }

    /// <summary>
    /// Performs a rate-limited, authenticated GET against oauth.reddit.com and returns the JSON body.
    /// </summary>
    /// <param name="pathAndQuery">Path and query, e.g. /r/anime/search.json?q=... .</param>
    /// <param name="cancellationToken">Cancellation token.</param>
    /// <returns>The parsed JSON document.</returns>
    public async Task<JsonDocument> GetJsonAsync(string pathAndQuery, CancellationToken cancellationToken = default)
    {
        var token = await GetAccessTokenAsync(cancellationToken).ConfigureAwait(false);

        for (var attempt = 0; attempt < 2; attempt++)
        {
            await _rateLimiter.WaitAsync(cancellationToken).ConfigureAwait(false);

            using var request = new HttpRequestMessage(HttpMethod.Get, OAuthBaseUrl + pathAndQuery);
            request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", token);
            request.Headers.UserAgent.ParseAdd(UserAgent);

            using var response = await _httpClient.SendAsync(request, cancellationToken).ConfigureAwait(false);

            if (response.Headers.Contains("x-ratelimit-remaining"))
            {
                _logger.LogDebug(
                    "Reddit rate limit: used={Used} remaining={Remaining} reset={Reset}s",
                    response.Headers.TryGetValues("x-ratelimit-used", out var used) ? string.Join(',', used) : "?",
                    response.Headers.TryGetValues("x-ratelimit-remaining", out var remaining) ? string.Join(',', remaining) : "?",
                    response.Headers.TryGetValues("x-ratelimit-reset", out var reset) ? string.Join(',', reset) : "?");
            }

            if (response.StatusCode == System.Net.HttpStatusCode.Unauthorized && attempt == 0)
            {
                // Token rejected: force a refresh and retry once.
                _logger.LogWarning("Reddit returned 401, refreshing access token");
                token = await GetAccessTokenAsync(cancellationToken, forceRefresh: true).ConfigureAwait(false);
                continue;
            }

            response.EnsureSuccessStatusCode();
            var stream = await response.Content.ReadAsStreamAsync(cancellationToken).ConfigureAwait(false);
            return await JsonDocument.ParseAsync(stream, cancellationToken: cancellationToken).ConfigureAwait(false);
        }

        throw new HttpRequestException("Reddit request failed after token refresh");
    }

    /// <summary>
    /// Verifies that the configured credentials can obtain an access token.
    /// </summary>
    /// <param name="cancellationToken">Cancellation token.</param>
    /// <returns>True if a token was obtained.</returns>
    public async Task<bool> TestCredentialsAsync(CancellationToken cancellationToken = default)
    {
        await GetAccessTokenAsync(cancellationToken, forceRefresh: true).ConfigureAwait(false);
        return !string.IsNullOrEmpty(_accessToken);
    }

    private async Task<string> GetAccessTokenAsync(CancellationToken cancellationToken, bool forceRefresh = false)
    {
        await _tokenLock.WaitAsync(cancellationToken).ConfigureAwait(false);
        try
        {
            if (!forceRefresh
                && !string.IsNullOrEmpty(_accessToken)
                && DateTime.UtcNow < _tokenExpiresAt)
            {
                return _accessToken;
            }

            var config = Plugin.Instance?.Configuration;
            if (config is null || string.IsNullOrWhiteSpace(config.RedditClientId) || string.IsNullOrWhiteSpace(config.RedditClientSecret))
            {
                throw new InvalidOperationException("Reddit API credentials are not configured. Open Dashboard → Plugins → Reddit Comments and enter a client id and secret.");
            }

            await _rateLimiter.WaitAsync(cancellationToken).ConfigureAwait(false);

            using var request = new HttpRequestMessage(HttpMethod.Post, TokenUrl);
            var credentials = Convert.ToBase64String(Encoding.UTF8.GetBytes(config.RedditClientId.Trim() + ":" + config.RedditClientSecret.Trim()));
            request.Headers.Authorization = new AuthenticationHeaderValue("Basic", credentials);
            request.Headers.UserAgent.ParseAdd(UserAgent);
            request.Content = new FormUrlEncodedContent(
            [
                new KeyValuePair<string, string>("grant_type", "client_credentials")
            ]);

            using var response = await _httpClient.SendAsync(request, cancellationToken).ConfigureAwait(false);
            if (!response.IsSuccessStatusCode)
            {
                var body = await response.Content.ReadAsStringAsync(cancellationToken).ConfigureAwait(false);
                throw new HttpRequestException($"Reddit token request failed with {(int)response.StatusCode}: {body}");
            }

            var stream = await response.Content.ReadAsStreamAsync(cancellationToken).ConfigureAwait(false);
            using var doc = await JsonDocument.ParseAsync(stream, cancellationToken: cancellationToken).ConfigureAwait(false);
            _accessToken = doc.RootElement.GetProperty("access_token").GetString();
            var expiresIn = doc.RootElement.TryGetProperty("expires_in", out var exp) ? exp.GetInt32() : 3600;
            _tokenExpiresAt = DateTime.UtcNow.AddSeconds(expiresIn - 60);

            _logger.LogInformation("Obtained Reddit access token, valid for {Seconds}s", expiresIn);
            return _accessToken!;
        }
        finally
        {
            _tokenLock.Release();
        }
    }
}