using System;
using System.Collections.Generic;
using System.Net.Http;
using System.Threading.Tasks;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Newtonsoft.Json;

namespace Microsoft.AspNetCore.Mvc.Filters
{
	/// <summary>
	/// Custom filter attribute to use Google's reCaptcha (v3).
	/// <br/>
	/// Usage: [ServiceFilter(typeof(GoogleReCaptchaAttribute))]
	/// </summary>
	/// <remarks>
	/// appsettings.json:
	/// <br/>
	/// <code>
	/// {<br/>
	///   [...]<br/>
	///   "Google": {<br/>
	///     "ReCaptcha": {<br/>
	///       "PrivateKey": "__private reCaptcha key__",<br/>
	///       "PublicKey": "__public reCaptcha key__"<br/>
	///     }<br/>
	///   }<br/>
	/// }
	/// </code>
	/// <br/>
	/// The score from google can be found on HttpContext.Items[GoogleReCaptchaAttribute.ScoreKey].
	/// </remarks>
	[System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage]
	public class GoogleReCaptchaAttribute : ActionFilterAttribute
	{
		/// <summary>
		/// The error key used in <see cref="ActionContext.ModelState"/>.
		/// </summary>
		public const string ErrorKey = "GoogleReCaptcha";

		/// <summary>
		/// The key used in forms submitted to the backend.
		/// </summary>
		public const string ResponseTokenKey = "g-recaptcha-response";

		/// <summary>
		/// The key used in <see cref="Http.HttpContext.Items"/> to transport the score (0 - bot, 1 - human).
		/// </summary>
		public const string ScoreKey = "GoogleReCaptchaScore";

		private const string VerificationUrl = "https://www.google.com/recaptcha/api/siteverify";

		private string privateKey;

		/// <summary>
		/// Executes the validattion in background.
		/// </summary>
		/// <param name="context">The action context.</param>
		/// <param name="next">The following action delegate.</param>
		/// <returns>An awaitable task.</returns>
		public override async Task OnActionExecutionAsync(ActionExecutingContext context, ActionExecutionDelegate next)
		{
			var configuration = context.HttpContext.RequestServices.GetService<IConfiguration>();
			privateKey = configuration?.GetValue<string>("Google:ReCaptcha:PrivateKey");

			if (string.IsNullOrWhiteSpace(privateKey))
				return;

			await DoValidation(context);
			await base.OnActionExecutionAsync(context, next);
		}

		private async Task DoValidation(ActionExecutingContext context)
		{
			if (!context.HttpContext.Request.HasFormContentType)
				return;

			var token = context.HttpContext.Request.Form[ResponseTokenKey];
			if (string.IsNullOrWhiteSpace(token))
			{
				context.ModelState.TryAddModelError(ErrorKey, "No token to validate Google reCaptcha");
				return;
			}

			await Validate(context, token);
		}

		private async Task Validate(ActionExecutingContext context, string token)
		{
			using var httpClient = new HttpClient();
			var param = new Dictionary<string, string>
			{
				{ "secret", privateKey },
				{ "response", token }
			};
			var response = await httpClient.PostAsync(VerificationUrl, new FormUrlEncodedContent(param));
			string json = await response.Content.ReadAsStringAsync();

			var result = JsonConvert.DeserializeObject<Response>(json);
			if (result?.Success != true)
			{
				context.ModelState.TryAddModelError(ErrorKey, "Google reCaptcha verification failed");
				context.HttpContext.Items[ScoreKey] = null;
			}
			else
			{
				context.HttpContext.Items[ScoreKey] = result.Score;
			}
		}

		private class Response
		{
			public bool Success { get; set; }

			public decimal Score { get; set; }

			public string Action { get; set; }

			[JsonProperty("challenge_ts")]
			public DateTime Timestamp { get; set; }

			public string Hostname { get; set; }

			[JsonProperty("error-codes")]
			public List<string> ErrorCodes { get; set; }
		}
	}
}