// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
 
using System.Linq;
using Microsoft.AspNetCore.Http;
using Microsoft.Net.Http.Headers;
 
namespace Microsoft.AspNetCore.Localization;
 
/// <summary>
/// Determines the culture information for a request via the value of the Accept-Language header.
/// </summary>
public class AcceptLanguageHeaderRequestCultureProvider : RequestCultureProvider
{
    /// <summary>
    /// The maximum number of values in the Accept-Language header to attempt to create a <see cref="System.Globalization.CultureInfo"/>
    /// from for the current request.
    /// Defaults to <c>3</c>.
    /// </summary>
    public int MaximumAcceptLanguageHeaderValuesToTry { get; set; } = 3;
 
    /// <inheritdoc />
    public override Task<ProviderCultureResult?> DetermineProviderCultureResult(HttpContext httpContext)
    {
        ArgumentNullException.ThrowIfNull(httpContext);
 
        var acceptLanguageHeader = httpContext.Request.GetTypedHeaders().AcceptLanguage;
 
        if (acceptLanguageHeader == null || acceptLanguageHeader.Count == 0)
        {
            return NullProviderCultureResult;
        }
 
        var languages = acceptLanguageHeader.AsEnumerable();
 
        if (MaximumAcceptLanguageHeaderValuesToTry > 0)
        {
            // We take only the first configured number of languages from the header and then order those that we
            // attempt to parse as a CultureInfo to mitigate potentially spinning CPU on lots of parse attempts.
            languages = languages.Take(MaximumAcceptLanguageHeaderValuesToTry);
        }
 
        var orderedLanguages = languages.OrderByDescending(h => h, StringWithQualityHeaderValueComparer.QualityComparer)
            .Select(x => x.Value).ToList();
 
        if (orderedLanguages.Count > 0)
        {
            return Task.FromResult<ProviderCultureResult?>(new ProviderCultureResult(orderedLanguages));
        }
 
        return NullProviderCultureResult;
    }
}