// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
 
using Microsoft.AspNetCore.Http.Features;
using Microsoft.Extensions.Primitives;
using Microsoft.Net.Http.Headers;
 
namespace Microsoft.AspNetCore.Http;
 
/// <summary>
/// Contains extension methods for modifying the `Trailer` response header
/// and trailing headers in an <see cref="HttpResponse" />.
/// </summary>
public static class ResponseTrailerExtensions
{
    /// <summary>
    /// Adds the given trailer name to the 'Trailer' response header. This must happen before the response headers are sent.
    /// </summary>
    /// <param name="response"></param>
    /// <param name="trailerName"></param>
    public static void DeclareTrailer(this HttpResponse response, string trailerName)
    {
        response.Headers.AppendCommaSeparatedValues(HeaderNames.Trailer, trailerName);
    }
 
    /// <summary>
    /// Indicates if the server supports sending trailer headers for this response.
    /// </summary>
    /// <param name="response"></param>
    /// <returns></returns>
    public static bool SupportsTrailers(this HttpResponse response)
    {
        var feature = response.HttpContext.Features.Get<IHttpResponseTrailersFeature>();
        return feature?.Trailers != null && !feature.Trailers.IsReadOnly;
    }
 
    /// <summary>
    /// Adds the given trailer header to the trailers collection to be sent at the end of the response body.
    /// Check <see cref="SupportsTrailers" /> or an InvalidOperationException may be thrown.
    /// </summary>
    /// <param name="response"></param>
    /// <param name="trailerName"></param>
    /// <param name="trailerValues"></param>
    public static void AppendTrailer(this HttpResponse response, string trailerName, StringValues trailerValues)
    {
        var feature = response.HttpContext.Features.Get<IHttpResponseTrailersFeature>();
        if (feature?.Trailers == null || feature.Trailers.IsReadOnly)
        {
            throw new InvalidOperationException("Trailers are not supported for this response.");
        }
 
        feature.Trailers.Append(trailerName, trailerValues);
    }
}