// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
 
using System.Diagnostics;
 
namespace System.Net.Http.Headers
{
    public sealed class HttpResponseHeaders : HttpHeaders
    {
        private const int AcceptRangesSlot = 0;
        private const int ProxyAuthenticateSlot = 1;
        private const int ServerSlot = 2;
        private const int VarySlot = 3;
        private const int WwwAuthenticateSlot = 4;
        private const int NumCollectionsSlots = 5;
 
        private object[]? _specialCollectionsSlots;
        private HttpGeneralHeaders? _generalHeaders;
        private readonly bool _containsTrailingHeaders;
 
        #region Response Headers
 
        private T GetSpecializedCollection<T>(int slot, Func<HttpResponseHeaders, T> creationFunc)
        {
            // 5 properties each lazily allocate a collection to store the value(s) for that property.
            // Rather than having a field for each of these, store them untyped in an array that's lazily
            // allocated.  Then we only pay for the 45 bytes for those fields when any is actually accessed.
            object[] collections = _specialCollectionsSlots ??= new object[NumCollectionsSlots];
            return (T)(collections[slot] ??= creationFunc(this)!);
        }
 
        public HttpHeaderValueCollection<string> AcceptRanges =>
            GetSpecializedCollection(AcceptRangesSlot, static thisRef => new HttpHeaderValueCollection<string>(KnownHeaders.AcceptRanges.Descriptor, thisRef));
 
        public TimeSpan? Age
        {
            get { return HeaderUtilities.GetTimeSpanValue(KnownHeaders.Age.Descriptor, this); }
            set { SetOrRemoveParsedValue(KnownHeaders.Age.Descriptor, value); }
        }
 
        public EntityTagHeaderValue? ETag
        {
            get { return (EntityTagHeaderValue?)GetSingleParsedValue(KnownHeaders.ETag.Descriptor); }
            set { SetOrRemoveParsedValue(KnownHeaders.ETag.Descriptor, value); }
        }
 
        public Uri? Location
        {
            get { return (Uri?)GetSingleParsedValue(KnownHeaders.Location.Descriptor); }
            set { SetOrRemoveParsedValue(KnownHeaders.Location.Descriptor, value); }
        }
 
        public HttpHeaderValueCollection<AuthenticationHeaderValue> ProxyAuthenticate =>
            GetSpecializedCollection(ProxyAuthenticateSlot, static thisRef => new HttpHeaderValueCollection<AuthenticationHeaderValue>(KnownHeaders.ProxyAuthenticate.Descriptor, thisRef));
 
        public RetryConditionHeaderValue? RetryAfter
        {
            get { return (RetryConditionHeaderValue?)GetSingleParsedValue(KnownHeaders.RetryAfter.Descriptor); }
            set { SetOrRemoveParsedValue(KnownHeaders.RetryAfter.Descriptor, value); }
        }
 
        public HttpHeaderValueCollection<ProductInfoHeaderValue> Server =>
            GetSpecializedCollection(ServerSlot, static thisRef => new HttpHeaderValueCollection<ProductInfoHeaderValue>(KnownHeaders.Server.Descriptor, thisRef));
 
        public HttpHeaderValueCollection<string> Vary =>
            GetSpecializedCollection(VarySlot, static thisRef => new HttpHeaderValueCollection<string>(KnownHeaders.Vary.Descriptor, thisRef));
 
        public HttpHeaderValueCollection<AuthenticationHeaderValue> WwwAuthenticate =>
            GetSpecializedCollection(WwwAuthenticateSlot, static thisRef => new HttpHeaderValueCollection<AuthenticationHeaderValue>(KnownHeaders.WWWAuthenticate.Descriptor, thisRef));
 
        #endregion
 
        #region General Headers
 
        public CacheControlHeaderValue? CacheControl
        {
            get { return GeneralHeaders.CacheControl; }
            set { GeneralHeaders.CacheControl = value; }
        }
 
        public HttpHeaderValueCollection<string> Connection
        {
            get { return GeneralHeaders.Connection; }
        }
 
        public bool? ConnectionClose
        {
            get { return HttpGeneralHeaders.GetConnectionClose(this, _generalHeaders); } // special-cased to avoid forcing _generalHeaders initialization
            set { GeneralHeaders.ConnectionClose = value; }
        }
 
        public DateTimeOffset? Date
        {
            get { return GeneralHeaders.Date; }
            set { GeneralHeaders.Date = value; }
        }
 
        public HttpHeaderValueCollection<NameValueHeaderValue> Pragma
        {
            get { return GeneralHeaders.Pragma; }
        }
 
        public HttpHeaderValueCollection<string> Trailer
        {
            get { return GeneralHeaders.Trailer; }
        }
 
        public HttpHeaderValueCollection<TransferCodingHeaderValue> TransferEncoding
        {
            get { return GeneralHeaders.TransferEncoding; }
        }
 
        public bool? TransferEncodingChunked
        {
            get { return HttpGeneralHeaders.GetTransferEncodingChunked(this, _generalHeaders); } // special-cased to avoid forcing _generalHeaders initialization
            set { GeneralHeaders.TransferEncodingChunked = value; }
        }
 
        public HttpHeaderValueCollection<ProductHeaderValue> Upgrade
        {
            get { return GeneralHeaders.Upgrade; }
        }
 
        public HttpHeaderValueCollection<ViaHeaderValue> Via
        {
            get { return GeneralHeaders.Via; }
        }
 
        public HttpHeaderValueCollection<WarningHeaderValue> Warning
        {
            get { return GeneralHeaders.Warning; }
        }
 
        #endregion
 
        internal HttpResponseHeaders(bool containsTrailingHeaders = false)
            : base(containsTrailingHeaders ? HttpHeaderType.All ^ HttpHeaderType.Request : HttpHeaderType.General | HttpHeaderType.Response | HttpHeaderType.Custom,
                  HttpHeaderType.Request)
        {
            _containsTrailingHeaders = containsTrailingHeaders;
        }
 
        internal bool ContainsTrailingHeaders => _containsTrailingHeaders;
 
        internal override void AddHeaders(HttpHeaders sourceHeaders)
        {
            base.AddHeaders(sourceHeaders);
            HttpResponseHeaders? sourceResponseHeaders = sourceHeaders as HttpResponseHeaders;
            Debug.Assert(sourceResponseHeaders != null);
 
            // Copy special values, but do not overwrite
            if (sourceResponseHeaders._generalHeaders != null)
            {
                GeneralHeaders.AddSpecialsFrom(sourceResponseHeaders._generalHeaders);
            }
        }
 
        internal override bool IsAllowedHeaderName(HeaderDescriptor descriptor)
        {
            if (!_containsTrailingHeaders)
                return true;
 
            KnownHeader? knownHeader = KnownHeaders.TryGetKnownHeader(descriptor.Name);
            if (knownHeader == null)
                return true;
 
            return (knownHeader.HeaderType & HttpHeaderType.NonTrailing) == 0;
        }
 
        private HttpGeneralHeaders GeneralHeaders => _generalHeaders ??= new HttpGeneralHeaders(this);
    }
}