// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
 
using System;
using System.Diagnostics.CodeAnalysis;
 
namespace System.Net.Http.Headers
{
    // Don't derive from BaseHeaderParser since empty values are not supported. After a ' ' separator a valid value
    // must follow. Also leading separators are not allowed.
    internal sealed class ProductInfoHeaderParser : HttpHeaderParser
    {
        // Unlike most other headers, User-Agent and Server use whitespace as separators
        private const string separator = " ";
 
        internal static readonly ProductInfoHeaderParser SingleValueParser = new ProductInfoHeaderParser(false);
        internal static readonly ProductInfoHeaderParser MultipleValueParser = new ProductInfoHeaderParser(true);
 
        private ProductInfoHeaderParser(bool supportsMultipleValues)
            : base(supportsMultipleValues, separator)
        {
        }
 
        public override bool TryParseValue([NotNullWhen(true)] string? value, object? storeValue, ref int index, [NotNullWhen(true)] out object? parsedValue)
        {
            parsedValue = null;
 
            if (string.IsNullOrEmpty(value) || (index == value.Length))
            {
                return false;
            }
 
            // Skip leading whitespace
            int current = index + HttpRuleParser.GetWhitespaceLength(value, index);
 
            if (current == value.Length)
            {
                return false; // whitespace-only values are not valid
            }
 
            int length = ProductInfoHeaderValue.GetProductInfoLength(value, current, out ProductInfoHeaderValue? result);
 
            if (length == 0)
            {
                return false;
            }
 
            // GetProductInfoLength() already skipped trailing whitespace. No need to do it here again.
            current += length;
 
            // If we have more values, make sure we saw a whitespace before. Values like "product/1.0(comment)" are
            // invalid since there must be a whitespace between the product and the comment value.
            if (current < value.Length)
            {
                // Note that for \r\n to be a valid whitespace, it must be followed by a space/tab. I.e. it's enough if
                // we check whether the char before the next value is space/tab.
                char lastSeparatorChar = value[current - 1];
                if ((lastSeparatorChar != ' ') && (lastSeparatorChar != '\t'))
                {
                    return false;
                }
            }
 
            // Separators for "User-Agent" and "Server" headers are whitespace. This is different from most other headers
            // where comma/semicolon is used as separator.
            index = current;
            parsedValue = result!;
            return true;
        }
    }
}