// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
 
using Microsoft.Extensions.Primitives;
 
namespace Microsoft.Net.Http.Headers;
 
internal abstract class BaseHeaderParser<T> : HttpHeaderParser<T>
{
    protected BaseHeaderParser(bool supportsMultipleValues)
        : base(supportsMultipleValues)
    {
    }
 
    protected abstract int GetParsedValueLength(StringSegment value, int startIndex, out T? parsedValue);
 
    public sealed override bool TryParseValue(StringSegment value, ref int index, out T? parsedValue)
    {
        parsedValue = default;
 
        // If multiple values are supported (i.e. list of values), then accept an empty string: The header may
        // be added multiple times to the request/response message. E.g.
        //  Accept: text/xml; q=1
        //  Accept:
        //  Accept: text/plain; q=0.2
        if (StringSegment.IsNullOrEmpty(value) || (index == value.Length))
        {
            return SupportsMultipleValues;
        }
 
        var current = HeaderUtilities.GetNextNonEmptyOrWhitespaceIndex(value, index, SupportsMultipleValues,
            out var separatorFound);
 
        if (separatorFound && !SupportsMultipleValues)
        {
            return false; // leading separators not allowed if we don't support multiple values.
        }
 
        if (current == value.Length)
        {
            if (SupportsMultipleValues)
            {
                index = current;
            }
            return SupportsMultipleValues;
        }
 
        var length = GetParsedValueLength(value, current, out var result);
 
        if (length == 0)
        {
            return false;
        }
 
        current = current + length;
        current = HeaderUtilities.GetNextNonEmptyOrWhitespaceIndex(value, current, SupportsMultipleValues,
            out separatorFound);
 
        // If we support multiple values and we've not reached the end of the string, then we must have a separator.
        if ((separatorFound && !SupportsMultipleValues) || (!separatorFound && (current < value.Length)))
        {
            return false;
        }
 
        index = current;
        parsedValue = result;
        return true;
    }
}