// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System.Collections;
using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.Text;

namespace System.Net.Http.Headers
{
    internal abstract class HttpHeaderParser
    {
        public const string DefaultSeparator = ", ";
        public static readonly byte[] DefaultSeparatorBytes = ", "u8.ToArray();

        public bool SupportsMultipleValues { get; }

        public string Separator { get; }

        public byte[] SeparatorBytes { get; }

        // If ValueType implements Equals() as required, there is no need to provide a comparer. A comparer is needed
        // e.g. if we want to compare strings using case-insensitive comparison.
        public virtual IEqualityComparer? Comparer => null;

        protected HttpHeaderParser(bool supportsMultipleValues)
        {
            SupportsMultipleValues = supportsMultipleValues;
            Separator = DefaultSeparator;
            SeparatorBytes = DefaultSeparatorBytes;
        }

        protected HttpHeaderParser(bool supportsMultipleValues, string separator) : this(supportsMultipleValues)
        {
            Debug.Assert(!string.IsNullOrEmpty(separator));
            Debug.Assert(Ascii.IsValid(separator));

            if (supportsMultipleValues)
            {
                Separator = separator;
                SeparatorBytes = Encoding.ASCII.GetBytes(separator);
            }
        }

        // If a parser supports multiple values, a call to ParseValue/TryParseValue should return a value for 'index'
        // pointing to the next non-whitespace character after a delimiter. E.g. if called with a start index of 0
        // for string "value , second_value", then after the call completes, 'index' must point to 's', i.e. the first
        // non-whitespace after the separator ','.
        public abstract bool TryParseValue(string? value, object? storeValue, ref int index, [NotNullWhen(true)] out object? parsedValue);

        public object ParseValue(string? value, object? storeValue, ref int index)
        {
            // Index may be value.Length (e.g. both 0). This may be allowed for some headers (e.g. Accept but not
            // allowed by others (e.g. Content-Length). The parser has to decide if this is valid or not.
            Debug.Assert((value == null) || ((index >= 0) && (index <= value.Length)));

            // If a parser returns 'null', it means there was no value, but that's valid (e.g. "Accept: "). The caller
            // can ignore the value.
            if (!TryParseValue(value, storeValue, ref index, out object? result))
            {
                throw new FormatException(SR.Format(System.Globalization.CultureInfo.InvariantCulture, SR.net_http_headers_invalid_value,
                    value == null ? "<null>" : value.Substring(index)));
            }
            return result;
        }

        // If ValueType is a custom header value type (e.g. NameValueHeaderValue) it already implements ToString() correctly.
        // However for existing types like int, byte[], DateTimeOffset we can't override ToString(). Therefore the
        // parser provides a ToString() virtual method that can be overridden by derived types to correctly serialize
        // values (e.g. byte[] to Base64 encoded string).
        public virtual string? ToString(object value)
        {
            Debug.Assert(value != null);

            return value.ToString();
        }
    }
}