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

using System.Threading;

using Internal.TypeSystem;

namespace System
{
    public partial class String
    {
        public static string Intern(string str)
        {
            ArgumentNullException.ThrowIfNull(str);

            return InternTable.GetOrCreateValue(str);
        }

        public static string IsInterned(string str)
        {
            ArgumentNullException.ThrowIfNull(str);

            string canonicalString;
            if (!InternTable.TryGetValue(str, out canonicalString))
                return null;
            return canonicalString;
        }

        private sealed class StringInternTable : LockFreeReaderHashtable<string, string>
        {
            protected sealed override bool CompareKeyToValue(string key, string value) => key == value;
            protected sealed override bool CompareValueToValue(string value1, string value2) => value1 == value2;
            protected sealed override string CreateValueFromKey(string key) => key;
            protected sealed override int GetKeyHashCode(string key) => key.GetHashCode();
            protected sealed override int GetValueHashCode(string value) => value.GetHashCode();
        }

        private static StringInternTable InternTable
        {
            get
            {
                if (s_lazyInternTable == null)
                {
                    StringInternTable internTable = new StringInternTable();
                    internTable.AddOrGetExisting(string.Empty);
                    Interlocked.CompareExchange<StringInternTable?>(ref s_lazyInternTable, internTable, null);
                }
                return s_lazyInternTable;
            }
        }

        private static volatile StringInternTable? s_lazyInternTable;
    }
}