// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
 
namespace Microsoft.AspNetCore.Components.Rendering;
 
/// <summary>
/// This comparer is optimized for use with dictionaries where the great majority of insertions/lookups
/// don't match existing entries. For example, when building a dictionary of almost entirely unique keys.
/// It's faster than the normal string comparer in this case because it doesn't use string.GetHashCode,
/// and hence doesn't have to consider every character in the string.
///
/// This primary scenario is <see cref="RenderTreeBuilder.ProcessDuplicateAttributes(int)"/>, which needs
/// to detect when one attribute is overriding another, but in the vast majority of cases attributes don't
/// actually override each other.
/// </summary>
internal sealed class SimplifiedStringHashComparer : IEqualityComparer<string>
{
    public static readonly SimplifiedStringHashComparer Instance = new SimplifiedStringHashComparer();
 
    public bool Equals(string? x, string? y)
    {
        return string.Equals(x, y, StringComparison.OrdinalIgnoreCase);
    }
 
    public int GetHashCode(string key)
    {
        var keyLength = key.Length;
        if (keyLength > 0)
        {
            // Consider just the length and middle and last characters.
            // This will produce a distinct result for a sufficiently large
            // proportion of attribute names.
            return unchecked(
                char.ToLowerInvariant(key[keyLength - 1])
                + 31 * char.ToLowerInvariant(key[keyLength / 2])
                + 961 * keyLength);
        }
        else
        {
            return default;
        }
    }
}