CachingComparer.cs

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using System.Collections.Generic;
 
namespace System.Linq;
 
internal abstract class CachingComparer<TElement>
{
    internal abstract int Compare(TElement element, bool cacheLower);
 
    internal abstract void SetElement(TElement element);
}
internal class CachingComparer<TElement, TKey> : CachingComparer<TElement>
{
    protected readonly Func<TElement, TKey> _keySelector;
 
    protected readonly IComparer<TKey> _comparer;
 
    protected readonly bool _descending;
 
    protected TKey _lastKey;
 
    public CachingComparer(Func<TElement, TKey> keySelector, IComparer<TKey> comparer, bool descending)
    {
        _keySelector = keySelector;
        _comparer = comparer;
        _descending = descending;
    }
 
    internal override int Compare(TElement element, bool cacheLower)
    {
        TKey val = _keySelector(element);
        int num = (_descending ? _comparer.Compare(_lastKey, val) : _comparer.Compare(val, _lastKey));
        if (cacheLower == num < 0)
        {
            _lastKey = val;
        }
        return num;
    }
 
    internal override void SetElement(TElement element)
    {
        _lastKey = _keySelector(element);
    }
}