CompareSets extensions, IEnumerable<T>.CompareToSet, FullOuterJoin extension

CompareSetsExtensions.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace CompareSetsExtensions
{
    public static class CompareSetsExtensions
    {
        public static IEnumerable<TResult> FullOuterJoin<TLeft, TRight, TKey, TResult>(
            IEnumerable<TLeft> left,
            IEnumerable<TRight> right,
            Func<TLeft,TKey> getKeyLeft,
            Func<TRight, TKey> getKeyRight,
            Func<(TKey key,TLeft left,TRight right, bool hasLeft, bool hasRight), TResult> getResult,
            IEqualityComparer<TKey> cmpByKey=null)
        {
            var lookup = left.Select(i=>(key:getKeyLeft(i), hasLeft:true, hasRight:false, left:i, right:default(TRight)))
                .Concat(
                         right.Select(i=>(key:getKeyRight(i), hasLeft:false, hasRight:true, left:default(TLeft), right:i))
                )
                .ToLookup(i=>i.key, i=>i, cmpByKey)
                ;
            foreach (var kvp in lookup)
            {
                var l = kvp.SingleOrDefault(i=>i.hasLeft);
                var r = kvp.SingleOrDefault(i=>i.hasRight);
                yield return getResult((kvp.Key, l.left, r.right, l.hasLeft, r.hasRight));
            }
        }


        public static IEnumerable<TResult> FullOuterJoin<T, TKey, TResult>(
            IEnumerable<T> left,
            IEnumerable<T> right,
            Func<T,TKey> getKey,
            Func<(TKey key,T left,T right, bool hasLeft, bool hasRight), TResult> getResult,
            IEqualityComparer<TKey> cmpByKey=null
            )
        {
            return FullOuterJoin<T, T, TKey, TResult>(left, right, getKey, getKey, getResult, cmpByKey);
        }

        public static IEnumerable<TResult> FullOuterJoin<T, TResult>(
            IEnumerable<T> left,
            IEnumerable<T> right,
            Func<(T left, T right, bool hasLeft, bool hasRight), TResult> resultSelector,
            IEqualityComparer<T> keyComparer=null)
        {
            return FullOuterJoin<T, T, TResult>(
                left,
                right,
                i=>i,
                r=>resultSelector((r.left, r.right, r.hasLeft, r.hasRight)),
                keyComparer);
        }

        public static void CompareToSet<T>(
            this IEnumerable<T> oldItems,
            IEnumerable<T> newItems,
            Action<T> onAddedItem,
            Action<T> onRemovedItem,
            Action<T, T> onExistInBothItem = null,
            IEqualityComparer<T> comparer = null
            )
        {
            var fullJoin = FullOuterJoin(oldItems, newItems, j=>j, comparer);
            foreach (var fj in fullJoin)
            {
                if(fj.hasLeft && fj.hasRight)
                    onExistInBothItem?.Invoke(fj.left, fj.right);
                else if(fj.hasLeft)
                    onRemovedItem?.Invoke(fj.left);
                else if(fj.hasRight)
                    onAddedItem?.Invoke(fj.right);
            }
        }
    }

}