Async variants of MinBy and MaxBy.

Ix.NET/Source/System.Interactive.Async/Max.cs

@@ -131,6 +131,50 @@ namespace System.Linq
             return ExtremaBy(source, keySelector, (key, minValue) => comparer.Compare(key, minValue), cancellationToken);
         }
 
+        public static Task<IList<TSource>> MaxBy<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, Task<TKey>> keySelector)
+        {
+            if (source == null)
+                throw new ArgumentNullException(nameof(source));
+            if (keySelector == null)
+                throw new ArgumentNullException(nameof(keySelector));
+
+            return source.MaxBy(keySelector, CancellationToken.None);
+        }
+
+        public static Task<IList<TSource>> MaxBy<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, Task<TKey>> keySelector, IComparer<TKey> comparer)
+        {
+            if (source == null)
+                throw new ArgumentNullException(nameof(source));
+            if (keySelector == null)
+                throw new ArgumentNullException(nameof(keySelector));
+            if (comparer == null)
+                throw new ArgumentNullException(nameof(comparer));
+
+            return source.MaxBy(keySelector, comparer, CancellationToken.None);
+        }
+
+        public static Task<IList<TSource>> MaxBy<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, Task<TKey>> keySelector, CancellationToken cancellationToken)
+        {
+            if (source == null)
+                throw new ArgumentNullException(nameof(source));
+            if (keySelector == null)
+                throw new ArgumentNullException(nameof(keySelector));
+
+            return MaxBy(source, keySelector, Comparer<TKey>.Default, cancellationToken);
+        }
+
+        public static Task<IList<TSource>> MaxBy<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, Task<TKey>> keySelector, IComparer<TKey> comparer, CancellationToken cancellationToken)
+        {
+            if (source == null)
+                throw new ArgumentNullException(nameof(source));
+            if (keySelector == null)
+                throw new ArgumentNullException(nameof(keySelector));
+            if (comparer == null)
+                throw new ArgumentNullException(nameof(comparer));
+
+            return ExtremaBy(source, keySelector, (key, minValue) => comparer.Compare(key, minValue), cancellationToken);
+        }
+
         private static async Task<TSource> Max_<TSource>(IAsyncEnumerable<TSource> source, IComparer<TSource> comparer, CancellationToken cancellationToken)
         {
             return (await MaxBy(source, x => x, comparer, cancellationToken).ConfigureAwait(false)).First();

Ix.NET/Source/System.Interactive.Async/Min.cs

@@ -131,6 +131,50 @@ namespace System.Linq
             return ExtremaBy(source, keySelector, (key, minValue) => -comparer.Compare(key, minValue), cancellationToken);
         }
 
+        public static Task<IList<TSource>> MinBy<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, Task<TKey>> keySelector)
+        {
+            if (source == null)
+                throw new ArgumentNullException(nameof(source));
+            if (keySelector == null)
+                throw new ArgumentNullException(nameof(keySelector));
+
+            return source.MinBy(keySelector, CancellationToken.None);
+        }
+
+        public static Task<IList<TSource>> MinBy<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, Task<TKey>> keySelector, IComparer<TKey> comparer)
+        {
+            if (source == null)
+                throw new ArgumentNullException(nameof(source));
+            if (keySelector == null)
+                throw new ArgumentNullException(nameof(keySelector));
+            if (comparer == null)
+                throw new ArgumentNullException(nameof(comparer));
+
+            return source.MinBy(keySelector, comparer, CancellationToken.None);
+        }
+
+        public static Task<IList<TSource>> MinBy<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, Task<TKey>> keySelector, CancellationToken cancellationToken)
+        {
+            if (source == null)
+                throw new ArgumentNullException(nameof(source));
+            if (keySelector == null)
+                throw new ArgumentNullException(nameof(keySelector));
+
+            return MinBy(source, keySelector, Comparer<TKey>.Default, cancellationToken);
+        }
+
+        public static Task<IList<TSource>> MinBy<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, Task<TKey>> keySelector, IComparer<TKey> comparer, CancellationToken cancellationToken)
+        {
+            if (source == null)
+                throw new ArgumentNullException(nameof(source));
+            if (keySelector == null)
+                throw new ArgumentNullException(nameof(keySelector));
+            if (comparer == null)
+                throw new ArgumentNullException(nameof(comparer));
+
+            return ExtremaBy(source, keySelector, (key, minValue) => -comparer.Compare(key, minValue), cancellationToken);
+        }
+
         private static async Task<TSource> Min_<TSource>(IAsyncEnumerable<TSource> source, IComparer<TSource> comparer, CancellationToken cancellationToken)
         {
             return (await MinBy(source, x => x, comparer, cancellationToken).ConfigureAwait(false)).First();
@@ -175,5 +219,45 @@ namespace System.Linq
 
             return result;
         }
+
+        private static async Task<IList<TSource>> ExtremaBy<TSource, TKey>(IAsyncEnumerable<TSource> source, Func<TSource, Task<TKey>> keySelector, Func<TKey, TKey, int> compare, CancellationToken cancellationToken)
+        {
+            var result = new List<TSource>();
+
+            var e = source.GetAsyncEnumerator();
+
+            try
+            {
+                if (!await e.MoveNextAsync(cancellationToken).ConfigureAwait(false))
+                    throw new InvalidOperationException(Strings.NO_ELEMENTS);
+
+                var current = e.Current;
+                var resKey = await keySelector(current).ConfigureAwait(false);
+                result.Add(current);
+
+                while (await e.MoveNextAsync(cancellationToken).ConfigureAwait(false))
+                {
+                    var cur = e.Current;
+                    var key = await keySelector(cur).ConfigureAwait(false);
+
+                    var cmp = compare(key, resKey);
+                    if (cmp == 0)
+                    {
+                        result.Add(cur);
+                    }
+                    else if (cmp > 0)
+                    {
+                        result = new List<TSource> { cur };
+                        resKey = key;
+                    }
+                }
+            }
+            finally
+            {
+                await e.DisposeAsync().ConfigureAwait(false);
+            }
+
+            return result;
+        }
     }
 }